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The "Campaign on Atmospheric Aerosol Research" network of China (CARE-China) is a long-term project for the study of the spatio-temporal distributions of physical aerosol characteristics as well as the chemical components and optical properties of aerosols over China. This study presents the first long-term data sets from this project, including 3 years of observations of online PM2.5 mass concentrations (2012-2014) and 1 year of observations of PM2.5 compositions (2012-2013) from the CARE-China network. The average PM2.5 concentration at 20 urban sites is 73.2 µg m-3 (16.8-126.9 µg m-3), which was 3 times higher than the average value from the 12 background sites (11.2-46.5 µg m-3). The PM2:5 concentrations are generally higher in east-central China than in the other parts of the country due to their relatively large particulate matter (PM) emissions and the unfavourable meteorological conditions for pollution dispersion. A distinct seasonal variability in PM2.5 is observed, with highs in the winter and lows during the summer at urban sites. Inconsistent seasonal trends were observed at the background sites. Bimodal and unimodal diurnal variation patterns were identified at both urban and background sites. The chemical compositions of PM2.5 were analysed at six paired urban and background sites located within the most polluted urban agglomerations -- North China Plain (NCP), Yangtze River delta (YRD), Pearl River delta (PRD), North-east China region (NECR), South-west China region (SWCR) -- and the cleanest region of China -- the Tibetan Autonomous Region (TAR). The major PM2.5 constituents across all the urban sites are organic matter (OM, 26.0 %), SO2-4 (17.7%, mineral dust (11.8 %), NO-3 (9.8 %), NH + 4 C4 (6.6 %), elemental carbon (EC) (6.0 %), Cl- (1.2 %) at 45% RH and unaccounted matter (20.7 %). Similar chemical compositions of PM2.5 were observed at background sites but were associated with higher fractions of OM (33.2 %) and lower fractions of NO-3 (8.6 %) and EC (4.1 %). Significant variations of the chemical species were observed among the sites. At the urban sites, the OM ranged from 12.6 µg m-3 (Lhasa) to 23.3 µg m-3 (Shenyang), the SO2-4 ranged from 0.8 µg m-3 (Lhasa) to 19.7 µg m-3 (Chongqing), the NO-3 ranged from 0.5 µg m-3(Lhasa) to 11.9 µg m-3 (Shanghai) and the EC ranged from 1.4 µg m-3 (Lhasa) to 7.1 µg m-3 (Guangzhou). The PM2.5 chemical species at the background sites exhibited larger spatial heterogeneities than those at urban sites, suggesting different contributions from regional anthropogenic or natural emissions and from long-range transport to background areas. Notable seasonal variations of PM2.5-polluted days were observed, especially for the megacities in east-central China, resulting in frequent heavy pollution episodes occurring during the winter. The evolution of the PM2.5 chemical compositions on polluted days was consistent for the urban and nearby background sites, where the sum of sulfate, nitrate and ammonia typically constituted much higher fractions (31-57 %) of PM2.5 mass, suggesting fine-particle pollution in the most polluted areas of China assumes a regional tendency, and the importance of addressing the emission reduction of secondary aerosol precursors including SO2 and NOx. Furthermore, distinct differences in the evolution of [NO-3]/[SO2-4] ratio and OC=EC ratio on polluted days imply that mobile sources and stationary (coal combustion) sources are likely more important in Guangzhou and Shenyang, respectively, whereas in Beijing it is mobile emission and residential sources. As for Chongqing, the higher oxidation capacity than the other three cities suggested it should pay more attention to the emission reduction of secondary aerosol precursors. This analysis reveals the spatial and seasonal variabilities of the urban and background aerosol concentrations on a national scale and provides insights into their sources, processes and lifetimes. [ABSTRACT FROM AUTHOR]

Recurrent smoke-haze episodes that occur in Southeast Asia (SEA) are of much concern because of their environmental and health impacts. These haze episodes are mainly caused by uncontrolled biomass and peat burning in Indonesia. Airborne particulate matter (PM) samples were collected in the southwest coast of Singapore from 16 August to 9 November in 2009 to assess the impact of smoke-haze episodes on the air quality due to the long-range transport of biomass and peat burning emissions. The physical and chemical characteristics of PM were investigated during pre-haze, smoke-haze, and post-haze periods. Days with PM mass concentrations of ≥35 μg m were considered as smoke-haze events. Using this criterion, out of the total 82 sampling days, nine smoke-haze events were identified. The origin of air masses during smoke-haze episodes was studied on the basis of HYSPLIT backward air trajectory analysis for 4 days. In terms of the physical properties of PM, higher particle surface area concentrations and particle gravimetric mass concentrations were observed during the smoke-haze period, but there was no consistent pattern for particle number concentrations during the haze period as compared to the non-haze period except that there was a significant increase at about 08:00, which could be attributed to the entrainment of PM from aloft after the breakdown of the nocturnal inversion layer. As for the chemical characteristics of PM, among the six key inorganic water-soluble ions (Cl, NO, nss-SO, Na, NH, and nss-K) measured in this study, NO, nss-SO, and NH showed a significant increase in their concentrations during the smoke-haze period together with nss-K. These observations suggest that the increased atmospheric loading of PM with higher surface area and increased concentrations of optically active secondary inorganic aerosols [(NH)SO or NHHSO and NHNO] resulted in the atmospheric visibility reduction in SEA due to the advection of biomass and peat burning emissions. [ABSTRACT FROM AUTHOR]

Volatile methyl siloxanes (VMS) have been widely used in personal care products and industrial applications, and are an important component of VOCs (volatile organic compounds) indoors. They have sufficiently long lifetimes to undergo long-range transport and to form secondary aerosols through atmospheric oxidation. To investigate these silicon-containing secondary organic aerosols (Si-SOA), we collected PM 2.5 samples during 8th-21st August 2018 (summer) and 3rd-23rd January 2019 (winter) at an urban site of Beijing. As the oxidation of VMS mainly results in hydrophilic polar semi-volatile and non-volatile oxidation products, the differences between total water-soluble Si and total water-soluble inorganic Si were used to estimate water-soluble organic Si, considered to be secondary organic Si (SO–Si). The average concentrations of SO–Si during the summer and winter campaigns were 4.6 ± 3.7 and 13.2 ± 8.6 ng m−3, accounting for approximately 80.1 ± 10.1% and 80.2 ± 8.7% of the total water-soluble Si, and 1.2 ± 1.2% and 5.0 ± 6.9% of total Si in PM 2.5 , respectively. The estimated Si-SOA concentrations were 12.7 ± 10.2 ng m−3 and 36.6 ± 23.9 ng m−3 on average in summer and winter, which accounted for 0.06 ± 0.07% and 0.16 ± 0.22% of PM 2.5 mass, but increased to 0.26% and 0.92% on certain days. We found that net solar radiation is positively correlated with SO–Si levels in the summer but not in winter, suggesting seasonally different formation mechanisms. [Display omitted] • This work provided a conservative estimation of SO–Si and Si-SOA. • Si-SOA levels were 12.7 and 36.6 ng m−3 in summer and winter, respectively. • High surface net solar radiation favors the formation of Si-SOA. • Gas-phase oxidation is the predominant Si-SOA formation pathway. [ABSTRACT FROM AUTHOR]

• PM 2.5 , SO 2 , NO x and carbonaceous species were constrained by control measures. • SNA concentrations increased due to favorable synoptic conditions. • Non-exhaust emission was not reduced during the control period. • Air masses from inland China had impacts on the study domain. A field campaign was conducted to study the PM 2.5 and atmospheric gases and aerosol's components to evaluate the efficacy of radical measures implemented by the Chinese government to improve air quality during the 2016 G20 Summit in Hangzhou China. The lower level of PM 2.5 (32.48 ± 11.03 µg/m3) observed during the control period compared to pre-control and post-control periods showed that PM 2.5 was alleviated by control policies. Based on the mass concentrations of particulate components, the emissions of PM 2.5 from local sources including fossil fuel, coal combustion, industry and construction were effectively reduced, but non-exhaust emission was not reduced as effectively as expected. The accumulation of SNA (SO 4 2−, NO 3 −, NH 4 +) was observed during the control period, due to the favourable synoptic weather conditions for photochemical reactions and heterogeneous hydrolysis. Because of transboundary transport during the control period, air masses from remote areas contributed significantly to local PM 2.5. Although, secondary organic carbon (OC sec) exhibited more sensitivity than primary organic carbon (OC pri) to control measures, and the increased nitrogen oxidation ratio (NOR) implied the regional transport of aged secondary aerosols to the study area. Overall, the results from various approaches revealed that local pollution sources were kept under control, indicating that the implementation of mitigation measures were helpful in improving the air quality of Hangzhou during G20 summit. To reduce ambient levels of PM 2.5 further in Hangzhou, regional control policies may have to be taken so as to reduce the impact of long-range transport of air masses from inland China. [Display omitted] [ABSTRACT FROM AUTHOR]