Aerosol light absorption alleviates particulate pollution during wintertime haze events.

Aerosol light absorption has been widely considered as a contributing factor to the worsening of particulate pollution in large urban areas, primarily through its role in stabilizing the planetary boundary layer (PBL). Here, we report that absorption-dominated aerosol-radiation interaction can decrease near-surface fine particulate matter concentrations ([PM]) at a large-scale during wintertime haze events. A "warm bubble" effect by the significant heating rate of absorbing aerosols above the PBL top generates a secondary circulation, enhancing the upward motion (downward motion) and the convergence (divergence) in polluted (relatively clean) areas, with a net effect of lowering near-surface [PM].

Urgency of controlling agricultural nitrogen sources to alleviate summertime air pollution in the North China Plain.

Agricultural nitrogen sources (ANS) have played an increasingly important role in the air quality since ANS emission controls are much weaker than those for fossil fuel combustion sources due to the increasing food demand. However, ANS emissions are highly uncertain due to stochastic agricultural management activities and limited field measurements, and impacts of ANS on the air quality remain elusive. In the study, the WRF-Chem model has been used to investigate ANS shares in near surface air pollutant concentrations during a growing season in the North China Plain (NCP), with ANS emissions constrained by satellite retrievals.

The relationship between the intensified heat waves and deteriorated summertime ozone pollution in the Beijing-Tianjin-Hebei region, China, during 2013-2017.

Summertime ozone (O) pollution has frequently occurred in the Beijing-Tianjin-Hebei (BTH) region, China, since 2013, resulting in detrimental impacts on human health and ecosystems. The contribution of weather shifts to O concentration variability owing to climate change remains elusive. By combining regional air chemistry model simulations with near-surface observations, we found that anthropogenic emission changes contributed to approximately 23% of the increase in maximum daily 8-h average O concentrations in the BTH region in June-July-August (JJA) 2017 (compared with that in 2013).

Why is the air humid during wintertime heavy haze days in Beijing?

Atmospheric humidity has been shown to promote haze formation, but it remains unclear why the air is humid during heavy haze days in winter. Here we combine water vapor isotope measurements with WRF-Chem simulations to elucidate increasing humidity with aggravation of haze during wintertime in urban Beijing. The vapor isotopic analysis in Beijing shows that the combustion-derived water (CDW) constitutes 11.

The Heavy Particulate Matter Pollution During the COVID-19 Lockdown Period in the Guanzhong Basin, China.

Nationwide restrictions on human activities (lockdown) in China since 23 January 2020, to control the 2019 novel coronavirus disease pandemic (COVID-19), has provided an opportunity to evaluate the effect of emission mitigation on particulate matter (PM) pollution. The WRF-Chem simulations of persistent heavy PM pollution episodes from 20 January to 14 February 2020, in the Guanzhong Basin (GZB), northwest China, reveal that large-scale emission reduction of primary pollutants has not substantially improved the air quality during the COVID-19 lockdown period. Simultaneous reduction of primary precursors during the lockdown period only decreases the near-surface PM mass concentration by 11.

Heterogeneous HONO formation deteriorates the wintertime particulate pollution in the Guanzhong Basin, China.

Despite implementation of strict emission mitigation measures since 2013, heavy haze with high levels of secondary aerosols still frequently engulfs the Guanzhong Basin (GZB), China, during wintertime, remarkably impairing visibility and potentially causing severe health issues. Although the observed low ozone (O) concentrations do not facilitate the photochemical formation of secondary aerosols, the measured high nitrous acid (HONO) level provides an alternate pathway in the GZB. The impact of heterogeneous HONO sources on the wintertime particulate pollution and atmospheric oxidizing capability (AOC) is evaluated in the GZB.

Effects of hydroperoxy radical heterogeneous loss on the summertime ozone formation in the North China Plain.

Hydroperoxy radical (HO) is a crucial oxidant participating in the oxidation of nitrogen oxide to nitrogen dioxide which constitutes one of the most important pathways for the ozone (O) photochemical formation in the troposphere. Laboratory experiments and field observations have revealed efficient HO heterogeneous uptake on wet aerosols, but its impact on the O formation remains controversial. A severe and persistent O pollution episode has been simulated using the WRF-Chem model to evaluate the impacts of the HO heterogeneous loss on the O formation in the North China Plain (NCP) during the summertime of 2018.

Cropland nitrogen dioxide emissions and effects on the ozone pollution in the North China plain.

Soil nitrogen dioxide (NO = NO + NO) emissions have been measured and estimated to be the second most significant contributor to the NO burden following the fossil fuel combustion source globally. NO emissions from croplands are subject to being underestimated or overlooked in air pollution simulations of regional atmospheric chemistry models. With constraints of ground and space observations of NO, the WRF-Chem model is used to investigate the cropland NO emission and its contribution to the near-surface ozone (O) pollution in North China Plain (NCP) during a growing season as a case study.

Local and transboundary transport contributions to the wintertime particulate pollution in the Guanzhong Basin (GZB), China: A case study.

Heavy haze with high levels of fine particulate matters (PM) frequently engulfs the Guanzhong Basin (GZB) in northwestern China during wintertime. Although it is an enclosed basin with a narrow opening to the east, prevailing easterly winds during heavy haze episodes have a large potential to bring air pollutants to the GZB from the two highly polluted neighboring provinces of Shanxi and Henan (SX&HN). The source-oriented WRF-Chem model simulations of a persistent and heavy haze episode that occurred in the GZB from December 6 to 21, 2016, reveal that local emissions dominate PM concentrations in the GZB, with an average near-surface PM contribution of about 56.

Increasing atmospheric oxidizing capacity weakens emission mitigation effort in Beijing during autumn haze events.

Although strict mitigation measures have been implemented since 2013 in Beijing-Tianjin-Hebei (BTH), China, air pollution still frequently occurs. Observations reveal that during pollution episodes in autumn, fine particulate matter (PM) concentrations have not decreased, and particularly, ozone (O) concentrations have increased remarkably from 2013 to 2015 in Beijing. Additionally, a concurrence of O and particulate pollution with high secondary aerosol contributions has been observed frequently, indicating high atmospheric oxidizing capacity (AOC) during particulate pollution.

Mitigating NO emissions does not help alleviate wintertime particulate pollution in Beijing-Tianjin-Hebei, China.

Stringent mitigation measures have reduced wintertime fine particulate matter (PM) concentrations by 42.2% from 2013 to 2018 in the Beijing-Tianjin-Hebei (BTH) region, but severe PM pollution still frequently engulfs the region. The observed nitrate aerosols have not exhibited a significant decreasing trend and constituted a major fraction (about 20%) of the total PM, although the surface-measured NO concentration has decreased by over 20%.

Wintertime nitrate formation pathways in the north China plain: Importance of NO heterogeneous hydrolysis.

Strict emission control measures have been implemented in the North China Plain (NCP) to improve air quality since 2013. However, heavy particulate matter (PM) pollution still frequently occurs in the region especially during wintertime, and the nitrate contribution to fine PM (PM) has substantially increased in recent several years. Nitrate aerosols, which are formed via nitric acid (HNO) to balance inorganic cations in the particle phase, have become a major fraction of PM during wintertime haze events in the NCP.

Aerosol-photolysis interaction reduces particulate matter during wintertime haze events.

Aerosol-radiation interaction (ARI) plays a significant role in the accumulation of fine particulate matter (PM) by stabilizing the planetary boundary layer and thus deteriorating air quality during haze events. However, modification of photolysis by aerosol scattering or absorbing solar radiation (aerosol-photolysis interaction or API) alters the atmospheric oxidizing capacity, decreases the rate of secondary aerosol formation, and ultimately alleviates the ARI effect on PM pollution. Therefore, the synergetic effect of both ARI and API can either aggravate or even mitigate PM pollution.

Impact of synoptic patterns and meteorological elements on the wintertime haze in the Beijing-Tianjin-Hebei region, China from 2013 to 2017.

Meteorological conditions play a key role in formation of air pollution, determining dispersion or accumulation of air pollutants. Aggressive emission mitigation measures have been taken recently in the Beijing-Tianjin-Hebei region (BTH), China, but pervasive and persistent haze still frequently engulfs this region during wintertime. Occurrence frequency of unfavorable meteorological conditions in winter is anticipated to constitute a significantly important factor in driving the heavy haze formation in BTH.

High time resolution observation of PM Brown carbon over Xi'an in northwestern China: Seasonal variation and source apportionment.

There is growing evidence suggesting the enhancement of brown carbon (BrC) in severe haze episodes. In this study, hourly measurements of BrC in PM were conducted in Xi'an, a typical city in northwestern China during winter and summer. The absorption coefficient for methanol exacts at 365 nm (b, which is typically used as a proxy of methanol-soluble BrC) in the winter sampling period was over 7 times than that in summer.

Wintertime nitrate formation during haze days in the Guanzhong basin, China: A case study.

In this study, the formation of nitrate aerosol from 16 to 24 December 2015 in the Guanzhong basin, China is simulated using the WRF-Chem model. The predicted near-surface O, NO, and fine particulate matters (PM) in the basin and inorganic aerosols and nitrous acid (HONO) in Xi'an are generally in good agreement with the observations. Sensitivity studies show that the heterogeneous HONO sources play an appreciable role in the nitrate formation in the basin, contributing 9.

Impacts of sea-land and mountain-valley circulations on the air pollution in Beijing-Tianjin-Hebei (BTH): A case study.

In the study, observational data analyses and the WRF-CHEM model simulations are used to investigate the role of sea-land and mountain-valley breeze circulations in a severe air pollution event occurred in Beijing-Tianjin-Hebei (BTH) during August 9-10, 2013. Both the wind observations and the model simulations have clearly indicated the evolution of the sea-land and mountain-valley breeze circulations during the event. The WRF-CHEM model generally reproduces the local meteorological circulations and also performs well in simulating temporal variations and spatial distributions of fine particulate matters (PM) and ozone (O) concentrations compared to observations in BTH.

Impacts of local circulations on the wintertime air pollution in the Guanzhong Basin, China.

Urbanization and industrialization in the recent 30years have caused frequent heavy haze pollution in the Guanzhong basin, China during wintertime. Based on the categorized large-scale synoptic situations, the local circulation and its impact on the air pollution in the basin have been investigated using observational data analyses and model simulations with the WRF-CHEM model. The simulated mountain-valley breeze circulations are well established under most of the large-scale synoptic situations in the south-north direction.

Critical role of meteorological conditions in a persistent haze episode in the Guanzhong basin, China.

In the present study, the critical role of the meteorological condition in a persistent extreme haze episode that occurred in Guanzhong basin of China on December 16 to 25, 2013 has been investigated. Analyses of the large-scale meteorological conditions on 850hPa during the episode have been performed using the NCEP FNL data set, indicating that synoptic situations generally facilitate the accumulation of pollutants either in horizontal or vertical directions in the basin. The FLEXPART model has been utilized to illustrate the pollutant transport patterns during the episode, further showing the dominant role of synoptic conditions in accumulation of pollutants in the basin.

Impacts of using an ensemble Kalman filter on air quality simulations along the California-Mexico border region during Cal-Mex 2010 field campaign.

The purpose of this study is to investigate the impact of using an ensemble Kalman filter (EnKF) on air quality simulations in the California-Mexico border region on two days (May 30 and June 04, 2010) during Cal-Mex 2010. The uncertainties in ozone (O3) and aerosol simulations in the border area due to the meteorological initial uncertainties were examined through ensemble simulations. The ensemble spread of surface O3 averaged over the coastal region was less than 10ppb.