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Atmospheric deposition is a source of potentially bioavailable iron (Fe) and thus can partially control biological productivity in large parts of the ocean. However, the explanation of observed high aerosol Fe solubility compared to that in soil particles is still controversial, as several hypotheses have been proposed to explain this observation. Here, a statistical analysis of aerosol Fe solubility estimated from four models and observations compiled from multiple field campaigns suggests that pyrogenic aerosols are the main sources of aerosols with high Fe solubility at low concentration. Additionally, we find that field data over the Southern Ocean display a much wider range in aerosol Fe solubility compared to the models, which indicate an underestimation of labile Fe concentrations by a factor of 15. These findings suggest that pyrogenic Fe-containing aerosols are important sources of atmospheric bioavailable Fe to the open ocean and crucial for predicting anthropogenic perturbations to marine productivity.

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]

Incomplete understanding of the sources of secondary organic aerosol (SOA) leads to large uncertainty in both air quality management and in climate change assessment. Chemical reactions occurring in the atmospheric aqueous phase represent an important source of SOA mass, yet, the effects of anthropogenic emissions on the aqueous SOA (aqSOA) are not well constrained. Here we use compound-specific dual-carbon isotopic fingerprints (δ13C and Δ14C) of dominant aqSOA molecules, such as oxalic acid, to track the precursor sources and formation mechanisms of aqSOA. Substantial stable carbon isotope fractionation of aqSOA molecules provides robust evidence for extensive aqueous-phase processing. Contrary to the paradigm that these aqSOA compounds are largely biogenic, radiocarbon-based source apportionments show that fossil precursors produced over one-half of the aqSOA molecules. Large fractions of fossil-derived aqSOA contribute substantially to the total water-soluble organic aerosol load and hence impact projections of both air quality and anthropogenic radiative forcing. Our findings reveal the importance of fossil emissions for aqSOA with effects on climate and air quality. Isotope fingerprinting is used to track precursor sources and formation pathways of aqueous SOA, such as oxalic acid, finding that fossil fuel precursors contributions have largely been underestimated. [ABSTRACT FROM AUTHOR]

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]

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]

Oxalic acid and the related organics are ubiquitous water-soluble constituents of ambient organic aerosols (OA). Their molecular distributions and relative abundances provide crucial insights on the secondary OA formation processes in the marine atmosphere and are important for understanding the cloud-aerosol interactions. Here, we measured the concentrations of dicarboxylic acids, pyruvic acid, ω-oxocarboxylic acids and α-dicarbonyls alongside aerosol chemical composition in the South Asian outflow to the Arabian Sea. Oxalic acid (C 2) was the most abundant dicarboxylic acid followed by succinic (C 4) and malonic (C 3) acids; this distribution is typical of "less aged OA". Oxalic acid levels correlated well with aerosol liquid water content (ALWC) and non-sea-salt (nss)-SO 4 2− (a tracer of coal combustion influence), suggesting an important aqueous phase oxidation of successive higher homologs. This was inferred based on the prevailing inverse relationships between the relative abundances of oxalic acid in total aliphatic dicarboxylic acids and those of malonic, succinic, glutaric, adipic, and azelaic acids. Likewise, the concentrations of glyoxal (Gly: a major compound emitted from vehicular exhaust) and glyoxylic acid (ωC 2) correlated strongly with C 4 , nss-Ca2+ (dust tracer) and elemental carbon (EC: a combustion tracer from biomass/fossil-fuels). These observations suggest gas/particle partitioning and subsequent aqueous phase oxidation of Gly to ωC 2 and then elongation (oligomerization) to succinic acid associated with the mineral dust particles. This means relative abundances of anthropogenic SO 4 2− in the dust-laden air masses govern the secondary OA formation over the Arabian Sea. [Display omitted] • Molecular distributions of diacids over Arabian Sea: oxalic > succinic > malonic. • Such molecular distribution is typical of less aged marine organic aerosols. • Glyoxal correlated with EC and succinic acid, suggesting vehicular exhaust origin. • Oxalic acid correlated with aerosol liquid water and anthropogenic SO 4 2−. • These relationships highlight the importance of aqueous SOA formation. [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]

Atmospheric 210 Pb and trace metals (Pb, Cd, Cu, Mn, Cr, Co, Ni and Zn) have been studied in fine mode aerosols (PM 2.5 ) from a sampling site (Kharagpur: 22.3°N, 87.3°E) in the Indo-Gangetic Plain (IGP) during the continental outflow (November–March) to the Bay of Bengal (BoB). The aerosol 210 Pb activity (1.3–6.6 mBq m −3 ) is significantly high in the wintertime (December–January) compared to model based data in the literature. The cause for higher 210 Pb activity is attributed to enhanced 222 Rn emanation from Alluvium in the IGP as well as lower boundary layer height. The trace metal concentrations (in ng m −3 ) also exhibit pronounced temporal variability (Pb: 8–296, Mn: 8–568, Cr: 4.5–33, Cu: 2.1–29.3, Ni: 2.3–14.3, Co: 0.5–1.6 and Cd: 1–29.5) and are of comparable magnitude with those documented over the BoB ( Srinivas and Sarin, 2013b ), suggesting dominant impact of IGP-outflow on marine atmospheric boundary layer. The enrichment factors (EF crust ) of Pb, Cd, Cu, Mn, Cr, Co and Ni in PM 2.5 , relative to upper continental crust, varied as 105–1561, 1265–24006, 13–87, 3–99, 7–27, 3–19 and 9–27, respectively. Significant linear relationship among trace metals and chemical species (non-sea-salt-K + , nss-SO 4 2− and EC) emphasizes their anthropogenic source. The high concentrations and EF crust of Pb, Cd and Cu, together with residence time of PM 2.5 (2–13 days, assessed from 210 Pb) in the IGP-outflow has implications to increase in the aerosol toxicity and their impact on biogeochemistry of ocean surface waters via air-sea deposition. [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’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 > Gal > Man;Lev/Gal: 5.5) and SEA-outflow (Lev > Man > 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]

Black carbon (BC) aerosols from incomplete combustion of biomass and fossil fuel exert asignificant influence on the climate system in South Asia, effecting in excess of 1.2 billionpeople. Models—seeking to advise mitigation policy—are constantly challenged inreproducing observations of seasonally varying BC concentrations in the densely populatedregions of South Asia e.g., over the Indo-Gangetic Plain (IGP). The systematicunderestimation of the absorption aerosol optical depth over South Asia by a factor of 2-3in climate models relative to observation-based estimates illustrates the currentlylarge uncertainties. This uncertainty may be related to several different factors,including inaccurate estimates of relative source contributions and emissions ofBC. As part of the South Asian Pollution Experiment 2016 (SAPOEX-16), we investigatedwintertime BC and its dual carbon-isotopic signature (δ13C/Δ14C) - diagnosed sources fromregional and large-scale background receptor sites — the Bangladesh Climate Observatory atBhola Island (BCOB), strategically located in the outflow region of the IGP to intercept theintegrated IGP emission signal and the remote Indian ocean-based Maldives ClimateObservatory at Hanimaadhoo Island (MCOH) to provide an integrated footprint of SouthAsia, respectively. Our results of "top-down" radiocarbon measurements of atmospheric BCfrom two regional-receptor sites show that fossil fuel combustion produces 55±3% (Δ14C:-440 ± 6 per mil) of BC emitted from South Asia. In contrast, the δ13C signalsfor BCOB-BC (∼ -27.2 ± 0.2) are more depleted relative to that of MCOH-BC(∼ −25 ± 0.2). Using a Bayesian statistical approach, we resolve the isotopicsignatures into three source classes namely biomass, coal and liquid fossil fuel.We find the liquid fossil fuel component of BC emissions is 49 ± 5% in the IGPversus the larger South Asian footprint of 23 ± 9%. Our observation is in starkcontrast to previous "Bottom-up" emission inventory-based modelling estimatessuggesting over ∼ 60% BC from biomass burning in the IGP. This finding brings outthe previously unknown sharp contrast in regional emission sources of BC fromanthropogenic combustion in South Asia. Observationally-constrained insights onBC sources allows for a more credible scientific underpinning of policy aimedat mitigating the acuteness and scale of ramifications due to BC in South Asia. [ABSTRACT FROM AUTHOR]

The article talks about pyrogenic iron as being the missing link to high iron solubility in aerosols. Topics discussed include atmospheric deposition being a source of potentially bioavailable iron (Fe) and hence being able to partially control biological productivity in ocean parts, and field data over the Southern Ocean displaying a wide range in aerosol Fe solubility.

Air-to-sea deposition of water-soluble total nitrogen (WSTN) can influence the primary productivity in the coastal oceans. Here, we assessed the concentrations of aerosol inorganic (WSIN: NH 4 + + NO 3 −) and WSTN over the Arabian Sea during winter season (SS379:6–24 December 2018). The mean concentrations of NH 4 + (109 ± 83 nmol m−3) overwhelm that of NO 3 − (32 ± 13 nmol m−3) and water-soluble organic nitrogen (WSON: WSTN-WSIN: 86 ± 81 nmol m−3), and contributing to ~50 ± 31% of WSTN mass. Significant linear relationships of WSON with water-soluble organic carbon and NH 4 + with non-sea-salt (nss)-K+ is observed suggesting their common origin from biomass burning and fertilizers. Backward air mass trajectories and satellite-based fire counts further revealed their provenance in the Indo-Gangetic Plain and southern India. The concentration of NO 3 − moderately correlated with nss Ca2+ (dust tracer), indicating heterogeneous reactive uptake on mineral aerosol surface. Despite high concentrations, the deposition fluxes of NH 4 + (~9.4 ± 7.1 μmol m−2 d−1) and WSON (7.4 ± 7.0 μmol m−2 d−1) are lower than NO 3 − (27 ± 11 μmol m−2 d−1) because of their predominant fine nature (i.e., strong correlation with nss-SO 4 2−). We also constrained the total annual atmospheric deposition rates of WSIN (0.94 Tg yr−1) and WSON (0.08 Tg yr−1) to the Arabian Sea during the continental outflow (November-April). The maximum dry-deposition of WSON to the Arabian Sea (0.24 Tg yr−1) is twice that of the riverine supply (0.11 Tg yr−1), highlighting the significance of aeolian sources. By using Redfield Stoichiometry, the WSTN deposition (21–73 μmol m−2 d−1) can account for <5.3% of fixed‑carbon by a primary production in the Arabian Sea. • High abundances of NH4+ and WSON in aerosols over Arabian Sea. • Aeolian supply of WSON is comparable to that of Riverine input. • Water soluble nitrogen in aerosols can support >3% of water column primary production. [ABSTRACT FROM AUTHOR]

We examined here the compositional characteristics of total suspended particulate (TSP) samples collected along the east coast of the Arabian Sea during a winter cruise (SS379: 6–24 December 2018) to understand the extent of influence of air pollutants from terrestrial sources. The TSP samples (N = 17) were analyzed for water-soluble inorganic species (WSIS), elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC). Backward air mass trajectories and satellite-based fire counts revealed an influence of long-range transport of biomass burning emissions in the Indo-Gangetic Plain and southern India. Anthropogenic (82–97%) SO 4 2− is the most abundant WSIS followed by Na+ and/or NH 4 +. Ambient EC loadings (0.5–2.4 μg m−3) and OC/EC ratios (2.5–10.5) are higher than usually expected for fossil-fuel combustion sources. Low mass ratios of NO 3 −/non-sea-salt SO 4 2− (0.08–0.49), as well as high ratios of nss-K+/EC (0.14–1.18) and WSOC/OC (0.30–0.74), highlight the significance of coal combustion and biomass burning emissions in the regional influx of air pollutants to the Arabian Sea. The multiple linear regression analysis has indicated strong correlations among sea salt components (Na+, Cl− and Mg2+), carbonaceous components (OC, EC and WSOC), and anthropogenic WSIS (NH 4 + and SO 4 2−). Furthermore, we used minimum R-squared technique based primary OC/EC ratio in the EC-tracer method to estimate the contribution of secondary organic carbon (SOC) to OC (7–61%) and TSP mass (0.7–11%). Overall, the composition of TSP is characterized by 21 ± 10% of particulate organic matter, 2.3 ± 1.1% of EC, 21 ± 9% of anthropogenic WSIS and 43 ± 15% of unidentified fraction including dust. This study has implications for understanding the evolving nature of secondary inorganic and organic aerosols in the continental outflows. • Chemical characterization of wintertime aerosols from the Arabian Sea. • Role of anthropogenic SO 4 2− in Cl-depletion of marine aerosols. • Large contribution of secondary organic aerosols to OC and TSP mass. • TSP contains high percentage of unidentified dust fraction followed by anthropogenic inorganic species and organic matter. [ABSTRACT FROM AUTHOR]