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Abstract Aerosol and particulate matter are playing significant roles in the regional climate and environment in the tropical rain forest of Southeast Asia. Both satellite and ground observations showed significant seasonal variations in the PM 2.5 concentration and the aerosol optical properties during 2012–2014 in the Xishuangbanna tropical rain forest. The annual mean values of the PM 2.5 , aerosol optical depth (AOD), and the Ångström exponent (α) were 34.3 ± 19.7 μg·m−3, 0.54 ± 0.37, and 1.36 ± 0.20 in the dry season and 16.90 ± 5.08 μg·m−3, 0.37 ± 0.10, and 1.07 ± 0.25 in the wet season, respectively. The results showed that 46.9% and 56.5% of the moderate-resolution imaging spectroradiometer (MODIS) C6 AOD data met the NASA accuracy requirements in the dry and wet season respectively and 17.1% and 17.7% of the seasonal mean systematically underestimated the ground-based data. There was a high correlation between PM 2.5 and AOD. The range of the correlation coefficient (R2) was 0.69–0.85 in the dry season and 0.33–0.39 in the wet season. Linear regression functions of PM 2.5 and MODIS AOD were developed and used to retrieve the spatial and temporal distributions of the PM 2.5 in the tropical rain forest over the last decade (2006–2015). The annual mean PM 2.5 increased slightly in the region. The range of the PM 2.5 was 20–40 μg·m−3 in the wet season and 25–80 μg·m−3 in the dry season. In northern Thailand, northern Vietnam and the central district of Laos, PM 2.5 was up to the range of 50–80 μg·m−3, which was mainly attributed to biomass burning in these areas. Highlights • MODIS C6 AOD was suitable to retrieve the PM 2.5 distribution over the tropical rain forest of Southeast Asia. • The linear fitting functions of PM 2.5 and AOD showed greatly seasonal difference. • The PM 2.5 distributions showed a slightly increased trend and exist three high-value areas. • PM 2.5 was mainly affected by the strong biomass combustion. [ABSTRACT FROM AUTHOR]

With a decrease in fine particulate matter (PM 2.5) due to the implementation of air pollution prevention-control measures, ozone (O 3) has risen in China especially in urban areas recently. It becomes more and more important to control air combined pollution with high concentration of PM 2.5 and O 3. We analyzed the characteristics of air combined pollution in Beijing from 2014 to 2019 in this paper. Results indicated that the trough O 3 showed an upward trend of 4.5 μg m−3 yr−1, while the peak decreased by −1.1 μg m−3 yr−1. There were three typical atmospheric chemical stages in the interaction between O 3 and PM 2.5 in summer through observations. The first was an intense photochemical stage with the synchronous and rapid growth of O 3 and PM 2.5. The O 3 concentration varied from 68 μg m−3 to 206 μg m−3 when PM 2.5 ≤ 35 μg m−3. While secondary inorganic ions (SNA) ranged from 2 to 21 μg m−3 especially SO 4 2− (0.8–12 μg m−3). The second was an aerosol chemical generation stage with the rapid increase of PM 2.5 and SNA under high O 3 concentration. O 3 was maintained at a high concentration between 198 μg m−3 and 241 μg m−3 when PM 2.5 was within the range of 35–125 μg m−3. SNA increased from 12 μg m−3 to 96 μg m−3. The third was a high PM 2.5 pollution stage suppressing the generation of O 3 when PM 2.5 ≥ 125 μg m−3. O 3 began to decline with the growth of PM 2.5 and decreased from 224 μg m−3 to 130 μg m−3. Ultraviolet radiation (UV) decreased from 13 MJ m−3 in the first to 8 MJ m−3 in this stage. [Display omitted] • The O 3 trough increased by 4.5 μg/m3 and the peak decreased by −1.1 μg/m3 per year in Beijing. • There were three typical processes in the interaction between O 3 and PM 2.5 in summer day. • The photochemical process was intense in dry-sunny days with the synchronous growth of O 3 and PM 2.5. • Aerosol chemical process intensified with a rapid growth of PM 2.5 and SNA in high O 3. • O 3 began to decline with the growth of PM 2.5 when PM 2.5 attenuated UV in a high level. [ABSTRACT FROM AUTHOR]

In order to achieve the targets specified in the Action Plan for Air Pollution Prevention and Control (APAPPC), a limited coal banning area (10,000 km2) was designated in the heavily polluted Beijing-Tianjin-Hebei region (BTH) for the first time in 2017. PM 2.5 and elements were sampled by the network of BTH to evaluate the effectiveness of this policy. This study found that the fine days with PM 2.5 < 75 μg m−3 accounted for 74.3% in the autumn and winter of 2017, which was significantly higher than that in 2016 (43%). The heavily polluted days (PM 2.5 > 150 μg m−3) also decreased from 32.2% in 2016 to 4.9% in 2017. Arsenic (As) is an important tracer in coal consumption, which can be used to reflect the influence of the establishment of coal banning areas on north China. The cluster analysis of air mass forward trajectory identified that the number of polluted trajectories with PM 2.5 and As in 2017 decreased by 47.6% and 49.7%, respectively. Under the implementation of the coal banning policy, the weighted concentration of PM 2.5 and As decreased by 94.2 μg m−3 and 5.1 ng m−3 in the coal banning area, 60.9 μg m−3 and 3.4 ng m−3 in the no coal banning area in BTH, respectively. The influence of weighted concentration of PM 2.5 and As in coal banning area on North China were 1.6–49.2 μg m−3 and 0.15–2.8 ng m−3, respectively, which was 38.8% and 29.7% lower than 2016. In coal banning area, BTH and other parts of North China, the reduction of the weight concentration of PM 2.5 in 2017 accounted for 41.4%, 26.8% and 31.8% of the total reduction, respectively, so was the As in 39%, 26.3% and 34.6%, indicating that setting up a coal banning area scientifically in limited areas can produce remarkable regional benefit. [Display omitted] • Coal banning area brought remarkable environmental benefit to the whole north China. • Fine days in the autumn and winter of BTH increased from 43% in 2016 to 74% in 2017. • Impact of PM 2.5 in coal banning area (BTH) reduced by 94.2 (60.9) μg m−3 in 2017. • The impact of coal banning area on PM 2.5 in North China decreased by 48.3% in 2017. [ABSTRACT FROM AUTHOR]

Measurements of fine particles (PM2.5), coarse particles (PM10) and aerosol optical properties were carried out at a background station-Dinghushan in the Pearl River Delta (PRD) region from 2009 to 2012. It showed that the long-term mean concentrations of PM2.5 and PM10 were as high as (51±31) μg/m3 and (76±43) μg/m3, respectively. Particulate pollution in the PRD region was mitigated over the last four years, and the annual mean concentrations of PM2.5 was decreased to (39±25) μg/m3 in 2012 from (65±33) μg/m3 in 2009. The concentrations of PM2.5 and PM10 were low in summer and high in the other three seasons. The correlation between the daily concentrations of PM2.5 and PM10 was generally high (R >0.90). The long-term mean PM2.5/PM10 ratio reached up to 0.67, the ratio was high in autumn (0.71) and low in summer (0.57). Atmospheric visibility was poor, the long-term mean of aerosol optical depth (AOD) at 500nm was 0.91±0.40, and Ångström exponent was 0.97±0.36. The values of AOD were low in winter and summer, and high in spring and autumn. The correlation between particulate matter (PM) concentrations and AOD was high. The correlation coefficient in dry season (October–next March) was better than that was in wet season (April–September). In addition, the correlation also demonstrated great differences for different air masses. [ABSTRACT FROM AUTHOR]

Abstract: Satellite observation has shown great promise in estimating the concentration of particulate matter at the surface. It is necessary to investigate the relationships between PM2.5 concentration and aerosol optical depth (AOD) to promote the accuracy of satellite remote sensing, especially in the high aerosol emitting region in North China. The annual concentration of PM2.5 was 33±10μg/m3 in the background of North China from 2009 to 2011, while the annual mean of AOD at 550nm was 0.23±0.10; there was a high correlation between the PM2.5 concentration and AOD in the region. The linear regression functions of the daily PM2.5 concentration (y) with the ground-observed AOD (x) and the MODIS AOD (x) were y=96.98x+6.61 (R2 =0.58) and y=100.10x+12.13 (R2 =0.57), respectively. The linear regression functions of the monthly PM2.5 concentration (y) with the ground-observed AOD (x) and the MODIS AOD (x) were y=91.66x+12.60 (R2 =0.75) and y=79.33x+16.33 (R2 =0.75), respectively. In addition, the results also demonstrated significant differences in the slopes and intercepts of the linear regression functions for different seasons. A large error resulted when any single function was used to retrieve the PM2.5 concentration using AOD. [Copyright &y& Elsevier]

Abstract: Evidence concerning the health risk of fine and coarse particles is limited in developing Asian countries. The modifying effect between particles and temperature and season also remains unclear. Our study is one of the first to investigate the acute effect of particles size fractions, modifying effects and interannual variations of relative risk in a developing megacity where particulate levels are extraordinarily high compared to other Asian cities. After controlling for potential confounding, the results of a time-series analysis during the period 2005&#x2013;2009 show that a 10 &#x03bc;g m&#x2212;3 increase in PM2.5 levels is associated with a 0.65% (95% CI: 0.29&#x2013;0.80%), 0.63% (95% CI: 0.25&#x2013;0.83%), and 1.38% (95% CI: 0.51&#x2013;1.71%) increase in non-accidental mortality, respiratory mortality, and circulatory mortality, respectively, while a 10 &#x03bc;g m&#x2212;3 increase in PM10 is similarly associated with increases of 0.15% (95% CI: 0.04&#x2013;0.22%), 0.08% (95% CI: 0.01&#x2013;0.18%), and 0.44% (95% CI: 0.12&#x2013;0.63%). We did not find a significant effect of PM2.5 &#x2013; 10 on daily mortality outcomes. Our analyses conclude that temperature and particulates, exposures to both of which are expected to increase with climate change, might act together to worsen human health in Beijing, especially in the cool seasons. The level of the estimated percentage increase assume an escalating tendency during the study period, in addition to having a low value in 2008, and after the Olympic Games, the values increased significantly as the temporary atmospheric pollution control measures were terminated mostly. [ABSTRACT FROM AUTHOR]

In 2017, a coal banning area of approximately 10,000 square kilometers was established in the Beijing-Tianjin-Hebei (BTH) region of China for the first time. This study compared the change of elemental concentrations during the autumn and winter of 2016 and 2017, and analyzed the environmental and health benefits of the coal banning policy. Result showed that compared to 2016, the reduction rate of atmospheric particulate matter with an aerodynamic diameter no larger than 2.5 μm (PM 2.5) in 2017 was as high as 63.5% (p < 0.01), and that of the total elements in PM 2.5 was up to 43% (p < 0.01). Coal consumption, transportation, industry, soil dust, and fuel consumption were the main sources of these elements in BTH. Coal consumption and industry accounted for 5.8–26.2% and 6.5–31.3%, respectively, of the total elements. After the coal banning area was established, the elements from coal consumption were reduced by 13.5–60.3% (the background station was reduced by 19.6%, p < 0.05), and those from industrial sources were reduced by 11–80.1%. The results of the health risk assessment showed that the carcinogenic risk of the coal consumption and industrial sources had been reduced by 30.7–48.3% in 2017. The policy of the coal banning is very effective in controlling the air pollution in urban agglomeration in developing countries and has resulted in great health and environmental benefits. Image 1 • Coal Banning Area is the most efficient pollution control measure in BTH in 2017. • The measure cut down the bulk coal consumption by 10.2% in the 10,000 km2 region. • The measure has resulted in the high benefits of environment and health in BTH. • PM 2.5 and total elements declined drastically by 63.5% and 43% in 2017. • The carcinogenic risk index of hazardous elements decreased by 36.3% in BTH. [ABSTRACT FROM AUTHOR]

Both PM 2.5 and visibility are important physical parameters to measure the air pollution condition. This paper first provided a new normalization approach for the relationship between PM 2.5 and visibility in heavy pollutant region, which realized a mathematical calculation for two different physical parameters based on non-dimensional data process (also called Min-Max normalization). The normalization relationship showed a power function (y = axb) with high correlation coefficients. The inflection points of the maximum power function curvature composed the theoretical threshold range of PM 2.5 for improving or deteriorating the visibility. In the case city of north China, the threshold range of PM 2.5 was 112–121 μg·m−3 corresponding to the visibility range 4.5–5.9 km. Within the threshold range, the mean concentration of main pollutant components was 106 μg·m−3. The mean extinction coefficient, b ext of 743.5 Mm−1, was mainly contributed by organic matter (34%), ammonium nitrate (19%), ammonium sulfate (18%) and elemental carbon (13%). When PM 2.5 was below the range, the visibility increased significantly with the slow decrease of PM 2.5. When PM 2.5 exceeded the range, the visibility reduced seriously due to the saturated extinction contribution of the main pollutants. The new normalization approach theoretically revealed the significance of threshold and how to improve visibility by controlling PM 2.5 at different pollution stages. Unlabelled Image • A new normalization approach was first applied to establish the mathematical relationship of PM 2.5 and visibility. • The normalization mathematical relationship was a power function with high correlation coefficients. • The maximum curvature of the function composed the theoretical threshold of PM 2.5 to improve or deteriorate visibility. • The extinction coefficients of aerosol components were quite different at different pollution stages. • The threshold range has guiding significance how to control PM 2.5 for the blue sky in heavy polluted cities. [ABSTRACT FROM AUTHOR]

PM2.5 data from major cities in the southern North China Plain during 2013–2015 were comprehensively analyzed relative to variation features, meteorology effects, and regional transport contributions. The annual average ranged from 87 to 123 μg m−3, with the highest in Baoding and Shijiazhuang, the moderate in Handan and Hengshui, and the lowest in Cangzhou, which revealed an evident concentration gradient with distance from the mountains. PM2.5 pollution indicated significantly regional characteristics and high correlations in daily PM2.5 changes and similar seasonal and diurnal variations in five cities. The highest concentrations mainly occurred in the winter, then autumn, spring, and summer, and the diurnal variations were bimodal with peaks during the morning rush hours and at night, which were mostly dominated by the differences in source emissions and the boundary layer. The PM2.5 concentrations were significantly positively correlated with relative humidity, especially during winter. The highest PM2.5 concentrations in all cities were associated with the south, southeast, and southwest pathways, while the short northwest pathway in the winter for Baoding and Shijiazhuang experienced the highest concentration. Regional contributions ranged from 19.6 to 33.7% annually, with the largest in Baoding and Shijiazhuang. These results provide a scientific basis for pollution forecasting and control in these heavily polluted cities. [ABSTRACT FROM AUTHOR]

Several air pollution episodes occurred in Beijing before and after the 2014 Asia-Pacific Economic Cooperation (APEC) summit, during which air-pollution control measures were implemented. Within this autumn-winter transit season, domestic heating started. Such interesting period merits comprehensive chemical characterization, particularly the organic species, to look into the influence of additional heating sources and the control measures on air pollution. Therefore, this study performed daily and 6h time resolved PM 2.5 sampling from the 24th October to 7th December, 2014, followed by comprehensive chemical analyses including water-soluble ions, elements and organic source-markers. Apparent alterations of chemical profiles were observed with the initiation of domestic heating. Through positive matrix factorization (PMF) source apportionment modeling, six PM 2.5 sources including secondary inorganic aerosol (SIA), traffic emission, coal combustion, industry emission, biomass burning and dust were separated and identified. Coal combustion was successfully distinguished from traffic emission by hopane diagnostic ratio. The result of this study reveals a gradual shift of dominating sources for PM pollution episodes from SIA to primary sources after starting heating. BaP eq toxicity from coal combustion increased on average by several to dozens of times in the heating period, causing both long-term and short-term health risk. Air mass trajectory analysis highlights the regional influence of the industry emissions from the area south to Beijing. Control measures taken during APEC were found to be effective for reducing industry source, but less effective in reducing the overall PM 2.5 level. These results provide implications for policy making regarding appropriate air pollution control measures. [Display omitted] • Source apportionment of PM 2.5 including organic source tracers in Beijing. • PM 2.5 composition and source changes with regard to domestic heating identified. • Effects of source strength and meteorological changes on PM pollution revealed. • Large increase of BaP eq toxicity in heating period and its main source identified. • Control measure taken during APEC evaluated with new perspective. [ABSTRACT FROM AUTHOR]

In recent decade, despite the implementation of strict emission reductions, PM 2.5 and surface ozone (O 3) in China are still at high pollution level. In this study, based on observations at air quality stations (∼1500 mainly in urban region), daily PM 2.5 and MDA8 (the daily maximum 8 h average) O 3 concentrations in high-density and uniform meteorological sites (∼2000) over China during 2017–2019 were retrieved using generalized additive models (GAMs) for different regions on a seasonal scale, in order to establish 0.1° gridded datasets and investigate their spatiotemporal characteristics, nonlinear effects of meteorology, and ambient air pollution-attributable health risks on a regional scale. This model can provide high accuracy in the estimation of daily PM 2.5 (O 3) in 2018 with overall cross-validation R2 and RMSE of 0.81 (0.80) and 12.73 (17.63) μg/m3 and predictive capability in 2017 and 2019 with R2 of 0.67 (0.70) for PM 2.5 (O 3). Temperature, relative humidity and visibility are three main meteorological factors driving PM 2.5 (O 3) response. PM 2.5 generally exhibits an exponential decrease with increasing visibility and an increase with increasing relative humidity, while is suppressed at the thresholds of 60% and 70%. For O 3 , in northern China, positive response of temperature is more important and O 3 suppression exists at 36 °C ∼ 41.5 °C, whereas negative response of relative humidity is dominating in southern China. Overall, annual averaged PM 2.5 (O 3) values across China reduced (increased) by 18.7% (11.2%) between 2017 and 2019, with the highest concentrations and variation in North China. These variations contributed to a 14.9% reduction in PM 2.5 attributed premature deaths, but a 10.4% increase in O 3 exposure related deaths. Generally, central-eastern China still exists greater health risk with per capita premature mortality ranging from 0.80‰ to 1.19‰ for PM 2.5 and 1.91‰ to 2.60‰ for O 3. Additionally, O 3 vegetation damages demonstrate a decline (increase) by 1.3% (21.7%) in North China with higher O 3 pollution (southern China with slight pollution) between 2017 and 2019. These findings highlight the essentiality for the synergistic control of PM 2.5 and O 3 based on regional differences in China. [Display omitted] • Daily PM 2.5 and O 3 at ∼2000 meteorological sites in China were constructed by GAM. • The PM 2.5 and O 3 models both shown a good performance and predictive capacity. • Northern China exhibits O 3 suppression, but more negative effect of humidity is in southern China. • PM 2.5 (O 3) levels and pollution- attributable premature deaths reduced (increased). • O 3 vegetation damages reduced (rose) in North (southern) China from 2017 to 2019. [ABSTRACT FROM AUTHOR]

Air quality in Beijing-Tianjin-Hebei and its surrounding regions has been significantly improved during the Three-Year Action Plan for Winning the Battle Against Air Pollution from 2018 to 2020. The PM 2.5 concentration can be significantly influenced by meteorological conditions, and the effectiveness of anthropogenic emissions reduction on PM 2.5 over the North China Plain remains unclear. In this study, the Nested Air Quality Model System (NAQPMS) and observations were used to quantitatively examine the contribution of meteorological and emission variations to autumn-winter (November to February) PM 2.5 concentrations in Beijing and its surrounding regions from 2018 to 2020. The model reproduced the temporal evolution of atmospheric pollutants and components well. The results showed that the observed mean PM 2.5 concentrations during the winter were reduced by 15% and 29% in 2019 and 2020, respectively, compared with that in 2018. In 2019, the meteorological conditions over the Beijing-Tianjin-Hebei (BTH) region were poor, causing an increase of PM 2.5 by 7.6% in BTH and 11% in Beijing; therefore, the control of air pollutant emission compensated for the unfavorable influence of meteorology and emission reduction was the decisive factor for PM 2.5 reduction; In 2020, the meteorological conditions were relatively favorable, and the changes in meteorological factors and emission reduction led to a decrease in PM 2.5 concentration by 18.6% and 10.5%, respectively, compared to 2018. Emission reduction also contributed to decrease in nitrate, ammonium, organic matter, element carbon and precursors. This work confirmed the obvious environmental benefit of pollution control measures from 2018 to 2020. There was obvious regional heterogeneity in the effect of meteorology and the impact of adverse meteorological conditions should be considered during the formulation of air pollution control measures. The results can serve as a reference for the formulation of air quality objectives or the evaluation of the environmental effect of pollution control schemes. • The effect of winter meteorology and control measures in 2018 to 2020 was assessed. • Observed winter PM 2.5 reduced by 15% and 29% in 2019 and 2020, respectively, compared to 2018. • Unfavorable meteorology in 2019 partially counteracted the emission control effects. • Emission mitigation played an important role in air quality improvement. [ABSTRACT FROM AUTHOR]

Rapid economic development had an important impact on the concentration of air pollutants and the aerosol properties in Taiyuan City, one of China's most important energy and heavy industry bases. Therefore, the long-term measurements of the optical and radiation properties of aerosols and air pollutants monitored by several urban sites in Taiyuan from 2013 to 2019 were analyzed. During the observation period, the multi-year mean concentrations of PM 2.5 , PM 10 , SO 2 , NO 2 and CO were 65 ± 7 μg/m3, 129 ± 14 μg/m3, 57 ± 21 μg/m3, 45 ± 6 μg/m3, 1.45 ± 0.31 mg/m3, respectively. The concentrations of various pollutants were highest in winter (PM 2.5 : 90 μg/m3; PM 10 : 149 μg/m3; SO 2 : 109 μg/m3; NO 2 : 49 μg/m3; CO: 2.03 mg/m3). After the implementation of air pollution control in 2013, the annual mean concentrations of PM 2.5 , PM 10 , SO 2 and CO in 2019 had been reduced by 28%, 29%, 73% and 47% respectively compared with those in 2013. But aerosol optical depth (AOD) and radiative forcing had no remarkable downtrend. Annual mean of AOD at 500 nm varied from 0.43 to 0.60 with high single scattering albedo (SSA), all above 0.91. Multi-year average radiative forcing at the top of atmosphere (TOA), interior of atmosphere (ATM), and bottom of the atmosphere (SFC) were − 2.68 ± 2.12 W/m2, 45.02 ± 3.14 W/m2, and − 47.46 ± 3.72 W/m2, respectively. And radiative forcing effects were strongest in winter (TOA: -5.74 ± 3.20 W/m2; ATM: 49.15 ± 3.85 W/m2; SFC: -54.83 ± 5.31 W/m2) due to the stronger anthropogenic emissions. The increase in humidity would increase the concentration of particulate matter and the extinction of the atmosphere. The increase of absorbing aerosols and scattering aerosols during the heating period in winter increased the aerosol radiative forcing, which would warm atmosphere and cool surface and then increase the stability of the atmosphere. Pollutants were difficult to diffuse and gradually accumulated thereby, and the radiative forcing effect continued to increase, forming positive feedback and making continuous pollution. • After air pollution control, particulate pollution in winter was still serious. • Particulate matter pollution in Taiyuan was mainly caused by man-made industrial scattering aerosols. • High PM 2.5 pollution and high humidity can reinforce aerosol radiative forcing (ARF). • The feedback of PM 2.5 and ARF would cause severe haze events especially in high humidity and stable weathers. [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]