From 2013 to 2017, with the implementation of the toughest-ever clean air policy in China, significant declines in fine particle (PM2.5) concentrations occurred nationwide. Here we estimate the drivers of the improved PM2.5 air quality and the associated health benefits in China from 2013 to 2017 based on a measure-specific integrated evaluation approach, which combines a bottom-up emission inventory, a chemical transport model, and epidemiological exposure-response functions. The estimated national population-weighted annual mean PM2.5 concentrations decreased from 61.8 (95%CI: 53.3-70.0) to 42.0 µg/m3 (95% CI: 35.7-48.6) in 5 y, with dominant contributions from anthropogenic emission abatements. Although interannual meteorological variations could significantly alter PM2.5 concentrations, the corresponding effects on the 5-y trends were relatively small. The measure-by-measure evaluation indicated that strengthening industrial emission standards (power plants and emission-intensive industrial sectors), upgrades on industrial boilers, phasing out outdated industrial capacities, and promoting clean fuels in the residential sector were major effective measures in reducing PM2.5 pollution and health burdens. These measures were estimated to contribute to 6.6- (95% CI: 5.9-7.1), 4.4- (95% CI: 3.8-4.9), 2.8- (95% CI: 2.5-3.0), and 2.2- (95% CI: 2.0-2.5) µg/m3 declines in the national PM2.5 concentration in 2017, respectively, and further reduced PM2.5-attributable excess deaths by 0.37 million (95% CI: 0.35-0.39), or 92% of the total avoided deaths. Our study confirms the effectiveness of China's recent clean air actions, and the measure-by-measure evaluation provides insights into future clean air policy making in China and in other developing and polluting countries.

https://www.pnas.org/content/pnas/116/49/24463.full.pdf

Drivers of improved PM2.5 air quality in China from 2013 to 2017.

Regional severe haze represents an enormous environmental problem in China, influencing air quality, human health, ecosystem, weather, and climate. These extremes are characterized by exceedingly high concentrations of fine particulate matter (smaller than 2.5 µm, or PM2.5) and occur with extensive temporal (on a daily, weekly, to monthly timescale) and spatial (over a million square kilometers) coverage. Although significant advances have been made in field measurements, model simulations, and laboratory experiments for fine PM over recent years, the causes for severe haze formation have not yet to be systematically/comprehensively evaluated. This review provides a synthetic synopsis of recent advances in understanding the fundamental mechanisms of severe haze formation in northern China, focusing on emission sources, chemical formation and transformation, and meteorological and climatic conditions. In particular, we highlight the synergetic effects from the interactions between anthropogenic emissions and atmospheric processes. Current challenges and future research directions to improve the understanding of severe haze pollution as well as plausible regulatory implications on a scientific basis are also discussed.

https://www.pnas.org/content/pnas/116/18/8657.full.pdf

Severe haze in northern China: A synergy of anthropogenic emissions and atmospheric processes.

. Severe regional haze pollution events occurred in eastern and central China in January 2013, which had adverse effects on the environment and public health. Extremely high levels of particulate matter with aerodynamic diameter of 2.5 μm or less (PM2.5) with dominant components of sulfate and nitrate are responsible for the haze pollution. Although heterogeneous chemistry is thought to play an important role in the production of sulfate and nitrate during haze episodes, few studies have comprehensively evaluated the effect of heterogeneous chemistry on haze formation in China by using the 3-D models due to of a lack of treatments for heterogeneous reactions in most climate and chemical transport models. In this work, the WRF-CMAQ model with newly added heterogeneous reactions is applied to East Asia to evaluate the impacts of heterogeneous chemistry and the meteorological anomaly during January 2013 on regional haze formation. As the parameterization of heterogeneous reactions on different types of particles is not well established yet, we arbitrarily selected the uptake coefficients from reactions on dust particles and then conducted several sensitivity runs to find the value that can best match observations. The revised CMAQ with heterogeneous chemistry not only captures the magnitude and temporal variation of sulfate and nitrate, but also reproduces the enhancement of relative contribution of sulfate and nitrate to PM2.5 mass from clean days to polluted haze days. These results indicate the significant role of heterogeneous chemistry in regional haze formation and improve the understanding of the haze formation mechanisms during the January 2013 episode.

/pdf/heterogeneous-chemistry-a-mechanism-missing-in-current-285nznliov.pdf

Heterogeneous chemistry: a mechanism missing in current models to explain secondary inorganic aerosol formation during the January 2013 haze episode in North China

Severe winter air pollution events, attributed to emissions from development, have increased in Beijing in recent decades. This study looks at how atmospheric conditions contribute and projects climate change will increase conditions favourable to such events. The frequency of Beijing winter severe haze episodes has increased substantially over the past decades1,2,3,4, and is commonly attributed to increased pollutant emissions from China’s rapid economic development5,6. During such episodes, levels of fine particulate matter are harmful to human health and the environment, and cause massive disruption to economic activities3,4,7,8,9,10,11,12,13,14,15,16, as occurred in January 201317,18,19,20,21. Conducive weather conditions are an important ingredient of severe haze episodes3,21, and include reduced surface winter northerlies3,21, weakened northwesterlies in the midtroposphere, and enhanced thermal stability of the lower atmosphere1,3,16,21. How such weather conditions may respond to climate change is not clear. Here we project a 50% increase in the frequency and an 80% increase in the persistence of conducive weather conditions similar to those in January 2013, in response to climate change. The frequency and persistence between the historical (1950–1999) and future (2050–2099) climate were compared in 15 models under Representative Concentration Pathway 8.5 (RCP8.5)22. The increased frequency is consistent with large-scale circulation changes, including an Arctic Oscillation upward trend23,24, weakening East Asian winter monsoon25,26, and faster warming in the lower troposphere27,28. Thus, circulation changes induced by global greenhouse gas emissions can contribute to the increased Beijing severe haze frequency.

Weather conditions conducive to Beijing severe haze more frequent under climate change

Abstract. Fine particulate matter (PM2.5) is a severe air pollution\nproblem in China. Observations of PM2.5 have been available since 2013\nfrom a large network operated by the China National Environmental Monitoring\nCenter (CNEMC). The data show a general 30 %–50 % decrease in annual mean\nPM2.5 across China over the 2013–2018 period, averaging at −5.2 µg m−3 a−1. Trends in the five megacity cluster regions targeted by\nthe government for air quality control are -9.3±1.8 µg m−3 a−1 (±95 % confidence interval) for Beijing–Tianjin–Hebei,\n-6.1±1.1 µg m−3 a−1 for the Yangtze River Delta, -2.7±0.8 µg m−3 a−1 for the Pearl River Delta, -6.7±1.3 µg m−3 a−1 for the Sichuan Basin, and -6.5±2.5 µg m−3 a−1 for the Fenwei Plain (Xi'an). Concurrent 2013–2018\nobservations of sulfur dioxide (SO2) and carbon monoxide (CO) show that\nthe declines in PM2.5 are qualitatively consistent with drastic\ncontrols of emissions from coal combustion. However, there is also a large\nmeteorologically driven interannual variability in PM2.5 that\ncomplicates trend attribution. We used a stepwise multiple linear regression\n(MLR) model to quantify this meteorological contribution to the PM2.5 trends across China. The MLR model correlates the 10 d PM2.5\nanomalies to wind speed, precipitation, relative humidity, temperature, and\n850 hPa meridional wind velocity (V850). The meteorology-corrected\nPM2.5 trends after removal of the MLR meteorological contribution can\nbe viewed as being driven by trends in anthropogenic emissions. The mean\nPM2.5 decrease across China is −4.6 µg m−3 a−1 in the\nmeteorology-corrected data, 12 % weaker than in the original data, meaning\nthat 12 % of the PM2.5 decrease in the original data is\nattributable to meteorology. The trends in the meteorology-corrected data\nfor the five megacity clusters are -8.0±1.1 µg m−3 a−1 for Beijing–Tianjin–Hebei (14 % weaker than in the original\ndata), -6.3±0.9 µg m−3 a−1 for the Yangtze River Delta\n(3 % stronger), -2.2±0.5 µg m−3 a−1 for the Pearl River\nDelta (19 % weaker), -4.9±0.9 µg m−3 a−1 for\nthe Sichuan Basin (27 % weaker), and -5.0±1.9 µg m−3 a−1 for the Fenwei Plain (Xi'an; 23 % weaker); 2015–2017 observations of\nflattening PM2.5 in the Pearl River Delta and increases in the Fenwei\nPlain can be attributed to meteorology rather than to relaxation of emission\ncontrols.\n

/pdf/fine-particulate-matter-pm-2-5-trends-in-china-2013-2018-4nxiozdatx.pdf

Fine particulate matter (PM 2.5 ) trends in China, 2013–2018: separating contributions from anthropogenic emissions and meteorology

Our analysis of fog and haze observations from the surface weather stations in China in recent 50 years (from 1961 to 2011) shows that the number of fog days has experienced two-stage variations, with an increasing trend before 1980 and a decreasing trend after 1990. Especially, an obvious decreasing trend after 1990 can be clearly seen, which is consistent with the decreasing trend of the surface relative humidity. However, the number of haze days has demonstrated an increasing trend. As such, the role of reduction of atmospheric relative humidity in the transition process from fog into haze has been further investigated. It is estimated that the mean relative humidity of haze days is about 69%, lower than previously estimated, which implies that it is more difficult for the haze particles to transform into fog drops. This is possibly one of the major environmental factors leading to the reduction of number of fog days. The threshold of the relative humidity for transition from fog into haze is about 82%, also lower than previously estimated. Thus, the reduction of the surface relative humidity in China mainly due to the increase of the surface temperature and the saturation specific humidity may exert an obvious impact on the environmental conditions for the formations of fog and haze. In addition, our investigation of the relationship between haze and visibility reveals that with the increase of haze days, the visibility has declined markedly. Since 1961, the mean visibility has dropped from 4-10 to 2-4 km, about a half of the previous horizontal distance of visibility.

Analysis of long-term variations of fog and haze in China in recent 50 years and their relations with atmospheric humidity

In 2013, China issued the “Action Plan for the Prevention and Control of Air Pollution” (“Ten Statements of Atmosphere”) and implemented a series of pollution reduction measures from 2013 to 2017. In key regions of China, the mass concentrations of particulate matter with aerodynamic equivalent diameters less than 2.5 μm (PM2.5) have dropped significantly. However, the contributions of meteorological changes to PM2.5 reduction are largely uncertain, which has attracted particular concern from the government and the public. Here, we investigated the impact of large-scale and boundary layer (BL) meteorological conditions on aerosol pollution and estimated the contributions of meteorological changes to PM2.5 reduction based on in-depth analysis and diagnosis of various observed meteorological elements and an integrated pollution-linked meteorological index (PLAM, which is approximately and linearly related to PM mass concentration). In this study, we found that the meteorological conditions worsened in 2014 and 2015 and improved in 2016 and 2017 relative to those in 2013 in key regions in China. In 2017 relative to 2013, only ~5% (approximately 13% of the total PM2.5 decline) of the 39.6% reduction in PM2.5 mass concentrations can be attributed to meteorological changes in the Beijing-Tianjin-Hebei (BTH) region, and only ~7% (approximately 20% of the total PM2.5 decline) of the 34.3% reduction can be attributable to meteorological changes in the Yangtze River Delta (YRD) region. Overall, the PM2.5 reduction due to meteorological improvement is much lower than the observed PM2.5 reduction in these areas, which indicates that emission reduction during the five-year implementation of the “Ten Statements of Atmosphere” is the dominant factor in the improvement in air quality. The changes in meteorology and climate are conducive to PM2.5 reduction but do not dominate the substantial improvement in air quality. Similar to the above regions, in the Pearl River Delta (PRD) region, the impact of meteorological changes on the annual averaged PM2.5 concentration from 2013 to 2017 was relatively weak, and the PM2.5 reduction was mainly due to emission reductions. During winter 2017 (January, February, and December of this year), the meteorological conditions improved ~20% in the BTH region (observed total PM2.5 reduction: 40.2%) and ~30% in the YRD region (observed total PM2.5 reduction: 38.2%) relative to those in 2013, showing the meteorological factors played more important role in the decrease of PM2.5 in winter of these years in the two regions, respectively. The meteorological conditions in winter 2016 were 14% better than those in winter 2017, but the PM2.5 reduction in winter 2016 was still less than that in winter 2017, reinforcing the significant contributions of the increasing efforts to reduce PM2.5 emissions in 2017. The substantial progress of strict emission measures was also confirmed by a comparison of several persistent heavy aerosol pollution episodes (HPEs) with similar meteorological conditions. It is found that the decrease of PM2.5 mass caused by emission reduction increases year by year, especially the decrease of PM2.5 concentration in 2016 and 2017. In China, HPEs mainly occur in winter, when meteorological conditions are approximately 40--100% worse than in other seasons. This worsening is partly due to the harbor effect of high topography, including downdrafts and the weak wind zone, and partly due to the increasingly stable regional BL structure caused by climate warming. For the formation of HPEs, it occurred under regional stagnant and stable conditions associated with upper-level circulation patterns, including the zonal westerly winds type and high-pressure ridges. After pollution formation, PM2.5 with mass accumulated to a certain degree can further worsen the BL meteorological conditions. The feedback effect associated with worsening conditions dominates PM2.5 mass explosive growth. In the context of high air pollutant emissions in China, unfavorable meteorological conditions are the necessary external conditions for the formation and accumulation of HPEs. Therefore, reducing aerosol pollution significantly during the earlier transport stage is critical in reducing persistent HPEs. Currently, even under favorable meteorological conditions, allowing emissions without restriction is also not advisable because aerosol pollution allowed to accumulate to a certain extent will significantly worsen the BL meteorological conditions and close the “meteorological channels” available for pollution dispersion.

The impact of meteorological changes from 2013 to 2017 on PM 2.5 mass reduction in key regions in China

. The weather conditions affecting aerosol pollution in Beijing and its
vicinity (BIV) in wintertime have worsened in recent years, particularly
after 2010. The relation between interdecadal changes in weather conditions
and climate warming is uncertain. Here, we analyze long-term variations of an
integrated pollution-linked meteorological index (which is approximately and
linearly related to aerosol pollution), the extent of changes in vertical
temperature differences in the boundary layer (BL) in BIV, and northerly
surface winds from Lake Baikal during wintertime to evaluate the potential
contribution of climate warming to changes in meteorological conditions
directly related to aerosol pollution in this area; this is accomplished
using NCEP reanalysis data, surface observations, and long-term vertical
balloon sounding observations since 1960. The weather conditions affecting
BIV aerosol pollution are found to have worsened since the 1960s as a whole.
This worsening is more significant after 2010, with PM 2.5 reaching
unprecedented high levels in many cities in China, particularly in BIV. The
decadal worsening of meteorological conditions in BIV can partly be
attributed to climate warming, which is defined by more warming in the higher
layers of the boundary layer (BL) than the lower layers. This worsening can
also be influenced by the accumulation of aerosol pollution, to a certain
extent (particularly after 2010), because the increase in aerosol pollution
from the ground leads to surface cooling by aerosol–radiation interactions,
which facilitates temperature inversions, increases moisture accumulations,
and results in the extra deterioration of meteorological conditions. If
analyzed as a linear trend, weather conditions have worsened by
 ∼  4 % each year from 2010 to 2017. Given such a deterioration rate,
the worsening of weather conditions may lead to a corresponding amplitude
increase in PM 2.5 in BIV during wintertime in the next 5 years (i.e.,
2018 to 2022). More stringent emission reduction measures will need to be
conducted by the government.

/pdf/the-interdecadal-worsening-of-weather-conditions-affecting-3sgp6w7imo.pdf

Yanju Liu

Papers

Analysis of long-term variations of fog and haze in China in recent 50 years and their relations with atmospheric humidity

The impact of meteorological changes from 2013 to 2017 on PM 2.5 mass reduction in key regions in China

The interdecadal worsening of weather conditions affecting aerosol pollution in the Beijing area in relation to climate warming