Secondary inorganic aerosols and aerosol acidity at different PM2.5 pollution levels during winter haze episodes in the Sichuan Basin, China

Wintertime fine particle (PM2.5) pollution remains to be perplexing air quality problems in many parts of China. In this study, PM2.5 compositions and aerosol acidity at different pollution levels at an urban cite in the southwest China's Sichuan Basin were investigated during a sustained winter haze episode. Organic matter was the most abundant component of PM2.5, followed by nitrate, sulfate and ammonium. Shares of organic aerosol in PM2.5 mass decreased with the elevated PM2.5 levels, while the enhancements of sulfate and secondary organic aerosol were much less than that of nitrate and ammonium during heavy pollution with increased ratios of nitrate to sulfate, implying a significant role of nitrate in the haze formation. Results also suggest the nighttime chemistry might contribute substantially to the formation of nitrate under severe pollutions. The daily average aerosol pH showed a decreasing trend with the elevated levels of PM2.5, and this increased aerosl acidity was mainly due to the fast rising secondary inorganic aerosol (SIA) concentration, with the increase in hydronium ion concentration in air (Hair+) surpassing the dilution effect of elevated aerosol liquid water content (LWC). Thermodynamic model calculations revealed that the air environment was NH3-rich with total NHx (NH3 + NH4+) greater than required NHx, and the aerosol pH exponentially declined with the decreasing excess NHx (p < 0.01). This study demonstrated that under air stagnation and NH3-rich environment during winter, the raised relative humidity (RH) would lead to an increase in LWC and thereby facilitate the aqueous chemistry processes with the neutralization capacity of NH3 to form sulfate and nitrate, which would further increase the LWC and lower the pH. This self-amplifying SIA formation might be crucial to the severe PM2.5 pollution and haze events during winter, and therefore cutting both NOx and NH3 emissions would benefit stopping the self-amplification.

Insights into the source contributions to the elevated fine particulate matter in Nigeria using a source-oriented chemical transport model

In 2021, Nigeria was ranked by the World Health Organization (WHO) as one of the top countries with highly deteriorating air quality in the world. To date, no study has elucidated the sources of elevated fine particulate matter (PM2.5) concentrations over the entire Nigeria. In this study, the Community Multiscale Air Quality (CMAQ) model was applied to quantify the contributions of seven emissions sectors to PM2.5 and its components in Nigeria in 2021. Residential, industry, and agriculture were the major sources of primary PM (PPM) during the four seasons, elemental carbon (EC) and primary organic carbon (POC) were dominated by residential and industry, while residential, industry, transportation, and agriculture were the important sources of secondary inorganic aerosols (SIA) and its components in most regions. PM2.5 was up to 150 μg/m3 in the north in all the seasons, while it reached ∼80 μg/m3 in the south in January. Residential contributed most to PM2.5 (∼80 μg/m3), followed by industry (∼40 μg/m3), transportation (∼20 μg/m3), and agriculture (∼15 μg/m3). The large variation in the sources of PM2.5 and its components across Nigeria suggests that emissions control strategies should be separately designed for different regions. The results imply that urgent control of PM2.5 pollution in Nigeria is highly necessitated.

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.

Causes of PM2.5 pollution in an air pollution transport channel city of northern China

To develop effective mitigation policies, a comprehensive understanding of the evolution of the chemical composition, formation mechanisms, and the contribution of sources at different pollution levels is required. PM_2.5 samples were collected for 1 year from August 2016 to August 2017 at an urban site in Zibo, then chemical compositions were analyzed. Secondary inorganic aerosols (SNA), anthropogenic minerals (MIN), and organic matter (OM) were the most abundant components of PM_2.5, but only the mass fraction of SNA increased as the pollution evolved, implying that PM_2.5 pollution was caused by the formation of secondary aerosols, especially nitrate. A more intense secondary transformation was found in the heating season (from November 15, 2016, to March 14, 2017), and a faster secondary conversion of nitrate than sulfate was discovered as the pollution level increased. The formation of sulfate was dominated by heterogeneous reactions. High relative humidity (RH) in polluted periods accelerated the formation of sulfate, and high temperature in the non-heating season also promoted the formation of sulfate. Zibo city was under ammonium-rich conditions during polluted periods in both seasons; therefore, nitrate was mainly formed through homogeneous reactions. The liquid water content increased significantly as the pollution levels increased when the RH was above 80%, indicating that the hygroscopic growth of aerosol aggravated the PM_2.5 pollution. Source apportionment showed that PM_2.5 was mainly from secondary aerosol formation, road dust, coal combustion, and vehicle emissions, contributing 36.6%, 16.5%, 14.7%, and 13.1% of PM_2.5 mass, respectively. The contribution of secondary aerosol formation increased remarkably with the deterioration of air quality, especially in the heating season.

Investigating the effect of sources and meteorological conditions on wintertime haze formation in Northeast China: A case study in Harbin

Heavy haze pollution has occurred frequently in the past few years in Northeast China during winters, which was distinct from other regions in China because of the particular meteorological conditions. In this study, we analyzed the temporal variation, source appointment, and influencing factors of PM_2.5 from December 1, 2018 to February 28, 2019 in Harbin. The results showed obvious differences between the non-haze and haze periods. The source appointment based on a single-particle aerosol mass spectrometer showed that coal combustion, vehicle emissions, biomass burning, and secondary inorganic aerosols (SIAs) were the major contributors of PM_2.5. It is interesting that from the non-haze to the haze period, contributions of coal combustion and SIAs increased (from 20.2% to 27.3%, and from 17.3% to 18.9%, respectively) while other sources decreased or increased little. It indicated the primary pollutants from heating supply were the most important contributor to haze formation due to the low temperature. Furthermore, from levels I (0 < PM_2.5 ≤ 75 μg m^-3) to III (115 < PM_2.5 ≤ 150 μg m^-3), SIAs increased from 15.3% to 19.4% (increased 4.1%), while coal combustion from 23.7% to 27.1% and increased 3.4%. It implied clearly that SIAs played a comparable role in the early stage of the evolution of haze episode as that of coal combustion. Combining data on prevailing winds and results of potential source contribution function indicated that PM_2.5 during the haze period was primarily influenced by the air masses originating from the southwestern areas via regional transport. A positive correlation was observed between relative humidity (RH) and haze pollution when RH ≥ 60%, indicating that hygroscopic growth may be the principal factor promoting secondary formation. CAPSULE: Coal combustion was the most important source in Harbin due to the low temperature, and secondary aerosols promoted the early stage of the haze evolution.

Atmospheric ammonia and its effect on PM2.5 pollution in urban Chengdu, Sichuan Basin, China

Controlling ammonia (NH₃) emissions has been proposed as a strategy to mitigate haze pollution. To explore the role of NH₃ in haze pollution in Sichuan Basin, where agricultural activities are intense, hourly in situ data of NH₃, as well as nitric acid and secondary inorganic aerosols (SIAs) were gathered in Chengdu from April 2017 to March 2018. We found that NH₃ had an annual mean concentration of 9.7 ± 3.5 (mean ± standard deviation) μg m^-3, and exhibited seasonal variations (spring > summer > autumn and winter) due to changes in emission sources and meteorological conditions (particularly temperature). Chengdu's atmosphere is generally NH₃-sufficient, especially in the warm seasons, implying that the formation of SIAs is more sensitive to the availability of nitric acid. However, an NH₃ "sufficient-to-deficient" transition was found to occur during winter pollution periods, and the frequency of NH₃ deficiency increased with the aggravation of pollution. Under NH₃-deficient conditions, the nitrogen oxidation ratio increased linearly with the increase in free NH₃, implying that NH₃ contributes appreciably to the formation of nitrate and thus to high PM_2.5 loadings. No relationships of NH₃ with fossil fuel combustion-related pollutants were found. The NH₃ emissions from farmland and livestock waste in the suburbs of Chengdu and regional transport from west of Chengdu probably contribute to the occurrence of high PM_2.5 loading in winter and spring, respectively. These results suggest that to achieve effective mitigation of PM_2.5 in Chengdu, local and regional emission control of NH₃ and NOx synergistically would be effective.

Nonlinear response of SIA to emission changes and chemical processes over eastern and central China during a heavy haze month

This study used a chemical transport model to investigate the response of secondary inorganic aerosols (SIA) to chemical processes and its precursor emissions over northern and southern city-clusters of China in January 2014. Unexpectedly, SIA concentrations with low levels of precursor emissions were much higher over the southern regions than those over the northern region with high levels of precursor emissions, based on ground observations and high-precision simulations. The sensitivity analysis of chemical processes suggests that the gas-phase chemistry was a critical factor determining the SIA pattern, especially the higher efficiency of nitrogen conversion to nitrate in southern cities controlled by favorable meteorological elements than that in northern city. However, the heterogeneous process led to the decrease of SIA in southern regions by 3% to 36% and the increasing of SIA in NCP by 26.9%, mainly attributing to the impact on nitrate. The reason was that sulfate enhancement by the heterogeneous reactions can compete ammonia (NH₃) and the excessive nitric acid converted into nitrogen oxide (NO_x), leading to nitrate decrease in southern regions under NH₃-deficient regimes. Moreover, through sensitivity experiments of precursor emission reduction by 20%, NH₃ control was found to be the most effective for reducing SIA concentrations comparing to sulfur dioxide (SO₂) and NO_x reduction and a more remarkable decrease of SIA was in southern regions by 10% to 15% than that in northern region by 6.7%. The effect of the synergy control of precursors emission varied in different city-clusters, inferring that the control strategy aimed at improving air quality should be implemented based on specific characteristics of precursors emission in different regions of China.

Sources of PM2.5-bound water soluble ions at EMEP's Auchencorth Moss (UK) supersite revealed by 3D-Concentration Weighted Trajectory (CWT) model

The Concentration Weighted Trajectory (CWT) model is a well-known tool which combines the residence time (trajectory points) of air masses over specific regions with ambient concentrations of air pollutants, aiming to identify potential long range transport impacts. An upgraded 3D-version of CWT model (3D-CWT), investigating not only the geographical origin of the exogenous emissions but also the altitudinal layers in which the transport occurs, was developed and coupled with PM_2.5-bound concentrations of water soluble ions (nss- SO₄^-2 (non-sea salt sulfates), NO₃^-, Cl^-, NH₄⁺, Na⁺, Mg⁺², Ca⁺² and K⁺) for the years 2017-2018, derived by the Auchencorth Moss supersite in Southeast Scotland, United Kingdom (UK). The 3D-CWT model was implemented in two distinct altitudinal layers above ground level (0 m ≤ Layer 1 < 1000 m, 1000 m ≤ Layer 2 < 2000 m), because few trajectory points exceeded the 2000 m limit. Transport of Secondary Inorganic Aerosols (SIA) from South - Southeast England were detected in both vertical layers, affecting SO₄^-2, NO₃^-, and NH₄⁺ levels, whilst SIA intrusions from Northwest Europe were detected in Layer 2. Sea salt particle transport from North Atlantic and the North Sea, comprising Cl^-, Na⁺ and Mg⁺², were detected in both layers whilst K⁺ contributions from Southeast England were also detected in both layers, suggesting also impacts from biomass burning. Moreover particle transport of a crustal origin, marked by Ca⁺² enhancement, mainly occurred in layer 1 and included soil/dust resuspension from areas around the station and infrequent dust intrusions from the Sahara desert.

Long-term residential exposure to PM2.5 constituents and mortality in a Danish cohort

Studies on health effects of long-term exposure to specific PM_2.5 constituents are few. Previous studies have reported an association between black carbon (BC) exposure and cardiovascular diseases (CVD) and a few studies have found an association between sulfate exposure and mortality. These studies, however, relied mainly on exposure data from centrally located air-monitoring stations, which is a crude approximation of personal exposure. We focused on specific chemical constituents of PM_2.5, i.e. elemental and primary organic carbonaceous particles (BC/OC), sea salt, secondary inorganic aerosols (SIA, i.e. NO₃^-, NH₄⁺, and SO₄^2-), and secondary organic aerosols (SOA), in relation to all-cause, CVD and respiratory disease mortality. We followed a Danish cohort of 49,564 individuals from enrollment in 1993-1997 through 2015. We combined residential address history from 1979 onwards with mean annual air pollution concentrations obtained by the AirGIS air pollution modelling system, lifestyle information from baseline questionnaires and socio-demography obtained by register linkage. During 895,897 person-years of follow-up, 10,193 deaths from all causes occurred - of which 2319 were CVD-related and 870 were related to respiratory disease. The 15-year time-weighted average concentrations of PM_2.5, BC/OC, sea salt, SIA and SOA were 13.8, 2.8, 3.4, 4.9, and 0.3 µg/m³, respectively. For all-cause mortality, a higher risk was observed with higher exposure to PM_2.5, BC/OC and SOA with adjusted hazard ratios of 1.03 (95% confidence intervals: 1.01, 1.05), 1.06 (1.03, 1.09), and 1.08 (1.03, 1.13) per interquartile range, respectively. The associations for BC/OC and SOA remained after adjustment for PM_2.5 in two-pollutant models. For CVD mortality, we observed elevated risks with higher exposure to PM_2.5, BC/OC and SIA. The results showed no clear relationship between sea salt and mortality. In this study, we observed a relationship between long-term exposure to PM_2.5, BC/OC, and SOA and all-cause mortality and between PM_2.5, BC/OC, and SIA and CVD mortality.

Increasing importance of nitrate formation for heavy aerosol pollution in two megacities in Sichuan Basin, southwest China

Secondary inorganic aerosols, including sulfate, nitrate, and ammonium contribute to a large extent to the severe haze pollution events in China. Understanding their formation mechanisms is critical for designing effective mitigation strategies to control haze pollution, especially as the role of nitrate seemed to become more important recently, especially in some megacities. In the present study, simultaneous observations were conducted in two megacities (Chengdu and Chongqing) in Sichuan Basin of southwest China, one of the regions suffering from severe aerosol pollution. One typical long-lasting pollution event in Chengdu and Chongqing was captured during wintertime from December 25, 2016 to January 5, 2017. The campaign-average of hourly concentrations of PM_2.5, sulfate, and nitrate, measured by an Aerosol Analyzer (ZSF) were 101 ± 73.8 μg/m³, 15.9 ± 11.8 μg/m³, and 24.9 ± 20.6 μg/m³, respectively, in Chengdu, and were 87.7 ± 53.8 μg/m³, 19.7 ± 13.5 μg/m³, and 15.1 ± 10.1 μg/m³, respectively, in Chongqing. Nitrate contributed substantially to PM_2.5 pollution when PM_2.5 was lower than 150 μg/m³, largely due to the strong secondary transformation of NO_X to nitrate during the occurrence of the pollution episode. Heterogeneous hydrolysis of N₂O₅ dominated nitrate formation during nighttime, while photochemical reactions and high-RH enhanced gas- to aqueous-phase dissolution of NH₃ and HNO₃ or cloud processes likely played important roles for nitrate formation during daytime. RH-dependent heterogeneous reactions contributed greatly to the formation of sulfate. NO_X is confirmed to play an important role as an oxidant in accelerating the secondary transformation of SO₂ to sulfate at high RH and low O₃ levels under neutralization condition during heavy PM_2.5 pollution episode. Results from this study identified the formation mechanism of nitrate, especially during the daytime, and addressed the importance of heterogeneous inorganic reactions in the formation of heavy aerosol pollution events.

Insight into the formation of secondary inorganic aerosol based on high-time-resolution data during haze episodes and snowfall periods in Zhengzhou, China

Episodic haze is frequently observed in Zhengzhou, China. Such haze typically contains secondary inorganic aerosols. In this paper, we explore the formation mechanisms of sulfate, nitrate, and ammonium (SNA) in Zhengzhou from January 3 to 25, 2018 based on the results of a series of online instruments and a size-segregated filter sampler. Our results document the remarkable contributions of SNA to winter haze episodes in Zhengzhou, where they account for about 50% of PM_2.5 mass concentration. SNA were mainly concentrated in droplet-mode particles, which increased remarkably with the aggravation of the haze episode. In addition, KNO₃ and NaNO₃ were formed in droplet-mode particles and coarse-mode particles respectively with increasing PM_2.5 concentration. The atmosphere during the observation period was ammonia-rich, and the aerosol was acidic under high PM_2.5 concentration. Homogeneous reactions dominated the formation of nitrate. HONO photolysis played a more important role in the origin of OH radicals when O₃ decreased during haze episode. Under high relative humidity (RH), nitrate formation was influenced by heterogeneous hydrolysis reactions of N₂O₅. Sulfates were mainly formed through aqueous-phase reactions, especially when the RH was higher than 60%. Under these conditions, there were amounts of liquid water content existed in aerosols. Finally, we observed enhanced conversion of SO₂ and NO₂ during snowfall periods. This effect may be attributable to the higher RH and O₃ levels despite the unfavorable effects of wet deposition and low concentrations of gaseous precursors.

Removal of PM2.5 and secondary inorganic aerosols in the North China Plain by dry deposition

The North China Plain (NCP) has experienced heavy air pollution in the past several decades featured by high levels of fine particulate matter (PM_2.5). PM_2.5 removal from the atmosphere in the NCP by dry deposition was estimated from 1999 through 2013 using the inferential method, which combined PM_2.5 air concentrations retrieved from satellite remote sensing and dry deposition velocities (V_d) calculated using a bulk particle dry deposition model. Dry deposition of the three major inorganic ions in PM_2.5, namely NH₄⁺ (ammonium), NO₃^- (nitrate), and SO₄^2- (sulfate), with their concentrations in 2000 and 2010 obtained from WRF-Chem model simulations, were also investigated considering their important roles in PM_2.5 formation and ecosystem health. High levels of modeled and satellite-retrieved PM_2.5 air concentrations, the secondary inorganic aerosols (the sum of NH₄⁺, NO₃^-, and SO₄^2-), and their respective deposition fluxes were identified from the southern NCP to Beijing-Tianjin metropolitans. The deposition fluxes derived from the inferential method and WRF-Chem increased considerably in the 2000s due to rising PM_2.5 atmospheric levels across the NCP. The enhancement of dry deposition velocities of PM_2.5 and three aerosol species in the NCP were associated nicely with increasing vegetation coverage and wind speed. We show that both air concentrations of PM_2.5 and secondary inorganic aerosols and rising dry deposition velocities related to extensive afforestation activities contributed to their deposition fluxes and an inclining trend of PM_2.5 removal from the atmosphere.

Composition, mixing state, and size distribution of single submicron particles during pollution episodes in a coastal city in southeast China

Size-resolved particle composition, size distribution, and mixing state were characterized at the single-particle level during two air pollution episodes during 12-25 January, 2016 in a coastal city in southeast China. The two pollution episodes occurred under distinct meteorological conditions (i.e., different wind speeds, relative humidity, and backward trajectories); thus, they were assigned to stagnation and transport episodes, respectively. Single-particle data, obtained from single-particle aerosol mass spectrometry (SPAMS), showed that carbonaceous particles were the predominant particles during the whole study period, accounting for more than 60% of the total particles. However, the number fractions of carbonaceous particles and nitrate-containing particles significantly increased in the stagnation episode, while the number fractions of sulfate- and ammonium-containing particles both increased in the transport episode compared to the levels over the whole study period. The potassium-rich (K-rich) particle class was more abundant and more strongly mixed with sulfate in the transport episode, which indicates the impact of biomass burning emissions and the subsequent aging process by acquiring sulfate during transport. The particle classes (e.g., carbonaceous and K-rich classes) had a broader size distribution during the pollution episodes than during the clean episode. The diameters of the size distribution peak for all particle classes (except for dust class) were observed to be larger in the transport episode than in the stagnation episode. This suggests that the particles underwent an extensive aging process through the addition of sulfate and ammonium during transport, leading to the growth of particles.

Source apportionment of fine particulate matter in China in 2013 using a source-oriented chemical transport model

China has been suffering high levels of fine particulate matter (PM_2.5). Designing effective PM_2.5 control strategies requires information about the contributions of different sources. In this study, a source-oriented Community Multiscale Air Quality (CMAQ) model was applied to quantitatively estimate the contributions of different source sectors to PM_2.5 in China. Emissions of primary PM_2.5 and gas pollutants of SO₂, NO_x, and NH₃, which are precursors of particulate sulfate, nitrate, and ammonium (SNA, major PM_2.5 components in China), from eight source categories (power plants, residential sources, industries, transportation, open burning, sea salt, windblown dust and agriculture) were separately tracked to determine their contributions to PM_2.5 in 2013. Industrial sector is the largest source of SNA in Beijing, Xi'an and Chongqing, followed by agriculture and power plants. Residential emissions are also important sources of SNA, especially in winter when severe pollution events often occur. Nationally, the contributions of different source sectors to annual total PM_2.5 from high to low are industries, residential sources, agriculture, power plants, transportation, windblown dust, open burning and sea salt. Provincially, residential sources and industries are the major anthropogenic sources of primary PM_2.5, while industries, agriculture, power plants and transportation are important for SNA in most provinces. For total PM_2.5, residential and industrial emissions are the top two sources, with a combined contribution of 40-50% in most provinces. The contributions of power plants and agriculture to total PM_2.5 are about 10%, respectively. Secondary organic aerosol accounts for about 10% of annual PM_2.5 in most provinces, with higher contributions in southern provinces such as Yunnan (26%), Hainan (25%) and Taiwan (21%). Windblown dust is an important source in western provinces such as Xizang (55% of total PM_2.5), Qinghai (74%), Xinjiang (59%). The large variation in sources of PM_2.5 across China suggests that PM_2.5 mitigation programs should be designed separately for different regions/provinces.

Role of secondary aerosols in haze formation in summer in the Megacity Beijing

A field experiment from 18 August to 8 September 2006 in Beijing, China, was carried out. A hazy day was defined as visibility<l0 km and RH (relative humidity)<90%. Four haze episodes, which accounted for ~60% of the time during the whole campaign, were characterized by increases of SNA (sulfate, nitrate, and ammonium) and SOA (secondary organic aerosol) concentrations. The average values with standard deviation of SO4(2-), NO3-, NH4+ and SOA were 49.8 (±31.6), 31.4 (±22.3), 25.8 (±16.6) and 8.9 (±4.1)μg/m3, respectively, during the haze episodes, which were 4.3, 3.4, 4.1, and 1.7 times those in the non-haze days. The SO4(2-), NO3-, NH4+, and SOA accounted for 15.8%, 8.8%, 7.3%, and 6.0% of the total mass concentration of PM10 during the non-haze days. The respective contributions of SNA species to PM10 rose to about 27.2%, 15.9%, and 13.9% during the haze days, while the contributions of SOA maintained the same level with a slight decrease to about 4.9%. The observed mass concentrations of SNA and SOA increased with the increase of PM10 mass concentration, however, the rate of increase of SNA was much faster than that of the SOA. The SOR (sulfur oxidation ratio) and NOR (nitrogen oxidation ratio) increased from non-haze days to hazy days, and increased with the increase of RH. High concentrations of aerosols and water vapor favored the conversion of SO2 to SO4(2-) and NO2 to NO3-, which accelerated the accumulation of the aerosols and resulted in the formation of haze in Beijing.