Particles In Winter Research Articles

New particle formation leads to enhanced cloud condensation nuclei concentrations on the Antarctic Peninsula

Abstract. Few studies have investigated the impact of new particle formation (NPF) on cloud condensation nuclei (CCN) in remote Antarctica, and none has elucidated the relationship between NPF and CCN production. To address that knowledge gap, we continuously measured the number size distribution of 2.5–300 nm particles and CCN number concentrations at King Sejong Station on the Antarctic Peninsula from 1 January to 31 December 2018. Ninety-seven NPF events were detected throughout the year. Clear annual and seasonal patterns of NPF were observed: high concentration and frequency of nucleation-mode particles in summer (December–February: 53 NPF cases) and undetected nucleation-mode particles in winter (June–August: no NPF cases). We estimated the spatial scale of NPF by multiplying the time during which a distinct nucleation mode can be observed at the sampling site by the locally measured wind speed. The estimated median spatial scale of NPF around the Antarctic Peninsula was found to be approximately 155 km, indicating the large scale of NPF events. Air back-trajectory analysis revealed that 80 cases of NPF events were associated with air masses originating over the ocean, followed by sea-ice (12 cases), multiple (3 cases), and land (2 cases) regions. We present and discuss three major NPF categories: (1) marine NPF, (2) sea-ice NPF, and (3) multiple NPF. Satellite estimates for sea-surface dimethylsulfoniopropionate (DMSP; a precursor of gaseous dimethyl sulfide) data showed that the production of oceanic biogenic precursors could be a key component in marine NPF events, whereas halogen compounds released from ice-covered areas could contribute to sea-ice NPF events. Terrestrial sources (wildlife colonies, vegetation, and meltwater ponds) from Antarctica could affect aerosol production in multiple air masses. Out of 97 observed NPF events, 83 cases were characterized by the simultaneous increase in the CCN concentration by 2 %–270 % (median 44 %) in the following 1 to 36 h (median 8 h) after NPF events. Overall, Antarctic NPF events were found to be a significant source of particles with different physical characteristics and related to biogenic sources in and around the Antarctic Peninsula, which subsequently grew to cloud condensation nuclei.

Sources and solubility of aerosol phosphorus at a coastal site in northern China: Coarse versus fine particles and spring versus winter

Atmospheric P is of great importance as it is a major source of phosphorus (P) in open oceans, which can limit the primary productivity of ocean ecosystems and indirectly affect global climate. However, the understanding on the sources of atmospheric P, especially water-soluble P in different particle size and the factors that dominate P solubility is limited. This study characterizes P in fine and coarse particles during winter and spring in Qingdao, a coastal city in North China. The average concentration of total P in fine particles was 23 ± 15 ng/m3 in winter and 28 ± 24 ng/m3 in spring, while in coarse particles it was 21 ± 15 ng/m3 in winter and 15 ± 12 ng/m3 in spring. In spring, fine particles had notably higher concentrations of total P than coarse particles, while in winter, the difference between the two particle sizes was not pronounced. Regardless of the concentration of total P, the sources of total P varied distinctly between fine and coarse particles in both seasons. The concentration of soluble P is obviously higher in spring than in winter for the fine particles but coarse particles show no significant difference between the two seasons. P solubility is always higher in spring compared to winter regardless of the particle size, which can be due to higher relative humidity. Both soluble P concentrations and P solubility were significantly higher in fine particles compared to coarse particles during winter and spring. A higher contribution of combustion emission and higher acidity can be the reasons for the higher P solubility in fine particles in winter. The findings suggest that a comprehensive evaluation of the ecological impacts of atmospheric phosphorus should consider its sources, aerosol acidity, humidity, and differences among fine and coarse particles.

Sediment Grain-Size Composition in the Permafrost Region of the Greater Khingan Range and Its Significance as a Material Source

Sediment grain-size distribution (GSD) provides rich information about sedimentary sources and can potentially do the same with regard to environmental and climatic changes. However, neither traditional descriptive statistics nor curving-fitting methods can fully address its complexity. We selected the Greater Khingan Range in northeastern China as the study area and used parameterized end-member analysis (EMA) of the GSD of four drilling cores to extract different end-member (EM) components. The results show that EM1 (mode particle size (Mo): 1.26–1.66 μm) originates from weathering and pedogenesis. The EMs with Mo values of 4.37–5.01 μm represent components transported by the upper westerly wind. EMs with Mo values of 7.58 μm and 11.48 μm represent wet dust deposition and dry dust deposition. The wind transport of particles in winter consists of low-level near-source transport and local-source transport (possibly from the flood plain of the Amur River). Due to the limitations of the EM model, the two sources have one or two EM components: AEM3 + AEM4, BEM3 + BEM4, CEM4, and DEM4. DEM5 is the only large particle-size component and may represent coarse-grained detritus generated via rock weathering. The components related to the winter monsoon and the 14C dating data suggest a weak-strong-weak-strong trend of the winter monsoon since the Marine Isotope Stage 3a (MIS 3a). Our results suggest that the strengthening of the winter monsoons in the previous few thousand years has caused the transportation of coarser grain sizes and further exacerbated permafrost degradation, providing a scientific reference for understanding climate change and the formation and evolution of permafrost in the Greater Khingan Mountains since the MIS 3a.

Characteristics, Origins, and Atmospheric Processes of Amines in Fine Aerosol Particles in Winter in China

AbstractAmines affect particle formation, aerosol acidity, nitrogen cycle, and climate change. However, little is known about the temporal and spatial differences in the composition, source, and formation process of amines in fine particulate matter (PM2.5) in different regions of China. Seven amines (amine salts) were investigated in PM2.5 collected in the Yangtze River Delta (YRD, Shanghai and Nanjing), Pearl River Delta (PRD, Guangzhou), northern China (Xi'an and Haerbin), and background (Puding) areas in winter 2017–2018. Methylamine was the dominant amine species at the YRD, northern China, and background sites, while diethylamine dominated at the PRD site; moreover, the mass concentration and fraction of diethylamine were the lowest at the background site. These results could be attributed to changes in sources and atmospheric processes. Further, coal combustion and biomass burning were found to be important contributors of amines in northern cities in winter. The contribution of biomass and coal combustion to amines was weakened in other urban sites. In contrast, the amine abundance at the background site was largely controlled by biogenic sources. Acid‐base chemistry and displacement mechanism were tightly associated with the formation of amine salts at all sites. In particular, the atmospheric degradation of amines by hydroxyl radical (•OH) was significant at the Nanjing, Guangzhou, Xi'an, and Haerbin sites. The insignificant photochemical degradation of amines at the Shanghai and background sites can be explained by differentiated amine sources, meteorological conditions, and •OH levels. Overall, our findings deepen the understanding of the origins and atmospheric processes of amines.

Molecular Composition, Seasonal Variation, and Size Distribution of n-Alkanes, PAHs, and Saccharides in a Medium-Sized City of Guanzhong Plain, Northwest China: Evaluation of Control Measures Executed in the Past Decade.

Baoji is a medium-sized city in the Guanzhong Plain of northwest China. The compositions of three important organic groups, namely n-alkanes, polycyclic aromatic hydrocarbons (PAHs), and saccharides in atmospheric aerosol with different aerodynamic diameters in power were determined. Both seasonal and daily trends of the target organic chemical groups were demonstrated. The concentration levels of total quantified n-alkanes and saccharides in total suspended particles (TSP) in winter were 541 ± 39 and 651 ± 74 ng·m-3, respectively, much higher than those of the other three seasons. A high total quantified PAHs concentration level of 59.6 ± 6.4 ng·m-3 was also seen in wintertime. n-Alkanes showed a bimodal percent distribution in spring, autumn, and winter. Two peaks were found with the particle sizes of 0.7 μm < Dp < 2.1 μm and 3.3 μm < Dp < 4.7 μm, respectively. In summer, a unimodal was seen with a peak of 4.7 μm < Dp < 5.8 μm. Dehydrated saccharides and PAHs present a unimodal size distribution peaking at the aerodynamic diameters of 0.7 µm < Dp < 2.1 µm. In contrast to glucose and fructose, they mainly exist in the coarse mode particles and have the highest concentrations at aerodynamic diameters of 4.7 µm < Dp < 9.0 µm. The geometric mean diameters (GMD) of n-alkanes and saccharides of the fine particles in winter were higher than in the other seasons. Compared with the data in 2008, the fossil fuel-derived n-alkanes and PAHs in winter decreased by nearly an order of magnitude in 2017. Both the carbon preference index (CPI) of n-alkanes and the diagnostic ratios of PAHs suggest that coal combustion and vehicle exhaust were the major pollution sources of the organic groups in the two decades. It should be noted that the contribution of traffic emissions greatly increased from 2008 to 2017, consistently with a large raise of registered vehicles in Baoji city. The overall results confirm that the control measures conducted by the local government in the recent decade mitigated the air pollution in this city.

Vertical Distribution of Atmospheric Ice Nucleating Particles in Winter over Northwest China Based on Aircraft Observations

The concentration of ice nucleating particles (INPs) in the cloud layer affects cloud processes more importantly than at the ground level. To make up for deficiencies in the observation of the vertical distribution of INPs over different background regions of China, airborne sampling of INPs was carried out at the altitudes of 2000–5500 m over Turpan, Xinjiang, northwest China on 29 December 2019, and the samples were analyzed in a static vacuum water vapor diffusion chamber. The sources and relationships of the INPs with meteorological conditions and the variation of the concentrations of aerosol particles of different sizes were explored. The results indicate that the concentration of INPs varies from 0.25 to 15.7 L−1 when the nucleation temperature changes from −17 to −26 °C and the relative humidity with respect to water (RHw) varies from 95 to 105%. The existence of an inversion layer near the planetary boundary layer (PBL) inhibits the vertical transport of aerosols, thus affecting the vertical distribution of INP concentration. INPs in the free atmosphere mainly originate from fine-mode aerosol particles transported from long distances by westerly winds and do not change significantly with height. The air mass in the PBL is short-range transported, and the INP concentration reaches 15.7 L−1 at T =−26 °C and RHw = 105%, which is obviously higher than that above the PBL. The analyses provide evidence that the meteorological conditions played an important part in regulating the vertical distribution of INPs.

Sources and processes of water-soluble and water-insoluble organic aerosol in cold season in Beijing, China

Abstract. Water-soluble and water-insoluble organic aerosol (WSOA and WIOA) constitute a large fraction of fine particles in winter in northern China, yet our understanding of their sources and processes are still limited. Here we have a comprehensive characterization of WSOA in cold season in Beijing. Particularly, we present the first mass spectral characterization of WIOA by integrating online and offline organic aerosol measurements from high-resolution aerosol mass spectrometer. Our results showed that WSOA on average accounted for 59 % of the total OA and comprised dominantly secondary OA (SOA, 69 %). The WSOA composition showed significant changes during the transition season from autumn to winter. While the photochemical-related SOA dominated WSOA (51 %) in early November, the oxidized SOA from biomass burning increased substantially from 8 % to 29 % during the heating season. Comparatively, local primary OA dominantly from cooking aerosol contributed the major fraction of WSOA during clean periods. WIOA showed largely different spectral patterns from WSOA which were characterized by prominent hydrocarbon ions series and low oxygen-to-carbon (O/C = 0.19) and organic mass-to-organic carbon (OM/OC = 1.39) ratios. The nighttime WIOA showed less oxidized properties (O/C = 0.16 vs. 0.24) with more pronounced polycyclic aromatic hydrocarbons (PAHs) signals than daytime, indicating the impacts of enhanced coal combustion emissions on WIOA. The evolution process of WSOA and WIOA was further demonstrated by the triangle plot of f44 (fraction of m/z 44 in OA) vs. f43, f44 vs. f60, and the Van Krevelen diagram (H/C vs. O/C). We also found more oxidized WSOA and an increased contribution of SOA in WSOA compared with previous winter studies in Beijing, indicating that the changes in OA composition due to clean air act have affected the sources and properties of WSOA.

Morphology, composition and mixing state of individual airborne particles: Effects of the 2017 Action Plan in Beijing, China

Beijing is one of the most important Chinese megacities with extremely serious air pollution problems and human health impacts. In response to the air pollution the central and municipal governments of China have implemented a series of actions; one of which is the “Action Plan for Comprehensive Prevention and Control of Autumn and Winter Air Pollution in Beijing-Tianjin-Hebei and Surrounding Areas 2017–2018” (the Action Plan) issued in 2017. The morphology, composition and mixing state of individual particles collected after the Action Plan was implemented were analyzed by high resolution-transmission electron microscopy coupled with an energy-dispersive X-ray spectrometer (TEM-EDX). The relative percentages of different individual particle types and the main sources of the particulate pollution before and after the Action Plan were compared. The results showed that sulfur was most frequently detected in the individual particles, and the particle types were mainly composed of soot aggregates, mineral particles, organic particles, metal particles, coal fly ashes, sulfate particles, and mixture particles. The mixture and sulfate particles dominated in the autumn samples, both for the haze and non-haze days. In winter the mineral and mixture particles dominated in samples for the non-haze days, while mixture particles and sulfate dominated in the samples for the haze days. The mixture particles in autumn were mainly the soot aggregates internally mixed with sulfate (S-soot type), while the mixture particles in winter were mainly the S-soot type and the mineral particles internally mixed with sulfate (S-mineral type). After the Action Plan, the relative percentages of sulfate particles, organic particles, and soot aggregates increased, while the relative percentages of mineral particles, metal particles and fly ashes decreased. The contribution from coal-fired sources was reduced significantly as evidenced by the decrease in the fly ash particles. The vehicle emissions and secondary reaction of particulate matter became the main sources of atmospheric particulate matters as evidenced by increase in sulfate particles, organic particles, and soot aggregates. The results presented in this study will assist in other plans to improve air quality and human health in megacities around the world.

Interannual evolution of elemental carbon-containing particles in winter in the atmosphere of Chengdu, China

As one of the important components of PM2.5, elemental carbon (EC) particles could quickly mix with other organic or inorganic species after entering the atmosphere, which directly affected its climate and environmental effects and impact on human health. In order to study the mixture and evolution of EC particles in the atmosphere, this study used single particle aerosol mass spectrometry to measure the EC-containing particles in three winters (2016/17, 2018/19 and 2019/20) in Chengdu, one of the most polluted megacities in China. The results showed that the average mass concentrations of PM2.5 in the three winters were 121 ± 58 μg m−3, 90 ± 32 μg m−3 and 79 ± 38 μg m−3, respectively, and the mass ratios of PM2.5/PM10 were 0.71, 0.78 and 0.78, respectively. EC-containing particles accounted for 40.7%–54.7% of all particles and could be divided into six categories: EC-dust (EC-D), fresh EC (ECf), EC-organic carbon (EC-OC), EC-nitrate (EC-N), EC-sulfate (EC-S) and EC-sulfate-nitrate (EC-SN). From winter 2016/17 to 2019/20, the contributions of EC-N and EC-OC particles increased, while the contributions of EC-S and EC-SN particles decreased. Meanwhile, EC-N particles became increasingly important for the formation of heavy pollution. Photochemical and liquid-phase processes dominated the formation of EC-N and EC-S particles, respectively, while the dominant formation mechanism of EC-SN particles changed from photochemical to liquid-phase processes from winter 2016/17 to 2019/20. Total EC and EC-S particles in Chengdu were mainly transported from the east and southeast areas to Chengdu, while ECf particles mainly derived from local emissions. Compared with the previous two winters, the potential source areas of the EC-containing particles in winter 2019/20 reduced significantly. These findings provide new insight into the atmospheric behavior of urban particles and are important reference for the study of EC particles in other regions in the world, especially in urban areas.

Characterization of submicron aerosol particles in winter at Albany, New York

A thorough understanding of chemical composition, particle pH, and pollutant emissions is essential to address the climate and human health effects of atmospheric particles. In this study, we used a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and Scanning Mobility Particle Sizer (SMPS) to characterize the composition of submicron particles. Moreover, we applied the ISORROPIA-II model to analyze the particle acidity effect on the compositional characterization of submicron particles from December 22, 2016 to January 7, 2017 in Albany, New York, USA. The results indicated that aerosols with mobility diameter from SMPS in the range 200–400 nm were the main contributors to the mass during the measurement period. The dominance of organics (47%) and sulfate (16%) was similar to previous observations in the eastern United States in Winter 2015, while the fraction of nitrate (23%) was much higher. Moreover, nitrate could easily form at colder temperatures and lower RH levels even when there were more acidic particle periods during the measurement period in Albany. The ISORROPIA-II model indicated that there were more acidic particles, which was estimated using pH values. Lower temperature conditions tended to favor nitrate formation. The nitrate concentration exceeded that of sulfate in the measurement period, even though the SO2 and NOx emissions were similar. The organics in submicron particles were strongly influenced by the local emissions in winter. However, the inorganic compounds in submicron particles could be derived from regional transport as their pollution sources originated from different directions. This may help strategize emission reductions in the future.

Analysis of Atmospheric Pollution Status over Nanjing Based on Multi-Source Detection

Nanjing, as one of the core cities in the Yangtze River Delta, has developed industries and serious air pollution. In order to further study the pollution status in Nanjing, MODIS and CALIPSO's aerosol products were used to analyze the aerosol characteristics of Nanjing from 2011 to 2019. Then, four kinds of common reactive gases (SO2, NO, NO2 and O3) in the atmosphere of Nanjing in winter were analyzed based on the data of ground-based reactive gas analyzer. The results shows that: (1) The AOD over Nanjing showed a downward trend in the whole nine years, and the AOD could reach 0.7 in winter. The pollution degree was the highest in the four seasons. The aerosol was mainly composed of coarse particles and absorbent particles in winter. (2) The near surface atmosphere (within 2 km) of Nanjing was dominated by large particles and irregular particles. Mainly, the proportion of fine particles and regular particles gradually increased within 2-6 km, and the volume of particles changed little within 2-6 km. (3) Among the four common reactive gases in winter, the φ(NO) in the atmosphere was the most significant, and the maximum value was 160×10-9, and the change trend of NO2 and O3 was opposite. Due to the emission limitation and the promotion of desulfurization measures, and the influence of photochemical reaction, the φ(SO2) was the lowest, only about 3×10-9. Combined with OMI data, the φ(NOx) over Nanjing remained at a low level in early 2020 due to the shutdown of factories because of COVID-19. Studies have shown that the particulate pollution in Nanjing has improved in recent years, but it is still necessary to pay attention to the emission of coarse particles, and it is necessary to strictly control the emission of gaseous pollutants from factories, especially NOx. © 2021, Editorial Board, Research of Environmental Sciences. All right reserved.

Size-resolved exposure risk of persistent free radicals (PFRs) in atmospheric aerosols and their potential sources

Abstract. Environmentally persistent free radicals (EPFRs) are a new type of substance with potential health risks. EPFRs are widely present in atmospheric particulates, but there is a limited understanding of the size-resolved health risks of these radicals. This study reports the exposure risks and source of EPFRs in atmospheric particulate matter (PM) of different particle sizes (&lt;10 µm) in Linfen, a typical coal-burning city in China. The type of EPFRs in fine particles (&lt; 2.1 µm) is different from that in coarse particles (2.1–10 µm) in both winter and summer. However, the EPFR concentration is higher in coarse particles than in fine particles in summer, and the opposite trend is found in winter. In both seasons, combustion sources are the main sources of EPFRs, with coal combustion as the major contributor in winter, while other fuels are the major source in summer. Dust contributes part of the EPFRs, and it is mainly present in coarse particles in winter and the opposite in summer. The upper respiratory tract was found to be the area with the highest risk of exposure to EPFRs of the studied aerosols, with an exposure equivalent to that of approximately 21 cigarettes per person per day. Alveolar exposure to EPFRs is equivalent to 8 cigarettes per person per day, with combustion sources contributing the most to EPFRs in the alveoli. This study helps us to better understand the potential health risks of atmospheric PM with different particle sizes.

Size Distribution of Particulate Chemical Components in Nanjing Jiangbei New Area

In order to explore the pollution characteristics of the chemical components of atmospheric particulate matter in Nanjing Jiangbei New Area, size-resolved samples were collected from 2013 to 2014. The size distribution and seasonal variation of the chemical components of the particles were studied. The results showed that the total concentration of nine water-soluble ions in fine particles was higher in autumn and winter than in spring and summer, while the concentration of coarse particles was highest in winter. The ratio of NO3-/SO42- for the fine particles in winter was slightly higher than in the other three seasons, and lowest in the coarse particles. The trend of particle size distribution in spring, summer and autumn was consistent. The distribution of water-soluble ions was bimodal, and NO3- peaked at 0.65-1.1 μm in all four seasons. The peak radius of SO42- in the fine particles in summer and autumn was 0.43-0.65 μm, and the peak value in winter moved towards the coarser particles, while Na+ and Cl- mainly existed in the coarse particles. The charge equivalent of anion and anion indicated that the coarse and fine particles were alkaline and weakly alkaline, respectively. Organic carbon (OC) and elemental carbon (EC) mainly existed in the fine particles, with a bimodal distribution. The secondary organic carbon (SOC) in the fine particles in autumn and winter was significantly higher than in spring and summer. The ratio method further indicated that the carbonaceous component of the particulate matter in Nanjing Jiangbei New Area is mainly from the emissions of coal and biomass combustion, and vehicle exhaust.

Multi-year ACSM measurements at the central European research station Melpitz (Germany) – Part 1: Instrument robustness, quality assurance, and impact of upper size cutoff diameter

Abstract. The aerosol chemical speciation monitor (ACSM) is nowadays widely used to identify and quantify the main components of fine particles in ambient air. As such, its deployment at observatory platforms is fully incorporated within the European Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS). Regular intercomparisons are organized at the Aerosol Chemical Monitoring Calibration Center (ACMCC; part of the European Center for Aerosol Calibration, Paris, France) to ensure the consistency of the dataset, as well as instrumental performance and variability. However, in situ quality assurance remains a fundamental aspect of the instrument's stability. Here, we present and discuss the main outputs of long-term quality assurance efforts achieved for ACSM measurements at the research station Melpitz (Germany) since 2012 onwards. In order to validate the ACSM measurements over the years and to characterize seasonal variations, nitrate, sulfate, ammonium, organic, and particle mass concentrations were systematically compared against the collocated measurements of daily offline high-volume PM1 and PM2.5 filter samples and particle number size distribution (PNSD) measurements. Mass closure analysis was made by comparing the total particle mass (PM) concentration obtained by adding the mass concentration of equivalent black carbon (eBC) from the multi-angle absorption photometer (MAAP) to the ACSM chemical composition, to that of PM1 and PM2.5 during filter weighing, as well as to the derived mass concentration of PNSD. A combination of PM1 and PM2.5 filter samples helped identifying the critical importance of the upper size cutoff of the ACSM during such exercises. The ACSM–MAAP-derived mass concentrations systematically deviated from the PM1 mass when the mass concentration of the latter represented less than 60 % of PM2.5, which was linked to the transmission efficiency of the aerodynamic lenses of the ACSM. The best correlations are obtained for sulfate (slope =0.96; R2=0.77) and total PM (slope =1.02; R2=0.90). Although, sulfate did not exhibit a seasonal dependency, total PM mass concentration revealed a small seasonal variability linked to the increase in non-water-soluble fractions. The nitrate suffers from a loss of ammonium nitrate during filter collection, and the contribution of organo-nitrate compounds to the ACSM nitrate signal make it difficult to directly compare the two methods. The contribution of m∕z 44 (f44) to the total organic mass concentration was used to convert the ACSM organic mass (OM) to organic carbon (OC) by using a similar approach as for the aerosol mass spectrometer (AMS). The resulting estimated OCACSM was compared with the measured OCPM1 (slope =0.74; R2=0.77), indicating that the f44 signal was relatively free of interferences during this period. The PM2.5 filter samples use for the ACSM data quality might suffer from a systematic bias due to a size truncation effect as well as to the presence of chemical species that cannot be detected by the ACSM in coarse mode (e.g., sodium nitrate and sodium sulfate). This may lead to a systematic underestimation of the ACSM particle mass concentration and/or a positive artifact that artificially decreases the discrepancies between the two methods. Consequently, ACSM data validation using PM2.5 filters has to be interpreted with extreme care. The particle mass closure with the PNSD was satisfying (slope =0.77; R2=0.90 over the entire period), with a slight overestimation of the mobility particle size spectrometer (MPSS)-derived mass concentration in winter. This seasonal variability was related to a change on the PNSD and a larger contribution of the supermicrometer particles in winter. This long-term analysis between the ACSM and other collocated instruments confirms the robustness of the ACSM and its suitability for long-term measurements. Particle mass closure with the PNSD is strongly recommended to ensure the stability of the ACSM. A near-real-time mass closure procedure within the entire ACTRIS–ACSM network certainly represents an optimal quality control and assurance of both warranting the quality assurance of the ACSM measurements as well as identifying cross-instrumental biases.

Contrasting mixing state of black carbon-containing particles in summer and winter in Beijing

Black carbon (BC) exerts a large impact on climate radiative forcing and public health, and such impacts depend strongly on chemical composition and mixing state. Here a single particle aerosol mass spectrometry (SPA-MS) along with an aerosol chemical speciation monitor was employed to characterize the composition and mixing state of BC-containing particles in summer and winter in Beijing. Approximately 2 million BC-containing particles were chemically analyzed, and the particles were classified into nine and eight different types in summer and winter, respectively, according to mass spectral signatures and composition. The BC-containing particles in summer were dominated by the type of nitrate-related BC (BC-N, 56.7%), while in winter the BC mixed with organic carbon (OC) and sulfate (BCOC-S), and OC and nitrate (BCOC-N) were two dominant types accounting for 44.9% and 16.6%, respectively. The number fractions of BC-N in summer, and BCOC-N and BC-SN in winter increased largely during periods with severe air pollution, suggesting the enhanced secondary formation on BC-containing particles. We also found that the primary emissions of the biomass burning and coal combustion can affect BC mixing state substaintially as indicated by the considerable fraction of BC mixed with levoglucosan and polycyclic aromatic hydrocarbons in winter. Bivariate polar plots and back trajectory analysis indicated that the sulfate-associated BC-containing particles were mostly from regional transport while the nitrate-related type was more from local production. The optical parameter of absorbing Ångström exponents (AAE) of BC was 1.2 and 1.5 in summer and winter, respectively, and the AAE dependence of BC mixing state was also different in the two seasons. While higher fractions of BC-N were observed during lower AAE periods in summer, the variations of dominant OC-related BC-containing particles in winter were fairly stable as a function of AAE.

Variation of the aerosol optical properties and validation of MODIS AOD products over the eastern edge of the Tibetan Plateau based on ground-based remote sensing in 2017

The aerosol change over the Tibetan Plateau (TP) is poorly understood due to the scarcity of aerosol measurements over the TP. The aerosol optical properties were measured with a CE-318 sun photometer from January to December in 2017 at the Litang station (30.00°N, 100.16°E, 3950.5 m a.s.l.) over the eastern edge of the TP to characterize the variations in the aerosol optical properties and assess the applicability of MODIS AOD products at the eastern edge of TP. The analyses show that the annual mean aerosol optical depth at 440 nm (AOD440nm) is 0.08 ± 0.03. The mean annual Angström exponent between 440 and 870 nm (AE440–870nm) is 0.72 ± 0.23. The monthly-averaged AOD440nm exhibits a maximum (0.11 ± 0.04) in July and minimum (0.05 ± 0.02) in January, while the monthly mean AE440–870nm shows a maximum (1.08 ± 0.19) in July and minimum (0.48 ± 0.32) in February. The findings based on measured AOD440nm and AE440–870nm indicate low AOD, dominated by coarse particles in winter and fine particles in summer over the eastern TP edge. Three datasets of MODIS AOD products with 3 km spatial resolution as well as with 10 km spatial resolution based on Dark Target (DT) and Deep Blue (DB) algorithms are validated using the ground-based measurements over the eastern edge of the TP. Although the validations show a low accuracy of MODIS AOD products of the three datasets over this TP region, the MODIS AOD products based on the DB algorithm with 10 km resolution could be used in the atmospheric environment over the eastern edge of the TP based on the best match between AOD data and ground-based measurements, the lowest retrieval error, and the largest percentage falling within the expected error envelope.

Size-fractionated water-soluble ions during autumn and winter: Insights into volatile ammonium formation mechanisms in Shanghai, a megacity of China

Abstract Size-fractionated aerosols were collected by a Micro-Orifice Uniform Deposit Impactor at an urban site in Shanghai during the autumn and winter of 2014. The results showed that the average concentration of PM1.8 increased from 49 μg m−3 in autumn to 74 μg m−3 in winter. By contrast, the relative contribution of the water-soluble inorganic species decreased from 40% in autumn to 31% in winter. This finding suggests that the increase of organic matter (OM) greatly contributed to the deterioration of particle pollution in winter. Compared to autumn, the concentration of sulfate decreased whereas the concentration of NH4Cl increased significantly and became an important constituent of the fine particles in winter. The formation of nitrate in autumn became significant at [NH4+]/[SO42−] > 2, indicating that H2SO4 was fully neutralized by NH3 to form (NH4)2SO4. Our results showed that the regression between [NO3−]/[SO42−] and [NH4+]/[SO42−] drawn an unrealistic nitrate formation mechanism when the particulate NH4Cl was abundant. Replacing [NO3−] with {[NO3−]+[Cl−]} in the regression between [NO3−]/[SO42−] and [NH4+]/[SO42−], it is established that (NH4)2SO4 is preferentially formed before the excess available NH3 can react with HNO3 and HCl to form NH4NO3 and NH4Cl during wintertime. The concentrations of particulate sulfate and nitrate were not positively correlated with the ambient relative humidity, indicating that heterogeneous reactions on wet particles were not the key to haze formation in Shanghai during winter. Our results suggest that more studies are needed to understand the complex haze formation mechanisms in China.

Phosphorus solubility in aerosol particles related to particle sources and atmospheric acidification in Asian continental outflow

Abstract. The continent-to-ocean supply of phosphorus (P) in the soluble state, recognized as bioavailable P, via the atmosphere is hypothesized to be crucial to the biological cycle in offshore surface seawater. To investigate the solubility of P in aerosol particles moving towards the northwestern Pacific from the Asian continent, we measured the total P (TP), total dissolved P (TDP) and dissolved inorganic P (DIP) in aerosols at Qingdao (36∘06′ N, 120∘33′ E), a coastal city in eastern China. The samples were collected in December 2012 and January 2013 (winter) and in March and April 2013 (spring), when the middle-latitude westerly wind was prevailing. On average, P solubility, i.e., the ratio of TDP to TP, was 32.9±16.7 % in winter and 21.3±9.8 % in spring, and the TP concentrations in the two seasons were similar. This seasonal solubility difference is attributed to the aerosol sources containing the P. Particles in winter were predominantly anthropogenic particles from local and regional areas, and particles in spring were significantly influenced by natural dust from the arid and semiarid areas in the inland part of the continent. Moreover, acidification processes associated with the formation of sulfate and nitrate in the winter samples enhanced P solubility, suggesting that the P in anthropogenic particles was more susceptible to the production of acidic species than that in natural dust particles. There was a strong positive correlation between P solubility and relative humidity (RH). P solubility was usually less than 30 % when RH was below 60 %, even when the content of acidic species and/or anthropogenic particles in the aerosols was high, suggesting humidity had a critical role in the production of TDP. In addition, the proportion of DIP in TDP was high when the particles were predominantly anthropogenic, and the proportion of dissolved organic P (DOP; quantified as TDP minus DIP) in TDP was high when the particles were dominated by natural dust. These results indicate that, as the contents of bioavailable P in Asian continent outflows are closely dependent on the aerosol particle origins, atmospheric acidic processes could convert P into a bioavailable state under certain meteorological conditions. Therefore, the recent severe air pollution over East Asia might have enhanced the input of bioavailable P to downwind marine areas.

Characteristics and Formation Mechanisms of Sulfate and Nitrate in Size-segregated Atmospheric Particles from Urban Guangzhou, China

ABSTRACT Various water-soluble inorganic compounds, including Na+, NH4+, K+, Ca2+, Mg2+, Cl–, NO3–, PO43– and SO42–, were analyzed in 130 sets of size-segregated (< 0.49, 0.49–0.95, 0.95–1.5, 1.5–3.0, 3.0–7.2 and 7.2–10.0 µm) aerosol samples collected from March 2013 to April 2014 in Guangzhou, China. SO42– was unimodally distributed and peaked during a typical droplet mode (0.49–0.95 µm). However, the distribution of NO3– significantly varied across the four seasons. It was unimodally distributed in summer and autumn, peaking in the coarse mode (3.0–7.2 µm), and bimodally distributed in winter and spring, peaking in the size ranges of 0.49–0.95 µm and 3.0–7.2 µm, respectively. The coarse-mode NO3– was mainly related to the influence of soil/dust. The additional mode during winter and spring was attributable to the formation of ammonium nitrate. Compared to clean days, polluted days favored the formation of SO42– in summer and autumn and NO3– in winter and spring. The sulfur oxidation ratios (SORs) for < 0.49, 0.49–0.95 and 0.95–1.5 µm particles were negatively correlated with the relative humidity (RH) in spring, summer and autumn, respectively. However, the SORs for 0.49–3.0 µm particles were positively correlated with the RH in winter, implying an important contribution from the aqueous oxidation of SO2. Further analysis shows that the SO42– in < 0.49 µm particles was formed primarily through gas-phase photochemical oxidation of SO2 during all four seasons. The formation of NO3– was mainly attributable to heterogeneous reactions for 1.5–3.0 µm particles year-round and homogeneous gas-phase reactions for < 0.49 µm particles in winter. Correlation analysis also indicates a positive influence from biomass burning on the formation of nitrate and sulfate. The average pH of PM3 was calculated to be 2.6–5.6. Thus, the aqueous oxidation of SO2 by NO2 plays a limited role in the formation of sulfate in the atmosphere of Guangzhou.

Variability of depolarization of aerosol particles in the megacity of Beijing: implications for the interaction between anthropogenic pollutants and mineral dust particles

Abstract. East Asia is suffering from severe air pollution problems due to intensive anthropogenic emissions and natural mineral dust aerosols. During transport, the aerosol particles undergo complex mixing processes, resulting in great impacts on regional air quality, human health and climate. In this study, we conducted a long-term observation using an optical particle counter equipped with a polarization detection module (POPC) at an urban site in Beijing. Mass concentrations of both PM2.5 and PM10 estimated from POPC compared well with ground-based measurements. The results revealed that the observed depolarization ratio (δ, termed as the ratio of the intensity of the s-polarized signal to the intensity of the 120∘ backward scattering signal [s/(s+p)]) for aerosol particles in the fine mode was generally much lower in summer than that in spring as a result of predominance of different aerosol types. Mineral dust particles in the coarse mode normally had a large δ value (0.3±0.05) owing to their nonspherical shape; however, particles in the fine mode mostly had water-soluble compositions, which led to an apparent decrease of their δ values in particular high relative humidity (RH) conditions. Because the observation site was subject to the impact of frequent dust events in spring, the δ value of particle at 1 µm was almost twice as high as that (0.07±0.01) in summer. Based on size-resolved δ values, anthropogenic pollutants, mineral dust and polluted mineral dust particles and their contribution to local air quality could be well distinguished. About 26.7 % of substandard days (daily averaged PM2.5 concentration larger than 75 µg m−3) in Beijing featured high atmospheric loading of coarse-mode particles in winter and springtime. In particular, during severe pollution episodes in winter, the δ values of coarse-mode particles decreased by 13 %, which implies a high possibility of dust-related heterogeneous processes in pollution formation. During dust events, δ values of particles with optical size (Dp) of 5 µm evidently decreased, with an increase of the PM2.5 ∕ PM10 ratio as well as RH, indicating the morphological changes of mineral dust. This study confirmed that high RH tends to promote water absorption processes on the dust surface as well as the coating of soluble compounds, and suggested that remote sensing techniques for aerosols may underestimate the impact of dust particles due to the complex mixing of dust and anthropogenic particles in urban areas, and the interaction between dust particles and pollutants should be considered well by the optical model.