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Nitrous acid (HONO), a key precursor of hydroxyl radicals (OH), is one of the factors affecting atmospheric chemistry and air quality. Currently, the proposed sources of HONO are not able to fully explain observed HONO concentrations. In this study, a comprehensive field observation of HONO was conducted in the autumn of 2021 in urban Beijing. The box model using a default Master Chemical Mechanism (MCM) was unable to reproduce the observed HONO concentrations with a normalized mean bias (NMB) of −92.8%. The NMB improved to −46.1% after the inclusion of seven additional HONO formation pathways. Several factors like vehicle emission factor (1.23%) and nocturnal NO2 heterogeneous uptake coefficient on the ground surface (8.25 × 10−6) were calculated based on observational data. The enhancement factor for nocturnal NO2 heterogeneous conversion was established as a function of relative humidity (RH) and incorporated into the model, which compensated for the missing nocturnal HONO sources and well-reproduced the observed HONO concentrations, with an NMB of −5.1%. The major source of HONO at night was found to be the heterogeneous reaction of NO2 on the ground surface, contributing up to 85.6%. During the daytime, it was the homogeneous reaction of NO with OH, accounting for 41.8%. The daytime primary source of OH was mainly the photolysis of HONO, which constituted 73.6% and therefore promoted the formation of secondary pollutants and exacerbated haze events. [ABSTRACT FROM AUTHOR]

Atmospheric nitrous acid (HONO) chemistry is of critical importance to air quality during polluted haze events, especially in China. However, current air quality models (such as WRF-CHEM, WRF-CMAQ, Box-MCM) generally underestimate the concentration of HONO, leading to a lack of fundamental understanding of haze pollution. Here, by combining field observations during haze events in Beijing and modeling results, we developed the new parameterization scheme for heterogeneous nitrogen dioxide (NO2) reaction on aerosol surfaces with the synergistic effects of relative humidity and ammonia, which has not been considered in existing air quality models. Including NO2 heterogeneous reactions into modeling significantly improves the estimation accuracy of HONO and OH levels, with the contribution reaching up to 91% and 78% during pollution episodes. The OH derived by HONO can partly explain high concentrations of particulate matter. Together, our work provides a new approach to illustrate the formation of HONO, OH, and haze with the consideration of heterogeneous NO2 → HONO chemistry. [ABSTRACT FROM AUTHOR]

Secondary inorganic aerosol (especially SNA: sulfate, nitrate, and ammonium) formed from different pathways, account for significant proportions of ambient particulate matter during haze events. For the first time, we quantitatively explored contributions from different SNA formation pathways by an online monitoring AMS instrument and 3‐dimension factor analysis PMF3 model, which is called "SNA formation pathway apportionment". Three formation process were identified: NH4NO3 heterogeneous phase reactions (NH4NO3‐heter), (NH4)2SO4 gas phase reactions ((NH4)2SO4‐gas), and mixed NH4NO3 and (NH4)2SO4 aqueous phase processes (S&N‐aque). Their overall contributions to SNA were 25.7%, 22.1%, and 52.2%, respectively. Moreover, (NH4)2SO4‐gas contributed mainly in clean (52.1%); S&N‐aque got the highest contribution in haze episode (69.6%); while sulfate gas phase formations were also found in haze stage (10.3%). The quantitative findings from this work provide hard evidence that aqueous phase processes (including sulfate aqueous phase reactions and nitrate gas‐particle partition) were major drivers for SNA increasing. Plain Language Summary: Secondary inorganic aerosol (particularly SNA: sulfate, nitrate and ammonium) was found to be the main component of particulate matter in northern China. SNA can be formed through different reaction pathways, such as gas phase, aqueous phase, and heterogeneous reactions pathways. However, quantitative assessments of different pathways are still lacking, especially during different pollution processes. In this work, AMS data with high time resolution and three‐dimension receptor model (PMF3) were used to study the characteristics of secondary aerosols in winter in Beijing and make SNA formation pathway contribution apportionment. Three possible formation pathways were identified by PMF3: Heterogeneous reactions pathway of NH4NO3, gas phase reactions pathway of (NH4)2SO4, and aqueous phase processes of mixed NH4NO3 & (NH4)2SO4, respectively. We found that the main dominant factors were diverse during different periods: (NH4)2SO4 gas phase reactions pathway contributed mainly to SNA in clean period; mixed NH4NO3 and (NH4)2SO4 aqueous phase processes contributed mostly in haze episode. The methods and findings of this work enable and further our understandings on the formations of secondary inorganic aerosol in haze. Key Points: Mixed NH4NO3 and (NH4)2SO4 (aqueous phase process) had the highest contribution during winter haze period (69.6%)(NH4)2SO4 (gas phase formation pathway) contributed mainly in clean (52.1%)(NH4)2SO4 (gas phase formation pathway) was also found during haze pollution stage (10.3%) [ABSTRACT FROM AUTHOR]

To provide a comprehensive understanding of carbonaceous aerosol and its role in the haze formation in the Central Plains Urban Agglomeration of China, size-segregated particulate matter samples (PM1, PM2.5 and PM10) were continually collected from 20 December 2017, to 17 January 2018, in Xinxiang, the third largest city of Henan province. The results showed that the mean mass concentrations of PM1, PM2.5 and PM10 were 63.20, 119.63 and 211.95 μg·m−3, respectively, and the organic carbon (OC) and elemental carbon (EC) were 11.37 (5.87), 19.24 (7.36), and 27.04 (10.27) μg·m−3, respectively. Four pollution episodes that were categorized by short evolution patterns (PE1 and PE3) and long evolution patterns (PE2 and PE4) were observed. Meteorological condition was attributed to haze episodes evolution pattern. Carbonaceous components contributed to PE1 and PE2 under drier condition through transportation and local combustion emission, while they were not main species in PE3 and PE4 for haze explosive growth under suitable RH, whatever for the short or long evolution pattern. The atmospheric self-cleaning processes were analyzed by a case study, which showed the wet scavenging effectively reduced the coarse particles with a removal rate of 73%, while it was not for the carbonaceous components in fine particles that is hydrophobic in nature. These results highlight that local primary emissions such as biomass combustion were the important sources for haze formation in Central China, especially in dry conditions. [ABSTRACT FROM AUTHOR]

Under a high atmospheric oxidization capacity, the synergistic effect of the physicochemical processes in the atmospheric boundary layer (ABL) caused summer haze pollution in Beijing. The southern and southwestern areas, generally 60–300 km away from Beijing, were seriously polluted in contrast to Beijing, which remained clean. Southerly winds moving faster than 20–30 km h -1 since the early morning primarily caused haze pollution initiation. The PM 2.5 (particulate matter with a dynamic equivalent diameter smaller than 2.5 µm) level increased to 75 µgm-3 over several hours during the daytime, which was simultaneously affected by the ABL structure. Additionally, the O3 concentration was quite high during the daytime (250 µgm-3), corresponding to a high atmospheric oxidation capacity. Much sulfate and nitrate were produced through active atmospheric chemical processes, with sulfur oxidation ratios (SORs) up to ∼0.76 and nitrogen oxidation ratios (NORs) increasing from 0.09 to 0.26, which further facilitated particulate matter (PM) level enhancement. However, the increase in sulfate was mainly linked to southerly transport. At midnight, the PM 2.5 concentration sharply increased from 75 to 150 µgm-3 over 4 h and remained at its highest level until the next morning. Under an extremely stable ABL structure, secondary aerosol formation dominated by nitrate was quite intense, driving the haze pollution outbreak. The PM levels in the southern and southeastern areas of Beijing were significantly lower than those in Beijing at this time, even below air quality standards; thus, the contribution of pollution transport had almost disappeared. With the formation of a nocturnal stable boundary layer (NSBL) at an altitude ranging from 0–0.3 km, the extremely low turbulence kinetic energy (TKE) ranging from 0 to 0.05 m 2 s -2 inhibited the spread of particles and moisture, ultimately resulting in elevated near-surface PM 2.5 and relative humidity (∼90 %) levels. Due to the very high humidity and ambient oxidization capacity, NOR rapidly increased from 0.26 to 0.60, and heterogeneous hydrolysis reactions at the moist particle surface were very notable. The nitrate concentration steeply increased from 11.6 to 57.8 µgm-3 , while the sulfate and organics concentrations slightly increased by 6.1 and 3.1 µgm-3 , respectively. With clean and strong winds passing through Beijing, the stable ABL dissipated with the potential temperature gradient becoming negative and the ABL height (ABLH) increasing to ∼2.5 km. The high turbulence activity with a TKE ranging from 3 to 5 m 2 s -2 notably promoted pollution diffusion. The self-cleaning capacity of the atmosphere is commonly responsible for air pollution dispersion. However, reducing the atmospheric oxidization capacity, through strengthening collaborative control of nitrogen oxide (NOx) and volatile organic compounds (VOCs), as well as continuously deepening regional joint air pollution control, is urgent. [ABSTRACT FROM AUTHOR]

Open biomass burning (OBB) has a high potential to trigger local and regional severe haze with elevated fine particulate matter (PM 2.5) concentrations and could thus deteriorate ambient air quality and threaten human health. Open crop straw burning (OCSB), as a critical part of OBB, emits abundant gaseous and particulate pollutants, especially in fields with intensive agriculture, such as in central and eastern China (CEC). This region includes nine provinces, i.e., Hubei, Anhui, Henan, Hunan, Jiangxi, Shandong, Jiangsu, Shanghai, and Fujian. The first four ones are located inland, while the others are on the eastern coast. However, uncertainties in current OCSB and other types of OBB emissions in chemical transport models (CTMs) lead to inaccuracies in evaluating their impacts on haze formations. Satellite retrievals provide an alternative that can be used to simultaneously quantify emissions of OCSB and other types of OBB, such as the Fire INventory from NCAR version 1.5 (FINNv1.5), which, nevertheless, generally underestimates their magnitudes due to unresolved small fires. In this study, we selected June 2014 as our study period, which exhibited a complete evolution process of OBB (from 1 to 19 June) over CEC. During this period, OBB was dominated by OCSB in terms of the number of fire hotspots and associated emissions (74 %–94 %), most of which were located at Henan and Anhui (> 60 %) with intensive enhancements from 5 to 14 June (> 80 %). OCSB generally exhibits a spatiotemporal correlation with regional haze over the central part of CEC (Henan, Anhui, Hubei, and Hunan), while other types of OBB emissions had influences on Jiangxi, Zhejiang, and Fujian. Based on these analyses, we establish a constraining method that integrates ground-level PM 2.5 measurements with a state-of-art fully coupled regional meteorological and chemical transport model (the two-way coupled WRF-CMAQ) in order to derive optimal OBB emissions based on FINNv1.5. It is demonstrated that these emissions allow the model to reproduce meteorological and chemical fields over CEC during the study period, whereas the original FINNv1.5 underestimated OBB emissions by 2–7 times, depending on specific spatiotemporal scales. The results show that OBB had substantial impacts on surface PM 2.5 concentrations over CEC. Most of the OBB contributions were dominated by OCSB, especially in Henan, Anhui, Hubei, and Hunan, while other types of OBB emissions also exerted an influence in Jiangxi, Zhejiang, and Fujian. With the concentration-weighted trajectory (CWT) method, potential OCSB sources leading to severe haze in Henan, Anhui, Hubei, and Hunan were pinpointed. The results show that the OCSB emissions in Henan and Anhui can cause haze not only locally but also regionally through regional transport. Combining with meteorological analyses, we can find that surface weather patterns played a cardinal role in reshaping spatial and temporal characteristics of PM 2.5 concentrations. Stationary high-pressure systems over CEC enhanced local PM 2.5 concentrations in Henan and Anhui. Then, with the evolution of meteorological patterns, Hubei and Hunan in the low-pressure system were impacted by areas (i.e., Henan and Anhui) enveloped in the high-pressure system. These results suggest that policymakers should strictly undertake interprovincial joint enforcement actions to prohibit irregular OBB, especially OCSB over CEC. Constrained OBB emissions can, to a large extent, supplement estimations derived from satellite retrievals as well as reduce overestimates of bottom-up methods. [ABSTRACT FROM AUTHOR]

Severe wintertime PM 2.5 pollution in Beijing has been receiving increasing worldwide attention, yet the decadal variations remain relatively unexplored. Combining field measurements and model simulations, we quantified the relative influences of anthropogenic emissions and meteorological conditions on PM 2.5 concentrations in Beijing over the winters of 2002–2016. Between the winters of 2011 and 2016, stringent emission control measures resulted in a 21 % decrease in mean mass concentrations of PM 2.5 in Beijing, with 7 fewer haze days per winter on average. Given the overestimation of PM 2.5 by the model, the effectiveness of stringent emission control measures might have been slightly overstated. With fixed emissions, meteorological conditions over the study period would have led to an increase in haze in Beijing, but the strict emission control measures have suppressed the unfavorable influences of the recent climate. The unfavorable meteorological conditions are attributed to the weakening of the East Asia winter monsoon associated particularly with an increase in pressure associated with the Aleutian Low. [ABSTRACT FROM AUTHOR]

Although much effort has been put into studying air pollution, our knowledge of the mechanisms of frequently occurring intense haze episodes in China is still limited. In this study, using 3 years of measurements of air pollutants at three different height levels on a 325 m Beijing meteorology tower, we found that a positive aerosol–boundary layer feedback mechanism existed at three vertical observation heights during intense haze polluted periods within the mixing layer. This feedback was characterized by a higher loading of PM 2.5 with a shallower mixing layer. Modelling results indicated that the presence of PM 2.5 within the boundary layer led to reduced surface temperature, relative humidity and mixing layer height during an intensive haze episode. Measurements showed that the aerosol–boundary layer feedback was related to the decrease in solar radiation, turbulent kinetic energy and thereby suppression of the mixing layer. The feedback mechanism can explain the rapid formation of intense haze episodes to some extent, and we suggest that the detailed feedback mechanism warrants further investigation from both model simulations and field observations. [ABSTRACT FROM AUTHOR]

Fine-particle pollution associated with haze threatens human health, especially in the North China Plain region, where extremely high PM 2.5 concentrations are frequently observed during winter. In this study, the Weather Research and Forecasting with Chemistry (WRF-Chem) model coupled with an improved integrated process analysis scheme was used to investigate the formation and evolution mechanisms of a haze event over the Beijing–Tianjin–Hebei (BTH) region in December 2015; this included an examination of the contributions of local emissions and regional transport to the PM 2.5 concentration in the BTH area, and the contributions of each detailed physical or chemical process to the variations in the PM 2.5 concentration. The mechanisms influencing aerosol radiative forcing (including aerosol direct and indirect effects) were also examined by using process analysis. During the aerosol accumulation stage (16–22 December, Stage 1), the near-surface PM 2.5 concentration in the BTH region increased from 24.2 to 289.8 µgm-3 , with the contributions of regional transport increasing from 12 % to 40 %, while the contribution of local emissions decreased from 59 % to 38 %. During the aerosol dispersion stage (23–27 December, Stage 2), the average concentration of PM 2.5 was 107.9 µgm-3 , which was contributed by local emissions (51 %) and regional transport (24 %). The 24 h change (23:00 minus 00:00 LST) in the near-surface PM 2.5 concentration was +43.9 µgm-3 during Stage 1 and -41.5 µgm-3 during Stage 2. The contributions of aerosol chemistry, advection, and vertical mixing to the 24 h change were +29.6 (+17.9) µgm-3 , -71.8 (-103.6) µgm-3 , and -177.3 (-221.6) µgm-3 during Stage 1 (Stage 2), respectively. Small differences in the contributions of other processes were found between Stage 1 and Stage 2. Therefore, the PM 2.5 increase over the BTH region during the haze formation stage was mainly attributed to strong production by the aerosol chemistry process and weak removal by the advection and vertical mixing processes. When aerosol radiative feedback was considered, the 24 h PM 2.5 increase was enhanced by 4.8 µgm-3 during Stage 1, which could be mainly attributed to the contributions of the vertical mixing process (+22.5 µgm-3), the advection process (-19.6 µgm-3), and the aerosol chemistry process (+1.2 µgm-3). The restrained vertical mixing was the primary reason for the enhancement in the near-surface PM 2.5 increase when aerosol radiative forcing was considered. [ABSTRACT FROM AUTHOR]

Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol–radiation feedback (ARF). Observations have revealed that the near-surface PM 2.5 concentrations ([PM 2.5 ]) generally exhibit an increasing trend with relative humidity (RH) in the North China Plain (NCP) during 2015 wintertime, indicating that the aerosol liquid water (ALW) caused by hygroscopic growth could play an important role in the PM 2.5 formation and accumulation. Simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the NCP were conducted using the WRF-Chem Model to comprehensively quantify contributions of the ALW effect to near-surface [PM 2.5 ]. The WRF-Chem Model generally performs reasonably well in simulating the temporal variations in RH against measurements in the NCP. The factor separation approach (FSA) was used to evaluate the contribution of the ALW effect on the ARF, photochemistry, and heterogeneous reactions to [PM 2.5 ]. The ALW not only augments particle sizes to enhance aerosol backward scattering but also increases the effective radius to favor aerosol forward scattering. The contribution of the ALW effect on the ARF and photochemistry to near-surface [PM 2.5 ] is not significant, being generally within 1.0 µ g m -3 on average in the NCP during the episode. Serving as an excellent substrate for heterogeneous reactions, the ALW substantially enhances the secondary aerosol (SA) formation, with an average contribution of 71 %, 10 %, 26 %, and 48 % to near-surface sulfate, nitrate, ammonium, and secondary organic aerosol concentrations. Nevertheless, the SA enhancement due to the ALW decreases the aerosol optical depth and increases the effective radius to weaken the ARF, reducing near-surface primary aerosols. The contribution of the ALW total effect to near-surface [PM 2.5 ] is 17.5 % on average, which is overwhelmingly dominated by enhanced SA. Model sensitivities also show that when the RH is less than 80 %, the ALW progressively increases near-surface [PM 2.5 ] but commences to decrease when the RH exceeds 80 % due to the high occurrence frequencies of precipitation. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols scatter or absorb a fraction of the incoming solar radiation to cool or warm the atmosphere, decreasing surface temperature and altering atmospheric stability to further affect the dispersion of air pollutants in the planetary boundary layer (PBL). In the present study, simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the North China Plain (NCP) were performed using the Weather Research and Forecasting model with Chemistry (WRF-Chem) to comprehensively quantify contributions of aerosol shortwave radiative feedback (ARF) to near-surface (around 15 m above the ground surface) PM 2.5 mass concentrations. The WRF-Chem model generally performs well in simulating the temporal variations and spatial distributions of air pollutants concentrations compared to observations at ambient monitoring sites in the NCP, and the simulated diurnal variations of aerosol species are also consistent with the measurements in Beijing. Additionally, the model simulates the aerosol radiative properties, the downward shortwave flux, and the PBL height against observations in the NCP well. During the episode, ARF deteriorates the haze pollution, increasing the near-surface PM 2.5 concentrations in the NCP by 10.2 µ g m -3 or with a contribution of 7.8 % on average. Sensitivity studies have revealed that high loadings of PM 2.5 attenuate the incoming solar radiation reaching the surface to cool the low-level atmosphere, suppressing the development of the PBL, decreasing the surface wind speed, further hindering the PM 2.5 dispersion, and consequently exacerbating the haze pollution in the NCP. Furthermore, when the near-surface PM 2.5 mass concentration increases from around 50 to several hundred µ g m -3 , ARF contributes to the near-surface PM 2.5 by more than 20 % during daytime in the NCP, substantially aggravating the heavy haze formation. However, when the near-surface PM 2.5 concentration is less than around 50 µ g m -3 , ARF generally reduces the near-surface PM 2.5 concentration due to the consequent perturbation of atmospheric dynamic fields. [ABSTRACT FROM AUTHOR]

Topic 3 of the Model Inter-Comparison Study for Asia (MICS-Asia) Phase III examines how online coupled air quality models perform in simulating high aerosol pollution in the North China Plain region during wintertime haze events and evaluates the importance of aerosol radiative and microphysical feedbacks. A comprehensive overview of the MICS-Asia III Topic 3 study design, including descriptions of participating models and model inputs, the experimental designs and results of model evaluation, are presented. Six modeling groups from China, Korea and the United States submitted results from seven applications of online coupled chemistry-meteorology models. Results are compared to meteorology and air quality measurements, including data from the Campaign on Atmospheric Aerosol Research Network of China (CARE-China) and the Acid Deposition Monitoring Network in East Asia (EANET). The correlation coefficients between the multi-model ensemble mean and the CARE-China observed near-surface air pollutants range from 0.51 to 0.94 (0.51 for ozone and 0.94 for PM2:5) for January 2010. However, large discrepancies exist between simulated aerosol chemical compositions from different models. The coefficient of variation (SD divided by the mean) can reach above 1.3 for sulfate in Beijing and above 1.6 for nitrate and organic aerosols in coastal regions, indicating that these compositions are less consistent from different models. During clean periods, simulated aerosol optical depths (AODs) from different models are similar, but peak values differ during severe haze events, which can be explained by the differences in simulated inorganic aerosol concentrations and the hygroscopic growth efficiency (affected by varied relative humidity). These differences in composition and AOD suggest that future models can be improved by including new heterogeneous or aqueous pathways for sulfate and nitrate formation under hazy conditions, a secondary organic aerosol (SOA) formation chemical mechanism with new volatile organic compound (VOCs) precursors, yield data and approaches and a more detailed evaluation of the dependence of aerosol optical properties on size distribution and mixing state. It was also found that using the ensemble mean of the models produced the best prediction skill. While this has been shown for other conditions (for example, the prediction of high-ozone events in the US (Mc- Keen et al., 2005)), this is to our knowledge the first time it has been shown for heavy haze events. [ABSTRACT FROM AUTHOR]

A continuous daily PM 2.5 sampling campaign from 10 April till 8 June 2013, including three haze episodes, was conducted in Beijing. Chemical species, including EC, OC, water-soluble ions and inorganic elements, were analysed by a thermal/optical carbon analyser, IC and ICP-MS, respectively. A comparison of air quality during such haze episodes in relation to clear air situations, as well as the differences between the haze episodes was emphasised. The results showed that the most important fractions of PM 2.5 during haze were SO 4 2− , NO 3 − and NH 4 + (namely, SNA) which together accounted for 54–61% of the total PM 2.5 mass. Estimated secondary organic carbon (SOC) was also found to be increased during haze, but the relative increase compared to clear days was much lower than for SNA, leading to a decrease in relative contribution of SOC to PM 2.5 in the observed haze events. Cluster analyses from back trajectories showed four air mass clusters during spring 2013 and air flow, which was from the south-easterly directions, might favour the accumulation of PM 2.5 , especially SNA and anthropogenic elements. All these results proved that the anthropogenic air pollution in the Southeast of Beijing was responsible for the formation of hazes in Beijing during spring 2013. [ABSTRACT FROM AUTHOR]

Extreme haze episodes repeatedly shrouded Beijing during the winter of 2012-2013, causing major environmental and health problems. To better understand these extreme events, particle number size distribution (PNSD) and particle chemical composition (PCC) data collected in an intensive winter campaign in an urban site of Beijing were used to investigate the sources of ambient fine particles. Positive matrix factorization (PMF) analysis resolved a total of eight factors: two traffic factors, combustion factors, secondary aerosol, two accumulation mode aerosol factors, road dust, and long-range transported (LRT) dust. Traffic emissions (54 %) and combustion aerosol (27 %) were found to be the most important sources for particle number concentration, whereas combustion aerosol (33 %) and accumulation mode aerosol (37 %) dominated particle volume concentrations. Chemical compositions and sources of fine particles changed dynamically in the haze episodes. An enhanced role of secondary inorganic species was observed in the formation of haze pollution. Regional transport played an important role for high particles, contribution of which was on average up to 24-49 % during the haze episodes. Secondary aerosols from urban background presented the largest contributions (45 %) for the rapid increase of fine particles in the severest haze episode. In addition, the invasion of LRT dust aerosols further elevated the fine particles during the extreme haze episode. Our results showed a clear impact of regional transport on the local air pollution, suggesting the importance of regional-scale emission control measures in the local air quality management of Beijing. [ABSTRACT FROM AUTHOR]

In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution episodes were observed by the 'Forming Mechanism and Control Strategies of Haze in China' group using an intensive aerosol and trace gases campaign that simultaneously obtained data at 11 ground-based observing sites in the CARE-China network. The characteristics and formation mechanism of haze pollution episodes were discussed. Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei (Jing-Jin-Ji) area; the two most severe episodes occurred during 9-15 January and 25-31 January. During these two haze pollution episodes, the maximum hourly PM mass concentrations in Beijing were 680 and 530 μg m, respectively. The process and evolution of haze pollution episodes in other major cities in the Jing-Jin-Ji area, such as Shijiazhuang and Tianjin were almost the same as those observed in Beijing. The external cause of the severe haze episodes was the unusual atmospheric circulation, the depression of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions. However, the internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols, which contributed to the 'explosive growth' and 'sustained growth' of PM. Particularly, the abnormally high amount of nitric oxide (NO) in the haze episodes, produced by fossil fuel combustion and vehicle emissions, played a direct or indirect role in the quick secondary transformation of coal-burning sulphur dioxide (SO) to sulphate aerosols. Furthermore, gaseous pollutants were transformed into secondary aerosols through heterogeneous reactions on the surface of fine particles, which can change the particle's size and chemical composition. Consequently, the proportion of secondary inorganic ions, such as sulphate and nitrate, gradually increased, which enhances particle hygroscopicity and thereby accelerating formation of the haze pollution. [ABSTRACT FROM AUTHOR]

Submicron aerosol is of extensive concern not only due to its significant impact on air quality but also because it is detrimental to human health. In this study, we investigated the characteristics, sources and chemical processes of submicron aerosol based on real-time online measurements of submicron aerosols (NR-PM 1) during December 2015 at an urban site in Beijing. The average mass concentration of NR-PM 1 was 92.5±84.9 µg/m3, the hourly maximum was 459.1 µg/m3 during the entire observation. The organic aerosol (OA) (55%) was the largest contributor to NR-PM 1. The average mass concentration of PAHs was 0.217±0.247 µg/m3, exhibiting the highest concentration at night and the lowest levels in the daytime. The average mass concentration of organic nitrate was 2.52±2.36 µg/m3 and that of inorganic nitrate was 7.62±8.22 µg/m3, accounting for 36% and 64%, respectively, of the total nitrate mass. Positive matrix factorization (PMF) differentiated the OA into five chemical components including LV-OOA, SV-OOA, COA, HOA and CCOA, accounting for 22%, 16%, 13%, 25% and 24% respectively, of the total OA. The average NR-PM 1 mass concentration on the heavy polluted days (HPD) was 182.8±70.2 µg/m3, which was approximately 9 times that on clean days (CD). The enhanced secondary formation of SNA was evident on HPD, especially the rapid increase of sulfate (23%) and nitrate (19%). [Display omitted] [ABSTRACT FROM AUTHOR]

Atmospheric aerosol particles exert significant influence on the earth's radiation budget, and accurate measurement of aerosol optical properties can help reduce the uncertainties in estimating aerosol radiative forcing. Therefore, year-round measurements of aerosol optical properties from August 2016 to July 2017 at an urban site in Beijing were analysed. The results show that the annual mean scattering coefficient (σ sp at 550 nm, 131 ± 135 Mm−1) was approximately half of the measurement in Beijing from 2008 to 2009 and from 2005 to 2006, mainly due to the reduction of PM2.5 and scattering aerosols in recent years. Meanwhile, annual mean of σ abs (637 nm) and SSA was 14 ± 11 Mm−1 and 0.85 ± 0.06, respectively. Seasonally, σ sp during winter was approximately two times higher than in other seasons, while σ abs was higher during autumn and winter, which may be attributed to biomass and fossil fuel burning under a shallow and stable boundary layer. Diurnal variation of σ sp showed an evening peak and a slight morning peak in spring and summer, as well as a night peak in autumn and winter. σ abs in four seasons were basically unimodal, with a morning peak in spring and summer, and a night peak in autumn and winter. In addition, the scattering Ångström exponent and backscattering Ångström exponent suggested the dominance of micro-size aerosols. Seasonal variations of SAE, BAE and the backscatter fraction indicated a larger proportion of isotropic scattering and fine-mode particles in summer and autumn, as well as a dominance of coarse particles in spring and a complicated aerosol type distribution and emission sources in winter. Furthermore, radiative forcing was estimated by a radiative transfer model. Radiative forcing varied between −6.69 and −235.67 Wm-2 (with the average of −54.93 ± 39.92 Wm-2) at the surface, with a mean of 35.56 ± 22.88 Wm-2 in the atmosphere. Image 1 • Year-round aerosol optical characteristics in Beijing are observed and analysed. • Seasonal and diurnal variation and the causes are discussed. • Changes are discovered compared with earlier years in Beijing and other regions. • Estimate the aerosols radiative forcing in Beijing. [ABSTRACT FROM AUTHOR]

Nitrate salts represent a major component of fine mode aerosols, which play an important role in air pollution worldwide. Based on on-line and off-line aerosol measurements in urban Beijing for both clean and haze conditions, we demonstrate that the absolute and relative concentrations of nitrate increased with visibility degradation (relative humidity), whereas the variations of organics tracked the patterns of mixing-layer height and temperature. We propose that the increase in the relative contribution of nitrate to PM 1 observed during the early stages of haze pollution was due to new particle formation, whereas the nitrate formed in PM 1-2.5 during the latter stages was due to heterogeneous formation and hygroscopic growth. The increasing trend of nitrate (and also sulfate and ammonium) but decreasing trends of organics during haze development, together with the increase of the NO 2 /SO 2 molar ratio with increasing proximity to downtown Beijing and with visibility degradation, provide further evidence that controlling NO x emissions should be a priority for improving air quality in mega cities. Additional large-scale investigation is required to adequately characterize the regional features of NO x -induced haze pollution in China. Such studies may provide insight into the formation of critical nuclei or the subsequent growth of freshly nucleated particles and advance our understanding of the role of nitrate in new particle formation. [ABSTRACT FROM AUTHOR]

Haze is a serious air pollution problem in China, especially in Beijing and surrounding areas, affecting visibility, public health and regional climate. In this study, the Weather Research and Forecasting-Chemistry (WRF-Chem) model was used to simulate PM 2.5 (particulate matters with aerodynamic diameter ≤ 2.5 μm) concentrations during the 2013 severe haze event in Beijing, and health impacts and health-related economic losses were calculated based on model results. Compared with surface monitoring data, the model results reflected pollution concentrations accurately (correlation coefficients between simulated and measured PM 2.5 were 0.7, 0.4, 0.5 and 0.6 in Beijing, Tianjin, Xianghe and Xinglong stations, respectively). Health impacts assessments show that the PM 2.5 concentrations in January might cause 690 (95% confidence interval (CI): (490, 890)) premature deaths, 45,350 (95% CI: (21,640, 57,860)) acute bronchitis and 23,720 (95% CI: (17,090, 29,710)) asthma cases in Beijing area. Results of the economic losses assessments suggest that the haze in January 2013 might lead to 253.8 (95% CI: (170.2, 331.2)) million US$ losses, accounting for 0.08% (95% CI: (0.05%, 0.1%)) of the total 2013 annual gross domestic product (GDP) of Beijing. [ABSTRACT FROM AUTHOR]

In this paper we investigate a severe pollution episode that occurred in the Yangtze River Delta (YRD) region in January 2013. The episode was caused by the combination of anthropogenic emissions and unusual atmospheric circulation, the depression of strong cold air activities and the very unfavorable dispersion. The episode contained three haze events (haze1: Jan. 4–9, haze2: Jan. 10–13, and haze3: Jan. 14–16). In Nanjing, aerosol size distributions from 10 nm to 10 μm and chemical components of single particles from 0.2 to 2 μm were measured with a Wide Range Particle Spectrometer (WPS) and a Single Particle Aerosol Mass Spectrometer (SPAMS), respectively. The results indicate that the mean PM 2.5 concentrations in the YRD region were greater than 110 μg · m − 3 . The highest PM 2.5 concentration of 175.6 μg · m − 3 occurred in Nanjing; the other cities had values in the range of 110.8–147.3 μg · m − 3 . The average PM 2.5 concentrations were 58.3, 122.7, 145.4 and 154.7 μg · m − 3 on clean and haze1, haze2 and haze3 days, respectively. The highest PM 2.5 values of 416.5, 415.5 and 300.5 μg · m − 3 in Nanjing occurred during the three haze events. The spectra of the aerosol number concentrations had unimodal distributions on clean and haze days. The maximum surface area peaks were located at 0.5–0.7 μm and had values of 419, 1397, 1309 and 1378 μm 2 · cm − 3 · nm − 1 on clean and haze1, haze2 and haze3 days, respectively. The number concentrations of biomass/biofuel burning-containing particles (biomass), organic carbon-containing particles (OC), elemental carbon-containing particles (EC), nitrate-containing particles (nitrate) and sulfate-containing particles (sulfate) increased significantly during the haze events. The chemical components of the aerosols during the haze1 and haze2 events were similar to those on clean days, and variations were caused by local particle accumulations under poor diffusion conditions. The high EC particle concentration of 24.76% during the haze3 event was impacted by the pollutants transported from surrounding cities. In addition, the different chemical components showed distinct size distributions. [ABSTRACT FROM AUTHOR]

Abstract: The analysis of air pollution episodes greatly aids in understanding air pollution and formulating policies to improve air quality. In this study, variations in the distribution of PM10 concentrations were analyzed using data collected from the air quality monitoring network in northern China set up by the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP, CAS) and the air quality monitoring network of the Beijing Municipal Environmental Protection Bureau (BJEPB). The results showed that there were two severe regional PM10 pollution episodes recorded from October 27, 2009, to November 10, 2009. During these pollution episodes, the highest daily PM10 concentration reached 600 μg m−3 in Shijiazhuang, which is far above the daily PM10 limit of 150 μg m−3. These aerosol pollution episodes were accompanied by regional haze and low visibility ranges. Synoptic patterns and long-range transport were the main factors controlling the regional air pollution processes. Transport patterns affected by synoptic cycle resulted in the spatial and temporal variations of PM10 and PM2.5 during the period studied. Conditions with light winds, temperature inversion and low mixed layer heights contributed to the buildup of PM10 and PM2.5 as well as gas-to-particle processing. In Beijing, the concentrations of sulfate and nitrate accounted for ∼1/3 of the PM10 mass and ∼2/3 of the PM2.5 mass, part of which is attributed to the additional secondary conversion of SO4 2− from SO2 and NO3 − from NOx during high-pollution days. Stationary anthropogenic sources were still a major contributor to pollution in these two episodes based on the NOx/SO2 ratios. To address regional air pollution, more attention should be paid to the increases in emissions and the higher frequency of haze days, which are associated with degraded visibility, that have accompanied the rapid economic development in northern China. [Copyright &y& Elsevier]

Aerosol size distribution and chemical composition are found to have significant effects on its hygroscopicity, acidity/alkalinity and light extinction. Heavy haze pollution occurred frequently in the wintertime of NCP, with long duration time and large impact area, which had important influences on air quality and human health. However, the study concerning the formation mechanism and physicochemical characteristics of aerosols in different haze stages have been rarely carried out. In this study, aerosol size distribution in the range of 10 nm - 10 μm, water soluble inorganic ions (WSIIs), PM (PM 2.5 , PM 10), trace gases, organic carbon (OC), elemental carbon (EC) and meteorological elements were derived during a regional haze pollution episode from Nov. 9 to 16, 2018. The aerosol water content and pH were further calculated using the ISORROPIA model. We divided the whole observation period into segments of clean days, fog processes, and haze processes, including three stages of I: accumulation, II: growth, and III: explosion, and a dry haze (D-haze) process based on the PM 2.5 concentration, visibility and RH. Given the shift of particle size to larger segment ascribed to the ageing process during fog/haze process, the size distribution peak in aerosol number concentration was located at 100 nm in the fog/haze episode and was 70 nm larger than that on clean days. The concentration descended significantly in stage III, peaking at 160 nm, suggesting a strong aging process of aerosols. The PM 2.5 increased sharply under high RH, static weather conditions and strengthening oxidation that favored liquid and heterogeneous reactions. The nitrate concentration was found to account for 20.5% (D-haze) - 29.0% (fog) of the total water soluble inorganic ions (WSIIs) during the fog/haze process, while sulfate constituted only 13.8% (stage III) - 21.3% (stage I), revealing dominant nitrate pollution. Hence, nitrate was considered to originate mainly from photochemical and heterogeneous reactions in haze episodes and was generated only by heterogeneous reactions. Higher (lower) aerosol water content made aerosols more acidic (alkaline) under similar chemical compositions. Therefore, the chemical reactions may differ under dry and wet haze pollution. Sources of carbonaceous aerosols changed in different haze stages with descending (enhancing) contributions from coal combustion (vehicle exhaust). Image 1 • Size distribution peaked at 30 nm (160 nm) in clean process (stage III: haze explosion). • Nitrate originated from photochemical (haze) and heterogeneous reactions (haze and fog). • Higher water content made aerosols more acidic under similar chemical compositions. • Contributions by coal combustion (vehicle exhausts) descended (enhanced) in haze episode. [ABSTRACT FROM AUTHOR]

Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol-radiation feedback (ARF). Observations have revealed that the near-surface PM2.5 concentrations ([PM2.5]) generally exhibits an increasing trend with the relative humidity (RH) in North China Plain (NCP) during 2015 wintertime, indicating that the aerosol liquid water (ALW) caused by hygroscopic growth might play an important role in the PM2.5 formation and accumulation. Simulations during a persistent and heavy haze pollution episode from 05 December 2015 to 04 January 2016 in NCP have been performed using the WRF-CHEM model to comprehensively quantify contributions of the ALW effect to near-surface [PM2.5]. The WRF-CHEM model generally performs reasonably in simulating the temporal variations of RH against measurements in NCP. The factor separation approach (FSA) is used to evaluate the contribution of the ALW effect on the ARF, photochemistry, and heterogeneous reactions to [PM2.5]. The ALW not only augments particle sizes to enhance aerosol backward scattering, but also increases effective radius to favor aerosol forward scattering. Therefore, the contribution of the ALW effect on the ARF and photochemistry to near [PM2.5] is not significant, generally within 1.0 μg m-3 on average in NCP during the episode. Serving as an excellent substrate for heterogeneous reactions, the ALW substantially enhances the secondary aerosol (SA) formation, with an average contribution of 71%, 10%, 26%, and 48% to near-surface sulfate, nitrate, ammonium, and secondary organic aerosol concentrations. Nevertheless, the SA enhancement due to the ALW decreases the aerosol optical depth and increases effective radius to weaken the ARF, reducing near-surface primary aerosols. The contribution of the ALW total effect to near-surface [PM2.5] is 17.5% on average, which is overwhelmingly dominated by enhanced SA. Model sensitivities also show that when the RH is less than 80%, the ALW progressively increases near-surface [PM2.5], but commences to decrease with the RH exceeding 80% due to the high occurrence frequencies of precipitation. [ABSTRACT FROM AUTHOR]