Search

Assessing aerosol mixing states, which primarily depend on aerosol chemical compositions, is indispensable to estimate direct and indirect effects of aerosols. The limitations of the direct measurements of aerosol chemical composition and mixing states necessitate modeling approaches to infer the aerosol mixing states. The Optical Properties of Aerosols and Clouds (OPAC) model has been extensively utilized to construct optically equivalent aerosol chemical compositions from measured aerosol optical properties using Mie inversion. However, the representation of real atmospheric aerosol mixing scenarios in OPAC has perennially been challenged by the exclusive assumption of external mixing. A Python successor to the aerosol module of the OPAC model is developed, named AeroMix, with novel capabilities to (1) model externally and core–shell mixed aerosols, (2) simulate optical properties of aerosol mixtures constituted by any number of aerosol components, and (3) define aerosol composition and relative humidity in up to six vertical layers. Designed as a versatile open-source aerosol optical model framework, AeroMix is tailored for sophisticated inversion algorithms aimed at modeling aerosol mixing states and also their physical and chemical properties. AeroMix's performance is demonstrated by modeling the probable aerosol mixing states over Kanpur (urban) and the Bay of Bengal (marine) in south Asia. The modeled mixing states are consistent with independent measurements using a single-particle soot photometer (SP2) and transmission electron microscopy (TEM), substantiating the potential capability of AeroMix to model complex aerosol mixing scenarios involving multiple internally mixed components in diverse environments. This work contributes a valuable tool for modeling aerosol mixing states to assess their impact on cloud-nucleating properties and radiation budget. [ABSTRACT FROM AUTHOR]

Assessing aerosol mixing states, which mainly depend on aerosol chemical compositions is indispensable to estimate aerosol direct and indirect effects. While the limitations in the measurements of aerosol chemical composition and mixing states persist globally, the Optical Properties of Aerosols and Clouds (OPAC) model has been widely used to construct optically equivalent aerosol chemical compositions from measured aerosol optical properties using Mie inversion. However, the representation of real atmospheric aerosol mixing scenarios in OPAC has perennially been challenged by the exclusive assumption of external mixing. A Python successor to the aerosol module of the OPAC model is developed, named 'AeroMix,' with novel capabilities to 1) model externally and core-shell mixed aerosols, 2) simulate optical properties of aerosol mixtures constituted by any number of aerosol components, 3) and define aerosol composition and relative humidity in up to 6 vertical layers. Designed as a versatile open-source aerosol optical model framework, AeroMix is tailored for sophisticated inversion algorithms aimed at modeling aerosol mixing states and also their physical and chemical properties. AeroMix's performance is demonstrated by modeling the probable aerosol mixing states over Kanpur (urban), India, and the Bay of Bengal (marine). The modeled mixing states are consistent with independent measurements using single-particle soot photometer (SP2) and transmission electron microscopy (TEM), substantiating the potential capability of AeroMix to model complex aerosol mixing scenarios involving multiple internally mixed components in diverse environments. This work contributes a valuable tool for modeling aerosol mixing states to assess their impact on cloud nucleating properties and radiation budget. [ABSTRACT FROM AUTHOR]

The continental outflow from the Indo-Gangetic Plain and Southeast Asia, during the late NE monsoon (January–March), dominates the transport of chemical constituents to the marine atmospheric boundary layer (MABL) of the Bay of Bengal (BoB). During the rest of the year, prevailing wind regimes and meteorological conditions do not favour the atmospheric transport of continental products. Here we report on the spatio-temporal variability of inorganic phosphorous (PInorg=PO3−4) in the MABL and its dry-deposition flux to the surface BoB. On the basis of the abundance of PInorg in PM2.5 (0.1–0.8 nmol m−3) and PM10 (0.3–2.8 nmol m−3), we document its dominant occurrence in the coarse mode (Da≥2.5 µm). The analytical data also provide evidence for the chemical processing of mineral dust by acidic species and mobilisation of PInorg during the long-range atmospheric transport. However, significantly high PInorg/non-sea-salt Ca2+ ratios over the BoB suggest dominant contribution from anthropogenic sources (fertilisers and biomass burning emissions). PInorg concentration over the Arabian Sea is about 4 to 5 times lower and is primarily associated with the mineral dust from desert regions. The dry-deposition flux of PInorg to the BoB varies by one order of magnitude (0.5–5.0 µmol P m−2 d−1; Av: 0.02 Tg P yr−1). These results have implications to the air-sea deposition of phosphorous over oceanic regions downwind of the pollution sources and impact on the biogeochemistry of surface waters.

Biomass burning (BB) significantly influences the chemical composition of organic aerosols (OAs) in the East Asian outflow. The source apportionment of BB-derived OA is an influential factor for understanding their regional emissions, which is crucial for reducing uncertainties in their projected climate and health effects. We analyzed here three different classes of atmospheric sugar compounds (anhydrosugars, primary sugars, and sugar alcohols) and two types of biogenic secondary organic aerosol (BSOA) tracers (isoprene- and monoterpene-derived SOA products) from a year-long study that collected total suspended particulate matter (TSP) from an island-based receptor site in Gosan, South Korea. We investigate the seasonal variations in the source emissions of BB-derived OA using mass concentrations of anhydrosugars and radiocarbon (14 C-) isotopic composition of organic carbon (OC) and elemental carbon (EC) in ambient aerosols. Levoglucosan (Lev) is the most abundant anhydrosugar, followed by galactosan (Gal), and mannosan (Man). Strong correlations of Lev with Gal and Man, along with their ratios (Lev / Gal is 6.65±2.26 ; Lev / Man is 15.1±6.76) indicate the contribution from hardwood burning emissions. The seasonal trends revealed that the BB impact is more pronounced in winter and fall, as evidenced by the high concentrations of anhydrosugars. Likewise, significant correlations among three primary sugars (i.e., glucose, fructose, and sucrose) emphasized the contribution of airborne pollen. The primary sugars showed higher concentrations in spring/summer than winter/fall. The fungal spore tracer compounds (i.e., arabitol, mannitol, and erythritol) correlated well with trehalose (i.e., a proxy for soil organic carbon), suggesting the origin from airborne fungal spores and soil microbes in the East Asian outflow. These sugar alcohols peaked in summer, followed by spring/fall and winter. Monoterpene-derived SOA tracers were most abundant compared to isoprene SOA tracers. Both BSOA tracers were dominant in summer, followed by fall, spring, and winter. The source apportionment based on multiple linear regressions and diagnostic mass ratios together revealed that BB emissions mostly contributed from hardwood and crop residue burning. We also found significant positive linear relationships of 14 C-based nonfossil- and fossil-derived organic carbon fractions with Lev C, along with the comparable regression slopes, suggesting the importance of BB and coal combustion sources in the East Asian outflow. [ABSTRACT FROM AUTHOR]

The marine atmosphere of the Bay of Bengal (BoB) is prone to get impacted by anthropogenic aerosols from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA), particularly during the northeast monsoon (NEM). In this study, we quantify and characterize carbonaceous aerosols and their absorption properties collected in two cruise campaigns onboard ORV Sindhu Sadhana during the continental outflow period over the BoB. Aerosol samples were classified based on the air mass back trajectory analyses, wherein samples were impacted by the continental air parcel (CAP), marine air parcel (MAP), and mix of both (CAP + MAP). Significant variability in the PM10 mass concentration (in μg m−3) is found with a maximum value for MAP samples (75.5 ± 36.4) followed by CAP + MAP (58.5 ± 27.3) and CAP (58.5 ± 27.3). The OC/EC ratio (>2) and diagnostic tracers i.e. nss-K+/EC (0.2–0.96) and nss-K+/OC (0.11–1.32) along with the absorption angstrom exponent (AAE: 4.31–6.02) and MODIS (Moderate Resolution Imaging Spectroradiometer) derived fire counts suggest the dominance of biomass burning emission sources. A positive correlation between OC and EC (i.e. r = 0.86, 0.70, and 0.42 for CAP, MAP, and CAP + MAP, respectively) further confirmed the similar emission sources of carbonaceous species. Similarly, a significant correlation between estimated secondary organic carbon (SOC) and water-soluble organic carbon (WSOC; r = 0.99, 0.96, and 0.97 for CAP, MAP, and CAP + MAP, respectively) indicate their similar chemical nature as well as dominant contribution of SOC to WSOC. The absorption coefficient (babs-365) and mass absorption efficiency (MAEBrC-365) of the soluble fraction were estimated at 365 nm wherein, babs-365 showed a linear relationship with WSOC and nss-K+, signifying the contribution of water soluble brown carbon from biomass burning emissions. The estimated MAEBrC-365 (0.30–0.93 m2 g−1), during this study, was consistent with the earlier observations over the BoB, particularly during the continental outflow season. [ABSTRACT FROM AUTHOR]

To understand the sources and transport pathways of organic compounds associated with soil microbes and higher plant waxes in the East Asian outflow, we assessed source‐specific tracers such as α‐, β‐ and ω‐hydroxy fatty acids (FAs) in remote marine aerosols collected at Chichijima Island in the western North Pacific (WNP) during 2001–2003. Molecular distributions of hydroxy FAs are characterized by strong even‐carbon numbered predominance, indicating biogenic sources. Hydroxy FAs showed a strong seasonality with higher loadings during winter/spring than summer/autumn. Cluster analysis of backward air mass trajectories, satellite‐based fire counts and dust extinction data reveal an impact of the East Asian outflow over the WNP in winter/spring. In the spring, there are larger relative abundances of short‐chain β‐hydroxy C10‐C18 FAs (a proxy for soil microbes), consistent with the higher loadings of non‐sea‐salt Ca2+ (dust tracer). The molecular distributions of β‐hydroxy FAs in spring are in agreement with those of the reference materials of Chinese loess (CJ‐1) and simulated Asian mineral dust (CJ‐2), suggesting their probable sources in East Asia. A comparison of relative abundances of short‐chain β‐hydroxy C10‐C18 FAs and long‐chain ω‐hydroxy C20‐C32 FAs (a proxy for higher plant metabolites) in Chichijima aerosols between this (2001–2003) and previous (1990–1993) studies have unveiled an increment of 20% and 30%, respectively. Such an increase was likely caused by the changes in source strength on a decadal‐scale and warrants further investigation. Furthermore, cluster analysis of trajectories and the overall distributions of hydroxy FAs between both datasets have shown their similar provenance in winter/spring. Plain Language Summary: Atmospheric transport is a major source of land‐derived lipid compounds in the deep‐sea sediments. Here, we assessed the source‐specific tracers (α‐, β‐ and ω‐hydroxy fatty acids [FAs]) of soil microbes and higher plant waxes in remote marine aerosols collected at Chichijima Island in the western North Pacific during 2001–2003 to understand their sources and transport patterns from East Asia. Molecular distributions of hydroxy FAs revealed characteristic even‐C number predominance, suggesting their origin from plant waxes/soil microbes. Besides, hydroxy FAs showed higher concentrations in winter/spring compared to summer/autumn. High concentrations of short‐chain β‐(C10‐C18) hydroxy FAs (tracers of soil‐microbes) coincide with the non‐sea‐salt Ca2+ loadings (dust tracer) in spring. Similar molecular distributions of β‐hydroxy FAs in the Chichijima aerosols and the reference materials of Chinese loess (CJ‐1) and simulated Asian mineral dust (CJ‐2), suggesting their probable sources in East Asia. A comparison of relative abundances of β‐(C10‐C18) and long‐chain ω‐(C20‐C32) hydroxy FAs (tracer of plant lipids) in Chichijima aerosols collected between 2001–2003 and 1990–1993 have unveiled an increment of ∼20% and 30%, respectively. Despite having similar sources in winter/spring between both these datasets, such an increase could only be explained by changes in source strength on decadal‐scale. Key Points: High concentrations of β/ω‐hydroxy fatty acids (FA) in winter/spring suggest the impact of plant/microbial lipids in the East Asian outflowDistributions of β/ω‐hydroxy FAs between 1990–93 and 2001‐03 clearly indicate their similar sources in East AsiaSignificant increases in β‐ (20%) and ω‐hydroxy FAs (30%) from 1990–93 to 2001–2003 suggest changes in source‐strength of the Asian outflow [ABSTRACT FROM AUTHOR]

The first field measurements of light absorbing water-soluble organic carbon (WSOC), referred as brown carbon (BrC), have been made in the marine atmospheric boundary layer (MABL) during the continental outflow to the Bay of Bengal (BoB) and the Arabian Sea (ARS). The absorption signal measured at 365 nm in aqueous extracts of aerosols shows a systematic linear increase with WSOC concentration, suggesting a significant contribution from BrC to the absorption properties of organic aerosols. The mass absorption coefficient ( b _abs ) of BrC shows an inverse hyperbolic relation with wavelength (from ∼300 to 700 nm), providing an estimate of the Angstrom exponent (α _P , range: 3–19; Av: 9 ± 3). The mass absorption efficiency of brown carbon (σ _abs−BrC ) in the MABL varies from 0.17 to 0.72 m ^2 g ^−1 (Av: 0.45 ± 0.14 m ^2 g ^−1 ). The α _P and σ _abs−BrC over the BoB are quite similar to that studied from a sampling site in the Indo-Gangetic Plain (IGP), suggesting the dominant impact of organic aerosols associated with the continental outflow. A comparison of the mass absorption efficiency of BrC and elemental carbon (EC) brings to focus the significant role of light absorbing organic aerosols (from biomass burning emissions) in atmospheric radiative forcing over oceanic regions located downwind of the pollution sources.

The near UV-visible light-absorbing organic carbon (OC) of ambient aerosols, referred to here as brown carbon (BrC), significantly influences the atmospheric radiative forcing on both regional and global scales. Here, we documented BrC absorption in the aqueous and methanol extracts of marine aerosols collected over the Bay of Bengal (BoB: September–October 2017) and a city, Visakhapatnam (May–June 2018), in southern India during the southwest monsoon (i.e., a transition period with weak continental impact). The absorption spectra of BrC over the BoB showed several peaks around 300–400 nm and differ from those observed over Visakhapatnam. The absorption coefficient of BrC over the BoB, unlike Visakhapatnam data, does not seem to covary with other chemical proxies of biomass burning (non-sea-salt or nss-K+) and coal combustion (nss-SO42−) in the continental outflows, suggesting a different source of BrC over the BoB. Besides, we observed higher proportions of water-insoluble organic carbon (WIOC/OC: 0.89 ± 0.02) and significant enrichment of Mg2+ over Na+ (i.e., relative to seawater) in BoB aerosols. This result and the backward air mass trajectories both hinted their major source of OC from marine-derived organic matter. In contrast, the absorption spectra of BrC over Visakhapatnam are like those from biomass burning emissions in the Indo-Gangetic Plain. This observation is further supported by the satellite-based fire counts and backward air mass trajectories. Therefore, our study underscores the BrC aerosols from the oceanic sources and southern India, hitherto unknown, and can improve our understanding of the regional climate effects of carbonaceous aerosols if included in models. [ABSTRACT FROM AUTHOR]

Brown carbon (BrC) is a source of light‐absorbing aerosols. The Arctic is more sensitive to emissions of light‐absorbing aerosols than lower latitudes. Knowledge of BrC in a historical period is beneficial to understand its role in a changing climate. Here, we present measurement of water‐soluble BrC (WS‐BrC) for the Arctic aerosols during late winter‐late spring in 1991. Mass absorption coefficient (0.07 ± 0.04 M/m) and efficiency (0.41 ± 0.21 m2/g) at 365 nm of WS‐BrC were lower than those in polluted urban and rural regions. WS‐BrC was mainly from biomass burning/combustion (dark winter to mid‐March) and marine sources connected with photochemical gas to particle conversion (after polar sunrise to June). Solar radiative absorption of WS‐BrC relative to elemental carbon was 5% on average in February to April and surged to 34% after mid‐May. This study helps in understanding the role of BrC in the Arctic climate. Key Points: During dark winter in February to mid‐March, the main contributors to water‐soluble brown carbon were biomass burning and biomass combustionAfter polar sunrise, marine sources of water‐soluble brown carbon connected with photochemical secondary production became dominantThe radiative absorption of WS‐BrC relative to EC was ~34% after mid‐May, much higher than that in dark winter (5%) and at lower latitudes [ABSTRACT FROM AUTHOR]

In this paper, we synthesize the size distribution and optical properties of the atmospheric water-soluble fraction of light-absorbing organic carbon (brown carbon; BrC) in the continental outflow from the Indo-Gangetic Plain (IGP) in South Asia to the North Indian Ocean. A comparison of the mass absorption coefficient of water-soluble BrC (babs-WSBrC-365nm) in PM2.5 with that in PM10 sampled over the Bay of Bengal reveals the dominance of BrC in fine mode. Furthermore, the babs-BrC-365nm shows a significant linear relationship with mass concentrations of airborne particulate matter, water-soluble organic carbon and non-sea-salt-K+ in the continental outflow from the IGP. This observation emphasizes the ubiquitous nature and significant contribution of water-soluble BrC from biomass burning emissions (BBEs). Comparing the absorption properties from this study with global datasets, it is discernible that BBEs dominate BrC absorption. Furthermore, the imaginary refractive index of water-soluble BrC (kWSBrC-365nm) in marine aerosols sampled over the North Indian Ocean during November is significantly higher than during December to January. Thus, significant temporal variability is associated with crop-residue burning emissions in the IGP on the composition of BrC over the North Indian Ocean. Our estimates show that the babs-WSBrC-365nm and kWSBrC-365nm from post-harvest crop-residue burning emissions in the IGP are much higher than the BBEs from the southeastern United States and Amazonian forest fires. Another major finding of this study is the lack of significant relationship between kWSBrC-365nm and the mass ratio of elemental carbon to particulate organic matter, as previously suggested by chamber experiments to model varying BrC absorption properties in ambient aerosols. Therefore, considerable spatio-temporal variability prevails among emission sources (wood burning vs. crop-residue burning), which needs to be considered when assessing the regional radiative forcing of BrC relative to major absorbing elemental carbon. [ABSTRACT FROM AUTHOR]

This study presents and evaluates the most comprehensive set to date of chemical, physical and optical properties of aerosols in the outflow from South Asia covering a full winter (Nov. 2014 - March 2015), here intercepted at the Indian Ocean receptor site of the Maldives Climate Observatory in Hanimaadhoo (MCOH). Cluster analysis of air-mass back trajectories for MCOH, combined with AOD and meteorological data, demonstrate that the wintertime northern Indian Ocean is strongly influenced by aerosols transported from source regions with three major wind regimes, originating from the Indo-Gangetic Plain (IGP), the Bay of Bengal (BoB) and the Arabian Sea (AS). As much as 97 ± 3% of elemental carbon (EC) in the PM10 was also found in the fine mode (PM2.5). Other mainly anthropogenic constituents such as organic carbon (OC), non-sea-salt (nss) -K+, nss-SO42− and NH4+ were also predominantly in the fine mode (70-95%), particularly in the air masses from IGP. The combination at this large-footprint receptor observatory of consistently low OC/EC ratio (2.0 ± 0.5), strong linear relationships between EC and OC as well as between nss-K+ and both OC and EC, suggest a predominance of primary sources, with a large biomass burning contribution. The particle number-size distributions for the air masses from IGP and BoB exhibited clear bimodal shapes within the fine fraction with distinct accumulation (0.1 μm < d < 1 μm) and Aitken (0.025 μm < d < 0.10 μm) modes. This study also supports that IGP is a key source region for the wider South Asia and nearby oceans, as defined by the criteria that anthropogenic AODs exceed 0.3 and absorption AOD > 0.03. Taken together, the aerosol pollution over the northern Indian Ocean in the dry season is dominated by a well-mixed long-range transported regime of the fine-mode aerosols largely from primary combustion origin. [ABSTRACT FROM AUTHOR]

This work reports on the current status of the global modeling of iron (Fe) deposition fluxes and atmospheric concentrations and the analyses of the differences between models, as well as between models and observations. A total of four global 3-D chemistry transport (CTMs) and general circulation (GCMs) models participated in this intercomparison, in the framework of the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) Working Group 38, “The Atmospheric Input of Chemicals to the Ocean". The global total Fe (TFe) emission strength in the models is equal to ∼ 72 TgFeyr-1 (38-134 TgFe yr-1) from mineral dust sources and around 2.1 TgFeyr-1 (1.8-2.7 TgFeyr-1) from combustion processes (the sum of anthropogenic combustion/ biomass burning and wildfires). The mean global labile Fe (LFe) source strength in the models, considering both the primary emissions and the atmospheric processing, is calculated to be 0.7 (-0:3) TgFeyr-1, accounting for both mineral dust and combustion aerosols. The mean global deposition fluxes into the global ocean are estimated to be in the range of 10-30 and 0.2-0.4 TgFeyr-1 for TFe and LFe, respectively, which roughly corresponds to a respective 15 and 0.3 TgFeyr-1 for the multi-model ensemble model mean. The model intercomparison analysis indicates that the representation of the atmospheric Fe cycle varies among models, in terms of both the magnitude of natural and combustion Fe emissions as well as the complexity of atmospheric processing parameterizations of Fe-containing aerosols. The model comparison with aerosol Fe observations over oceanic regions indicates that most models overestimate surface level TFe mass concentrations near dust source regions and tend to underestimate the low concentrations observed in remote ocean regions. All models are able to simulate the tendency of higher Fe concentrations near and downwind from the dust source regions, with the mean normalized bias for the Northern Hemisphere (∼ 14), larger than that of the Southern Hemisphere (∼ 2:4) for the ensemble model mean. This model intercomparison and model-observation comparison study reveals two critical issues in LFe simulations that require further exploration: (1) the Fe-containing aerosol size distribution and (2) the relative contribution of dust and combustion sources of Fe to labile Fe in atmospheric aerosols over the remote oceanic regions. [ABSTRACT FROM AUTHOR]

This work reports on the current status of global modelling of iron (Fe) deposition fluxes and atmospheric concentrations and analyses of the differences between models, as well as between models and observations. A total of four global 3-D chemistry-transport (CTMs) and general circulation (GCMs) models have participated in this intercomparison, in the framework of the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) Working Group 38, "The Atmospheric Input of Chemicals to the Ocean". The global total Fe (TFe) emissions strength in the models is equal to ~ 72 Tg-Fe yr-1 (38-134 Tg-Fe yr-1) from mineral dust sources and around 2.1 Tg-Fe yr-1 (1.8-2.7 Tg-Fe yr-1) from combustion processes (sum of anthropogenic combustion/biomass burning and wildfires). The mean global labile Fe (LFe) source strength in the models, considering both the primary emissions and the atmospheric processing, is calculated to be 0.7 (±0.3) Tg-Fe yr-1, accounting for mineral dust and combustion aerosols together. The multi model ensemble global TFe and LFe deposition fluxes into the global ocean are calculated to be ~ 15 Tg-Fe yr-1 and ~ 0.3 Tg-Fe yr-1, respectively. The model intercomparison analysis indicates that the representation of the atmospheric Fe cycle varies among models, in terms of both the magnitude of natural and combustion Fe emissions as well as the complexity of atmospheric processing parametrizations of Fe-containing aerosols. The model comparison with aerosol Fe observations over oceanic regions indicate that most models overestimate surface level TFe mass concentrations near the dust source regions and tend to underestimate the low concentrations observed in remote ocean regions. All models are able to simulate the tendency of higher Fe loading near and downwind from the dust source regions, with the mean normalized bias for the Northern Hemisphere (~ 14), larger than the Southern Hemisphere (~ 2.4) for the ensemble model mean. This model intercomparison and model--observation comparison study reveals two critical issues in LFe simulations that require further exploration: 1) the Fe-containing aerosol size distribution and 2) the relative contribution of dust and combustion sources of Fe to labile Fe in atmospheric aerosols over the remote oceanic regions. [ABSTRACT FROM AUTHOR]

This review aims to update our understanding on molecular distributions of water-soluble dicarboxylic acids and related compounds in atmospheric aerosols with a focus on their geographical variability, size distribution, sources and formation pathways. In general, molecular distributions of diacids in aerosols from the continental sites and over the open ocean waters are often characterized by the predominance of oxalic acid (C 2 ) followed by malonic acid (C 3 ) and/or succinic acid (C 4 ), while those sampled over the polar regions often follow the order of C 4 ≥ C 2 and C 3 . The most abundant and ubiquitous diacid is oxalic acid, which is principally formed via atmospheric oxidation of its higher homologues of long chain diacids and other pollution-derived organic precursors (e.g., olefins and aromatic hydrocarbons). However, its occurrence in marine aerosols is mainly due to the transport from continental outflows (e.g., East Asian outflow during winter/spring to the North Pacific) and/or governed by photochemical/aqueous phase oxidation of biogenic unsaturated fatty acids (e.g., oleic acid) and isoprene emitted from the productive open ocean waters. The long-range atmospheric transport of pollutants from mid latitudes to the Arctic in dark winter facilitates to accumulate the reactants prior to their intense photochemical oxidation during springtime polar sunrise. Furthermore, the relative abundances of C 2 in total diacid mass showed similar temporal trends with downward solar irradiation and ambient temperatures, suggesting the significance of atmospheric photochemical oxidation processing. Compound-specific isotopic analyses of oxalic acid showed the highest δ 13 C among diacids whereas azelaic acid showed the lowest value, corroborating the significance of atmospheric aging of oxalic acid. On the other hand, other diacids gave intermediate values between these two diacids, suggesting that aging of oxalic acid is associated with 13 C enrichment. [ABSTRACT FROM AUTHOR]

Oxidation of isoprene, a major biogenic volatile organic compound emitted from forest canopies, is a potential source of oxalic acid; the dominant species in organic aerosols. We evaluated here ozonolysis of isoprene in dry darkness as a source of oxalic (C 2), malonic (C 3) and succinic (C 4) acids. We found that oxalic acid and methylglyoxal are dominant products within 10 min of reaction followed by glyoxylic, malonic or succinic acids. Interestingly, molecular distributions of oxidation products from early reactions (9–29 min) were characterized by the predominance of methylglyoxal followed by C 2 , which became dominant after 30 min. The isoprene-derived secondary organic aerosols (SOAs) showed chemical evolution with reaction time towards the molecular characteristics of dicarboxylic acids similar to those of ambient aerosols (C 2 >C 3 ≥C 4). The carbon-based relative abundances of methylglyoxal decreased steadily (40%→30%), while those of C 2 increased with reaction time (15%→25%), but no such variations persisted for glyoxal (6–10%). This finding means that methylglyoxal is more important intermediate of oxalic acid than glyoxal. In contrast, smaller variability and lower concentrations of pyruvic and glyoxylic acids than other intermediates indicate that oxalic acid formation under dry conditions follows a different pathway than in aqueous-phase heterogeneous chemistry usually invoked for cloud/fog/atmospheric waters. Here, we propose new reaction schemes for high levels of methylglyoxal and oxalic acid via gas-phase chemical reactions with ozone and OH radicals to better interpret the ambient SOA composition. Furthermore, the relative abundances of C 2 exhibit small variability from 1 to 8 h, suggesting its stable character towards the oxidation by hydroxyl radicals. Unlabelled Image • Major secondary organic aerosol (SOA) from the isoprene ozonolysis are oxalic, succinic and glyoxylic acids, methylglyoxal • Time-series batch experiments revealed rapid evolution of oxalic acid and other SOA within 10 min • Relative abundance of methylglyoxal decreased with an increase in that of oxalic acid, suggesting gas-phase transformation [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosols affect climate, especially in high aerosol loading regions such as South Asia. A key uncertainty for the climate effects of BC is the evolution of light‐absorbing properties in the atmosphere. Here, we present a year‐round comparison of the mass absorption cross section (MAC; 678 nm) of BC in air (PM10) and rain, for samples collected at the Maldives Climate Observatory at Hanimaadhoo. We develop a filter‐loading correction scheme for estimating BC absorption on filters used in high‐volume samplers. The year‐round average MAC678 of BC in the rain is almost twice (13.3 ± 4.2 m2/g) compared to the PM10 aerosol (7.2 ± 2.6 m2/g). A possible explanation is the elevated ratio of organic carbon (OC) to BC observed in rain particulate matter (9.4 ± 6.3) compared to in the aerosols (OC/BC 2.6 ± 1.4 and water‐insoluble organic carbon/BC 1.2 ± 0.8), indicating a coating‐enhancement effect. In addition to BC, we also investigated the MAC365 of water‐soluble brown carbon in PM10 (0.4 ± 0.4 m2/g, at 365 nm). In contrast to BC, MAC365brown carbon relates to air mass history, showing higher values for samples from air originating over the South Asian landmass. Furthermore, calculated washout ratios are much lower for BC compared to OC and inorganic ions such as sulfate, implying a longer atmospheric lifetime for BC. The wet deposition flux for BC during the high loading winter was 3 times higher than during the wet summer, despite much less precipitation in the winter. Key Points: Year‐round black carbon mass absorption cross sections were simultaneously constrained for rainwater and aerosolsThe mass absorption cross section of black carbon in rainwater was twice as large as in PM10 aerosolsThe washout ratio for black carbon is an order of magnitude lower than, for example, organic and sulfate, suggesting greater atmospheric longevity [ABSTRACT FROM AUTHOR]

Abstract Organic aerosols (OA), a ubiquitous component of ambient particulate matter (accountingfor a mass fraction as high as 90%)1 can have multiple effects on Earth&#x2019;s system, includingthose "direct" (scattering/absorption properties) and "indirect" ones (for example,their ability to act as cloud condensation nuclei: CCN or influence the hygroscopicproperties, cloud albedo)1,2. Understanding the sources and formation pathways ofOA is an essential prerequisite for reducing their associated uncertainties in thepredicted climate effects. These impacts could be more severe for the tropical regionswith intense anthropogenic activities such as S. Asia. Our study here focuses oncharacterizing the OA in the S. and SE. Asian outflow to a coastal Ocean, the Bay of Bengal(BoB). We demonstrate the simultaneous use of stable carbon isotopic composition of totalcarbon (δ13CTC) and anhydrosugars, tracers of biomass burning (BB) emissions,to elucidate the relative significance of primary (POA) versus secondary organicaerosols (SOA). We assessed the concentrations of anhydrosugars (levoglucosan:Lev, mannosan: Man and galactosan: Gal) and δ13CTC over the BoB, influencedby the continental outflow from the Indo-Gangetic Plain (IGP) and forest fires inSouth-East Asia (SEA). We observed notable differences in the molecular distributionsand diagnostic mass ratios of anhydrosugars between IGP- (Lev &#x003E; Gal &#x003E; Man;Lev/Gal: 5.5) and SEA-outflow (Lev &#x003E; Man &#x003E; Gal; Lev/Gal: 26). The positivelinear/nonlinear relationship of δ13CTC with Lev, K+, and total carbon (TC) in thewintertime SEA-outflow are in sharp contrast to those in summer from Mt. Tai,China, and Rondônia, Brazil. A negative linear relationship of reciprocal values ofmass concentrations of TC, water-soluble organic carbon (WSOC), and Lev withδ13CTCin the SEA-outflow suggests a mixing of 13C-enriched POA of BB origin and"fresh or less-aged SOA" formed by the oxidation of 13C-depleted volatile organiccompounds. References 1 Kanakidou, M. et al. Organic aerosol and global climate modelling: a review. Atmos.Chem. Phys. 5, 1053-1123, doi:10.5194/acp-5-1053-2005 (2005). 2 Hallquist, M. et al. The formation, properties and impact of secondary organic aerosol:current and emerging issues. Atmospheric Chemistry and Physics 9, 5155-5236 (2009). [ABSTRACT FROM AUTHOR]