Showing papers in "Atmospheric Chemistry and Physics in 2020"
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Research Triangle Park1, Foundation for Research & Technology – Hellas2, University of Washington3, University of Michigan4, National Center for Atmospheric Research5, University of East Anglia6, Colorado State University7, University of Maryland, Baltimore County8, Leibniz Association9, University of Crete10, Columbia University11, University of Illinois at Urbana–Champaign12, Nankai University13, Georgia Institute of Technology14, Tsinghua University15, Pacific Northwest National Laboratory16, McGill University17
TL;DR: This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets, including recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.
Abstract: . Acidity, defined as pH, is a central component of aqueous
chemistry. In the atmosphere, the acidity of condensed phases (aerosol
particles, cloud water, and fog droplets) governs the phase partitioning of
semivolatile gases such as HNO3 , NH3 , HCl, and organic acids and
bases as well as chemical reaction rates. It has implications for the
atmospheric lifetime of pollutants, deposition, and human health. Despite
its fundamental role in atmospheric processes, only recently has this field
seen a growth in the number of studies on particle acidity. Even with this
growth, many fine-particle pH estimates must be based on thermodynamic model
calculations since no operational techniques exist for direct measurements.
Current information indicates acidic fine particles are ubiquitous, but
observationally constrained pH estimates are limited in spatial and temporal
coverage. Clouds and fogs are also generally acidic, but to a lesser degree
than particles, and have a range of pH that is quite sensitive to
anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient
ammonia. Historical measurements indicate that cloud and fog droplet pH has
changed in recent decades in response to controls on anthropogenic
emissions, while the limited trend data for aerosol particles indicate
acidity may be relatively constant due to the semivolatile nature of the
key acids and bases and buffering in particles. This paper reviews and
synthesizes the current state of knowledge on the acidity of atmospheric
condensed phases, specifically particles and cloud droplets. It includes
recommendations for estimating acidity and pH, standard nomenclature, a
synthesis of current pH estimates based on observations, and new model
calculations on the local and global scale.
305 citations
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TL;DR: In this paper, the space-time extremely randomized trees (STET) model was enhanced by integrating updated spatiotemporal information and additional auxiliary data to improve the spatial resolution and overall accuracy of PM 2.5 estimates across mainland China.
Abstract: . Fine particulate matter with aerodynamic diameters ≤2.5 µ m
(PM 2.5 ) has adverse effects on human health and the atmospheric
environment. The estimation of surface PM 2.5 concentrations has made
intensive use of satellite-derived aerosol products. However, it has been a great challenge to obtain high-quality and high-resolution PM 2.5 data from both ground and satellite observations, which is essential to monitor air pollution over small-scale areas such as metropolitan regions. Here, the space–time
extremely randomized trees (STET) model was enhanced by integrating updated
spatiotemporal information and additional auxiliary data to improve the
spatial resolution and overall accuracy of PM 2.5 estimates across
China. To this end, the newly released Moderate Resolution Imaging
Spectroradiometer Multi-Angle Implementation of Atmospheric Correction AOD product, along with meteorological, topographical and land-use data and
pollution emissions, was input to the STET model, and daily 1 km PM 2.5
maps for 2018 covering mainland China were produced. The STET model performed
well, with a high out-of-sample (out-of-station) cross-validation coefficient
of determination ( R2 ) of 0.89 (0.88), a low root-mean-square error of
10.33 (10.93) µ g m −3 , a small mean absolute error of 6.69 (7.15) µ g m −3 and a small mean relative error of 21.28 % (23.69 %).
In particular, the model captured well the PM 2.5 concentrations at both
regional and individual site scales. The North China Plain, the Sichuan
Basin and Xinjiang Province always featured high PM 2.5 pollution
levels, especially in winter. The STET model outperformed most models
presented in previous related studies, with a strong predictive power (e.g.,
monthly R2=0.80 ), which can be used to estimate historical
PM 2.5 records. More importantly, this study provides a new approach
for obtaining high-resolution and high-quality PM 2.5 dataset across mainland
China (i.e., ChinaHighPM 2.5 ), important for air pollution studies
focused on urban areas.
240 citations
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TL;DR: In this paper, the authors used a multiple linear regression model to fit ozone to meteorological variables and found that meteorology played a significant but not dominant role in the 2013-2019 ozone trend.
Abstract: . Surface ozone data from the Chinese Ministry of Ecology
and Environment (MEE) network show sustained increases across the country
over the 2013–2019 period. Despite Phase 2 of the Clean Air Action Plan targeting
ozone pollution, ozone was higher in 2018–2019 than in previous years. The
mean summer 2013–2019 trend in maximum 8 h average (MDA8) ozone was 1.9 ppb a −1 ( p ) across China and 3.3 ppb a −1 ( p ) over the North China Plain (NCP). Fitting ozone to meteorological
variables with a multiple linear regression model shows that meteorology
played a significant but not dominant role in the 2013–2019 ozone trend,
contributing 0.70 ppb a −1 ( p ) across China and 1.4 ppb a −1 ( p=0.02 ) over the NCP. Rising June–July temperatures over the NCP
were the main meteorological driver, particularly in recent years
(2017–2019), and were associated with increased foehn winds. NCP data for
2017–2019 show a 15 % decrease in fine particulate matter (PM 2.5 )
that may be driving the continued anthropogenic increase in ozone, as well as
unmitigated emissions of volatile organic compounds (VOCs). VOC emission
reductions, as targeted by Phase 2 of the Chinese Clean Air Action Plan, are
needed to reverse the increase in ozone.
228 citations
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TL;DR: A long-term historical emission inventory of air and climate pollutants in East, Southeast, and South Asia from 1950-2015 was developed as the Regional Emission inventory in ASIA version 3.1.
Abstract: . A long-term historical emission inventory of air and climate pollutants in East, Southeast, and South Asia from 1950–2015 was developed as the Regional Emission inventory in ASia version 3.1 (REASv3.1). REASv3.1 provides details of emissions from major anthropogenic sources for each country and its sub-regions and also provides monthly gridded data with 0.25° × 0.25° resolution. The average total emissions in Asia during 1950–1955 and from 2010–2015 (growth rates in these 60 years) are as follows: SO2: 3.15 Tg, 42.4 Tg (13.5); NOx: 1.83 Tg, 47.6 Tg (26.0); CO: 62.2 Tg, 319 Tg (5.13); non-methane volatile organic compounds: 9.14 Tg, 61.8 Tg (6.77); NH3: 7.99 Tg, 31.3 Tg (3.92); CO2: 1.12 Pg, 18.3 Pg (16.3); PM10: 5.76 Tg, 28.4 Tg (4.92); PM2.5: 4.52 Tg, 20.3 Tg (4.50); black carbon: 0.751 Tg, 3.38 Tg (4.51); and organic carbon: 2.62 Tg, 6.92 Tg (2.64). Clearly, all the air pollutant emissions in Asia increased significantly during these six decades, but situations were different among countries and regions. Due to China's rapid economic growth in recent years, its relative contribution to emissions in Asia has been the largest. However, most pollutant species reached their peaks by 2015 and the growth rates of other species was found to be reduced or almost zero. On the other hand, air pollutant emissions from India showed an almost continuous increasing trend. As a result, the relative ratio of emissions of India to that of Asia have increased recently. The trend observed in Japan was different from the rest of Asia. In Japan, emissions increased rapidly during 1950s–1970s, which reflected the economic situation of the period; however, most emissions decreased from their peak values, which were approximately 40 years ago, due to the introduction of regulations and laws for air pollution. Similar features were found in the Republic of Korea and Taiwan. In the case of other Asian countries, air pollutant emissions generally showed an increase along with economic growth and motorization. Trends and spatial distribution of air pollutants in Asia are becoming complicated. Datasets of REASv3.1, including table of emissions by countries and sub-regions for major sectors and fuel types, and monthly gridded data with 0.25° × 0.25° resolution for major source categories are available through the following URL: http://www.nies.go.jp/REAS/ .
180 citations
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TL;DR: In this article, the authors conducted numerical experiments with an up-to-date regional chemical transport model to assess the contribution of changes in meteorological conditions and anthropogenic emissions to the summer ozone level from 2013 to 2017 in various regions of China.
Abstract: . China has suffered from increasing levels of ozone pollution in
urban areas despite the implementation of various stringent emission
reduction measures since 2013. In this study, we conducted numerical
experiments with an up-to-date regional chemical transport model to assess
the contribution of the changes in meteorological conditions and
anthropogenic emissions to the summer ozone level from 2013 to 2017 in
various regions of China. The model can faithfully reproduce the observed
meteorological parameters and air pollutant concentrations and capture the
increasing trend in the surface maximum daily 8 h average (MDA8) ozone
( O3 ) from 2013 to 2017. The emission-control measures implemented by
the government induced a decrease in MDA8 O3 levels in rural areas but
an increase in urban areas. The meteorological influence on the ozone trend
varied by region and by year and could be comparable to or even more
significant than the impact of changes in anthropogenic emissions.
Meteorological conditions can modulate the ozone concentration via direct
(e.g., increasing reaction rates at higher temperatures) and indirect (e.g.,
increasing biogenic emissions at higher temperatures) effects. As an
essential source of volatile organic compounds that contributes to ozone
formation, the variation in biogenic emissions during summer varied across
regions and was mainly affected by temperature. China's midlatitude areas
(25 to 40 ∘ N) experienced a significant decrease in
MDA8 O3 due to a decline in biogenic emissions, especially for the
Yangtze River Delta and Sichuan Basin regions in 2014 and 2015. In contrast,
in northern (north of 40 ∘ N) and southern (south of 25 ∘ N) China, higher temperatures after 2013 led to an increase in MDA8 O3
via an increase in biogenic emissions. We also assessed the individual
effects of changes in temperature, specific humidity, wind field, planetary
boundary layer height, clouds, and precipitation on ozone levels from 2013
to 2017. The results show that the wind field change made a significant
contribution to the increase in surface ozone over many parts of China. The
long-range transport of ozone and its precursors from outside the modeling
domain also contributed to the increase in MDA8 O3 in China, especially
on the Qinghai–Tibetan Plateau (an increase of 1 to 4 ppbv). Our study
represents the most comprehensive and up-to-date analysis of the impact of
changes in meteorology on ozone across China and highlights the importance
of considering meteorological variations when assessing the effectiveness of
emission control on changes in the ozone levels in recent years.
166 citations
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International Institute for Applied Systems Analysis1, University of Leeds2, Goddard Space Flight Center3, Universities Space Research Association4, University of Reading5, École Polytechnique6, University of Paris7, University of Toulouse8, Environment Canada9, Geophysical Fluid Dynamics Laboratory10, National Institute for Environmental Studies11, University of Tokyo12, Met Office13, National Center for Atmospheric Research14, Goddard Institute for Space Studies15, Norwegian Meteorological Institute16, University of Cologne17, Stockholm University18, Commonwealth Scientific and Industrial Research Organisation19, National Oceanic and Atmospheric Administration20, Cooperative Institute for Research in Environmental Sciences21
TL;DR: In this article, the authors evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Comparisons Project (RFMIP).
Abstract: The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (±0.23) W m−2, comprised of 1.81 (±0.09) W m−2 from CO2, 1.08 (± 0.21) W m−2 from other well-mixed greenhouse gases, −1.01 (± 0.23) W m−2 from aerosols and −0.09 (±0.13) W m−2 from land use change. Quoted uncertainties are 1 standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W m−2. The majority of the remaining 0.21 W m−2 is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol–cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from −0.63 to −1.37 W m−2, exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4×CO2 forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming.
142 citations
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TL;DR: In this article, the seasonality and trends of tropospheric NO2 columns over central-eastern China were analyzed using the GEOS-Chemchemical transport model, and they found no significant trend of the NOx lifetime in summer, supporting the emission trend reported by the MEIC.
Abstract: . Satellite observations of tropospheric NO2 columns
are extensively used to infer trends in anthropogenic emissions of nitrogen
oxides ( NO x ≡ NO + NO 2 ), but this may be complicated by
trends in NOx lifetime. Here we use 2004–2018 observations from the
Ozone Monitoring Instrument (OMI) satellite-based instrument (QA4ECV and POMINO v2 retrievals) to examine
the seasonality and trends of tropospheric NO2 columns over
central–eastern China, and we interpret the results with the GEOS-Chem
chemical transport model. The observations show a factor of 3 increase in
NO2 columns from summer to winter, which we explain in GEOS-Chem as
reflecting a longer NOx lifetime in winter than in summer (21 h versus 5.9 h in 2017). The 2005–2018 summer trends of OMI NO2 closely follow the trends in the Multi-resolution Emission Inventory for China (MEIC), with a rise over the 2005–2011 period and a 25 % decrease since. We find in GEOS-Chem no significant trend of the NOx lifetime in summer, supporting the emission trend reported by the MEIC. The winter trend of OMI
NO2 is steeper than in summer over the entire period, which we
attribute to a decrease in NOx lifetime at lower NOx emissions. Half of the NOx sink in winter is from N2O5 hydrolysis, which counterintuitively becomes more efficient as NOx emissions decrease due
to less titration of ozone at night. The formation of organic nitrates also
becomes an increasing sink of NOx as NOx emissions decrease but emissions of volatile organic compounds (VOCs) do not.
136 citations
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TL;DR: In this article, the authors evaluated the effect of changes in multi-pollutant emissions from anthropogenic activities on surface ozone (O3) levels during the same period by using an up-to-date regional chemical transport model (WRF-CMAQ)driven by an interannual anthropogenic emission inventory.
Abstract: . The Chinese government launched the Air Pollution Prevention and
Control Action Plan in 2013, and various stringent measures have since been
implemented, which have resulted in significant decreases in emissions and
ambient concentrations of primary pollutants such as SO2 , NOx , and
particulate matter (PM). However, surface ozone ( O3 ) concentrations
have still been increasing in urban areas across the country. In a previous
analysis, we examined in detail the roles of meteorological variation during
2013–2017 in the summertime surface O3 trend in various regions of
China. In this study, we evaluated the effect of changes in multi-pollutant
emissions from anthropogenic activities on O3 levels during the same
period by using an up-to-date regional chemical transport model (WRF-CMAQ)
driven by an interannual anthropogenic emission inventory. The Community Multiscale Air
Quality (CMAQ) model
was improved with regard to heterogeneous reactions of reactive gases on
aerosol surfaces, which led to better model performance in reproducing the
ambient concentrations of those gases. The model simulations showed that the
maximum daily 8 h average (MDA8) O3 mixing ratio in urban areas
increased by 0.46 ppbv per year ( ppbv a−1 ) ( p=0.001 ) from 2013 to
2017. In contrast, a slight decrease in MDA8 O3 by 0.17 ppbv a−1
( p=0.005 ) in rural areas was predicted, mainly attributable to the
NOx emission reduction. The effects of changes in individual pollutant
emissions on O3 were also simulated. The reduction of NOx emission
increased the O3 levels in urban areas due to the nonlinear
NOx and volatile organic compound (VOC) chemistry and decreasing aerosol
effects; the slight increase in VOC emissions enhanced the O3 levels;
the reduction of PM emissions increased the O3 levels by enhancing the
photolysis rates and reducing the loss of reactive gases on aerosol
surfaces; and the reduction of SO2 emissions resulted in a drastic
decrease in sulfate concentrations, which increased O3 through aerosol
effects. In contrast to the unfavorable effect of the above changes in
pollutant emissions on efforts to reduce surface O3 , the reduction of
CO emissions did help to decrease the O3 level in recent years. The
dominant cause of increasing O3 due to changes in anthropogenic
emissions varied geographically. In Beijing, NOx and PM emission
reductions were the two largest causes of the O3 increase; in Shanghai,
the reduction of NOx and increase in VOC emissions were the two major
causes; in Guangzhou, NOx reduction was the primary cause; in
Chengdu, the PM and SO2 emission decreases contributed most to the
O3 increase. Regarding the effects of decreasing concentrations of
aerosols, the drop in heterogeneous uptake of reactive gases – mainly
HO2 and O3 – was found to be more important than the increase in
photolysis rates. The adverse effect of the reductions of NOx ,
SO2 , and PM emissions on O3 abatement in Beijing, Shanghai,
Guangzhou, and Chengdu would have been avoided if the anthropogenic VOCs
emission had been reduced by 24 %, 23 %, 20 %, and 16 %,
respectively, from 2013 to 2017. Our analysis revealed that the NOx
reduction in recent years has helped to contain the total O3 production
in China. However, to reduce O3 levels in major urban and industrial
areas, VOC emission controls should be added to the current
NOx - SO2 -PM policy.
122 citations
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TL;DR: Wang et al. as mentioned in this paper used regression analysis and weather classification to assess the impacts of local and synoptic meteorology on daily variance in surface ozone in eastern China in summer during 2013-2018.
Abstract: . Ozone pollution in China is influenced by meteorological processes on
multiple scales. Using regression analysis and weather classification, we
statistically assess the impacts of local and synoptic meteorology on daily
variability in surface ozone in eastern China in summer during 2013–2018. In
this period, summertime surface ozone in eastern China (20–42 ∘ N, 110–130 ∘ E) is among the highest in the world, with regional means of 73.1
and 114.7 µ g m −3 , respectively, in daily mean and daily maximum
8 h average. Through developing a multiple linear regression (MLR) model
driven by local and synoptic weather factors, we establish a quantitative
linkage between the daily mean ozone concentrations and meteorology in the
study region. The meteorology described by the MLR can explain
∼43 % of the daily variability in summertime surface ozone
across eastern China. Among local meteorological factors, relative humidity
is the most influential variable in the center and south of eastern China,
including the Yangtze River Delta and the Pearl River Delta regions, while
temperature is the most influential variable in the north, covering the
Beijing–Tianjin–Hebei region. To further examine the synoptic influence of
weather conditions explicitly, six predominant synoptic weather patterns
(SWPs) over eastern China in summer are objectively identified using the
self-organizing map clustering technique. The six SWPs are formed under the
integral influence of the East Asian summer monsoon, the western Pacific
subtropical high, the Meiyu front, and the typhoon activities. On average,
regionally, two SWPs bring about positive ozone anomalies (1.1 µ g m −3
or 1.7 % and 2.7 µ g m −3 or 4.6 %), when eastern China is under a weak cyclone system or under the prevailing
southerly wind. The impact of SWPs on the daily variability in surface ozone
varies largely within eastern China. The maximum impact can reach ±8 µ g m −3 or ±16 % of the daily mean in some areas. A combination of the regression and the clustering approaches suggests a strong performance of the MLR in predicting the sensitivity of surface ozone in eastern China to the variation of synoptic weather. Our assessment highlights the importance of meteorology in modulating ozone pollution over China.
111 citations
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TL;DR: In this article, the authors explored the molecular composition of light-absorbing organic aerosol, or brown carbon (BrC), sampled at the Missoula Fire Sciences laboratory as a part of the FIREX Fall 2016 lab intensive.
Abstract: . To better understand the effects of wildfires on air quality and
climate, it is important to assess the occurrence of chromophoric compounds
in smoke and characterize their optical properties. This study explores the
molecular composition of light-absorbing organic aerosol, or brown carbon
(BrC), sampled at the Missoula Fire Sciences laboratory as a part of the
FIREX Fall 2016 lab intensive. A total of 12 biomass fuels from different plant
types were tested, including gymnosperm (coniferous) and angiosperm
(flowering) plants and different ecosystem components such as duff, litter,
and canopy. Emitted biomass burning organic aerosol (BBOA) particles were
collected onto Teflon filters and analyzed offline using high-performance
liquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer
(HPLC–PDA–HRMS). Separated BrC chromophores were classified by their
retention times, absorption spectra, integrated absorbance in the near-UV
and visible spectral range (300–700 nm), and chemical formulas from the
accurate m∕z measurements. BrC chromophores were grouped into the following
classes and subclasses: lignin-derived products, which include lignin pyrolysis
products; distillation products, which include coumarins and flavonoids;
nitroaromatics; and polycyclic aromatic hydrocarbons (PAHs). The observed
classes and subclasses were common across most fuel types, although specific BrC
chromophores varied based on plant type (gymnosperm or angiosperm) and
ecosystem component(s) burned. To study the stability of the observed BrC
compounds with respect to photodegradation, BBOA particle samples were
irradiated directly on filters with near UV (300–400 nm) radiation, followed
by extraction and HPLC–PDA–HRMS analysis. Lifetimes of individual BrC
chromophores depended on the fuel type and the corresponding combustion
condition. Lignin-derived and flavonoid classes of BrC generally had
the longest lifetimes with respect to UV photodegradation. Moreover,
lifetimes for the same type of BrC chromophores varied depending on biomass
fuel and combustion conditions. While individual BrC chromophores
disappeared on a timescale of several days, the overall light absorption by
the sample persisted longer, presumably because the condensed-phase
photochemical processes converted one set of chromophores into another
without complete photobleaching or from undetected BrC chromophores that
photobleached more slowly. To model the effect of BrC on climate, it is
important to understand the change in the overall absorption coefficient
with time. We measured the equivalent atmospheric lifetimes of the overall
BrC absorption coefficient, which ranged from 10 to 41 d, with subalpine
fir having the shortest lifetime and conifer canopies, i.e., juniper, having
the longest lifetime. BrC emitted from biomass fuel loads encompassing
multiple ecosystem components (litter, shrub, canopy) had absorption
lifetimes on the lower end of the range. These results indicate that
photobleaching of BBOA by condensed-phase photochemistry is
relatively slow. Competing chemical aging mechanisms, such as heterogeneous
oxidation by OH, may be more important for controlling the rate of BrC
photobleaching in BBOA.
109 citations
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TL;DR: In this paper, an updated comparison of modeled and observed Δ17O (nitrate) and a reassessment of modeled nitrateformation pathways is presented. But the authors do not consider the effect of photolysis of aerosol nitrate on the relative importance of nitrate formation pathways.
Abstract: . The formation of inorganic nitrate is the main sink for nitrogen
oxides ( NOx = NO + NO2 ). Due to the importance of NOx for
the formation of tropospheric oxidants such as the hydroxyl radical (OH) and
ozone, understanding the mechanisms and rates of nitrate formation is
paramount for our ability to predict the atmospheric lifetimes of most
reduced trace gases in the atmosphere. The oxygen isotopic composition of
nitrate ( Δ17O (nitrate)) is determined by the relative
importance of NOx sinks and thus can provide an observational
constraint for NOx chemistry. Until recently, the ability to utilize
Δ17O (nitrate) observations for this purpose was hindered by our
lack of knowledge about the oxygen isotopic composition of ozone ( Δ17O(O3) ). Recent and spatially widespread observations of Δ17O(O3) motivate an updated comparison of modeled and
observed Δ17O (nitrate) and a reassessment of modeled nitrate
formation pathways. Model updates based on recent laboratory studies of
heterogeneous reactions render dinitrogen pentoxide ( N2O5 )
hydrolysis as important as NO2 + OH (both 41 %) for global
inorganic nitrate production near the surface (below 1 km altitude). All
other nitrate production mechanisms individually represent less than 6 %
of global nitrate production near the surface but can be dominant locally.
Updated reaction rates for aerosol uptake of NO2 result in significant
reduction of nitrate and nitrous acid (HONO) formed through this pathway in
the model and render NO2 hydrolysis a negligible pathway for nitrate
formation globally. Although photolysis of aerosol nitrate may have
implications for NOx , HONO, and oxidant abundances, it does not
significantly impact the relative importance of nitrate formation pathways.
Modeled Δ17O (nitrate) ( 28.6±4.5 ‰)
compares well with the average of a global compilation of observations ( 27.6±5.0 ‰) when assuming Δ17O(O3) = 26 ‰, giving confidence in the model's
representation of the relative importance of ozone versus HOx ( = OH + HO2 + RO2 ) in NOx cycling and nitrate formation on the
global scale.
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TL;DR: In this paper, the authors compared six biomass burning (BB) emissions datasets for 2008 globally as well as in 14 regions in Southern Hemisphere Africa (SHAF) and South America (SHSA), and compared the simulated AOD with observed AOD from the AErosol-Obotic NETwork (AERONET) and the Multiangle Imaging Spectro-Radiometer (MISR) in the 14 regions during 2008.
Abstract: . Aerosols from biomass burning (BB) emissions are poorly constrained in
global and regional models, resulting in a high level of uncertainty in
understanding their impacts. In this study, we compared six BB aerosol
emission datasets for 2008 globally as well as in 14 regions. The six BB
emission datasets are (1) GFED3.1 (Global Fire Emissions Database version 3.1), (2) GFED4s (GFED version 4 with small fires), (3) FINN1.5 (FIre
INventory from NCAR version 1.5), (4) GFAS1.2 (Global Fire Assimilation
System version 1.2), (5) FEER1.0 (Fire Energetics and Emissions Research
version 1.0), and (6) QFED2.4 (Quick Fire Emissions Dataset version 2.4).
The global total emission amounts from these six BB emission datasets
differed by a factor of 3.8, ranging from 13.76 to 51.93 Tg for organic
carbon and from 1.65 to 5.54 Tg for black carbon. In most of the regions,
QFED2.4 and FEER1.0, which are based on satellite observations of fire
radiative power (FRP) and constrained by aerosol optical depth (AOD) data
from the Moderate Resolution Imaging Spectroradiometer (MODIS), yielded
higher BB aerosol emissions than the rest by a factor of 2–4. By comparison, the BB
aerosol emissions estimated from GFED4s and GFED3.1, which are based on satellite
burned-area data, without AOD constraints, were at the low end of the range.
In order to examine the sensitivity of model-simulated AOD to the different
BB emission datasets, we ingested these six BB emission datasets separately
into the same global model, the NASA Goddard Earth Observing System (GEOS)
model, and compared the simulated AOD with observed AOD from the AErosol
RObotic NETwork (AERONET) and the Multiangle Imaging SpectroRadiometer
(MISR) in the 14 regions during 2008. In Southern Hemisphere Africa (SHAF)
and South America (SHSA), where aerosols tend to be clearly dominated by
smoke in September, the simulated AOD values were underestimated in almost all
experiments compared to MISR, except for the QFED2.4 run in SHSA. The
model-simulated AOD values based on FEER1.0 and QFED2.4 were the closest to the
corresponding AERONET data, being, respectively, about 73 % and 100 % of
the AERONET observed AOD at Alta Floresta in SHSA and about 49 % and
46 % at Mongu in SHAF. The simulated AOD based on the other four BB
emission datasets accounted for only ∼50 % of the AERONET
AOD at Alta Floresta and ∼20 % at Mongu. Overall, during
the biomass burning peak seasons, at most of the selected AERONET sites in
each region, the AOD values simulated with QFED2.4 were the highest and closest to
AERONET and MISR observations, followed closely by FEER1.0. However, the
QFED2.4 run tends to overestimate AOD in the region of SHSA, and the QFED2.4
BB emission dataset is tuned with the GEOS model. In contrast, the FEER1.0
BB emission dataset is derived in a more model-independent fashion and is
more physically based since its emission coefficients are independently
derived at each grid box. Therefore, we recommend the FEER1.0 BB emission
dataset for aerosol-focused hindcast experiments in the two biomass-burning-dominated regions in the Southern Hemisphere, SHAF, and SHSA (as well as in
other regions but with lower confidence). The differences between these six
BB emission datasets are attributable to the approaches and input data used
to derive BB emissions, such as whether AOD from satellite observations is
used as a constraint, whether the approaches to parameterize the fire
activities are based on burned area, FRP, or active fire count, and which
set of emission factors is chosen.
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TL;DR: In this article, the microbial community associated with primary sugar compounds (SCs) in PM10 (particles smaller than 10 µm) and their main sources in the surrounding environment (soils and vegetation) were investigated.
Abstract: . Primary biogenic organic aerosols (PBOA) represent a major fraction of coarse organic matter (OM) in air. Despite their implication in many atmospheric processes and human health problems, we surprisingly know little about PBOA characteristics (i.e., composition, dominant sources, and contribution to airborne-particles). In addition, specific primary sugar compounds (SCs) are generally used as markers of PBOA associated with Bacteria and Fungi but our knowledge of microbial communities associated with atmospheric particulate matter (PM) remains incomplete. This work aimed at providing a comprehensive understanding of the microbial fingerprints associated with SCs in PM10 (particles smaller than 10 µm) and their main sources in the surrounding environment (soils and vegetation). An intensive study was conducted on PM10 collected at rural background site located in an agricultural area in France. We combined high-throughput sequencing of Bacteria and Fungi with detailed physicochemical characterization of PM10, soils and plant samples, and monitored meteorology and agricultural activities throughout the sampling period. Results shows that in summer SCs in PM10 are a major contributor of OM in air, representing 0.8 to 13.5 % of OM mass. SCs concentrations are clearly determined by the abundance of only a few specific airborne Fungi and Bacteria Taxa. These microbial are significantly enhanced in leaf over soil samples. Interestingly, the overall community structure of Bacteria and Fungi are similar within PM10 and leaf samples and significantly distinct between PM10 and soil samples, indicating that surrounding vegetation are the major source of SC-associated microbial taxa in PM10 in rural area.
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TL;DR: In this article, the Dynamic Projection model for Emissions in China (DPEC) is developed to explore China's future anthropogenic emission pathways. But, the model is designed to integrate the energy system model, emission inventory model, dynamic projection model, and parameterized scheme of Chinese policies.
Abstract: . Future trends in air pollution and greenhouse gas (GHG)
emissions for China are of great concern to the community. A set of global
scenarios regarding future socio-economic and climate developments, combining
shared socio-economic pathways (SSPs) with climate forcing outcomes as
described by the Representative Concentration Pathways (RCPs), was created
by the Intergovernmental Panel on Climate Change (IPCC). Chinese researchers have also developed various emission scenarios by considering detailed local environmental and climate policies. However, a comprehensive scenario set connecting SSP–RCP scenarios with local policies and representing dynamic emission changes under local policies is still missing. In this work, to fill this gap, we developed a dynamic projection model, the Dynamic Projection model for Emissions in China (DPEC), to explore China's
future anthropogenic emission pathways. The DPEC is designed to
integrate the energy system model, emission inventory model, dynamic
projection model, and parameterized scheme of Chinese policies. The model
contains two main modules, an energy-model-driven activity rate projection
module and a sector-based emission projection module. The activity rate
projection module provides the standardized and unified future energy
scenarios after reorganizing and refining the outputs from the energy system
model. Here we use a new China-focused version of the Global Change
Assessment Model (GCAM-China) to project future energy demand and supply in
China under different SSP–RCP scenarios at the provincial level. The
emission projection module links a bottom-up emission inventory model, the
Multi-resolution Emission Inventory for China (MEIC), to GCAM-China and
accurately tracks the evolution of future combustion and production technologies
and control measures under different environmental policies. We developed
technology-based turnover models for several key emitting sectors (e.g.
coal-fired power plants, key industries, and on-road transportation
sectors), which can simulate the dynamic changes in the unit/vehicle fleet
turnover process by tracking the lifespan of each unit/vehicle on an annual
basis. With the integrated modelling framework, we connected five SSP scenarios
(SSP1–5), five RCP scenarios (RCP8.5, 7.0, 6.0, 4.5, and 2.6), and three
pollution control scenarios (business as usual, BAU; enhanced control
policy, ECP; and best health effect, BHE) to produce six combined emission
scenarios. With those scenarios, we presented a wide range of China's future
emissions to 2050 under different development and policy pathways. We found
that, with a combination of strong low-carbon policy and air pollution
control policy (i.e. SSP1-26-BHE scenario), emissions of major air
pollutants (i.e. SO2 , NOx , PM 2.5 , and non-methane volatile organic compounds – NMVOCs) in China will
be reduced by 34 %–66 % in 2030 and 58 %–87 % in 2050 compared to 2015. End-of-pipe control measures are more effective for reducing air pollutant emissions before 2030, while low-carbon policy will play a more important role
in continuous emission reduction until 2050. In contrast, China's emissions
will remain at a high level until 2050 under a reference scenario without active
actions (i.e. SSP3-70-BAU). Compared to similar scenarios set from the
CMIP6 (Coupled Model Intercomparison Project Phase 6), our estimates of
emission ranges are much lower than the estimates from the harmonized CMIP6 emissions dataset in
2020–2030, but their emission ranges become similar in the year 2050.
••
TL;DR: In this article, an extensive, quality checked data archive, the Cirrus Guide II in-situ data base, is created from airborne in-Situ measurements during 150 flights in 24 campaigns and contains meteorological parameters, IWC, Nice, Rice, RHice and H2O for each of the flights.
Abstract: . This study presents airborne in-situ and satellite remote sensing climatologies of cirrus clouds and humidity. The climatologies serve as a guide to the properties of cirrus clouds, with the new in-situ data base providing detailed insights into boreal mid-latitudes and the tropics, while the satellite-borne data set offers a global overview. To this end, an extensive, quality checked data archive, the Cirrus Guide II in-situ data base, is created from airborne in-situ measurements during 150 flights in 24 campaigns. The archive contains meteorological parameters, IWC, Nice, Rice, RHice and H2O for each of the flights (IWC: ice water content, Nice: number concentration of ice crystals, Rice: ice crystal mean mass radius, RHice: relative humidity with respect to ice, H2O: water vapor mixing ratio). Depending on the specific parameter, the data base has extended by about a factor of 5–10 compared to the previous studies of Schiller et al. (2008), JGR, and Kramer et al. (2009), ACP. One result of our investigations is, that across all latitudes, the thicker liquid origin cirrus predominate at lower altitudes, while at higher altitudes the thinner in-situ cirrus prevail. Further, exemplary investigations of the radiative characteristics of in-situ and liquid origin cirrus show that the in-situ origin cirrus only slightly warm the atmosphere, while liquid origin cirrus have a strong cooling effect. An important step in completing the Cirrus Guide II is the provision of the global cirrus Nice climatology, derived by means of the retrieval algorithm DARDAR-Nice from ten years of cirrus remote sensing observations from satellite. The in-situ data base has been used to evaluate and adjust the satellite observations. We found that the global median Nice from satellite observations is almost two times higher than the in-situ median and increases slightly with decreasing temperature. Nice medians of the most frequentl occuring cirrus sorted by geographical regions are highest in the tropics, followed by austral/boreal mid-latitudes, Antarctica and the Arctic. Since the satellite climatologies enclose the entire spatial and temporal Nice occurrence, we could deduce that half of the cirrus are located in the lowest, warmest cirrus layer and contain a significant amount of liquid origin cirrus. A specific highlight of the study is the in-situ observations of tropical tropopause layer (TTL) cirrus and humidity in the Asian monsoon anticyclone and the comparison to the surrounding tropics. In the convectively very active Asian monsoon, peak values of Nice and IWC of 30 ppmv and 1000 ppmv are detected around the cold point tropopause (CPT). Above the CPT, ice particles that are convectively injected can locally add a significant amount of water available for exchange with the stratosphere. We found IWCs of up to 8 ppmv in the Asian monsoon in comparison to only 2 ppmv in the surrounding tropics. Also, the highest RHice inside of the clouds as well as in clear sky (120–150 %) are observed around and above the CPT. We attribute this to the high amount of H2O (3–5 ppmv) in comparison to 1.5–3 ppmv in other tropical regions. The supersaturations above the CPT suggest that the water exchange with the stratosphere is 10–20 % higher than expected in regions of weak convective activity and up to about 50 % in the Asian monsoon.
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Met Office1, University of California, Riverside2, Goddard Institute for Space Studies3, Columbia University4, National Center for Atmospheric Research5, Geophysical Fluid Dynamics Laboratory6, University of Toulouse7, ETH Zurich8, Norwegian Meteorological Institute9, Kyushu University10, China Meteorological Administration11
TL;DR: In this article, the 6th Coupled Model Intercomparison Project (CMIP6) presents an opportunity to analyse the change in air pollutants simulated by the current generation of climate and Earth system models that include a representation of chemistry and aerosols (particulate matter).
Abstract: . Poor air quality is currently responsible for large impacts on human health across the world. In addition, the air pollutants, ozone (O3) and particulate matter less than 2.5 microns in diameter (PM2.5), are also radiatively active in the atmosphere and can influence Earth’s climate. It is important to understand the effect of air quality and climate mitigation measures over the historical period and in different future scenarios to ascertain any impacts from air pollutants on both climate and human health. The 6th Coupled Model Intercomparison Project (CMIP6) presents an opportunity to analyse the change in air pollutants simulated by the current generation of climate and Earth system models that include a representation of chemistry and aerosols (particulate matter). The shared socio-economic pathways (SSPs) used within CMIP6 encompass a wide range of trajectories in precursor emissions and climate change, allowing for an improved analysis of future changes to air pollutants. Firstly, we conduct an evaluation of the available CMIP6 models against surface observations of O3 and PM2.5. CMIP6 models show a consistent overestimation of observed surface O3 concentrations across most regions and in most seasons, with a large diversity in simulated values over northern hemisphere continental regions. Conversely, observed surface PM2.5 concentrations are consistently underestimated by CMIP6 models, particularly for the northern hemisphere winter months, with the largest model diversity near natural emission source regions. Over the historical period (1850–2014) large increases in both surface O3 and PM2.5 are simulated by the CMIP6 models across all regions, particularly over the mid to late 20th Century when anthropogenic emissions increase markedly. Large regional historical changes are simulated for both pollutants, across East and South Asia, with an increase of up to 40 ppb for O3 and 12 µg m-3 for PM2.5. In future scenarios containing strong air quality and climate mitigation measures (ssp126), air pollutants are substantially reduced across all regions by up to 15 ppb for O3 and 12 µg m-3 for PM2.5. However, for scenarios that encompass weak action on mitigating climate and reducing air pollutant emissions (ssp370), increases of both surface O3 (up 10 ppb) and PM2.5 (up to 8 µg m-3) are simulated across most regions. Although, for regions like North America and Europe small reductions in PM2.5 are simulated in this scenario. A comparison of simulated regional changes in both surface O3 and PM2.5 from individual CMIP6 models highlights important differences due to the interaction of aerosols, chemistry, climate and natural emission sources within models. The prediction of regional air pollutant concentrations from the latest climate and Earth system models used within CMIP6 shows that the particular future trajectory of climate and air quality mitigation measures could have important consequences for regional air quality, human health and near-term climate. Differences between individual models emphasises the importance of understanding how future Earth system feedbacks influence natural emission sources.
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TL;DR: In this article, two different model OA schemes within the standard GEOS-Chemchemical transport model were evaluated against a suite of 15 globally distributed airborne campaigns from 2008 to 2017, primarily in the spring and summer seasons.
Abstract: . Chemical transport models have historically struggled to
accurately simulate the magnitude and variability of observed organic
aerosol (OA), with previous studies demonstrating that models significantly
underestimate observed concentrations in the troposphere. In this study, we
explore two different model OA schemes within the standard GEOS-Chem
chemical transport model and evaluate the simulations against a suite of 15
globally distributed airborne campaigns from 2008 to 2017, primarily in the
spring and summer seasons. These include the ATom, KORUS-AQ, GoAmazon,
FRAPPE, SEAC4RS, SENEX, DC3, CalNex, OP3, EUCAARI, ARCTAS and ARCPAC
campaigns and provide broad coverage over a diverse set of
atmospheric composition regimes – anthropogenic, biogenic, pyrogenic and
remote. The schemes include significant differences in their treatment of
the primary and secondary components of OA – a “simple scheme” that models
primary OA (POA) as non-volatile and takes a fixed-yield approach to
secondary OA (SOA) formation and a “complex scheme” that simulates POA as
semi-volatile and uses a more sophisticated volatility basis set approach
for non-isoprene SOA, with an explicit aqueous uptake mechanism to model
isoprene SOA. Despite these substantial differences, both the simple and
complex schemes perform comparably across the aggregate dataset in their
ability to capture the observed variability (with an R2 of 0.41 and
0.44, respectively). The simple scheme displays greater skill in minimizing
the overall model bias (with a normalized mean bias of 0.04 compared to 0.30 for the
complex scheme). Across both schemes, the model skill in reproducing
observed OA is superior to previous model evaluations and approaches the
fidelity of the sulfate simulation within the GEOS-Chem model. However,
there are significant differences in model performance across different
chemical source regimes, classified here into seven categories.
Higher-resolution nested regional simulations indicate that model resolution
is an important factor in capturing variability in highly localized
campaigns, while also demonstrating the importance of well-constrained
emissions inventories and local meteorology, particularly over Asia. Our
analysis suggests that a semi-volatile treatment of POA is superior to a
non-volatile treatment. It is also likely that the complex scheme
parameterization overestimates biogenic SOA at the global scale. While this
study identifies factors within the SOA schemes that likely contribute to OA
model bias (such as a strong dependency of the bias in the complex scheme on
relative humidity and sulfate concentrations), comparisons with the skill of
the sulfate aerosol scheme in GEOS-Chem indicate the importance of other
drivers of bias, such as emissions, transport and deposition, that are
exogenous to the OA chemical scheme.
••
TL;DR: In this article, the optical properties and molecular characteristics of water-soluble and methanol-ssoluble organic carbon (OC; MSOC) emitted from simulated combustion of biomass and coal and vehicle emissions were investigated using ultraviolet-visible (UV-vis)Spectroscopy, excitation-emission matrix (EEM) spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry.
Abstract: . Brown carbon (BrC) plays an essential impact on radiative forcing
due to its ability to absorb sunlight. In this study, the optical properties
and molecular characteristics of water-soluble and methanol-soluble organic
carbon (OC; MSOC) emitted from the simulated combustion of biomass and coal
fuels and vehicle emissions were investigated using ultraviolet–visible (UV–vis)
spectroscopy, excitation–emission matrix (EEM) spectroscopy, and
Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)
coupled with electrospray ionization (ESI). The results showed that these
smoke aerosol samples from biomass burning (BB) and coal combustion (CC) had
a higher mass absorption efficiency at 365 nm (MAE 365 ) than vehicle
emission samples. A stronger MAE 365 value was also found in MSOC than
water-soluble organic carbon (WSOC), indicating low polar compounds would
possess a higher light absorption capacity. Parallel factor (PARAFAC) analysis
identified six types of fluorophores (P1–6) in WSOC including two
humic-like substances (HULIS-1) (P1 and P6), three protein-like substances
(PLOM) (P2, P3, and P5), and one undefined substance (P4). HULIS-1 was mainly from
aging vehicle exhaust particles; P2 was only abundant in BB aerosols; P3 was
ubiquitous in all tested aerosols; P4 was abundant in fossil burning
aerosols; and P5 was more intense in fresh vehicle exhaust particles. The
MSOC chromophores (six components; C1–6) exhibited consistent
characteristics with WSOC, suggesting the method could be used to indicate
the origins of chromophores. FT-ICR mass spectra showed that CHO and CHON
were the most abundant components of WSOC, but S-containing compounds
appeared in a higher abundance in CC aerosols and vehicle emissions than BB
aerosols, while considerably fewer S-containing compounds largely with CHO
and CHON were detected in MSOC. The unique formulas of different sources in the van Krevelen (VK) diagram presented different molecular distributions.
To be specific, BB aerosols with largely CHO and CHON had a medium H ∕ C and
low O ∕ C ratio, while CC aerosols and vehicle emissions largely with
S-containing compounds had an opposite H ∕ C and O ∕ C ratio. Moreover, the
light absorption capacity of WSOC and MSOC was positively associated with
the unsaturation degree and molecular weight in the source aerosols. The
above results are potentially applicable to further studies on the EEM-based or
molecular-characteristic-based source apportionment of chromophores in
atmospheric aerosols.
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TL;DR: In this paper, the authors used machine-learning models to estimate the business-as-usual NO2 mixing ratios that would have been observed in the absence of lockdown, and quantified the so-called meteorology-normalized NO2 reductions induced by the lockdown measures by comparing the estimated business as-usual values with the observed NO 2 mixing ratios.
Abstract: . The spread of the new coronavirus SARS-CoV-2 that causes COVID-19 forced the Spanish Government to implement extensive lockdown measures to reduce the number of hospital admissions, starting on 14 March 2020. Over the following days and weeks, strong reductions in nitrogen dioxide ( NO2 ) pollution were reported in many regions of Spain. A substantial part of these reductions was obviously due to decreased local and regional anthropogenic emissions. Yet, the confounding effect of meteorological variability hinders a reliable quantification of the lockdown's impact upon the observed pollution levels. Our study uses machine-learning (ML) models fed by meteorological data along with other time features to estimate the “business-as-usual” NO2 mixing ratios that would have been observed in the absence of the lockdown. We then quantify the so-called meteorology-normalized NO2 reductions induced by the lockdown measures by comparing the estimated business-as-usual values with the observed NO2 mixing ratios. We applied this analysis for a selection of urban background and traffic stations covering the more than 50 Spanish provinces and islands. The ML predictive models were found to perform remarkably well in most locations, with an overall bias, root mean square error and correlation of +4 %, 29 % and 0.86, respectively. During the period of study, from the enforcement of the state of alarm in Spain on 14 March to 23 April, we found the lockdown measures to be responsible for a 50 % reduction in NO2 levels on average over all provinces and islands. The lockdown in Spain has gone through several phases with different levels of severity with respect to mobility restrictions. As expected, the meteorology-normalized change in NO2 was found to be stronger during phase II (the most stringent phase) and phase III of the lockdown than during phase I. In the largest agglomerations, where both urban background and traffic stations were available, a stronger meteorology-normalized NO2 change is highlighted at traffic stations compared with urban background sites. Our results are consistent with foreseen (although still uncertain) changes in anthropogenic emissions induced by the lockdown. We also show the importance of taking the meteorological variability into account for accurately assessing the impact of the lockdown on NO2 levels, in particular at fine spatial and temporal scales. Meteorology-normalized estimates such as those presented here are crucial to reliably quantify the health implications of the lockdown due to reduced air pollution.
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TL;DR: In this article, the formation pathways of nitrate and sulfate in different polluted cases, hourly samples of PM 2.5 were collected continuously in Beijing during the wintertime of 2016.
Abstract: . A vast area in China is currently going through severe haze episodes with
drastically elevated concentrations of PM 2.5 in winter. Nitrate and
sulfate are the main constituents of PM 2.5 , but their formations via
NO2 and SO2 oxidation are still not comprehensively understood,
especially under different pollution or atmospheric relative humidity (RH)
conditions. To elucidate formation pathways of nitrate and sulfate in
different polluted cases, hourly samples of PM 2.5 were collected
continuously in Beijing during the wintertime of 2016. Three serious
pollution cases were identified reasonably during the sampling period, and
the secondary formations of nitrate and sulfate were found to make a
dominant contribution to atmospheric PM 2.5 under the relatively high RH
condition. The significant correlation between NOR, NOR = NO 3 - / ( NO 3 - + NO 2 ) , and [NO2]2 × [O3] during the nighttime under the RH≥60 % condition indicated
that the heterogeneous hydrolysis of N2O5 involving aerosol
liquid water was responsible for the nocturnal formation of nitrate at the
extremely high RH levels. The more often coincident trend of NOR and [HONO] × [DR] (direct radiation) × [ NO2 ] compared to its occurrence with [Dust] × [ NO2 ] during the daytime under the 30 % RH 60 % condition provided convincing evidence that the gas-phase
reaction of NO2 with OH played a pivotal role in the diurnal formation
of nitrate at moderate RH levels. The extremely high mean values of SOR, SOR = SO 4 2 - / ( SO 4 2 - + SO 2 ) , during the whole day
under the RH≥60 % condition could be ascribed to the evident
contribution of SO2 aqueous-phase oxidation to the formation of sulfate
during the severe pollution episodes. Based on the parameters measured in
this study and the known sulfate production rate calculation method, the
oxidation pathway of H2O2 rather than NO2 was found to
contribute greatly to the aqueous-phase formation of sulfate.
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TL;DR: In this paper, a gridded monthly AOD merged product for the period 1995-2017 is presented, which provides a long-term perspective on AOD changes over different regions of the world and users are encouraged to use this dataset.
Abstract: . Satellite instruments provide a vantage point for studying aerosol
loading consistently over different regions of the world. However, the
typical lifetime of a single satellite platform is on the order of 5–15 years; thus, for climate studies, the use of multiple satellite sensors
should be considered. Discrepancies exist between aerosol optical depth
(AOD) products due to differences in their information content, spatial and
temporal sampling, calibration, cloud masking, and algorithmic assumptions.
Users of satellite-based AOD time-series are confronted with the challenge
of choosing an appropriate dataset for the intended application. In this
study, 16 monthly AOD products obtained from different satellite sensors and
with different algorithms were inter-compared and evaluated against Aerosol
Robotic Network (AERONET) monthly AOD. Global and regional analyses
indicate that products tend to agree qualitatively on the annual, seasonal
and monthly timescales but may be offset in magnitude. Several approaches
were then investigated to merge the AOD records from different satellites
and create an optimised AOD dataset. With few exceptions, all merging
approaches lead to similar results, indicating the robustness and stability
of the merged AOD products. We introduce a gridded monthly AOD merged
product for the period 1995–2017. We show that the quality of the merged
product is as least as good as that of individual products. Optimal
agreement of the AOD merged product with AERONET further demonstrates the
advantage of merging multiple products. This merged dataset provides a
long-term perspective on AOD changes over different regions of the world,
and users are encouraged to use this dataset.
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TL;DR: In this paper, a regional fully coupled meteorology-chemistry model, Weather Research and Forecasting model with Chemistry (WRF-Chem), was employed to study the seasonality of ozone pollution and its sources in both China and India.
Abstract: . A regional fully coupled meteorology–chemistry model, Weather Research and
Forecasting model with Chemistry (WRF-Chem), was employed to study the
seasonality of ozone ( O3 ) pollution and its sources in both China and
India. Observations and model results suggest that O3 in the North
China Plain (NCP), Yangtze River Delta (YRD), Pearl River Delta (PRD), and
India exhibit distinctive seasonal features, which are linked to the
influence of summer monsoons. Through a factor separation approach, we
examined the sensitivity of O3 to individual anthropogenic, biogenic,
and biomass burning emissions. We found that summer O3 formation in
China is more sensitive to industrial and biogenic sources than to other
source sectors, while the transportation and biogenic sources are more
important in all seasons for India. Tagged simulations suggest that local
sources play an important role in the formation of the summer O3 peak
in the NCP, but sources from Northwest China should not be neglected to
control summer O3 in the NCP. For the YRD region, prevailing winds and
cleaner air from the ocean in summer lead to reduced transport from polluted
regions, and the major source region in addition to local sources is
Southeast China. For the PRD region, the upwind region is replaced by
contributions from polluted PRD as autumn approaches, leading to an autumn
peak. The major upwind regions in autumn for the PRD are YRD (11 %) and
Southeast China (10 %). For India, sources in North India are more
important than sources in the south. These analyses emphasize the relative
importance of source sectors and regions as they change with seasons,
providing important implications for O3 control strategies.
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TL;DR: In this paper, the authors compare the CMIP5 and CMIP6 model ensembles and find a systematic shift between CMIP eras to be unexplained as a process of random sampling from modeled forcing and feedback distributions.
Abstract: . The Earth's equilibrium climate sensitivity (ECS) to a doubling of atmospheric CO2 , along with the transient climate response (TCR) and
greenhouse gas emissions pathways, determines the amount of future warming. Coupled climate models have in the past been important tools to estimate
and understand ECS. ECS estimated from Coupled Model Intercomparison Project Phase 5 (CMIP5) models lies between 2.0 and 4.7 K (mean of
3.2 K ), whereas in the latest CMIP6 the spread has increased to 1.8–5.5 K (mean of 3.7 K ), with 5 out of 25 models exceeding
5 K . It is thus pertinent to understand the causes underlying this shift. Here we compare the CMIP5 and CMIP6 model ensembles and find
a systematic shift between CMIP eras to be unexplained as a process of random sampling from modeled forcing and feedback distributions. Instead,
shortwave feedbacks shift towards more positive values, in particular over the Southern Ocean, driving the shift towards larger ECS values in many
of the models. These results suggest that changes in model treatment of mixed-phase cloud processes and changes to Antarctic sea ice representation
are likely causes of the shift towards larger ECS. Somewhat surprisingly, CMIP6 models exhibit less historical warming than CMIP5 models, despite an
increase in TCR between CMIP eras (mean TCR increased from 1.7 to 1.9 K ). The evolution of the warming suggests, however, that several of
the CMIP6 models apply too strong aerosol cooling, resulting in too weak mid-20th century warming compared to the instrumental
record.
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TL;DR: In this article, a new identification of mechanisms of secondary ice production (SIP) based on the observation of small faceted ice crystals with typical sizes smaller than 100 µm was proposed.
Abstract: . This study attempts a new identification of mechanisms of
secondary ice production (SIP) based on the observation of small faceted ice
crystals (hexagonal plates or columns) with typical sizes smaller than 100 µm . Due to their young age, such small ice crystals can be used as
tracers for identifying the conditions for SIP. Observations reported here
were conducted in oceanic tropical mesoscale convective systems (MCSs) and midlatitude frontal clouds in the temperature range from 0 to
−15 ∘ C and heavily seeded by aged ice particles. It was found that
in both MCSs and frontal clouds, SIP was observed right above the melting
layer and extended to higher altitudes with colder temperatures. The roles
of six possible mechanisms to generate the SIP particles are assessed using
additional observations. In most observed SIP cases, small secondary ice
particles spatially correlated with liquid-phase, vertical updrafts and aged
rimed ice particles. However, in many cases, neither graupel nor liquid
drops were observed in the SIP regions, and therefore, the conditions for an
active Hallett–Mossop process were not met. In many cases, large
concentrations of small pristine ice particles were observed right above the melting layer, starting at temperatures as warm as −0.5 ∘ C. It is
proposed that the initiation of SIP above the melting layer is stimulated by
the recirculation of large liquid drops through the melting layer with
convective turbulent updrafts. After re-entering a supercooled environment
above the melting layer, they impact with aged ice, freeze, and shatter. The size of the splinters generated during SIP was estimated as 10 µm or
less. A principal conclusion of this work is that only the freezing-drop-shattering mechanism could be clearly supported by the airborne in situ
observations.
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TL;DR: In this article, the authors investigated the relationship between aerosol water uptake and p NO 3 -enhancement, further impacting on visibility degradation, based on field observations and theoretical calculations in Beijing.
Abstract: . As has been the case in North America and western Europe,
the SO2 emissions have substantially reduced in the North China Plain (NCP) in
recent years. Differential rates of reduction in SO2 and NOx
concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate
( p NO 3 - ) over the NCP. In this
study, we observed a polluted episode with the particulate nitrate mass
fraction in nonrefractory PM 1 (NR-PM 1 ) being up to 44 % during
wintertime in Beijing. Based on this typical p NO 3 - -dominated haze
event, the linkage between aerosol water uptake and p NO 3 -
enhancement, further impacting on visibility degradation, has been
investigated based on field observations and theoretical calculations.
During haze development, as ambient relative humidity (RH) increased from
∼10 % to 70 %, the aerosol particle liquid water
increased from ∼1 µg m−3 at the beginning to
∼75 µg m−3 in the fully developed haze period. The
aerosol liquid water further increased the aerosol surface area and volume,
enhancing the condensational loss of N2O5 over particles. From the
beginning to the fully developed haze, the condensational loss of
N2O5 increased by a factor of 20 when only considering aerosol
surface area and volume of dry particles, while increasing by a factor of 25 when
considering extra surface area and volume due to water uptake. Furthermore,
aerosol liquid water favored the thermodynamic equilibrium of HNO3 in
the particle phase under the supersaturated HNO3 and NH3 in the
atmosphere. All the above results demonstrated that p NO 3 - is
enhanced by aerosol water uptake with elevated ambient RH during haze
development, in turn facilitating the aerosol take-up of water due to the
hygroscopicity of particulate nitrate salt. Such mutual promotion between
aerosol particle liquid water and particulate nitrate enhancement can
rapidly degrade air quality and halve visibility within 1 d. Reduction
of nitrogen-containing gaseous precursors, e.g., by control of traffic
emissions, is essential in mitigating severe haze events in the NCP.
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TL;DR: In this article, the authors present a critical review of the laboratory studies related to secondary ice production and identify gaps in our knowledge of SIP as well as to stimulate further laboratory studies focused on obtaining a quantitative description of efficiencies for each SIP mechanism.
Abstract: . Secondary ice production (SIP) plays a key role in the formation of ice
particles in tropospheric clouds. Future improvement of the accuracy of
weather prediction and climate models relies on a proper description of SIP
in numerical simulations. For now, laboratory studies remain a primary tool
for developing physically based parameterizations for cloud modeling. Over
the past 7 decades, six different SIP-identifying mechanisms have
emerged: (1) shattering during droplet freezing, (2) the rime-splintering
(Hallett–Mossop) process, (3) fragmentation due to ice–ice collision, (4) ice particle fragmentation due to thermal shock, (5) fragmentation of
sublimating ice, and (6) activation of ice-nucleating particles in transient
supersaturation around freezing drops. This work presents a critical review
of the laboratory studies related to secondary ice production. While some of
the six mechanisms have received little research attention, for others
contradictory results have been obtained by different research groups.
Unfortunately, despite vast investigative efforts, the lack of consistency
and the gaps in the accumulated knowledge hinder the development of
quantitative descriptions of any of the six SIP mechanisms. The present work
aims to identify gaps in our knowledge of SIP as well as to stimulate
further laboratory studies focused on obtaining a quantitative description
of efficiencies for each SIP mechanism.
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TL;DR: In this article, the uncertainties associated with fire emissions and their air quality and radiative impacts from underlying dry matter consumed and emissions factors are explored and compared with a variety of fire emission inventories with surface and airborne observations of black carbon (BC) and organic aerosol (OA) concentrations and satellite-derived aerosol optical depth.
Abstract: . Fires and the aerosols that they emit impact air quality, health, and climate, but the abundance and properties of carbonaceous aerosol (both black carbon and organic carbon) from biomass burning (BB) remain uncertain and poorly constrained. We aim to explore the uncertainties associated with fire emissions and their air quality and radiative impacts from underlying dry matter consumed and emissions factors. To investigate this, we compare model simulations from a global chemical transport model, GEOS-Chem, driven by a variety of fire emission inventories with surface and airborne observations of black carbon (BC) and organic aerosol (OA) concentrations and satellite-derived aerosol optical depth (AOD). We focus on two fire-detection-based and/or burned-area-based (FD-BA) inventories using burned area and active fire counts, respectively, i.e., the Global Fire Emissions Database version 4 (GFED4s) with small fires and the Fire INventory from NCAR version 1.5 (FINN1.5), and two fire radiative power (FRP)-based approaches, i.e., the Quick Fire Emission Dataset version 2.4 (QFED2.4) and the Global Fire Assimilation System version 1.2 (GFAS1.2). We show that, across the inventories, emissions of BB aerosol (BBA) differ by a factor of 4 to 7
over North America and that dry matter differences, not emissions factors,
drive this spread. We find that simulations driven by QFED2.4 generally
overestimate BC and, to a lesser extent, OA concentrations observations from two fire-influenced aircraft campaigns in North America (ARCTAS and DC3) and from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, while simulations driven by FINN1.5 substantially underestimate concentrations. The GFED4s and GFAS1.2-driven simulations provide the best agreement with OA and BC mass concentrations at the surface (IMPROVE), BC observed aloft (DC3 and ARCTAS), and AOD observed by MODIS over North America. We also show that a sensitivity simulation including an enhanced source of secondary organic aerosol (SOA) from fires, based on the NOAA Fire Lab 2016 experiments, produces substantial additional OA; however, the spread in the primary emissions estimates implies that this magnitude of SOA can be neither confirmed nor ruled out when comparing the simulations against the observations explored here. Given the substantial uncertainty in fire emissions, as represented by these four emission inventories, we find a sizeable range in 2012 annual BBA PM 2.5 population-weighted exposure over Canada and the contiguous US (0.5 to 1.6 µ g m −3 ). We also show that the range in the estimated global direct radiative effect of carbonaceous aerosol from fires ( −0.11 to −0.048 W m −2 ) is large and comparable to the direct radiative forcing of OA ( −0.09 W m −2 ) estimated in the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC). Our analysis suggests that fire emissions uncertainty challenges our ability to accurately characterize the impact of smoke on air quality and climate.
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TL;DR: In this article, the role of aerosol vertical distribution in thermodynamic stability and PBL development was investigated by jointly using micropulse lidar, sun photometer, and radiosonde measurements taken in Beijing.
Abstract: . The aerosol–planetary boundary layer (PBL) interaction was
proposed as an important mechanism to stabilize the atmosphere and
exacerbate surface air pollution. Despite the tremendous progress made in
understanding this process, its magnitude and significance still have large
uncertainties and vary largely with aerosol distribution and meteorological
conditions. In this study, we focus on the role of aerosol vertical
distribution in thermodynamic stability and PBL development by jointly using
micropulse lidar, sun photometer, and radiosonde measurements taken in
Beijing. Despite the complexity of aerosol vertical distributions,
cloud-free aerosol structures can be largely classified into three types:
well-mixed, decreasing with height, and inverse structures. The aerosol–PBL
relationship and diurnal cycles of the PBL height and PM 2.5 associated with these different aerosol vertical structures show
distinct characteristics. The vertical distribution of aerosol radiative
forcing differs drastically among the three types, with strong heating in the
lower, middle, and upper PBL, respectively. Such a discrepancy in the heating
rate affects the atmospheric buoyancy and stability differently in the three
distinct aerosol structures. Absorbing aerosols have a weaker effect of
stabilizing the lower atmosphere under the decreasing structure than under
the inverse structure. As a result, the aerosol–PBL interaction can be
strengthened by the inverse aerosol structure and can be potentially
neutralized by the decreasing structure. Moreover, aerosols can both enhance
and suppress PBL stability, leading to both positive and negative
feedback loops. This study attempts to improve our understanding of the
aerosol–PBL interaction, showing the importance of the observational
constraint of aerosol vertical distribution for simulating this interaction
and consequent feedbacks.
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TL;DR: In this paper, the authors derived a global biomass burning emission inventory on the basis of the Global Fire Emissions Database version 4 (GFED4), and developed a module to simulate the light absorption of BrC in the CommunityAtmosphere Model version 5 (CAM5) of the Community Earth System Model (CESM).
Abstract: . Carbonaceous aerosols significantly affect global
radiative forcing and climate through absorption
and the scattering of sunlight. Black carbon (BC) and brown carbon (BrC) are
light-absorbing carbonaceous aerosols. The direct radiative effect (DRE) of
BrC is uncertain. A recent study suggests that BrC absorption is comparable
to BC in the upper troposphere over biomass burning regions and that the
resulting radiative heating tends to stabilize the atmosphere. Yet current
climate models do not include proper physical and chemical treatments of
BrC. In this study, we derived a BrC global biomass burning emission
inventory on the basis of the Global Fire Emissions Database version 4 (GFED4),
developed a module to simulate the light absorption of BrC in the Community
Atmosphere Model version 5 (CAM5) of the Community Earth System Model (CESM), and investigated the photobleaching effect and convective transport
of BrC on the basis of Studies of Emissions, Atmospheric Composition, Clouds
and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective
Clouds and Chemistry Project (DC3) measurements. The model simulations of BC
were also evaluated using HIAPER (High-Performance Instrumented Airborne
Platform for Environmental Research) Pole-to-Pole Observations (HIPPO)
measurements. We found that globally BrC is a significant absorber, the DRE
of which is 0.10 W m −2 , more than 25 % of BC DRE ( +0.39 W m −2 ).
Most significantly, model results indicated that BrC atmospheric heating in
the tropical mid and upper troposphere is larger than that of BC. The source
of tropical BrC is mainly from wildfires, which are more prevalent in the
tropical regions than higher latitudes and release much more BrC relative to
BC than industrial sources. While BC atmospheric heating is skewed towards the northern mid-latitude lower atmosphere, BrC heating is more centered in the
tropical free troposphere. A possible mechanism for the enhanced convective
transport of BrC is that hydrophobic high molecular weight BrC becomes a
larger fraction of the BrC and less easily activated in a cloud as the
aerosol ages. The contribution of BrC heating to the Hadley circulation and
latitudinal expansion of the tropics is likely comparable to BC heating.
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TL;DR: In this paper, a new implementation of the volatility basis set (VBS) that explicitly resolves peroxy radical (RO2 ) products formed via autoxidation is presented, and the model includes a strong temperature dependence for RO2 as well as explicit termination of RO2, including reactions with NO, HO2, and other RO2.
Abstract: . Gas-phase autoxidation of organics can generate highly oxygenated organic molecules (HOMs) and thus increase secondary organic aerosol production and enable new-particle formation.
Here we present a new implementation of the volatility basis set (VBS) that explicitly resolves peroxy radical ( RO2 ) products formed via autoxidation.
The model includes a strong temperature dependence for autoxidation as well as explicit termination of RO2 , including reactions with NO, HO2 , and other RO2 .
The RO2 cross-reactions can produce dimers (ROOR).
We explore the temperature and NOx dependence of this chemistry, showing that temperature strongly influences the intrinsic volatility distribution and that NO can suppress autoxidation under conditions typically found in the atmosphere.