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The use of electron microscopy to analyze the morphology, composition, and sources of atmospheric particles has been studied extensively worldwide. However, in China, there are few studies on single-particle source analysis based on computer-controlled scanning electron microscopy (CCSEM) technology for a large number of particles, and the related technical methods need to be established and improved. In this study, ambient particulate matter (PM) was collected simultaneously from urban, suburban, and background areas of Beijing in spring 2018 and subsequently characterized using the IntelliSEM-EPAS™ technology (an advanced CCSEM software). The deposition velocity model was used to deduce the size distribution and calculate the concentration of ambient PM. Based on the k-means algorithm and empirical rules, all particles investigated were quantitatively apportioned to nine major sources, including soil/road dust, carbonates-silicates, carbonates, irregular carbonaceous particles, irregular iron oxides, combustion/industry, calcium sulfate, secondary particles, and salt-related particles. The size-resolved contributions (mass and number) of different sources were calculated. For example, soil/road dust (65.1 %), carbonate-silicate (16.1 %), and carbonate (7.1 %) were the top three sources contributing to PM

To better understand the characteristics and transformation mechanisms of secondary inorganic aerosols, hourly mass concentrations of water-soluble inorganic ions (WSIIs) in PM2.5 and their gaseous precursors were measured online from 2016 to 2018 at an urban site in Beijing. Seasonal and diurnal variations in water-soluble ions and gaseous precursors were discussed and their gas-particle conversion and partitioning were also examined, some related parameters were characterized. The (TNH3) Rich was also defined to describe the variations of the excess NH3 in different seasons. In addition, a sensitivity test was carried out by using ISORROPIA II to outline the driving factors of gas-particle partitioning. In Beijing, the relative contribution of nitrate to PM2.5 has increased markedly in recent years, especially under polluted conditions. In the four seasons, only a small portion of NO2 in the atmosphere was converted into total nitrate (TNO3), and more than 80% of TNO3 occurred in the form of nitrate due to the abundant ammonia. The concentration of total ammonia (TNH3) was much higher than that required to neutralize acid gases, and most of the TNH3 occurred as gaseous NH3. The nitrous acid (HONO) concentration was highly correlated with NH3 concentration and had increased significantly in Beijing compared with previous studies. The total chloride (TCl) was the highest in winter, and e(Cl−) was more sensitive to variations in the ambient temperature (T) and relative humidity (RH) than e(NO3−).

Atmospheric nitrous acid (HONO) chemistry is of critical importance to air quality during polluted haze events. However, current air quality models generally underestimate the concentration of HONO, indicating that a fundamental understanding of haze pollution is lacking. 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, ammonia, and sulfur dioxide], 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.

Stringent emission controls effectively brought down particulate mass concentrations by >30% in the North China Plain (NCP) and Pearl River Delta (PRD) regions. However, the low-visibility (

Aerosol liquid water content (ALWC) has important influences on atmospheric radiation and aerosol chemical processes. In this work, the changes in ALWC of PM

China's "Blue Sky Action Plan" aimed at tremendous improvements in atmospheric visibility. While stringent emission control policies have substantially brought down PM

Size-resolved hygroscopic growth factors of urban aerosol during a haze episode were measured using a Humidified Tandem Differential Mobility Analyzer (HTDMA) (gm(RH)). These factors were also derived from size-resolved particulate chemical composition combined with the κ-Kohler theory (gκ(RH)) and the thermodynamic model ISORROPIA-II running in forward mode (giso-f(RH)) and reverse mode (giso-r(RH)), respectively. In terms of agreement among these hygroscopic growth factors, gκ(RH) matched gm(RH) best, followed by giso-r(RH). In contrast, giso-f(RH) demonstrated a poorer agreement with gm(RH). The good consistency among gm(RH), gκ(RH), and giso-r(RH) was because they only focus on the physical hygroscopic process, whereas giso-f(RH) contains not only the direct influence of relative humidity (RH) on particle size but also the influence of gaseous precursor on the particle chemical composition, which indirectly affects the hygroscopicity of the particles. In this sense, size-resolved gκ(RH) and giso-r(RH) in a wide size range are more adequate to investigate the impact of RH on light scattering and aerosol radiative forcing. At RH = 80%, gκ(RH) for accumulation mode particles was 1.30–1.45 on polluted days and higher than that on clean days (1.2–1.3). Whereas on both polluted and clean days, gκ(RH) of ultrafine and coarse mode particles were generally lower than 1.25. The strong hygroscopicity of accumulation mode particles observed on polluted days can deteriorate visibility due to their high extinction efficiency.

To better understand the characteristics and transformation mechanisms of secondary inorganic aerosols, hourly mass concentrations of water-soluble inorganic ions (WSIIs) in PM

Comprehensive observations have been carried out in Beijing to investigate the impact of the Clean Air Action implemented in 2013 on changes in aerosol chemistry characteristics in heating seasons of 2016-2017 and 2017-2018. Results showed that PM

Nitrous Acid (HONO) is an important precursor of hydroxyl radical (OH) and has significant impacts on the formation of Ozone (O3) and Secondary Organic Aerosol (SOA). The atmospheric concentrations of HONO were measured during early autumn in downtown, Beijing (China). This study investigated HONO pollution characteristics and potential sources during day and night. The maximum hourly HONO levels reached 5.16 ppb, with 1.23 ppb on average. HONO concentration exhibited typical diurnal variation characteristics, with maximum at nighttime and minimum at daytime. The potential sources mainly included vehicle emission, heterogeneous reaction of NO2 on aerosol surfaces (Photo-enhanced at the daytime) and photolysis of particulate nitrate (NO3−) in Beijing. Vehicle emission was an important HONO source, particular at the morning rush period and lower HONO concentration. The simulated results highlighted that the main contribution of HONO was NO2 heterogeneous reaction on aerosol surfaces. The photolysis of particulate NO3− was also an important daytime HONO source, particularly in the pollution period. The main loss routine was the photolysis of HONO and dry deposition at day and night, respectively.

For aerosol related studies, the size distribution and hygroscopicity of chemical components are very important information. In order to characterize the distributions of chemical compositions and the water absorption ability for ambient aerosols of Beijing, a MOUDI-120 sampler was used to collected size-resolved samples in three seasons. All the samples were analyzed in the laboratory for water-soluble inorganic ions and carbon fractions. The size-resolved aerosol liquid water content (ALWC) of the sampled particles was modeled by the ISORROPIA II. The distributions of the chemical compositions, the ALWC, and the charge balance conditions were all discussed for three different pollution levels. During the sampling, the aerosols in stages 6–10 (< 1.0 μm) were under relatively dry conditions due to the significant pressure drops. Three modes (condensation mode, droplet mode, and coarse mode) could be identified from the distributions of the main chemical components and the ALWC. For the droplet mode, the ammonium was not enough to balance NO3– and SO42– during the heavily polluted period. The influence of relative humidity on ALWC is greater than that of the chemical compositions.

A MOUDI-120 sampler was used in Beijing to collect multi-stage samples in the summer and winter of 2013 to 2015. Thirty-three sample sets were collected during the daytime, nighttime, and different pollution levels. The actual relative humidity in the impactors was calculated for the first time. The carbonaceous components (organic and elemental carbon, OC and EC, respectively) and water-soluble inorganic ions (Na + , NH 4 + , K + , Mg 2 + , Ca 2 + , Cl − , NO 3 − , and SO 4 2 − ) were analyzed in each sample. The characteristics of the mass concentration distribution and charge balance were discussed. On the basis of relative humidity in the impactors, aerosols less than 1.0 μm were sampled under relatively dry conditions in most cases. The concentration levels for the chemical species were higher in the winter than in the summer. Three modes (condensation mode, droplet mode, and coarse mode) could be identified from the distributions of NH 4 + , NO 3 − , SO 4 2 − , Cl − , K + , OC and EC. The distribution characteristics for the pollution dissipation process were different from the pollution accumulation process. NO 3 − and NO 2 − contributed most of the negative electric charges in the stage below 0.1 μm. In the condensation mode, the cations were dominated by NH 4 + , which was sufficient to balance the anions. In the droplet mode of the heavily polluted samples, the ammonium was not sufficient to balance the anions. In the coarse mode, the positive electric charges were primarily composed of metal cations. The analyzed anions were not sufficient to neutralize the measured cations.

Since the 1980s, air pollution has become a major problem in northern China. Exposure to the extremely high concentrations of aerosols and trace gases might lead to important human health outcomes, including respiratory, cardiovascular and cerebrovascular diseases and malignant tumours. In this study, we collected data on mortality, visibility and the concentrations of certain air pollutants in Beijing from 1949 to 2011. Our goal was to investigate the mortality trends of different types of diseases and the relationship between mortality and air pollution. Based on the chemical compositions in particles and satellite formaldehyde, we found that mortality due to circulatory diseases was correlated with sulphate, nitrate and formaldehyde, whereas respiratory diseases were correlated with calcium, sulphate and nitrate, and malignant tumours was correlated with ammonium, nitrate and formaldehyde with an 11-year lag. The different responses to different air pollutants for different diseases are primarily a result of energy usage.

To improve the operational air quality forecasting over China, a new aerosol/gas phase pollutants assimilation capability is developed within the WRFDA system using 3DVAR algorithm. In this first application, the interface for MOSAIC aerosol scheme is built with flexible extending potentials. Based on the new WRFDA-Chem system, five experiments assimilating different surface observations, including PM2.5, PM10, SO2, NO2, O3, and CO are conducted for January 2017 along with a control experiment without DA. Results exhibit that the WRFDA-Chem system evidently improves the air quality forecasting. On the analysis aspect, the assimilation of surface observations reduces the bias and RMSE in the initial condition (IC) remarkably; on the forecast aspect, better forecast performances are acquired up to 24-h, in which the experiment assimilating the six pollutants simultaneously displays the best forecast skill overall. With respect to the impact of DA cycling frequency, the responses toward IC updating are found out to be different among the pollutants. For PM2.5, PM10, SO2 and CO, the forecast skills increase with the DA frequency; for O3, although improvements are acquired at the 6-h cycling frequency, the advantage of more frequent DA could be consumed by the disadvantage of unbalanced photochemistry (due to inaccurate precursor NOx/VOC ratios) from assimilating the existing observations (only O3 and NO2, but no VOC). Considering after one aspect (IC) in the model is corrected by DA, the deficiencies from other aspects (e.g., chemical reactions) could be more evident, this study further explores the model deficiencies by investigating the effects of assimilating gaseous precursors on the forecast of related aerosols. Results exhibit that the parameterization (uptake coefficients) in the newly added Sulfate-Nitrate-Ammonium (SNA) relevant heterogeneous reactions in the model are not fully appropriate although it best simulates observed SNA aerosols without DA; since the uptake coefficients were originally tuned under the inaccurate gaseous precursor scenarios without DA, the biases from the two aspects (SNA reactions and IC DA) were just compensated. In the future chemistry development, parameterizations (such as uptake coefficients) for different gaseous precursor scenarios should be adjusted and verified with the help of DA technique. According to these results, DA ameliorates certain aspects by using observation as constraints, and thus provides an opportunity to identify and diagnose the model deficiencies; it is useful especially when the uncertainties of various aspects are mixed up and the reaction paths are not clearly revealed. In the future, besides being used to improve the forecast through updating IC, DA could be treated as another approach to explore necessary developments in the model.

The hygroscopicity of organic aerosols (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter κOrg remains poorly characterized, especially in the relatively polluted environment on the North China Plain (NCP). Here we conducted simultaneous wintertime measurements of bulk aerosol chemical compositions of PM2.5 and PM1 and bulk aerosol hygroscopicity of PM10 and PM1 on the NCP using a capture vaporizer time-of-flight aerosol chemical speciation monitor (ToF-ACSM) and a humidified nephelometer system which measures aerosol light scattering enhancement factor f(RH). A method for calculating κOrg based on f(RH) and bulk aerosol chemical composition measurements was developed. We found that κOrg varied in a wide range with significant diurnal variations. The derived κOrg ranged from almost 0.0 to 0.25 with an average (± 1σ) of 0.08 (± 0.06) for the entire study. The derived κOrg was highly correlated with f44 (fraction of m/z 44 in OA), an indicator of oxidation degree of OA (R = 0.79), and the relationship can be parameterized as κOrg = 1.04 × f44 − 0.02. On average, κOrg reached the minimum (0.02) in the morning near 07:30 and then increased rapidly reaching the peak value of 0.16 near 14:30. The diurnal variations of κOrg were highly and positively correlated with those of mass fractions of oxygenated OA (R = 0.95), indicating that photochemical processing played a dominant role for the increase of κOrg in winter on NCP. Results in this study demonstrate the potential wide applications of humidified nephelometer system together with aerosol composition measurements for investigating the hygroscopicity of OA in various environments, and highlight that the parameterization of κOrg as a function of OA aging processes needs to be considered in chemical transport models for better evaluating the impacts of OA on cloud formation, atmospheric chemistry and radiative forcing.

To investigate the cause of fine particulate matter (particles with an aerodynamic diameter less than 2.5 µm, PM

Online measurements for chemical components in PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 μm) were carried out in Qinhuangdao (QHD), Tangshan (TS), Shijiazhuang (SJZ), and Zhengzhou (ZZ), China, from December 14, 2016 to February 2, 2017 to assess the impact of the fireworks control on air quality during the Spring Festival. Pre-fireworks, fireworks, and post-fireworks periods were all assessed. Results show that average concentrations of PM2.5 during fireworks periods were in the following order of SJZ > ZZ > TS > QHD. Fireworks displays have great impacts on concentrations of K+, SO42−, Cl−, Mg2+, and OC. According to results of PM2.5 source apportionment, fireworks were the largest PM2.5 source during fireworks periods in QHD, TS, SJZ, and ZZ, accounting for 68.9%, 53.8%, 61.1%, and 51.0% of PM2.5, respectively. This result was similar to the result of the relative ratio method (68.6% in QHD, 57.2% in TS, 63.0% in SJZ, 53.9% in ZZ). Based on results of PM2.5 source apportionment, the contribution of fireworks from 19:00 on January 27 to 8:00 on January 28 to PM2.5 in ZZ (44.1%) was the lowest in comparison with QHD (75.7%), TS (66.0%), and SJZ (63.8%), which can be ascribed to stringent fireworks control measures in ZZ. Although fireworks displays were banned in QHD and SJZ during the Spring Festival, contributions of fireworks to PM2.5 in these two cities were higher or slightly lower than that of TS during the special period when fireworks were allowed to be set off in TS, indicating that no stringent measures were taken to effectively control fireworks displays in QHD and SJZ. These results highlight the necessity of limiting fireworks emissions during the Spring Festival.

Comprehensive observations have been carried out in Beijing to investigate the impact of the Clean Air Action implemented in 2013 on changes in aerosol chemistry characteristics in heating seasons of 2016-2017 and 2017-2018. Results showed that PM2.5, SO2, NO2, NH3, O3 and CO concentrations decreased by 40.9%, 46.0%, 29.0%, 40.6%, 11.0% and 44.4%, respectively. Significant decreases were also observed for NO3- (32.5%), SO42- (52.9%), NH4+ (56.0%), Cl- (64.6%) and K+ (68.2%), on average. Enhanced PM2.5 pollution has changed from sulfate-driven to nitrate-driven. The decrease in SO2 was more significant than NO2 as a response to one reason of the larger decrease in SO42- concentration. The formation of sulfate was dominated by heterogeneous reactions in two heating seasons. Low pH could facilitate more efficient conversion of SO2 to sulfate. Photochemical reactions played a much more important role in the formation of nitrate in the second heating season, especially in the daytime. The major source regions for sulfate and nitrate were identified by back trajectories and the potential source function (PSCF). More nitrate was brought into Beijing when air masses coming from polluted regions in the southwest prevailed in 2017-2018 heating season. Thus, regional joint prevention and control are of great importance in the achievement of an effective reduction in PM2.5 pollution in the future.

Hygroscopicity is an important property of aerosols, which cannot be obtained for a wide range of particle sizes by online observation. A cascade impactor sampler, an essential method for obtaining the size-resolved chemical compositions of ambient aerosols, enables the acquisition of the size-resolved hygroscopicity for particles in multiple modes. A micro-orifice uniform deposit impactor (MOUDI-122, MSP) was used to collect the size-resolved aerosol samples during 2016 and 2017. The water-soluble components in these samples were analysed for different pollution levels in two periods. Then, the hygroscopicity parameter (κ) was calculated. The changing hygroscopicity in different size ranges was directly influenced by variations in the distribution of the water-soluble components. The contribution of sulfate to the κ was much higher in the summer period than that in the winter period due to the higher SO42− concentration in the summer. During the summer period, the contribution of nitrate to the κ of particle sizes above 0.56 μm was significantly higher than that of particles smaller than 0.56 μm, while in the winter period, the contribution of nitrate in finer particles with sizes below 1.8 μm was much higher than that in coarse particles. The contribution of chloride to the κ increased significantly in the winter period due to the influence of heating coal emissions. For particles below 1.0 μm, the contribution and fraction of water-soluble organic compounds (WSOCs) to the aerosol hygroscopicity increased with decreasing particle size. Compared with the aerosol hygroscopicity during the first stage of a pollution episode, the hygroscopicity of particles above 0.18 μm was significantly enhanced during the stages of pollution accumulation and formation. The results in this study were in good agreement with the results of other similar studies or data derived by other methods, indicating that the hygroscopicity based on size-resolved water-soluble components is reliable and can be used in the study of activation, radiation force, and heterogeneous reaction mechanism of particles with multiple sizes.

A suitable crop rotation with optimum fertilizer application can balance the need for high crop yields with minimal environmental risk. This has not yet been achieved in China, and the Chinese Government now requires rotations that include a fallow period with the aim of improving soil quality. As a step towards achieving these aims, a 5-year field experiment was carried out to evaluate the effects of different rates of nitrogen (N) fertilizer in three typical and acceptable crop rotations on grain yield, nitrogen use efficiency (NUE), N balance and economic benefit at Jiangjin, Chongqing, China. The rotations were rice-flooded fallow (RF), rice-wheat (RW) and maize-wheat (MW). Three fertilizer treatments were tested over five years. The RW rotation with reduced fertilizer application had the highest grain yield and generated the highest economic benefit. The MW rotation was not suitable for the region that we studied because high precipitation can cause serious disease problems for the wheat. The RF rotation with reduced fertilizer application proved to be the best system for ensuring food security, increasing economic benefit and reducing environmental risk.

To better characterize anthropogenic emission-relevant aerosol species, the Gridpoint Statistical Interpolation (GSI) and Weather Research and Forecasting with Chemistry (WRF/Chem) data assimilation system was updated from the GOCART aerosol scheme to the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) 4-bin (MOSAIC-4BIN) aerosol scheme. Three years (2015–2017) of wintertime (January) surface PM2.5 (fine particulate matter with an aerodynamic diameter smaller than 2.5 µ m) observations from more than 1600 sites were assimilated hourly using the updated three-dimensional variational (3DVAR) system. In the control experiment (without assimilation) using Multi-resolution Emission Inventory for China 2010 (MEIC_2010) emissions, the modeled January averaged PM2.5 concentrations were severely overestimated in the Sichuan Basin, central China, the Yangtze River Delta and the Pearl River Delta by 98–134, 46–101, 32–59 and 19–60 µ g m −3 , respectively, indicating that the emissions for 2010 are not appropriate for 2015–2017, as strict emission control strategies were implemented in recent years. Meanwhile, underestimations of 11–12, 53–96 and 22–40 µ g m −3 were observed in northeastern China, Xinjiang and the Energy Golden Triangle, respectively. The assimilation experiment significantly reduced both high and low biases to within ±5 µ g m −3 . The observations and the reanalysis data from the assimilation experiment were used to investigate the year-to-year changes and the driving factors. The role of emissions was obtained by subtracting the meteorological impacts (by control experiments) from the total combined differences (by assimilation experiments). The results show a reduction in PM2.5 of approximately 15 µ g m −3 for the month of January from 2015 to 2016 in the North China Plain (NCP), but meteorology played the dominant role (contributing a reduction of approximately 12 µ g m −3 ). The change (for January) from 2016 to 2017 in NCP was different; meteorology caused an increase in PM2.5 of approximately 23 µ g m −3 , while emission control measures caused a decrease of 8 µ g m −3 , and the combined effects still showed a PM2.5 increase for that region. The analysis confirmed that emission control strategies were indeed implemented and emissions were reduced in both years. Using a data assimilation approach, this study helps identify the reasons why emission control strategies may or may not have an immediately visible impact. There are still large uncertainties in this approach, especially the inaccurate emission inputs, and neglecting aerosol–meteorology feedbacks in the model can generate large uncertainties in the analysis as well.

Hygroscopicity is an important feature of ambient aerosols, which is very crucial to the study of light extinction, radiation force, and formation mechanism. The light scattering hygroscopic growth factor (f(RH)) is an important parameter which is usually measured by the humidified nephelometer system and could better describe the aerosol hygroscopicity under wide particle size range and continuous relative humidity (RH). The f(RH) can be applied to the establishment of a parameterization scheme for light extinction, the calculation of hygroscopicity parameter (κ), and also the estimation of aerosol liquid water content (ALWC). However, the humidified nephelometer system in the previous studies could only observe the f(RH) below 90 % due to the larger error of the sensor under high RH (> 90 %). Furthermore, the f(RH) observations in North China Plain needs to be greatly strengthened both in the temporal resolution and the observation duration. In view of this, an improved high-resolution humidified nephelometer system was established to observe the f(RH) of PM2.5 for a wide RH range between 30 %–96 % in the urban area of Beijing over three seasons (winter, summer, and autumn) in 2017. It was found that the f(80 %) at 525 nm of PM2.5 was evidently higher under the polluted conditions and highly correlated with the fractions of all the water-soluble ions. A two-parameter fit equation was selected to fit the observed f(RH) data. For each season, the fitting curve under the very clean condition was lower than that of other conditions. And the f(RH) points of polluted conditions were more concentrated with higher fitting R2 for summer and autumn data. The hygroscopicity of aerosol under higher RH was probably enhanced when compared with the data in the previous study conducted in NCP. In summer, the fitting f(RH) showed a significant dependence on wavelength for each pollution condition. However, there was an opposite performance in the f(RH) curves of different wavelengths for the very clean condition in winter. It was showed that The simulation showed that the maximum uncertainty of f(RH) was less than 10 %.

Acidity (pH) plays a key role in the physical and chemical behavior of aerosol and cannot be measured directly. In this work, aerosol liquid water content (ALWC) and size-resolved pH are predicted by thermodynamic model (ISORROPIA-II) in 2017 of Beijing. The mean aerosol pH over four seasons is 4.3±1.6 (spring), 4.5±1.1 (winter), 3.9±1.3 (summer), 4.1±1.0 (autumn), respectively, showing the moderate aerosol acidity. The aerosol pH in fine mode is in the range of 1.8 ~ 3.9, 2.4 ~ 6.3 and 3.5 ~ 6.5 for summer, autumn and winter, respectively. And coarse particles are generally neutral or alkaline. Diurnal variation of aerosol pH follows both aerosol components (especially the sulfate) and ALWC. For spring, summer and autumn, the averaged nighttime pH is 0.3~0.4 unit higher than that on daytime. Whereas in winter, the aerosol pH is relatively low at night and higher at sunset. SO42− and RH are two crucial factors affecting aerosol pH. For spring, winter and autumn, the effect of SO42− on aerosol pH is greater than RH, and it is comparable with RH in summer. The aerosol pH decreases with elevated SO42− concentration. As the NO3− concentration increases, the aerosol pH firstly increases and then decreases. Sulfate-dominant aerosols are more acidic with pH lower than 4, whereas nitrate-dominated aerosols are weak in acidity with pH ranges 3~5. In recent years, the dominance of NO3− in inorganic ions may be another reason responsible for the moderately acidic aerosol. ALWC has a different effect on aerosol pH in different seasons. In winter, the increasing RH could reduce the aerosol pH whereas it shows a totally reverse tendency in summer, and the elevated RH has little effect on aerosol pH in spring and autumn when the RH is between 30 % and 80 %. The dilution effect of ALWC on Hair+ is only obvious in summer. The elevated NH3 and NH4+ could reduce aerosol acidity by decreasing Hair+ concentration exponentially.

Continuous measurements of nitrous acid (HONO) were performed from December 12 to December 22, 2015 in both urban and suburban areas of Beijing to study the formation mechanism of HONO. The measurement campaign in both sites included a clean–haze–clean transformation process. HONO concentrations showed similar variations in the two sites, while they were always higher in the urban area. Moreover, correlations of HONO with NOx, NO2, NO, PM2.5and relative humidity (RH) were studied to explore possible HONO formation pathways, and the contributions of direct emissions, heterogeneous reactions, and homogeneous reactions were also calculated. This showed that HONO in urban and suburban areas underwent totally different formation procedures, which were affected by meteorological conditions, PM2.5concentrations, direct emissions, homogeneous reactions and heterogeneous reactions. PM2.5concentrations and RH would influence the NO2conversion efficiency. Heterogeneous reactions of NO2were more efficient in suburban areas and in clean periods while direct emissions and homogeneous reactions contributed more in urban areas and in polluted periods when the concentrations of NOxand NO were at a high level.

To study the HONO formation mechanisms during a pollution period, a continuous measurement was performed in both urban and suburban aeras of Beijing. During this period, the PM2.5 concentrations increased to 201 and 137 mu g/m(3) in urban and suburban areas, respectively. The concentrations of HONO, CO, SO2, O-3, NO, NO2, NO (x) were 1.45 ppbv, 0.61 ppmv, 8.7 ppbv, 4.3 ppbv, 44.4 ppbv, 37.4 ppbv, 79.4 ppbv and 0.72 ppbv, 1.00 ppmv, 1.2 ppbv, 7.9 ppbv, 3.7 ppbv, 8.2 ppbv, 11.9 ppbv, in urban and suburban areas, respectively. To compare possible pathways of HONO formation in both sites, the contributions of direct emissions, heterogeneous formations, and homogeneous productions were studied. HONO/NO2 ratios in the two sites indicated that heterogeneous reactions of NO2 were more efficient in suburban areas. And in both urban and suburban areas, the increase of PM2.5 concentrations and RH would promote the conversion efficiency in RH that ranged from 0% to 85%. However, when RH was above 85%, the HONO formation slowed down. Moreover, the study of direct emissions and homogeneous reactions showed that they contributed to a majority of HONO increase in urban areas than the 20% contributions in suburban areas. It implied that the high NO (x) concentrations and NO concentrations in urban areas or in pollution periods would make direct emissions and homogeneous reactions become dominant in HONO formations.

The light scattering hygroscopic growth factor (f(RH)), which quantifies the hygroscopicity of polydisperse ambient aerosols at continuous relative humidity (RH) levels, is usually measured by a humidified nephelometer system. The hygroscopicity of aerosol will change evidently under high RH conditions (>90%). However, in previous studies, only the aerosol hygroscopicity below 90% RH was generally obtained and discussed due to the difficulty in creating a stable high RH and measuring it accurately. In view of these conditions, an improved high-resolution humidified nephelometer system was established to observe the f(RH) of PM2.5 for a wide RH range between 30 and 96% in an urban area of Beijing during three seasons (winter, summer, and autumn) in 2017. Two sensors were used to calculate the humidified RHs, which made the uncertainty in the f(RH) at a high RH much lower than that in previous studies (lower than 10% for the maximum value). It was found that the f(80%) at 525 nm of PM2.5 was evidently higher under polluted conditions and highly correlated with the fractions of all of the water-soluble ions. A two-parameter fitting equation was selected to fit the observed f(RH) data. The f(RH) data under polluted conditions were more uniform with higher fitting R2 values during the summer and autumn. The hygroscopicity of aerosols has probably increased compared with that in the previous study conducted in the NCP. The fitted curves of the seasonal f(RH) data showed a significant dependence on the wavelength and increased with increasing wavelength. The hygroscopicity of PM2.5 for RH > 90% was definitely lower than that for 80%

More than 400 PM2.5 samples were collected at four urban sites in Beijing (BJ), Tianjin (TJ), Shijiazhuang (SJZ), and Chengde (CD), and also one site in Shangdianzi (SDZ), which was used as a regional background station, over four seasons from 2009 to 2010. The organic carbon (OC) and elemental carbon (EC) in each sample were analyzed. The average annual concentrations were 71.8–191.2 μg m−3 for PM2.5, 10.8–26.4 μg m−3 for OC, and 3.9–9.7 μg m−3 for EC at the five sites. OC and EC concentrations were lower in the spring and summer and much higher in the autumn and winter, mainly due to aerosol emissions from additional fuel combustion for heating. OC/EC ratios were lowest in the summer and highest in the winter at SDZ, BJ, TJ, and SJZ. These seasonal trends indicate that the characteristics of carbonaceous aerosol pollution were spatially similar and season-dependent in the plain area of Beijing, Tianjin, and Hebei (BTH). An EC tracer method was used to calculate the concentrations for secondary organic carbon (SOC); SOC concentrations were also higher in the autumn and winter and lowest during the summer at all five sites. A stable atmosphere and low temperatures, which were more frequent during the winter and autumn, facilitated the accumulation of air pollutants and accelerated the condensation or adsorption of volatile organic compounds in the BTH area. Over the past ten years (1999–2009), Beijing had observed a decrease in the EC concentrations during every season and a remarkable reduction in aerosol emissions from coal combustion for heating.

In order to study the temporal and spatial variations of PM2.5 and its chemical compositions in the region of Beijing, Tianjin, and Hebei (BTH), PM2.5 samples were collected at four urban sites in Beijing (BJ), Tianjin (TJ), Shijiazhuang (SJZ), and Chengde (CD), and also one site at Shangdianzi (SDZ) regional background station over four seasons from 2009 to 2010. The samples were weighted for mass concentrations and analyzed in the laboratory for chemical profiles of 19 elements (Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Ni, P, Pb, Sr, Ti, V, and Zn), eight water-soluble inorganic ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−, and carbon fractions (OC and EC). The concentrations of PM2.5 and its major chemical species were season dependent and showed spatially similar characteristics in the plain area of BTH. The average annual concentrations of PM2.5 were 71.8–191.2 μg m−3 at the five sites, with more than 90% of sampling days exceeding 50 μg m−3 at BJ, TJ, and SJZ. PM2.5 pollution was most serious at SJZ, and the annual concentrations of PM2.5, secondary inorganic ions, OC, EC, and most crustal elements were all highest. Due to stronger photochemical oxidation, the sum of concentrations of secondary inorganic ions (NH4+, NO3−, and SO42− was highest in the summer at SDZ, BJ, TJ, and CD. Analysis of electric charges of water-soluble inorganic ions indicated the existence of nitric acid or hydrochloric acid in PM2.5. For all five sites, the concentrations of OC, EC and also secondary organic carbon (SOC) in the spring and summer were lower than those in the autumn and winter. SOC had more percentages of increase than primary organic carbon (POC) during the winter. The sums of crustal elements (Al, Ca, Fe, Mg, Ti, Ba, and Sr) were higher in the spring and autumn owing to more days with blowing or floating dust. The concentrations of heavy metals were at higher levels in the BTH area by comparison with other studies. In Shijiazhuang and Chengde, the PM2.5 pollution was dominated by coal combustion. Motor vehicle exhausts and coal combustion emissions both played important roles in Tianjin PM2.5 pollution. However, motor vehicle exhausts had played a more important role in Beijing owing to the reduction of coal consumption and sharp increase of cars in recent years. At SDZ, regional transportation of air pollutants from southern urban areas was significant.

In China, visibility condition has become an important issue that concerns both society and the scientific community. In order to study visibility condition trends and characteristics for the BTH (Beijing, Tianjin, and Hebei) area, meteorological data (1980–2008) were collected from over 100 ground stations. The 29-year average visibilities were 21.7, 13.6, and 18.0 km for Beijing, Tianjin, and Hebei, respectively, and the long-term visibility of BTH exhibited decreasing trend before 1998, slight fluctuations between 1998 and 2006, and increasing trend between 2006 and 2008. Most of the visibility trend lines depicted the same fluctuations and converged to a very narrow range for the urban stations in each area. Average visibility at non-urban stations was apparently higher than that at urban stations, with 29-year average of 19.4 and 14.4 km respectively. Visibility was higher in the spring and autumn for Tianjin and Hebei, but best in the winter for Beijing probably due to higher wind speed and lower relative humidity. The visibility spatial distribution showed lower visibility that primarily occurred in the urban areas of Beijing, Tianjin, and Shijiazhuang. The trends regarding days with haze, good visibility, and fog for the urban and non-urban areas of the BTH region were also discussed. There was an increasing trend for days with haze but no apparent trend for days with good visibility. On average, there were no more than nine foggy days per year in the BTH area. Visibility and PM2.5 were observed to negatively correlate at two stations in Beijing, where PM2.5 data are available, and aerosols could cause a greater degree of visibility impairment in the summer months according to the correlations between visibility and PM2.5 concentration.

Particulate matter is the chief pollutant of urban ambient air in North China, and the contribution of resuspended dust to urban ambient PM10 is very high. Studies on source apportionment for urban ambient particulate matter and resuspended dust were carried out in six north cities (Tianjin, Jinan, Shijiazhuang, Taiyuan, Urumqi and Yinchuan) during 2000 and 2002. About 150 resuspended dust samples were collected in these cities and then resampled PM10 from them in resuspension chamber in the lab. Samples were analyzed by ICP and X-ray fluorescence for 19 elements (Na, Mg, Al, Si, P, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, Ba and Pb), ion chromatography for 4 ions (Cl−, NO3−, SO42− and NH4+) and carbon elemental analyzer for TC and OC. Crustal elements and TC are in abundance in resuspended dust. Zn, Cu, and Cr are enriched. According to the results of CMB receptor model, resuspended dust was the greatest source type of ambient PM10 in six cities, and soil, cement and coal combustion were the main contributors to resuspended dust.