Severe Haze Episodes Research Articles

Synergistic effect of reductions in multiple gaseous precursors on secondary inorganic aerosols in winter under a meteorology-based redistributed daily NH3 emission inventory within the Beijing-Tianjin-Hebei region, China

Secondary inorganic aerosols (SIA) account for 20–60% of the total fine particulates in the Beijing-Tianjin-Hebei (BTH) region of China, indicating an urgent need to clarify the relationship among such compounds. The purpose of this study was to quantify the relationship between emissions of NH3, NOx, SO2, VOCs and SIA concentrations during a severe winter haze episode using an air quality model and a meteorology-based redistributed NH3 emission inventory within the BTH region. The results showed that the model performance regarding the NH3 simulations in January by the four emission inventories improved after the redistribution of daily NH3 emissions, with an increase of 0.02–0.13 in R, a 9–56% decrease in NMB, and a 7–51% decrease in NME. The updated simulations reproduced the daily observations of SIA, SO2, and NO2 well. A total of 125 sets of sensitivity simulations showed that a synergistic reduction in NH3 and VOCs was more efficient in terms of SIA control than simply reducing SO2 or NOx in the BTH region. If only NOx emissions were reduced, the SIA concentration would first increase and then decrease, and it could decline by another 0.86–8.03% in parallel with an equal NH3 emission cut. SIA could be reduced by approximately 22.68% with the most stringent inorganic precursors' control. Moreover, VOCs emission reductions could lead to a decrease in SIA, and the impact of VOCs on SIA was similar to that of NH3. The collaborative control of both inorganic precursors and VOCs was more effective than single-factor control measures for decreasing SIA, and the decline rate was approximately 29.26% under minimum emission conditions. This improved effectiveness was obtained because VOCs mitigation effectively decreases the ozone concentration, which in turn influences SIA formation. Finally, on the premise of a 60% SO2 cut, the reduction scheme NH3:VOCs:NOx = 4:4:1 was suggested for SIA control.

Investigation of sources and formation mechanisms of fine particles and organic aerosols in cold season in Fenhe Plain, China

Air quality has been improved significantly from 2017 to 2020 in Fenhe Plain, one of the most polluted regions in China. However, severe haze episodes from days to weeks were still frequently observed in cold seasons, and the sources and formation mechanisms especially organic aerosols are poorly understood due to the limited highly time-resolved measurements. Here we have a comprehensive characterization of chemical composition, variations and sources of fine particles in fall and winter in Fenhe Plain mainly using a PM2.5 Aerosol Chemical Speciation Monitor. Our results showed that organics and nitrate were two major species of PM2.5, on average accounting for 35.4% and 27.0%, respectively, and they played consistently important roles across different pollution levels. Positive matrix factorization revealed three primary sources from traffic, biomass burning and coal combustion emissions, and two secondary factors that are associated with photochemical and aqueous-phase processing. While secondary OA (SOA) dominated OA for the entire study (63%), large increases in primary OA from 19% to 41% were observed during heating period. Particularly, we found a high contribution of photochemical OOA (45%) during severely polluted periods in heating season demonstrating the presence of strong photochemical production in winter. Most aerosol species presented similarly pronounced diurnal cycles with low concentrations during daytime indicating the major influences from planetary boundary layer and primary emissions. The bivariate polar plots further illustrated the different sources of primary and secondary species, i.e., local emissions versus regional transport from the south and southeast. By analyzing four case studies, we found three major formation mechanisms of severe haze episodes that were driven by photochemical processing, aqueous-phase production, and local emissions, respectively. Therefore, different mitigating strategies are suggested in Fenhe Plain depending on the types of severe haze episodes.

Molecular Composition of Oxygenated Organic Molecules and Their Contributions to Organic Aerosol in Beijing.

The understanding at a molecular level of ambient secondary organic aerosol (SOA) formation is hampered by poorly constrained formation mechanisms and insufficient analytical methods. Especially in developing countries, SOA related haze is a great concern due to its significant effects on climate and human health. We present simultaneous measurements of gas-phase volatile organic compounds (VOCs), oxygenated organic molecules (OOMs), and particle-phase SOA in Beijing. We show that condensation of the measured OOMs explains 26-39% of the organic aerosol mass growth, with the contribution of OOMs to SOA enhanced during severe haze episodes. Our novel results provide a quantitative molecular connection from anthropogenic emissions to condensable organic oxidation product vapors, their concentration in particle-phase SOA, and ultimately to haze formation.

Precursors and Pathways Leading to Enhanced Secondary Organic Aerosol Formation during Severe Haze Episodes.

Molecular analyses help to investigate the key precursors and chemical processes of secondary organic aerosol (SOA) formation. We obtained the sources and molecular compositions of organic aerosol in PM2.5 in winter in Beijing by online and offline mass spectrometer measurements. Photochemical and aqueous processing were both involved in producing SOA during the haze events. Aromatics, isoprene, long-chain alkanes or alkenes, and carbonyls such as glyoxal and methylglyoxal were all important precursors. The enhanced SOA formation during the severe haze event was predominantly contributed by aqueous processing that was promoted by elevated amounts of aerosol water for which multifunctional organic nitrates contributed the most followed by organic compounds having four oxygen atoms in their formulae. The latter included dicarboxylic acids and various oxidation products from isoprene and aromatics as well as products or oligomers from methylglyoxal aqueous uptake. Nitrated phenols, organosulfates, and methanesulfonic acid were also important SOA products but their contributions to the elevated SOA mass during the severe haze event were minor. Our results highlight the importance of reducing nitrogen oxides and nitrate for future SOA control. Additionally, the formation of highly oxygenated long-chain molecules with a low degree of unsaturation in polluted urban environments requires further research.

Impacts of secondary aerosol formation and long range transport on severe haze during the winter of 2017 in the Seoul metropolitan area

Severe haze episodes occur frequently in the Seoul Metropolitan Area (SMA) and throughout East Asian countries, especially during the winter and early spring. We investigated the sources and chemistry of particulate matter (PM) during three winter haze episodes in Seoul that occurred between January 1st and February 10th in 2017 using a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and positive matrix factorization (PMF) analysis.The average concentration of sub-micrometer aerosol (PM1 = NR-PM1 + black carbon (BC)) was 32.6 μg/m3, which was composed of 42% organics, 27% nitrate, 11% sulfate, 13% ammonium and 4% BC by mass. Six distinct sources of organic aerosol (OA) were identified: vehicle emitted hydrocarbon-like OA (HOA), cooking OA (COA), biomass burning OA (BBOA), and 3 different types of secondary OA (SOA) with varying degrees of oxidation and temporal trends.The nitrate mass fraction increased during the three haze episodes, with nitrate accounting for 27-33% of PM1 mass. Enhanced nitrate concentrations and higher nitrate oxidation ratios (NOR), despite lower enhancement in relative humidity (RH) than the low PM loading period, suggest that regional transport of nitrate contributed to the nitrate mass during the haze periods.Lower HOA and COA concentrations during the high PM loading periods further confirm that local emissions or stagnant meteorological conditions were not the main reason for the severe haze. Residence time analysis (RTA), concentration field analysis (CFA), and column-CFA results from FLEXPART also showed that the measurement period was accompanied by atmospheric transport of nitrate, sulfate, and ammonium from eastern China. Nevertheless, we found that NO2, a precursor of nitrate, was predominantly from local emissions. These findings suggest that nitrate in Seoul is not only locally formed, but also transported from upwind areas.

Local emissions and secondary pollutants cause severe PM2.5 elevation in urban air at the south edge of the North China Plain: Results from winter haze of 2017–2018 at a mega city

Severe haze occurrence in the north of the North China Plain (NCP) is recognized as a consequence of the regional transport of pollutants initially from the south and then the rapid formation of secondary pollutants in the local air. However, the origin of pollutants causing haze in the southern NCP has not yet been elucidated even through careful data observation. Based on the contents of water-soluble inorganic ions in PM2.5 samples collected during two severe haze episodes in Zhengzhou, a mega city located on the southern edge of the NCP, we estimated the contributions of local primary emissions and secondary pollutants to haze occurrence. On average, Na+, K+, and Ca2+ mainly originated from anthropogenic sources, and their anthropogenic fractions had proportions of 97.5%, 93.9%, and 76.5% in their respective total mass. Anions Cl− and SO42− substantially originated from not only produced substantially via secondary formation but also from primary emissions, and their primary proportions in their respective total mass were 51.1% and 30.8%. In contrast, NH4+ and NO3− were dominated by secondary formation. The increase in PM2.5 was mainly caused by the formation of secondary inorganic (29.1%) and organic species (57.2%) and the primary anthropogenic emissions (12.5%). These results indicated that the haze at the southern edge of the NCP was mainly caused by pollutants in the local areas. Compared to the haze in the northern NCP, the haze in the southern NCP edge had a higher PM2.5 mass concentration and a higher proportion of secondary species, but a lower proportion of primary species, indicating the high heterogeneity of winter haze over the NCP.

Haze episodes before and during the COVID-19 shutdown in Tianjin, China: Contribution of fireworks and residential burning

Potential health benefits from improved ambient air quality during the COVID-19 shutdown have been recently reported and discussed. Despite the shutdown measures being in place, northern China still suffered severe haze episodes (HE) that are not yet fully understood, particularly how the source emissions changed. Thus, the meteorological conditions and source emissions in processing five HEs occurred in Beijing-Tianjin-Hebei area were investigated by analyzing a comprehensive real-time measurement dataset including air quality data, particle physics, optical properties, chemistry, aerosol lidar remote sensing, and meteorology. Three HEs recorded before the shutdown began were related to accumulated primary pollutants and secondary aerosol formation under unfavorable dispersion conditions. The common “business as usual” emissions from local primary sources in this highly polluted area exceeded the wintertime atmospheric diffusive capacity to disperse them. Thus, an intensive haze formed under these adverse meteorological conditions such as in the first HE, with coal combustion to be the predominant source. Positive responses to the shutdown measures were demonstrated by reduced contributions from traffic and dust during the final two HEs that overlapped the Spring and Lantern Festivals, respectively. Local meteorological dispersion during the Spring Festival was the poorest among the five HEs. Increased residential burning plus fireworks emissions contributed to the elevated PM2.5 with the potential of enhancing the HEs. Our results highlight that reductions from shutdown measures alone do not prevent the occurrence of HEs. To further reduce air pollution and thus improve public health, abatement strategies with an emphasis on residential burning are needed.

Vertical Characteristics of Pollution Transport in Hong Kong and Beijing, China

Transported pollution plays an important role in the atmospheric environment of eastern China. This study analyzed the characteristics of surface winds at different air quality levels using meteorological station observations of both wind and mass concentrations of particulate matter with aerodynamic diameters <2.5 μm (PM2.5) over Hong Kong and Beijing. In recent decades, wind directions at the surface exhibit a similar pattern for both good and poor air quality levels at all three stations, indicating a weak relationship between surface winds and air quality in Hong Kong. However, winds at a height of 1–2 km govern pollution accumulation. This dominant role is illustrated by a sudden change in wind direction within this layer and a simultaneous pollution accumulation stage on 8 January 2014. The controlling influence of winds at 1–2 km on both the deterioration and improvement of air quality is also supported by a distinct vertical wind distribution for all 21 monotonic increasing stages and 17 decreasing stages of PM2.5. In contrast, air pollution is transported to Beijing throughout the atmospheric layer that extends from the surface to a height of more than 3 km. This key difference may be due to variations in meteorology, topography, and emission sources between Hong Kong and Beijing. The results that layer of 1–2 km in Hong Kong and of surface to 3 km in Beijing is the height where pollution transport is most likely to occur are critical for forecasting severe haze episodes in eastern China.

Effects of Aerosol Water Content on the formation of secondary inorganic aerosol during a Winter Heavy PM2.5 Pollution Episode in Xi'an, China

Sulfate, nitrate and ammonium are the most abundant secondary inorganic aerosols (SIA) in atmospheric fine particle matter (PM2.5). Meteorological conditions, gas-particle transportation process, and aerosol acidity (pH) can influence SIA formation. In this study, we conducted semi-continuous measurements of water-soluble inorganic ions during a winter extreme pollution event (from January 9th to January 17th, 2015, average PM2.5 concentrations of ~250 μg m−3 and peak concentrations of ~700 μg m−3) in Xi'an to elucidate on the SIA formation mechanism. The hourly mean level of the total water-soluble ion was 137.4 μg m−3, accounting for 55.3% of PM2.5 on average. The dominant ions concentrations of SO42−, NO3− and NH4+ accounted for 15.8%, 19.0% and 13.2% of PM2.5 mass, respectively. PM2.5 was moderately acidic, with an average pH of 4.8 ± 0.4. Compared to the clean periods, sulfate content decreased by 6.9% during the polluted periods, while those of nitrate and ammonium increased by 2.2% and 5.0%, respectively. The increase in nitrogen oxidation ratios (NOR) and ammonia conversion ratio (NHR) from normal days to haze days were greater when comparison to sulfur oxidation ratios (SOR). In the polluted periods, sulfate and nitrate formations were facilitated by water content increase. Strong linear correlation coefficients between SOR (R2 = 0.81) and NOR (R2 = 0.55) with aerosol water content indicated that the gas-liquid reaction of SO2 and NO2 is the major pathway of sulfate and nitrate formation during severe haze episodes. In contrast, the NHR and aerosol water content exhibited a logarithmic relationship, which reveals that when water content was greater than 100 μg m−3, the gas-particle partitioning ratio of ammonium was basically unchanged following an increase in water content.

Observational Analysis of Aerosol–Meteorology Interactions for the Severe Haze Episode in Korea

Korea has occasionally suffered from various kinds of severe hazes such as long-range transported aerosol (LH), yellow sand (YS), and urban haze (UH). We classified haze days into LH, YS, and UH and analyzed the characteristics of its associated meteorological conditions for 2011–2016 using reanalysis data and surface observations. The results show that higher boundary layer height and stronger wind speed were found for the LH and YS hazes relative to those for UH. Intensive analysis on a golden episode of 10–18 January 2013 indicates that the cloud fraction increased along with extended light precipitation at a weaker rate by enhanced aerosol loading for an unprecedented LH event, which in turn brought about a decrease in boundary layer height (BLH) with less irradiance, that is, much stronger stability. Later, the intensified stability after the LH event accumulated and increased domestic aerosols, and eventually resulted in the longer-lasting severe haze. This study suggests that aerosol–meteorology interactions play an important role in both short-term weather and fine particle forecasts, especially on polluted days.

Inversion breakup over different shapes of urban areas

Severe haze episodes and extreme heat events are both related to the strong inversion in the atmospheric boundary layer. Inversion breakup is the main mechanism to alleviate the accumulation of heat or pollutants, especially in urban areas. However, the physical process and turbulent flow structures of inversion breakup remain unclear. In this study, the characteristics of mean flow structures and turbulence statistics of inversion breakup at the two-dome stage were investigated by conducting reduced-scale water tank experiments. A two-layer density profile (i.e. a lower-level stable layer and an upper-level neutral layer) was created to simulate the surface-based inversion covered by a neutral residual layer. Various shapes of urban areas were tested. The results showed that the lower-level dome in the stable layer (Dome 1) and the upper-level dome in the neutral layer (Dome 2) are both substantially affected by the urban shape. In Dome 1, the convergent inflow is mainly along the angular bisector across the urban corners, whereas the divergent outflow is perpendicular to the edges. In Dome 2, the divergent outflow is on the same vertical plane with the convergent inflow (i.e. along the angular bisector). Furthermore, the turbulence production and momentum transfer are mainly induced by the thermal plume structures near the urban centre, indicating the strong influences of buoyancy. In divergent outflow regions, both shear and buoyancy play important roles.

Improving PM2.5 forecast during haze episodes over China based on a coupled 4D-LETKF and WRF-Chem system

To improve the PM2.5 forecast during severe haze episodes, we developed a data assimilation system based on the four-dimensional local ensemble transform Kalman filter (4D-LETKF) and the WRF-Chem model to assimilate surface PM2.5 observations. The data assimilation system was successful in optimizing the initial PM2.5 mass concentrations. The root-mean-square error (RMSE) of the initial PM2.5 concentrations after assimilation decreased at 76.75% of the stations and the RMSE reduction exceeds 30% at 20.7% of the stations. The correlation coefficients for the PM2.5 analyses increased by more than 0.3 at 33% of the stations. The forecasts for the spatial distribution and evolution of the haze were improved remarkably after assimilation while the forecasts without assimilation usually significantly underestimated the PM2.5 mass concentrations during the severe haze episodes. The RMSE of the 24-h forecasts after assimilation can be reduced by 32.02% in the polluted regions. During haze episodes, the 48-h forecasts after assimilation can benefit from the assimilation to a similar extent with the 24-h forecasts. Both the forecast accuracy and the duration of assimilation benefits were improved remarkably which demonstrate the effectiveness of the 4D-LETKF-PM2.5 data assimilation system, and further experiments are to be conducted to improve its performance.

Long-term characterization of aerosol chemistry in cold season from 2013 to 2020 in Beijing, China.

Severe haze episodes in cold season in Beijing have been mitigated greatly during the last decade. However, the changes in aerosol chemistry as responses to the large reductions in gaseous precursors during the two phases of clean air action, i.e., phase Ⅰ (2013-2017) and phase Ⅱ (2018-2020), are less understood. Here we characterized such changes in cold season (January-March) by using five-year real-time aerosol particle composition measurements. Our results showed consistently large reductions for all chemical species from 2013 to 2020 with the largest decreases being chloride (95%) and organics (74%) followed by sulfate (69%), while the decreases in nitrate were comparatively small (44%). However, the contributions of sulfate were fairly stable despite the increased nitrate contributions from 18% in 2013 to 30% in 2020. Organic aerosol (OA) composition also changed significantly since 2018 with large increases in the contributions of secondary OA and corresponding decreases in primary OA from fossil fuel combustion and cooking emissions. The changes in aerosol chemistry were closely related to the different reductions in gaseous precursors, e.g., SO2 vs. NO2, and the enhanced secondary processes, e.g., the increases in O3, sulfur and nitrogen oxidation efficiency. Further, we found that the changes in aerosol chemistry in cold season during the phase Ⅱ of clean air action (2018-2020) started to slow down with relatively small changes in PM2.5 and secondary inorganic species. Our results point towards a future challenge in mitigating air pollution in cold season, and the need of more stringent and scientific strategies to control secondary aerosol pollution in an environment with enhanced oxidation capacity and high precursors.

Chemical formation pathways of secondary organic aerosols in the Beijing-Tianjin-Hebei region in wintertime

A regional air quality model system (RAQMS) was developed by incorporating an aqueous reaction mechanism for secondary organic aerosol (SOA) formation and primary semi-volatile (SVOC) and intermediate volatile organic compounds (IVOC) precursors to investigate various chemical pathways for SOA formation in the Beijing-Tianjin-Hebei (BTH) region in wintertime. Model comparison against observations demonstrates that the model is able to well reproduce meteorological variables and major aerosol chemical components, and the model improves SOA simulation significantly by including primary S/IVOCs (SVOC + IVOC) and aqueous reactions into the model. SVOC and IVOC emissions were relatively higher over the regions from southern Hebei province to Beijing and Tianjin, with the emission rates up to 0.15 mg m−2 hr−1 and 0.3 mg m−2 hr−1, respectively. The glyoxal (GLY) and methylglyoxal (MGLY) emissions were similar in distribution to those of S/IVOC emissions, with the maximum of 0.08 mg m −2 hr−1. The simulated SOA concentrations exhibited a southwest-northeast belt extending from southern Hebei to Beijing, with the maximum of 35 μg m−3 in southern Hebei province. The average contributions from various precursors or chemical pathways to SOA formation during the study period were estimated, in which AVOCs (anthropogenic VOCs), SVOCs, IVOCs, BVOCs (biogenic VOCs), GLY and MGLY contributed 38.4%, 24.9%, 28.4%, 0.2% and 8.1% of SOA mass concentration, respectively, in the BTH region. The maximum average contribution of GLY and MGLY to SOA reached 16% in southern Hebei province during a severe haze episode. From the clean to haze periods, concentrations of all SOA components apparently increased along with increasing atmospheric stability and weakening dispersion, but the increasing rates of SOA concentration produced by AVOCs and aqueous reactions were larger than those by primary S/IVOCs due to different chemical processes, and the AVOC-produced SOA dominated over the other SOA components during the haze periods.

Measurement report: Characterization of severe spring haze episodes and influences of long-range transport in the Seoul metropolitan area in March 2019

Abstract. Severe haze episodes have occurred frequently in the Seoul metropolitan area (SMA) and throughout East Asian countries, especially during winter and early spring. Although notable progress has been attained in understanding these issues, the causes of severe haze formation have not yet been fully investigated. SMA haze is especially difficult to understand, because the area is impacted by both local emissions from anthropogenic and biogenic activities and emissions transported from upwind sources. Here, we investigated the emission sources and formation processes of particulate matter (PM) during three haze episodes measured in early spring of 2019, from 22 February to 2 April, using a high-resolution aerosol mass spectrometer (HR-AMS). Overall, the average concentration of nonrefractory submicron aerosol (NR-PM1) + BC (black carbon) was 35.1 µg m−3, which was composed of 38 % organics, 12 % SO4, 30 % NO3, 13 % NH4, and 5 % BC. The organics had an average oxygen-to-carbon ratio (O∕C) of 0.52 and an average organic mass to organic carbon ratio (OM∕OC) of 1.86. Seven distinct sources of organic aerosols (OAs) were identified via positive matrix factorization (PMF) analysis of the HR-AMS data: vehicle-emitted hydrocarbon-like OA (HOA), cooking OA (COA), solid-fuel-burning emitted OA (SFOA), and four different types of oxidized secondary OA with varying oxidation degrees and temporal trends. Of the 40 d of the measurement period, 23 were identified as haze days (daily average: >35 µg m−3), during which three severe haze episodes were recorded. In particular, PM1 concentration exceeded 100 µg m−3 during the first episode when an alert was issued, and strict emission controls were implemented in the SMA. Our results showed that nitrate dominated during the three haze episodes and accounted for 39 %–43 % of the PM1 concentration on average (vs. 21 %–24 % during the low-loading period), for which there were indications of regional-transport influences. Two regional-transport-influenced oxidized organic aerosols (OOAs), i.e., less oxidized OOA2 (LO-OOA2) and more oxidized OOA2 (MO-OOA2), contributed substantially to the total PM1 during the haze period (12 %–14 % vs. 7 % during the low-loading period), as well. In contrast, HOA and COA only contributed little (4 %–8 % vs. 4 %–6 % during the low-loading period) to the PM1 concentration during the haze days, indicating that local emissions were likely not the main reason for the severe haze issues. Hence, from simultaneous downwind (SMA) and upwind (Beijing) measurements using HR-AMS and ACSM (aerosol chemical speciation monitor) over the same period, the temporal variations in PM1 and each chemical species showed peak values on the order of Beijing (upwind) to the SMA for approximately 2 d. Furthermore, lead (Pb) derived from HR-AMS measurements was observed to increase significantly during the haze period and showed good correlations with MO-OOA2 and LO-OOA2, which is consistent with regional sources. A multiple linear regression model indicated that the transported regionally processed air masses contributed significantly to Pb in the SMA (31 %), especially during the haze period, although local burning was also important by contributing 38 %. The above results suggest that regional transport of polluted air masses might have played an important role in the formation of the haze episodes in the SMA during early spring.

Impacts of PBL schemes on PM2.5 simulation and their responses to aerosol-radiation feedback in GRAPES_CUACE model during severe haze episodes in Jing-Jin-Ji, China

Using the online-coupled Global/Regional Assimilation and Prediction System coupled with the Chinese Unified Atmospheric Chemistry Environment (GRAPES_CUACE) model, numerical experiments are conducted to compare the impacts of three planetary boundary layer (PBL) schemes—Medium Range Forecast (MRF), Yonsei University (YSU), and Mellor-Yamada-Janjic (MYJ)—on PM2.5 simulations and their responses to aerosol-radiation feedback (ARF), under different terrain and weather conditions. The baseline experiments without ARF with all three PBL schemes can reproduce the spatial and temporal distribution characteristics of surface PM2.5 on the whole and show that the model can capture the PM2.5 variation better in unstable conditions (clean days) than in stable conditions (pollution days). The three PBL schemes show differences in the simulations of meteorological factors, e.g., the MRF scheme produces the best 10-m wind speed, the MYJ scheme produces the best 2-m temperature, and the YSU scheme produces the lowest PBL height (PBLH) and vertical diffusivity. Consequently, there is no significant difference among the three PBL schemes in surface PM2.5 simulations due to the combined action of the surface and the PBL structure. However, all the baseline experiments massively underestimate PM2.5 during severe pollution days. The online computing of ARF in the three PBL schemes can substantially improve the underestimation of PM2.5 by reducing sensible heat fluxes, 2-m temperature, friction velocity, and the PBL height. More importantly, the response intensity of three PBL schemes to ARF is very different. For PM2.5 simulations, MYJ has the weakest response, whereas MRF and YSU have similar responses. During the heavy pollution days with more intense aerosol-radiation effect, the increase of PM2.5 induced by ARF in MRF and YSU is about twice that in MYJ, making the PM2.5 simulated by MRF and YSU more consistent with the observations. This result shows the ARF dependence on the PBL processes and suggests the importance of PBL-ARF interaction to the PM2.5 numerical prediction during haze episodes.

Projection of weather potential for winter haze episodes in Beijing by 1.5 °C and 2.0 °C global warming

Haze episodes become very frequent in Beijing over the past decade, and such trend is related to favorable weather conditions. Here, we project the changes of weather conditions conducive to winter haze episodes in Beijing by 1.5 °C and 2.0 °C global warming using Haze Weather Index (HWI) and data of ensemble simulations from the Community Earth System Model (CESM) low-warming experiment. Compared to present day (2006–2015), the frequency in winter season is projected to increase by 14% for regular haze episodes (HWI > 0) and 21% for severe haze episodes (HWI > 1) at the 1.5 °C global warming. Projections shows larger increases of 27% for regular and 18% for severe haze events at the 2 °C global warming. The additional warming of 0.5 °C largely enhances the persistence of weather conditions conducive to haze episodes. The increased temperature contrast between near-surface and mid-troposphere in eastern Asia accounts for 57% and 81% of the change in HWI by 1.5 °C and 2 °C warming, respectively. Considering increased haze weather potential caused by climate warming, we suggest that additional efforts in emission reductions of carbon dioxide and air pollution are necessary to mitigate haze episodes in Beijing.

A modeling study of PM2.5 transboundary transport during a winter severe haze episode in southern Yangtze River Delta, China

The WRF-CHEM (Weather Research Forecast-Chemistry) model was used to investigate a severe haze episode in Zhejiang province in southern Yangtze River Delta, China, during 28 November to 10 December 2013 to obtain the characteristics of PM2.5 transboundary transport and its contributions to haze formation. The predicted temporal variations of PM2.5 and meteorological variables are in good agreement with in-situ observations in Zhejiang. Simulated wind direction and pollution levels in upwind areas have significant effects on modeled near-surface PM2.5 concentrations. Both the large-scale atmospheric circulations and local weather conditions contributed to the accumulation of pollutants in Zhejiang, featuring the weakened East Asia trough, the dominance of a weak high pressure system, a low wind speed and a low planetary boundary layer height (PBLH) during the heavily polluted period from 4 to 7 December. The near-surface PM2.5 concentration was highly positively correlated with the accumulated PM2.5 transport mass (R = 0.88 at a lag of 7 h) and moderately negatively correlated with the PBLH (R = −0.39) during this period, indicating that the horizontal transport across Zhejiang's boundary under stable meteorological condition played important roles in the high PM2.5 concentrations. The analysis of PM2.5 transboundary transport flux shows that the main transport pathway for outside pollutants affecting Zhejiang was mainly the northerly flow. There were strong input fluxes mainly within the planetary boundary layer on 4 and 6 December before the maximum concentration occurred in the early morning on 7 December. The contribution of PM2.5 transboundary transport to the aerosol pollution in the study domain was determined by a receptor-oriented model based on the conservation of mass formula for PM2.5. The PM2.5 transboundary transport across the Zhejiang's boundary contributed 62% to the total PM2.5 increase during the severe haze episode, and the primary emission and secondary PM2.5 formation contributed 15% and 23%, respectively. This result suggests that the joint efforts with neighboring provinces to mitigate pollutant emissions are important to improve air quality in Zhejiang during winter.

Aerosol radiative effects and feedbacks on boundary layer meteorology and PM<sub>2.5</sub> chemical components during winter haze events over the Beijing-Tianjin-Hebei region

Abstract. An online coupled regional climate–chemistry–aerosol model (RIEMS-Chem) was developed and utilized to investigate the mechanisms of haze formation and evolution and aerosol radiative feedback during winter haze episodes in February–March 2014 over the Beijing-Tianjin-Hebei (BTH) region in China. Model comparison against a variety of observations demonstrated a good ability of RIEMS-Chem in reproducing meteorological variables, planetary boundary layer (PBL) heights, PM2.5, and its chemical components, as well as aerosol optical properties. The model performances were remarkably improved for both meteorology and chemistry by taking aerosol radiative feedback into account. The domain-average aerosol radiative effects (AREs) were estimated to be −57 W m−2 at the surface, 25 W m−2 in the atmosphere, and −32 W m−2 at the top of atmosphere (TOA) during a severe haze episode (20–26 February), with the maximum hourly surface ARE reaching −384 W m−2 in southern Hebei province. The average feedback-induced changes in 2 m air temperature (T2), 10 m wind speed (WS10), 2 m relative humidity (RH2), and PBL height over the BTH region during the haze episode were −1.8 ∘C, −0.5 m s−1, 10.0 %, and −184 m, respectively. The BTH average changes in PM2.5 concentration due to the feedback were estimated to be 20.0 µg m−3 (29 %) and 45.1 µg m−3 (39 %) for the entire period and the severe haze episode, respectively, which demonstrated a significant impact of aerosol radiative feedback on haze formation. The relative changes in secondary aerosols were larger than those in primary aerosols due to enhanced chemical reactions by aerosol feedback. The feedback-induced absolute change in PM2.5 concentrations was largest in the haze persistence stage, followed by those in the growth stage and dissipating stage. Process analyses on haze events in Beijing revealed that local emission, chemical reaction, and regional transport mainly contributed to haze formation in the growth stage, whereas vertical processes (diffusion, advection, and dry deposition) were major processes for PM2.5 removals. Chemical processes and local emissions dominated the increase in PM2.5 concentrations during the severe haze episode, whereas horizontal advection contributed to the PM2.5 increase with a similar magnitude to local emissions and chemical processes during a moderate haze episode on 1–4 March. The contributions from physical and chemical processes to the feedback-induced changes in PM2.5 and its major components were explored and quantified through process analyses. For the severe haze episode, the increase in the change rate of PM2.5 (9.5 µg m−3 h−1) induced by the feedback in the growth stage was attributed to the larger contribution from chemical processes (7.3 µg m−3 h−1) than that from physical processes (2.2 µg m−3 h−1), whereas, during the moderate haze episode, the increase in the PM2.5 change rate (2.4 µg m−3 h−1) in the growth stage was contributed more significantly by physical processes (1.4 µg m−3 h−1) than by chemical processes (1.0 µg m−3 h−1). In general, the aerosol–radiation feedback increased the accumulation rate of aerosols in the growth stage through weakening vertical diffusion, promoting chemical reactions, and/or enhancing horizontal advection. It enhanced the removal rate through increasing vertical diffusion and vertical advection in the dissipation stage, and had little effect on the change rate of PM2.5 in the persistence stage.

Seasonal characterization of aerosol composition and sources in a polluted city in Central China

We characterized the aerosol composition and sources of particulate matter (PM) in Sanmenxia, a polluted city located in the Fen-Wei Plain region of Central China. The PM2.5 concentration decreased by 18% from 72 μg m−3 in 2014 to 59 μg m−3 in 2019. All chemical species presented pronounced seasonal variations, with their highest concentrations in winter due to enhanced emissions and the frequent stagnant meteorological conditions. Nitrate was the major fraction of PM2.5 during all seasons (35–41%) except summer (25%), while sulfate was a dominant species in summer (29%) compared to other seasons (16–18%) from July 2018 to June 2019. The detailed analysis of a wintertime severe haze episode that lasted for approximately half a month demonstrated that secondary aerosols, including secondary organic aerosol, sulfate, nitrate, and ammonium, contributed 89% to non-refractory PM1 (NR-PM1), indicating the remarkable role of secondary aerosol formation in air pollution in Sanmenxia. Positive matrix factorization analysis further showed considerably enhanced low-volatility oxygenated organic aerosol (OA) and hydrocarbon-like OA during severe haze episodes, while significant contributions in semi-volatile oxygenated OA and coal combustion OA during clean periods. Severe pollution events in the city were generally associated with air masses from the southwest, and we also found that aerosol species, especially secondary aerosol species, showed distinct forenoon increases that were caused by the subsidence of air pollutants aloft. Our results highlight that future air quality improvement would benefit substantially from a more efficient control of gaseous precursors, particularly the NOx emissions from industry and vehicle emissions.