Sulfate alters aerosol absorption properties in East Asian outflow

Size distributions, mixing state, and morphology of refractory black carbon in an urban atmosphere of northeast Asia during summer

Black carbon (BC) exerts profound impacts on air quality, human health, and climate. Here, we investigated concentrations and size distributions of refractory BC (rBC) and mixing state and morphology of rBC-containing particles in urban Seoul for 2019 summer. Mass concentrations of rBC ranged from 0.02 μgm^-3 to 2.89 μgm^-3, and daily maximums of rBC mass, daily minimums of rBC mass median diameter (MMD) (110-130 nm), and shell-to-core ratio (R_shell/core) occurred with NO₂ maximums during morning rush hour. As the first report of ground observations on rBC mixing state, these results indicate that vehicle emission is a major local source of rBC in Seoul. MMDs of 127-146 nm and the greatest mass loadings of ≥1 μg m^-3 were accompanied by high O₃ and PM_2.5 concentrations, in contrast to the largest MMDs (135-165 nm) associated with transport from upstream regions. The average R_shell/core was 1.25 for the rBC mass-equivalent diameter (D_rBC) of 140-220 nm. R_shell/core increased gradually through the day and was positively correlated with O_x concentration, indicating photochemical aging of rBC particles. Co-emissions of rBC and volatile organic compounds from vehicles facilitated internal mixing during the daytime. However, R_shell/core tended to be low at temperature >∼30 °C, while 58 % of rBC particles with R_shell/core exceeding 1.25 were found at nighttime under relative humidity >75 %. These results demonstrate that the mixing state of freshly-emitted rBC particles was altered through coating by photochemically oxidized vapors during the day and hygroscopic growth at night. Additionally, the delay-time approach revealed rBC morphological characteristics, the most common being the bare type (74 %), and the attached type (6 %) was relatively large in numbers during morning rush hour. Therefore, it is suggested that during summer, rBC particles from traffic emissions should be considered in parallel to winter pollution mitigation strategies in urban atmosphere of northeast Asia.

Insights into aerosol chemical composition and optical properties at Lulin Atmospheric Background Station (2862 m asl) during two contrasting seasons

Continental outflows from peninsular Southeast Asia and East Asia dominate the widespread dispersal of air pollutants over subtropical western North Pacific during spring and autumn, respectively. This study analyses the chemical composition and optical properties of PM₁₀ aerosols during autumn and spring at a representative high-altitude site, viz., Lulin Atmospheric Background Station (23.47°N, 120.87°E; 2862 m a.s.l.), Taiwan. PM₁₀ mass was reconstructed and the contributions of major chemical components were also delineated. Aerosol scattering (σ_sp) and absorption (σ_ap) coefficients were regressed on mass densities of major chemical components by assuming external mixing between them, and the site-specific mass scattering efficiency (MSE) and mass absorption efficiency (MAE) of individual components for dry conditions were determined. NH₄NO₃ exhibited the highest MSE among all components during both seasons (8.40 and 12.58 m² g^-1 at 550 nm in autumn and spring, respectively). (NH₄)₂SO₄ and organic matter (OM) accounted for the highest σ_sp during autumn (51%) and spring (50%), respectively. Mean MAE (mean contribution to σ_ap) of elemental carbon (EC) at 550 nm was 2.51 m² g^-1 (36%) and 7.30 m² g^-1 (61%) in autumn and spring, respectively. Likewise, the mean MAE (mean contribution to σ_ap) of organic carbon (OC) at 550 nm was 0.84 m² g^-1 (64%) and 0.83 m² g^-1 (39%) in autumn and spring, respectively. However, a classification matrix, based on scattering Ångström exponent, absorption Ångström exponent, and single scattering albedo (ω), demonstrated that the composite absorbing aerosols were EC-dominated (with weak absorption; ω = 0.91-0.95) in autumn and a combination of EC-dominated and EC/OC mixture (with moderate absorption; ω = 0.85-0.92) in spring. This study demonstrates a strong link between chemical composition and optical properties of aerosol and provides essential information for model simulations to assess the imbalance in regional radiation budget with better accuracy over the western North Pacific.

Physicochemical and isotopic properties of ambient aerosols and precipitation particles during winter in Seoul, South Korea

The aim of this study was to characterize the physicochemical properties and microbial communities of particulate matter (PM) in Seoul, Korea. We collected long-term (2017-2019) precipitation samples and PM₁₀ and PM_2.5 monitoring data to determine the impact of soluble and insoluble chemical species on the soil surface. Ambient PM₁₀ concentrations were higher than PM_2.5 concentrations during the monitoring period, but both decreased during rainfall due to the washing effect of precipitation. PM_2.5 particles had a "fluffy" shape and contained sulfur (0.2%), but suspended particles (SPs) contained many carbon particles (approximately 60%). Spherical particles containing metal oxides, Fe and Al, might be originated from coal combustion, wild fires, and metal-refining processes under high-temperature conditions. Dissolved ions in precipitation included those eluted from salts and coal combustion based on the correlation coefficients of Na and Cl (R = 0.953) and F and NO₃ (R = 0.706). The δ¹⁵N-NO₃ and δ³⁴S-SO₄ of precipitation were enriched as the atmospheric temperature decreased from 9.8 to -1.6°C, implying the influence of domestic coal combustion. Backward trajectories showed that, in winter, air parcels passed through industrialized cities from China to South Korea. The microbial communities associated with PM were strongly influenced by atmospheric conditions. Proteobacteria (range from 4.6 to 76.7%) and Firmicutes (range from 6.0 to 91.4%) were the most dominant phyla and were significantly affected by changes in the PM_2.5 environment. The results indicate that the acidity of precipitation and the composition of aerosols were affected by fossil fuel combustion and mineral dust, and that atmospheric conditions may change as PM_2.5 concentrations increase.

Enhanced mixing state of black carbon with nitrate in single particles during haze periods in Zhengzhou, China

Black carbon (BC) plays an important role in air quality and climate change, which is closely associated with its mixing state and chemical compositions. In this work the mixing state of BC-containing single particles was investigated to explore the evolution process of ambient BC particles using a single particle aerosol mass spectrometer (SPAMS) in March 2018 in Zhengzhou, China. The BC-containing particles accounted for 61.4% of total detected ambient single particles and were classified into five types including BC-nitrate (BC-N, 52.3%) as the most abundant species, followed by BC-nitrate-sulfate (BC-NS, 22.4%), BCOC (16.8%), BC-fresh (BC-F, 4.5%) and BC-sulfate particles (BC-S, 4.0%). With enhancement of the ambient nitrate concentration, the relative peak area (RPA) of nitrate in BC-N and BCNS particles both increased, yet only the number fraction (N_f) of BCN particles increased while the N_f of BC-NS particles decreased, suggesting that the enhanced mixing state of BC with nitrate was mainly due to the increase in the ambient nitrate mass concentration. In addition, the N_f of BC-N decreased from 65.3% to 28.4% as the absorbing Ångström exponents (AAE) of eBC increased from 0.75 to 1.45, which indicated the reduction of light absorption ability of aged BC particles with the enhanced formation of BC-N particles. The results of this work indicated a change in the mixing state of BC particles due to the dominance of nitrate in PM_2.5, which also influenced the optical properties of aged BC particles.

Light-absorption enhancement of black carbon in the Asian outflow inferred from airborne SP2 and in-situ measurements during KORUS-AQ

We investigated the changes in the size distribution, coating thickness, and mass absorption cross-section (MAC) of black carbon (BC) with aging and estimated the light absorption enhancement (E_abs) in the Asian outflow from airborne in-situ measurements during 2016 KORUS-AQ campaign. The BC number concentration decreased, but mass mean diameter increased with increasing altitude in the West Coast (WC) and Seoul Metropolitan Area (SMA), reflecting the contrast between freshly emitted BC-containing particles at the surface and more aged aerosol associated with aggregation during vertical mixing and transport. Contradistinctively, BC number and mass size distributions were relatively invariant with altitude over the Yellow Sea (YS) because sufficiently aged BC from eastern China were horizontally transported to all altitudes over the YS, and there are no significant sources at the surface. The averaged inferred MAC of refractory BC in three regions reflecting differences in their size distributions increased to 9.8 ± 1.0 m² g^-1 (YS), 9.3 ± 0.9 m² g^-1 (WC), and 8.2 ± 0.9 m² g^-1 (SMA) as BC coating thickness increased from 20 nm to 120 nm. The absorption coefficient of BC calculated from the coating thickness and MAC were highly correlated with the filter-based absorption measurements with the slope of 1.16 and R² of 0.96 at 550 nm, revealing that the thickly coated BC had a large MAC and absorption coefficient. The E_abs due to the inferred coatings was estimated as 1.0-1.6, which was about 30% lower than those from climate models and laboratory experiments, suggesting that the increase in the BC absorption by the coatings in the Asian outflow is not as large as calculated in the previous studies. Organics contributed to the largest E_abs accounting for 69% (YS), 61% (WC), and 64% (SMA). This implies that organics are largely responsible for the lensing effect of BC rather than sulfates in the Asian outflow.

Comparison and evaluation of the simulated annual aerosol characteristics over China with two global aerosol models

In this study, simulations of the annual mean aerosol budget, aerosol optical properties, and surface mass concentration in 2006 in China are performed with two aerosol interactive global atmosphere models, namely, the Nonhydrostatic ICosahedral Atmospheric Model (NICAM) coupled with the Spectral Radiation Transport Model for Aerosol Species (SPRINTARS) and the Beijing Climate Center Atmospheric General Circulation Model (BCC_AGCM) coupled with the Canadian Aerosol Module (CAM) online. The observed and simulated aerosol optical depths (AODs) exhibit similar horizontal distributions with large values over eastern and central China, and sulfate aerosols contribute the main differences between the AODs simulated by NICAM and BCC_AGCM. The simulated sulfate and dust surface concentrations are more consistent with observations compared with the simulated carbonaceous surface concentrations, and both models can reproduce the decreasing tendency of the sulfate surface concentration from urban sites to rural sites. However, the dust emission and deposition levels in China simulated by BCC_AGCM are three times as high as those simulated by NICAM, and the major sink processes of the anthropogenic sulfate, black carbon (BC), and organic carbon (OC) aerosols over China are very different between the two models. The emission and deposition results, which are closely related to the model-assumed aerosol particle size distribution, indicate that the current aerosol size distribution used in the two models should be further improved. The differences in dust emission parameterizations also lead significant discrepancies in aerosol cycles and the dust emission scheme is an important factor determining the magnitudes of global and regional dust emission fluxes.

Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN): integrated analysis and intensive winter campaign 2018

Fine-particle pollution associated with winter haze threatens the health of more than 400 million people in the North China Plain. The Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN) investigated the physicochemical mechanisms leading to haze formation with a focus on the contributions of multiphase processes in aerosols and fogs. We integrated observations on multiple platforms with regional and box model simulations to identify and characterize the key oxidation processes producing sulfate, nitrate and secondary organic aerosols. An outdoor twin-chamber system was deployed to conduct kinetic experiments under real atmospheric conditions in comparison to literature kinetic data from laboratory studies. The experiments were spanning multiple years since 2017 and an intensive field campaign was performed in the winter of 2018. The location of the site minimizes fast transition between clean and polluted air masses, and regimes representative for the North China Plain were observed at the measurement location in Gucheng near Beijing. The consecutive multi-year experiments document recent trends of PM_2.5 pollution and corresponding changes of aerosol physical and chemical properties, enabling in-depth investigations of established and newly proposed chemical mechanisms of haze formation. This study is mainly focusing on the data obtained from the winter campaign 2018. To investigate multiphase chemistry, the results are presented and discussed by means of three characteristic cases: low humidity, high humidity and fog. We find a strong relative humidity dependence of aerosol chemical compositions, suggesting an important role of multiphase chemistry. Compared with the low humidity period, both PM₁ and PM_2.5 show higher mass fraction of secondary inorganic aerosols (SIA, mainly as nitrate, sulfate and ammonium) and secondary organic aerosols (SOA) during high humidity and fog episodes. The changes in aerosol composition further influence aerosol physical properties, e.g., with higher aerosol hygroscopicity parameter κ and single scattering albedo SSA under high humidity and fog cases. The campaign-averaged aerosol pH is 5.1 ± 0.9, of which the variation is mainly driven by the aerosol water content (AWC) concentrations. Overall, the McFAN experiment provides new evidence of the key role of multiphase reactions in regulating aerosol chemical composition and physical properties in polluted regions.

Aerosol-induced atmospheric heating rate decreases over South and East Asia as a result of changing content and composition

Aerosol emissions from human activities are extensive and changing rapidly over Asia. Model simulations and satellite observations indicate a dipole pattern in aerosol emissions and loading between South Asia and East Asia, two of the most heavily polluted regions of the world. We examine the previously unexplored diverging trends in the existing dipole pattern of aerosols between East and South Asia using the high quality, two-decade long ground-based time series of observations of aerosol properties from the Aerosol Robotic Network (AERONET), from satellites (Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI)), and from model simulations (Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). The data cover the period since 2001 for Kanpur (South Asia) and Beijing (East Asia), two locations taken as being broadly representative of the respective regions. Since 2010 a dipole in aerosol optical depth (AOD) is maintained, but the trend is reversed—the decrease in AOD over Beijing (East Asia) is rapid since 2010, being 17% less in current decade compared to first decade of twenty-first century, while the AOD over South Asia increased by 12% during the same period. Furthermore, we find that the aerosol composition is also changing over time. The single scattering albedo (SSA), a measure of aerosol’s absorption capacity and related to aerosol composition, is slightly higher over Beijing than Kanpur, and has increased from 0.91 in 2002 to 0.93 in 2017 over Beijing and from 0.89 to 0.92 during the same period over Kanpur, confirming that aerosols in this region have on an average become more scattering in nature. These changes have led to a notable decrease in aerosol-induced atmospheric heating rate (HR) over both regions between the two decades, decreasing considerably more over East Asia (− 31%) than over South Asia (− 9%). The annual mean HR is lower now, it is still large (≥ 0.6 K per day), which has significant climate implications. The seasonal trends in AOD, SSA and HR are more pronounced than their respective annual trends over both regions. The seasonal trends are caused mainly by the increase/decrease in anthropogenic aerosol emissions (sulfate, black carbon and organic carbon) while the natural aerosols (dust and sea salt) did not change significantly over South and East Asia during the last two decades. The MERRA-2 model is able to simulate the observed trends in AODs well but not the magnitude, while it also did not simulate the SSA values or trends well. These robust findings based on observations of key aerosol parameters and previously unrecognized diverging trends over South and East Asia need to be accounted for in current state-of-the-art climate models to ensure accurate quantification of the complex and evolving impact of aerosols on the regional climate over Asia.

The Key Role of Sulfate in the Photochemical Renoxification on Real PM2.5

The active nitrogen species (HONO, NO, and NO₂) have important impacts on the atmospheric oxidative capacity and the transformation of many atmospheric species. In this study, a fast photochemical renoxification rate of adsorbed HNO₃/NO₃^- to active nitrogen species (HONO, NO, and NO₂) was detected on real urban PM_2.5, and sulfate was found to play a key role in this process. Different from the reported direct photolysis pathway, the photochemical reaction of HNO₃/NO₃^- on PM_2.5 is dominated by a photosensitizing mechanism. Acidic protons are proved to be essential for this pathway. The role of sulfate, because of the nonvolatility of its conjugated acid, is to conserve the necessary acidic protons when interacting with HNO₃ and thus maintain its photoreactivity. This work implies that sulfate will have important implications in atmospheric nitrogen cycling by accelerating the release of nitrogen oxides from photochemical renoxification of HNO₃/NO₃^- adsorbed on ambient particulates and thus can cause major environmental problems.