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The refractive index (RI) of atmospheric aerosols is a critical parameter in aerosol direct radiative forcing (DRF) calculation that exhibits considerable uncertainties owing partly to substantial spatiotemporal changes in the RI. A tandem system comprising a single-particle soot photometer connected to a differential mobility analyzer is developed. Two field campaigns are conducted to derive the size-resolved real part of the RI of scattering (i.e., BC-free) particles at 1064 nm (RRIsp1064nm) in urban Beijing. Measurements of the RRIsp1064nm within a mobility diameter (Dmob) range of approximately 200 nm to 400 nm in summer and winter clearly show seasonal variations in the RRIsp1064nm, including magnitude and size dependence. A decreasing RRIsp1064nm trend is observed in summer as the Dmob enlarges from 237 nm to 422 nm, whereas the RRIsp1064nm increases in winter as the Dmob grows. This finding likely indicates varying aerosol physicochemical properties with particle size in summer and winter. Size-resolved RRIsp1064nm values within the Dmob range are generally larger in summer than in winter, thereby resulting in a larger bulk RRIsp1064nm of 1.50 ± 0.01 in summer compared with winter (1.47 ± 0.02). The low RRIsp1064nm in winter is likely attributable to the high contribution of organic aerosols (OA), particularly less oxidized OA, which generally have lower RRI values than secondary inorganic aerosols. Pronounced low RRIsp1064nm values are observed during polluted episodes in winter, implying significant OA contribution. Photooxidation is likely an important regulator of the RRIsp1064nm in summer. This inference is supported by the fact that RRIsp1064nm values in the afternoon are larger than those at night, which corresponds well to diurnal variations in Ox (Ox=O3+NO2) concentrations. Results indicate that seasonal variations in the RRIsp1064nm should be considered carefully in research, such as in DRF assessments, aerosol property parameterization in chemical models, and aerosol optical property and composition remote sensing. Copyright © 2021 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

To evaluate the effectiveness of regional pollution control measures for improving visibility imposed during the Asia-Pacific Economic Cooperation (APEC) period, day- and nighttime PM 2.5 and PM 10 samples were collected at an urban site in Beijing from October to November, 2014. PM 2.5 and PM 10 samples were subject to chemical analysis for major water-soluble ions, organic carbon (OC), element carbon (EC), and biomass burning tracers – anhydrosugar levoglucosan (LG). In addition, aerosol scattering coefficient (b sp ) and aerosol absorption coefficient (b ap ) at dry condition were measured. PM 2.5 mass concentration was 190 ± 125, 88 ± 60, 199 ± 142 μg m −3 during the pre-, during- and post-APEC period, respectively, while the concentration of the sum of (NH 4 ) 2 SO 4 and NH 4 NO 3 was 75 ± 69, 19 ± 22 and 40 ± 46 μg m −3 , respectively. The sum of (NH 4 ) 2 SO 4 and NH 4 NO 3 accounted for 49 ± 24%, 19 ± 12% and 24 ± 12% of b ext (the sum of b sp and b ap ) at ambient condition during the pre-, during- and post-APEC period, respectively, and the corresponding numbers are 39 ± 18%, 62 ± 8% and 61 ± 10% for the sum of OM and EC. Reduction of secondary inorganic aerosols played a key role in the “APEC blue”, especially under moisture conditions due to their hygroscopic properties. As a result, visibility was improved significantly during the APEC period with five out of the 12 days having a visibility higher than 20 km. Control of biomass burning, especially during the nighttime, was not performed well during the APEC period, which should be paid more attention in making future emission control measures. [ABSTRACT FROM AUTHOR]

Daily PM 2.5 samples were collected at an urban site in Beijing during four one-month periods in 2009–2010, with each period in a different season. Samples were subject to chemical analysis for various chemical components including major water-soluble ions, organic carbon (OC) and water-soluble organic carbon (WSOC), element carbon (EC), trace elements, anhydrosugar levoglucosan (LG), and mannosan (MN). Three sets of source profiles of PM 2.5 were first identified through positive matrix factorization (PMF) analysis using single or combined biomass tracers — non-sea salt potassium (nss-K + ), LG, and a combination of nss-K + and LG. The six major source factors of PM 2.5 included secondary inorganic aerosol, industrial pollution, soil dust, biomass burning, traffic emission, and coal burning, which were estimated to contribute 31 ± 37%, 39 ± 28%, 14 ± 14%, 7 ± 7%, 5 ± 6%, and 4 ± 8%, respectively, to PM 2.5 mass if using the nss-K + source profiles, 22 ± 19%, 29 ± 17%, 20 ± 20%, 13 ± 13%, 12 ± 10%, and 4 ± 6%, respectively, if using the LG source profiles, and 21 ± 17%, 31 ± 18%, 19 ± 19%, 11 ± 12%, 14 ± 11%, and 4 ± 6%, respectively, if using the combined nss-K + and LG source profiles. The uncertainties in the estimation of biomass burning contributions to WSOC due to the different choices of biomass burning tracers were around 3% annually and up to 24% seasonally in terms of absolute percentage contributions, or on a factor of 1.7 annually and up to a factor of 3.3 seasonally in terms of the actual concentrations. The uncertainty from the major source (e.g. industrial pollution) was on a factor of 1.9 annually and up to a factor of 2.5 seasonally in the estimated WSOC concentrations. [ABSTRACT FROM AUTHOR]

To evaluate PM 2.5 contributions to light scattering under different air pollution levels, PM 2.5 and its major chemical components, PM 10 , size-segregated water-soluble ions, and aerosol scattering coefficient (b sp ) under dry conditions were measured at an urban site in Beijing in January 2013 when heavy pollution events frequently occurred. Measurements were categorized into three pollution levels including heavy-polluted (Air Quality Index (AQI) ≥ 200), light-polluted (200 > AQI ≥ 100) and clean periods (AQI < 100). The average PM 2.5 mass concentration was 248 μg m −3 during the heavy-polluted period, which was 2.4 and 5.6 times of those during the light-polluted (104 μg m −3 ) and clean (44 μg m −3 ) periods, respectively. The concentrations of SO 4 2− , NO 3 − and NH 4 + increased much more than those of OC and EC during the heavy-polluted period compared with those during the light-polluted and clean periods. Good correlations between PM 2.5 and b sp were found (R 2 > 0.95) during the different pollution levels. The mass scattering efficiency (MSE) of PM 2.5 was 4.9 m 2 g −1 during the heavy-polluted period, which was higher than those during the light-polluted (4.3 m 2 g −1 ) and clean periods (3.6 m 2 g −1 ). To further evaluate the impact of individual chemical components of PM 2.5 on light scattering, a multiple linear regression equation of measured b sp against the mass concentration of (NH 4 ) 2 SO 4 , NH 4 NO 3 , Organic Matter (OM), EC, Fine Soil (FS), Coarse Matter (CM) and Other chemical compounds were performed. (NH 4 ) 2 SO 4 , NH 4 NO 3 and OM were the dominant species contributing to b sp under both dry and ambient conditions. OM contributed more to b sp than the sum of (NH 4 ) 2 SO 4 and NH 4 NO 3 did under the dry condition during all the pollution periods and this was also the case under the ambient condition during the light-polluted and clean periods. However, the total contributions of (NH 4 ) 2 SO 4 and NH 4 NO 3 to b sp under the ambient condition was 55%, much more than the 29% contribution from OM during the heavy-polluted period. High (NH 4 ) 2 SO 4 and NH 4 NO 3 concentrations and their hygroscopicity were the main reasons causing visibility degradation during the heavy-polluted period, and the effect can be enhanced under high RH conditions. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is the most important aerosol light-absorbing component, and its effect on radiation forcing is determined by its microphysical properties. In this study, two microphysical parameters of refractory BC (rBC), namely, size distribution and mixing state, in urban Beijing from 2013 to 2019 were investigated to understand the effects of source changes over the past years. The mass equivalent diameter of rBC (D c) exhibited bimodal lognormal distributions in all seasons, with the major modes accounting for most (>85%) of the rBC masses. The mass median diameter (MMD) was obviously larger in winter (209 nm) than in summer (167 nm) likely due to the contribution of more rBC with larger D c from solid fuel combustion and enhanced coagulation of rBC in polluted winter. More rBC particles were thickly coated in winter, with the number fraction of thickly coated rBC (f coatBC) ranging within 29%–48% compared with that of 12%–14% in summer. However, no evidential increase in BC light-absorption capability was observed in winter. This finding was likely related to the lower absorption efficiency of larger rBC in winter, which partly offset the coating-induced light enhancement. Two stage of decreases in MMD and f coatBC were observed, accompanied with a persistent decrease in rBC loading, thereby reflecting the discrepant effects of source control measures on rBC loading and physical properties. The control measures in the earlier stage before 2016 was more efficient to reduce the rBC loading but slightly influenced the microphysical properties of rBC. As of 2016, the reduction in rBC concentration slowed down because of its low atmospheric loading. However, rBC showed a more obvious decrease in its core size and became less coated. The decrease in f coatBC may have weakened the BC absorption and accelerated the decrease in light absorption resulting from the reduction in rBC loading. Image 1 Decreases in BC core size and coating fraction were observed in Beijing from 2013 to 2019 in response to clean air actions. [ABSTRACT FROM AUTHOR]

Carbonaceous aerosol, mainly comprising elemental carbon (EC) and organic carbon (OC), plays an important role in aerosol–climate interaction owing to its light-absorbing feature. The radiative forcing of carbonaceous aerosol is largely determined by its mass absorption efficiency (MAE). To depict the MAE of carbonaceous aerosol and its variations with aerosol size and chemical composition, light absorption coefficients and the main chemical composition of PM 1 and PM 2.5 were comparably measured in a winter field campaign in urban Beijing. On average, aerosol absorption by PM 1 at 880 nm wavelength contributes to approximately half of aerosol absorption by PM 2.5 , mainly because aerosol absorption at this wavelength was dominated by EC and nearly half of the total EC mass in PM 2.5 existed in PM 1. The average MAE of EC at 880 nm (MAE EC,880nm) was 3.9 ± 0.7 m2 g−1 for PM 2.5 , lower than that for PM 1 (4.4 ± 0.8 m2 g−1) likely because of the larger EC cores and lower coating degrees of EC particles in PM 1–2.5. Variation in MAE EC,880nm was related to the bulk mass fractions of nitrate in PM 1 and PM 2.5 , implying the important impact of secondary nitrate formation on the modification of EC mixing states and enhancing EC absorption efficiency in Beijing. Absorption by OC took up about 40% of the absorption coefficients for PM 1 and PM 2.5 at 370 nm. The mean MAE of OC at 370 nm (MAE OC,370nm) was 2.4 ± 0.9 m2 g−1 for PM 1 , higher than that of for PM 2.5 (1.9 ± 0.6 m2 g−1). The high value of MAE OC,370nm might be associated with regionally transported aerosols during clean and polluted periods. Enlarged particle sizes might have considerably weakened MAE OC,370nm for PM 2.5 while exerting negligible impact on MAE OC,370nm for PM 1. • Light absorption capacity of elemental carbon and organic carbon aerosols was compared between PM 1 and PM 2.5. • Nitrate was found to be the main aerosol chemical composition affecting the mass absorption efficiency (MAE) of EC. • MAE of EC in PM 2.5 was lower than that in PM 1 owing to large core sizes and low coating degrees of EC particles in PM 1–2.5. • Enlarged particle sizes in polluted periods might have weakened MAE of OC for PM 2.5. [ABSTRACT FROM AUTHOR]

Black carbon (BC) and its mixing state were measured with a ground-based single particle soot photometer in urban Beijing during the extremely polluted winter of 2013. Up to 70 ± 14% of the BC-containing particles were thickly-coated during periods of haze, compared to 37 ± 9% on non-hazy days. The thickly-coated number fraction (NF BC-thick ) increased with increasing BC, reaching a plateau at ∼80–90% when BC concentrations were ≥15 μg m −3 and visibility was ≤2 km. Regional inflows brought more aged, highly thickly-coated BC to Beijing during haze. The absorption coefficient showed a distinct linear correlation with BC concentration; the mass absorption efficiency (MAE) of BC was acquired, with an overall mean of 4.2 ± 0.01 m 2 g −1 at 870 nm. The MAE of BC amplified with increasing ambient relative humidity. This was largely explained by the increase in NF BC-thick , which was likely due to the enhanced production of secondary aerosol under humid conditions. [ABSTRACT FROM AUTHOR]

Beijing suffered from serious air pollution in October, 2011 with the occurrence of three continuous episodes. Here we analyze the pollution status of particulate matter, the relationship between the gaseous pollutants, physical and chemical properties of single particles, and the profile of water-soluble ions in PM 2.5 during the three episodes. Regional and photochemically aged air masses, which were characterized as having high values of O 3 and SO 2 , were hypothesized to have played a dominant role in the first episode. After mixing local air masses with freshly-emitted primary pollutants, the concentration of NO x continued to increase and the size of SO 4 2– NO 3 − and NH 4 + in the particle population continued to become smaller. The amount of elemental carbon-rich and organic carbon-rich particles in the scaled single particles (0.2–2 μm) and water-soluble K + in PM 2.5 also increased in the episodes. All the available information suggests that the biomass or fuel burning sources in or around Beijing may have had a huge impact on the last two episodes. [ABSTRACT FROM AUTHOR]

Coal-to-clean energy programs (coal bans) can improve air quality and public health and welfare. However, the field measurements taken in rural areas are still insufficient to evaluate the efficacy of coal bans in China. In this study, more than three years of observations were made on organic carbon (OC), elemental carbon (EC, also called black carbon, BC) and PM 2.5 at a rural site (Xianghe) in the Beijing-Tianjin-Hebei region (BTH) of China in combination with auxiliary measurements in urban areas (Beijing, Tianjin, Langfang, Baoding and Shijiazhuang). As a result of the coal ban, the annual average PM 2.5 concentrations decreased in rural areas. Accordingly, the annual average concentrations of OC declined from 19.1 to 12.3 μg/m3, while those of EC declined from 4.7 to 3.0 μg/m3. The amplitudes of the decrease of total carbon (the sum of OC and EC) and PM 2.5 in the areas with implementation of coal ban were greater than those in the non-coal banned areas. Based on the simulation performed using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem), the quantitative contributions ascribed to coal ban policy and meteorology are discussed. In contrast to those observed in the winter of 2016/2017, the declines in the observed EC concentration could be attributed to meteorological impact (45% and 15%), changes in other emissions (29% and 40%) and coal bans (26% and 45%) for the winter of 2017/2018 and 2018/2019, respectively. Besides, in primary OC and PM 2.5 concentrations, 50% and 33% of fractional changes respectively were resulted from the reduced residential emissions associated with coal ban in the winter of 2017/2018; and more surprisingly, they rose to 66% and 57% in the winter of 2018/2019, respectively. In summary, coal ban policy had worked effectively in the BTH of China and should be widely implemented in this region if the supply of natural gas and electricity is sufficient and affordable to the local residents. • An obvious decline in OC, EC and PM 2.5 concentrations was observed. • TC and PM 2.5 declined more obviously in the coal ban areas. • The quantitative contributions of coal bans and meteorology are assessed. [ABSTRACT FROM AUTHOR]

Central North China Plain (NCP) is one of the most important source region of air pollutants over the Beijing-Tianjin-Hebei (BTH) region. The national government has issued abatement measures to improve the air quality in this area from 2017. To examine the effects of control measures, observational analysis on PM 2.5 characteristics was performed in a city of central NCP during 2017–2019 to investigate the variation in mass concentration, chemical composition, and emission source of PM 2.5. Annual PM 2.5 concentration significantly reduced by 16% from 2017 to 2019, implying substantial improvements in air quality. PM 2.5 enriched in autumn-winter seasons was dominated by SNA (sum of sulfate, nitrate and ammonium; ~38%), followed by organic carbon matters (OM; ~24%) and fine soil (FS; ~12%). This chemical composition was different from that in a megacity in NCP (Beijing) where OM accounted for a comparable fraction to SNA. Approximately half of SNA was attributed to nitrate, indicating that SNA changed from sulfate-driven to nitrate-driven, and the considerable effects of coal combustion cutoff, in which sulfate was concentrated. Decreased mass fraction of SNA and increased OM fraction in PM 2.5 were observed in 2018–2019 partly contributed to the decrease in PM 2.5. A progressive increase in the contribution of heterogeneous formed SNA whilst a decrease in OM was observed as the pollution elevated from clean to heavily polluted. Six sources (soil dust, biomass burning, secondary emission, road traffic, coal combustion and industry) were identified by the Positive Matrix Factorization (PMF) model in both years and dominated by secondary aerosols, respectively contributing 39% and 41% to PM 2.5. The decreasing concentrations (with reductions of 17%–61%) of the secondary source, coal combustion, soil dust and biomass burning largely accounted for the reduction in PM 2.5 , as a consequence of the recent abatement measures. By contrast, contributions of vehicle-related emissions, similar to the increasing contribution of vehicles at sites in NCP after 2013, should receive increased attention. [Display omitted] • Analysis of variations in PM 2.5 in central North China Plain during 2017–2019 were performed. • Decreasing PM 2.5 was due to the decrease in secondary inorganic aerosols. • The reduced sources tracked the mitigation measures well. [ABSTRACT FROM AUTHOR]