. Compared to the severe winter haze episodes in the North China Plain (NCP),
haze pollution during summertime has drawn little public attention. In this
study, we present the highly time-resolved chemical composition of submicron
particles (PM 1) measured in Beijing and Xinxiang in the NCP region
during summertime to evaluate the driving factors of aerosol pollution.
During the campaign periods (30 June to 27 July 2015, for Beijing and 8 to
25 June 2017, for Xinxiang), the average PM 1 concentrations were 35.0
and 64.2  µ g m −3 in Beijing and Xinxiang. Pollution episodes
characterized with largely enhanced nitrate concentrations were observed at
both sites. In contrast to the slightly decreased mass fractions of sulfate,
semivolatile oxygenated organic aerosol (SV-OOA), and low-volatility
oxygenated organic aerosol (LV-OOA) in PM 1 , nitrate displayed a
significantly enhanced contribution with the aggravation of aerosol
pollution, highlighting the importance of nitrate formation as the driving
force of haze evolution in summer. Rapid nitrate production mainly occurred
after midnight, with a higher formation rate than that of sulfate, SV-OOA, or
LV-OOA. Based on observation measurements and thermodynamic modeling, high
ammonia emissions in the NCP region favored the high nitrate production in
summer. Nighttime nitrate formation through heterogeneous hydrolysis of
dinitrogen pentoxide (N 2 O 5) enhanced with the development of haze
pollution. In addition, air masses from surrounding polluted areas during
haze episodes led to more nitrate production. Finally, atmospheric
particulate nitrate data acquired by mass spectrometric techniques from
various field campaigns in Asia, Europe, and North America uncovered a higher
concentration and higher fraction of nitrate present in China. Although
measurements in Beijing during different years demonstrate a decline in the
nitrate concentration in recent years, the nitrate contribution in PM 1 
still remains high. To effectively alleviate particulate matter pollution in
summer, our results suggest an urgent need to initiate ammonia emission
control measures and further reduce nitrogen oxide emissions over the NCP
region.

/pdf/nitrate-driven-urban-haze-pollution-during-summertime-over-fa1tg89n64.pdf

Nitrate-driven urban haze pollution during summertime over the North China Plain

. Organic aerosol (OA) is an important fraction of submicron aerosols. However,
it is challenging to predict and attribute the specific organic compounds and
sources that lead to observed OA loadings, largely due to contributions from
secondary production. This is especially true for megacities surrounded by
numerous regional sources that create an OA background. Here, we utilize
in situ gas and aerosol observations collected on board the NASA DC-8 during
the NASA–NIER KORUS-AQ (Korea–United States Air Quality) campaign to
investigate the sources and hydrocarbon precursors that led to the secondary
OA (SOA) production observed over Seoul. First, we investigate the
contribution of transported OA to total loadings observed over Seoul by
using observations over the Yellow Sea coupled to FLEXPART Lagrangian
simulations. During KORUS-AQ, the average OA loading advected into Seoul was
 ∼1 –3  µg sm−3 . Second, taking this background into
account, the dilution-corrected SOA concentration observed over Seoul was
 ∼140 µ g sm - 3 ppmv - 1 at 0.5 equivalent photochemical
days. This value is at the high end of what has been observed in other
megacities around the world (20–70  µ g sm - 3 ppmv - 1 at 0.5
equivalent days). For the average OA concentration observed over Seoul
(13  µg sm−3 ), it is clear that production of SOA from locally
emitted precursors is the major source in the region. The importance
of local SOA production was supported by the following observations.
(1) FLEXPART source contribution calculations indicate any
hydrocarbons with a lifetime of less than 1 day, which are shown to dominate the
observed SOA production, mainly originate from South Korea. (2) SOA
correlated strongly with other secondary photochemical species, including
short-lived species (formaldehyde, peroxy acetyl nitrate, sum of acyl peroxy
nitrates, dihydroxytoluene, and nitrate aerosol). (3) Results from
an airborne oxidation flow reactor (OFR), flown for the first time, show a
factor of 4.5 increase in potential SOA concentrations over Seoul versus over
the Yellow Sea, a region where background air masses that are advected into
Seoul can be measured. (4) Box model simulations reproduce SOA
observed over Seoul within 11 % on average and suggest that short-lived
hydrocarbons (i.e., xylenes, trimethylbenzenes, and semi-volatile and intermediate-volatility compounds) were the main SOA precursors over Seoul. Toluene
alone contributes 9 % of the modeled SOA over Seoul. Finally, along with
these results, we use the metric ΔOA / ΔCO2 to
examine the amount of OA produced per fuel consumed in a megacity, which
shows less variability across the world than ΔOA∕ΔCO .

/pdf/secondary-organic-aerosol-production-from-local-emissions-z9q9wagkrv.pdf

Secondary organic aerosol production from local emissions dominates the organic aerosol budget over Seoul, South Korea, during KORUS-AQ

The Korea-United States Air Quality (KORUS-AQ) field study was conducted during May-June 2016. The effort was jointly sponsored by the National Institute of Environmental Research of South Korea and the National Aeronautics and Space Administration of the United States. KORUS-AQ offered an unprecedented, multi-perspective view of air quality conditions in South Korea by employing observations from three aircraft, an extensive ground-based network, and three ships along with an array of air quality forecast models. Information gathered during the study is contributing to an improved understanding of the factors controlling air quality in South Korea. The study also provided a valuable test bed for future air quality-observing strategies involving geostationary satellite instruments being launched by both countries to examine air quality throughout the day over Asia and North America. This article presents details on the KORUS-AQ observational assets, study execution, data products, and air quality conditions observed during the study. High-level findings from companion papers in this special issue are also summarized and discussed in relation to the factors controlling fine particle and ozone pollution, current emissions and source apportionment, and expectations for the role of satellite observations in the future. Resulting policy recommendations and advice regarding plans going forward are summarized. These results provide an important update to early feedback previously provided in a Rapid Science Synthesis Report produced for South Korean policy makers in 2017 and form the basis for the Final Science Synthesis Report delivered in 2020.

/pdf/the-korea-united-states-air-quality-korus-aq-field-study-18c697j34q.pdf

The Korea–United States Air Quality (KORUS-AQ) field study

In an environment with many local, remote, persistent, and episodic sources of pollution, meteorology is the primary factor that drives periods of unhealthy air quality and reduced visibility. The 2016 Korea-United States Air Quality (KORUS-AQ) field study provides a unique opportunity to examine the impact of meteorology on the relative influence of local and transboundary pollution. Much of the KORUS-AQ campaign can be grouped into four distinct research periods based on observed synoptic meteorology, including a period of complex aerosol vertical profiles driven by dynamic meteorology, stagnation under a persistent anticyclone, low-level transport and haze development, and a blocking pattern. These episodes are examined using a diverse archive of ground, airborne, and satellite data. While frontal boundaries are recognized as the primary mechanism driving pollution transport in eastern Asia, results show that they are not always related to sustained periods of hazardous air quality and reduced visibility at the surface. Significant long-range transport of pollution and dust was constrained to a few short events, suggesting that the majority of pollutants sampled during KORUS-AQ originated from local sources. A severe regional pollution episode is examined in detail, featuring dense haze and significant secondary particle formation within a shallow moist boundary layer. Observations during KORUS-AQ also highlight a rapid, 40 ppbv increase in ozone pollution as a strong sea breeze front traversed the Seoul Metropolitan Area. Representativeness of meteorology and pollution conditions measured by KORUS-AQ is considered by comparison with climatology. This analysis is an essential step toward improved local and regional forecasting of air quality and visibility.

Meteorology influencing springtime air quality, pollution transport, and visibility in Korea

Anthropogenic emissions in Asia have significantly increased during the last two decades; as a result, the induced air pollution and its influences on radiative forcing and public health are becoming increasingly prominent The Aerodyne Aerosol Mass Spectrometer (AMS) has been widely deployed in Asia for real-time characterization of aerosol chemistry In this paper, we review the AMS measurements in Asia, mainly in China, Korea, Japan, and India since 2001 and summarize the key results and findings The mass concentrations of non-refractory submicron aerosol species (NR-PM1) showed large spatial distributions with high mass loadings occurring in India and north and northwest China (602-813 μg m-3), whereas much lower values were observed in Korea, Japan, Singapore and regional background sites (75-151 μg m-3) Aerosol composition varied largely in different regions, but was overall dominated by organic aerosols (OA, 32-75%), especially in south and southeast Asia due to the impact of biomass burning While sulfate and nitrate showed comparable contributions in urban and suburban regions in north China, sulfate dominated inorganic aerosols in south China, Japan and regional background sites Positive matrix factorization analysis identified multiple OA factors from different sources and processes in different atmospheric environments, eg, biomass burning OA in south and southeast Asia and agricultural seasons in China, cooking OA in urban areas, and coal combustion in north China However, secondary OA (SOA) was a ubiquitous and dominant aerosol component in all regions, accounting for 43-78% of OA The formation of different SOA subtypes associated with photochemical production or aqueous-phase/fog processing was widely investigated The roles of primary emissions, secondary production, regional transport, and meteorology on severe haze episodes, and different chemical responses of primary and secondary aerosol species to source emission changes and meteorology were also demonstrated Finally, future prospects of AMS studies on long-term and aircraft measurements, water-soluble OA, the link of OA volatility, oxidation levels, and phase state were discussed

A review of aerosol chemistry in Asia: insights from aerosol mass spectrometer measurements

. Non-refractory submicrometer particulate matter (NR-PM1) was measured in the Seoul Metropolitan Area (SMA), Korea, using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) from 14 April to 15 June 2016, as a part of the Korea-US Air Quality Study (KORUS-AQ) campaign. This was the first highly time-resolved, real-time measurement study of springtime aerosol in SMA and the results reveal valuable insights into the sources and atmospheric processes that contribute to PM pollution in this region. The average concentration of submicrometer aerosol (PM1 =  NR-PM1 + black carbon (BC)) was 22.1 µg m−3, which was composed of 44 % organics, 20 % sulfate, 17 % nitrate, 12 % ammonium, and 7 % BC. Organics had an average atomic oxygen-to-carbon (O ∕ C) ratio of 0.49 and an average organic mass-to-carbon (OM/OC) ratio of 1.82. Four distinct sources of OA were identified via positive matrix factorization (PMF) analysis of the HR-ToF-AMS data: vehicle emissions represented by a hydrocarbon-like OA factor (HOA; O ∕ C  =  0.15; 17 % of OA mass), food cooking activities represented by a cooking-influenced OA factor (COA; O ∕ C  =  0.19; 22 % of OA mass), and secondary organic aerosol (SOA) represented by a semi-volatile oxygenated OA factor (SV-OOA; O ∕ C  =  0.44; 27 % of OA mass) and a low-volatility oxygenated OA factor (LV-OOA; O ∕ C  =  0.91; 34 % of OA mass). Our results indicate that air quality in SMA during KORUS-AQ was influenced strongly by secondary aerosol formation, with sulfate, nitrate, ammonium, SV-OOA, and LV-OOA together accounting for 76 % of the PM1 mass. In particular, the formation of LV-OOA and sulfate was mainly promoted by elevated ozone concentrations and photochemical reactions during daytime, whereas SV-OOA and nitrate formation was contributed by both nocturnal processing of VOC and nitrogen oxides, respectively, and daytime photochemical reactions. In addition, lower nighttime temperature promoted gas-to-particle partitioning of semivolatile species and formation of SV-OOA and nitrate. During a period of 4 days (from 20 to 23 May ), LV-OOA increased dramatically and accounted for up to 41 % of the PM1 mass. This intense LV-OOA formation event was associated with large enhancements of both anthropogenic and biogenic VOCs (e.g., isoprene and toluene), high concentration of Ox ( =  O3 + NO2), strong solar radiation, and stagnant conditions, suggesting that it was mainly driven by local photochemical formation. We have also investigated the formation and evolution mechanisms of severe haze episodes. Unlike the winter haze events which were mainly caused by intense local emissions coupled with stagnant meteorological conditions, the spring haze events appeared to be influenced by both regional and local factors. For example, there were episodes of long-range transport of plumes followed by calm meteorology conditions, which promoted the formation and accumulation of local secondary species, leading to high concentrations of PM. Overall, our results indicate that PM pollutants in urban Korea originate from complex emission sources and atmospheric processes and that the concentrations and composition of PM are controlled by various factors, including meteorological conditions, local anthropogenic emissions, and upwind sources.

/pdf/influence-of-intense-secondary-aerosol-formation-and-long-4whfos9erp.pdf

Influence of intense secondary aerosol formation and long-range transport on aerosol chemistry and properties in the Seoul Metropolitan Area during spring time: results from KORUS-AQ

Jongbae Heo

Papers

Influence of intense secondary aerosol formation and long-range transport on aerosol chemistry and properties in the Seoul Metropolitan Area during spring time: results from KORUS-AQ