Black carbon (BC) aerosols are short-lived climate pollutants with important, but uncertain, climate impacts. In this Review, we synthesize observations of atmospheric BC concentrations, sources, optical properties, lifetimes and climate effects, drawing comparisons with atmospheric model simulations. Isotopic fingerprinting reveals regional differences in BC sources, with biomass burning contributing 93 ± 3% in sub-Saharan Africa, 56 ± 7% in South Asia and 28 ± 5% in East Asia. Atmospheric BC loadings have declined in South America, East Asia, Europe and North America, and stabilized in Africa and South Asia owing to clean air policies and advances in technology and practices. The optical properties of BC influence its climate effects. The global-mean mass absorption coefficient (MAC550) of atmospheric BC is 12.3 ± 5.8 m2 g−1, being highest in Africa, Europe and South Asia. MAC550 is enhanced near universally by 1.6 ± 0.4 owing to ageing during long-range transport. In major emission regions, the aerosol absorption optical depth and the direct aerosol radiative forcing ratio between the bottom and the top of the atmosphere are lower in model simulations than in observations by factors of 2 and 1.5, respectively. Relative to long-term observations, model simulations estimate higher BC deposition fluxes but lower concentrations and sunlight absorption. These discrepancies have implications for the accuracy of model representations of humidity, clouds, precipitation and climate forcing. Future research should prioritize comparisons of emission inventory and model estimates with observations to enhance model accuracy and guide mitigation efforts. Black carbon is a short-lived climate pollutant with regionally variable sources, properties and climate effects. This Review synthesizes global observations and atmospheric model simulations, showing declining black carbon emissions owing to clean air policies and technological advances, and substantial model–observation discrepancies in absorption, deposition and radiative forcing estimates.

Abstract. Microplastics (MPs) are increasingly detected in marine and atmospheric particulate matter, but their sources, transport, and air–sea exchange remain poorly constrained in remote marine regions. We investigated airborne microplastics (AMPs) and marine microplastics (MMPs) during a 1-month campaign at the Maldives Climate Observatory Hanimaadhoo and in adjacent coastal waters. Airborne particles were collected with a deposition box, and seawater was sampled at coastal sites. Particle morphology, size, and polymer composition were determined by optical microscopy and µ-Raman spectroscopy and evaluated alongside aerosol scattering and equivalent black carbon (eBC). AMP concentrations ranged from 0 to 10.3 MP m-3 (mean: 3.1 ± 2.8 MP m-3). MMP concentrations ranged from 120 to 2656 MP m-3 (mean: 876 ± 804 MP m-3), exceeding typical open-ocean values and indicating substantial local influence from inhabited islands and maritime activity. Significant differences in size distributions and pronounced compositional differences of AMPs and MMPs suggest weak coupling between the atmospheric and marine reservoirs. Low dry-season wind speeds probably suppressed wave breaking, bubble bursting, and sea-spray-mediated transfer from ocean to atmosphere. Instead, AMP variability coincided with polluted air-mass advection, elevated eBC, and enhanced aerosol scattering, indicating stronger control by regional atmospheric transport than by local marine emissions. These results show that remote island atmospheres are exposed to anthropogenic plastic aerosol transport and demonstrate the value of combining MP observations with established aerosol tracers to resolve plastic-particle pathways in marine environments.

Sulfate aerosols cool the climate and thus temporarily mask climate warming, but at a cost to air quality. Their short atmospheric lifetime leads to heterogeneous global coverage, with sulfate concentrations over South Asia being especially elevated and continuing to increase. It remains challenging to constrain the relative importance of different emission sources due to poor observational coverage and uncertainties in bottom-up technology-based emission estimates. The stable sulfur isotope composition (delta 34S-SO42-) quantitatively distinguishes natural and anthropogenic sources. This study aimed to constrain the sources of sulfate arriving at the Maldives Climate Observatory Hanimaadhoo (MCOH), which is ideally situated for intercepting the outflow from airsheds over the Indian subcontinent. The results show that anthropogenic sources of sulfate contributed 93 +/- 14 %, 87 +/- 10 %, and 66 +/- 12 % in winter (post-monsoon), spring (pre-monsoon), and summer (monsoon), respectively. There was also a moderate to strong correlation (r2= 0.75, p << 0.05, n=7) between continental anthropogenic (winter and spring) sulfate (delta 34S) and black carbon aerosols from fossil fuel combustion (pinpointed by Delta 14C). This study provides improved constraints on sulfate sources for South Asia - a key region for aerosol pollution and aerosol masking of climate warming.

Carbon monoxide (CO) is an indirect short-lived climate forcer with uncertainties both in sources and its role in atmospheric oxidation. Based on nine winter-long dual-isotope campaigns at two South Asian receptor sites intercepting the continental outflow, we quantified CO source contributions and emission–sink dynamics. Combustion accounts for 68–74% of South Asia regional CO (including 34–37% from biomass burning) with secondary atmospheric oxidation contributing 26–32% (dominated by oxidation of non-methane volatile organic compounds NMVOCs at 21–26% with methane oxidation contributing 5.5–6.4%). These isotope-observational constraints suggest a twice higher role for atmospheric oxidation than in model estimates. Spatially, the absolute contributions of both primary and secondary CO decrease from the Indo-Gangetic Plain (IGP) to the northern Indian Ocean, indicating enhanced oxidation near source regions, while the relative contribution of secondary CO increases. Observation-model comparison suggests that continental transport dominates CO over adjacent oceanic regions, while local production is minor. During the COVID-19 pandemic, combustion-derived CO fell sharply, NMVOC-derived CO rose, and CH4-derived CO remained stable, suggesting enhanced oxidation from reduced competition among precursors. Our results reveal a far greater contribution of CO from atmospheric oxidation in South Asia than in current model estimates, highlighting the need for sustained emission controls to deliver concurrent climate and health benefits.

Abstract. Each year during the winter period, a persistent haze forms over the Indian subcontinent and the northern Indian Ocean. This has been shown to influence regional warming, rainfall patterns and air quality. Previous studies have demonstrated that this haze is largely anthropogenic in origin, with its composition dominated by sulfate, organic compounds and black carbon. Nevertheless, to date, information about its composition has largely been limited to bulk chemical composition and low time-resolution data. Here, we aim to characterise aerosol composition over the Indian Ocean on the molecular level, in order to identify the impact of different sources and processing in this region. High-time-resolution measurements were conducted at the Maldives Climate Observatory, Hanimaadhoo (MCOH) using a Time-of-Flight Aerosol Mass Spectrometer (AMS) and Chemical Ionisation Mass Spectrometer with a Filter Inlet for Gases and Aerosols (FIGAERO-CIMS). Results showed a remarkably uniform composition, despite variability in source regions and total concentration, indicating strong regional mixing of air masses. Sulfate accounted for approximately 52 % of non-refractory sub-micron particulate mass. Eighteen sulfur-containing organic compounds were identified, some for the first time in this location. Tracers of some sources, particularly biomass burning, were identified in the organic mass spectrum. However, the majority of organic mass was dominated by highly-processed compounds such as dicarboxylic acids. Our results underscore the impact of long-range transport and heterogeneous sulfate-driven chemistry on aerosol composition over the Indian Ocean, with important implications for understanding radiative forcing, aerosol-cloud interactions, and regional climate feedbacks in South Asia.

Sulfate aerosols are short lived climate forcers that cool the climate, but at the cost of human health and the environment. Their short lifetime leads to an unequal global distribution, with massive emissions in South Asia, resulting in some of the highest atmospheric loadings. These emissions originate from natural and anthropogenic sources, with their relative contributions uncertain, due to emissions being short lived and diffuse. However, the stable isotopic composition (δ34S), holds some promise of improved apportionment of sulfate sources. The aim was to leverage this isotopic composition to distinguish sources of sulfate aerosols intercepted at the Maldives Climate Observatory Hanimaadhoo (MCOH). This site is strategically located to intercept a wide footprint of the outflow from South Asia.The results demonstrated that non-sea salt sulfate was largely of anthropogenic origin, contributing 93±21%, 85±14%, 61±20% in winter, spring, and summer, respectively. This study also found a moderate to strong correlation (r2 = 0.68) between continental anthropogenic (winter and spring) sulfate (δ34S) and fossil fuel black carbon (δ13C, Δ14C). This study provides improved constraints on sulfate sources in South Asia using stable δ34S isotopic analysis, which builds a foundation for future investigations aimed at unravelling the nexus of sulfate emissions in South Asia.

Black carbon (BC) aerosols perturb the climate and affect air quality/human health. In the highly populated and heavily polluted South Asian region, the wintertime modeled atmospheric abundance of BC has remained underestimated relative to surface observations. We hypothesize this is linked to underestimated (i) atmospheric lifetime (tau BC) and/or (ii) regional emission fluxes of BC. To address this hypothesis, we developed a novel inversion framework combining multiwinter (2018-2020) hourly resolved BC and carbon monoxide (CO) measurements from a wide footprint site in the North Indian Ocean, intercepting wintertime South Asian outflow. The average Delta BC/Delta CO ratio in this continental outflow of 14 +/- 5 ng m-3 ppb-1 was 2-3 times higher than in East Asian outflow and shows a profound regional wintertime presence of BC. The empirically derived tau BC of 8 +/- 0.5 days was higher than global-mean tau BC of 5.5 days employed in climate models and suggests greater regional longevity of wintertime BC. The Delta BC/Delta CO inversion-estimated 'top-down' BC emission flux of similar to 200 Gg/month was in fact higher by a factor of similar to 1.5 than wintertime monthly BC emission flux from scaled 'bottom-up' emission inventory (similar to 125 Gg/month). Taken together, assimilating higher BC emissions with greater longevity seems promising to reconcile the model-observation offset of wintertime BC abundance for South Asia.

ABSTRACT In South Asia, our understanding of atmospheric aerosols and their optical properties is limited, posing a challenge to comprehending climate change dynamics. This study characterises aerosol optical properties, radiative properties, black carbon (BC) and ozone (O 3 ) at seven South Asian locations, including Nam Co (Tibetan Plateau, TP), Dhaka, Bhola (Bangladesh), and Hanimaadhoo, Kashidhoo, Male' and Gan (Maldives). The study utilises columnar aerosol data from the Aerosol Robotic Network (AERONET) and reanalysis data from Modern‐Era Retrospective Analysis for Research and Applications (MERRA‐2) from 2001 to 2020. Notably, during the winter, the highest Aerosol optical depth (AOD) levels were observed in Dhaka (1.0 ± 0.5) and Bhola (0.8 ± 0.4) among these seven locations. BC concentrations in Dhaka ranged from 2.1 to 2.8 μg m −3 , while Bhola recorded concentrations between 1.4 and 2.1 μg m −3 . O 3 levels across Maldives sites remained consistent, with values ranging between 314 and 345 dobson units (DU), surpassing those in Bangladesh and TP. The analysis shows a significant difference in the rate at which the atmosphere heats (HR) up due to aerosols. Higher heating rates were observed over Kashidhoo during the post‐monsoon and winter seasons, while lower values were seen during the pre‐monsoon and monsoon seasons, compared with Hanimaadhoo and Male'. It is important to note that Bangladesh had higher HR values than the Maldives. This study helps us better understand the impact of atmospheric aerosols on South Asia's climate and the different seasonal patterns.

Black Carbon (BC), formed by incomplete combustion, absorbs solar radiation and heats the atmosphere. We investigated the enhancement in optical absorption of BC due to coatings of water-soluble (WS) species in the polluted South Asian atmosphere. The BC Mass Absorption Cross-section (MAC; 678 nm) was estimated before and after removal of the WS components. Wintertime samples were collected from three South Asian receptor observatories intercepting large-footprint outflow: Bangladesh Climate Observatory Bhola (BCOB; integrating outflow of the Indo-Gangetic Plain), Maldives Climate Observatories at Hanimaadhoo (MCOH) and at Gan (MCOG), both reflecting outflow from the South Asian region. The ambient MAC observed at BCOB, MCOH and MCOG were 4.2 +/- 1.4, 7.9 +/- 1.9 and 7.1 +/- 1.5 m(2) g(-1), respectively. The average enhancement of the BC MAC due to WS coatings (i.e., ws-E-MAC) was identical at all three sites (1.6 +/- 0.5) indicating that the anthropogenic aerosols had already evolved to a fully coated morphology at BCOB and/or that subsequent aging involved two compensating evolution processes of the coating. Inspecting the key coating component sulfate; the sulfate-to-BC ratio increased threefold when transitioning from BCOB to MCOH and by about 1.5 times from BCOB to MCOG. Conversely, both WS organic carbon (WSOC)/BC and water-insoluble OC (WIOC)/BC ratios declined with distance: WSOC/BC diminished by 84 % from BCOB to MCOH and by 80 % from BCOB to MCOG, while WIOC/BC dropped by about 63 % and 59 %, respectively. Such declines in WSOC and WIOC reflect a combination of photochemical oxidation and more efficient washout of OC compared to BC. The observed changes in the SO42-/BC and WSOC/BC ratios across South Asia highlight the significant impact of aerosol composition on the optical properties of Black Carbon (BC). These findings emphasize the need for detailed studies on aerosol composition to improve climate models and develop effective strategies for reducing the impact of anthropogenic aerosols on the climate.

Rainwater samples collected at Sinhagad on 65 rain occasions during monsoon season (June – September 2005) and on 23 rain occasions during post-monsoon season (October-November 2005) with standard rain collection instruments, i.e. wet-only (WO) and bulk collectors (BC), were considered for the present study. Sinhagad is a hill station on a mountaintop in the Western Ghats, located about 40 km southwest of Pune. The following ionic components were determined: H+, NH4+, Ca2+, Mg2+, K+, Na+, SO42−, NO32−, Cl− and F−. The pH analyses showed that rainwater in both the seasons were alkaline. The major neutralizing component was associated with Ca2+. In monsoon rains, the major anion was Cl− and the major cation was Na+; whereas in post-monsoon the major anion was SO42− and the major cation was Ca2+. The concentrations of nss SO42−, NO32− and NH4+ were found to be higher during post-monsoon than monsoon. Surprisingly high concentrations of Ca2+ and SO42− were found during the monsoon season. Since no large upwind sources of these compounds are expected, one is led to conclude that long-range transport may be involved, possibly even including African sources. Since the local sources didn’t seem to influence the samples significantly, this rural site is useful for obtaining regionally representative precipitation data.

Wet deposition (WD) and Dry deposition (DD) samples were collected during a period of 4 year (2006 to 2009), at four different sites representing different surroundings around Pune city in southwest India. The samples were collected on a daily basis for WD and weekly basis for DD. These samples were analyzed for major ionic components e.g., Cl−, NO3−, SO42−, Na+, K+, NH4+, Ca2+ and Mg2+. Both the WD and DD were alkaline (pH > 5.6) at all the four sites. The WD fluxes of all the ionic components were higher than the DD fluxes, except at the traffic junction Swargate, where majority of the species appeared with much higher DD fluxes than WD fluxes (68

Snow samples and aerosol samples were collected at coastal Antarctica near Larsemann Hills and Maitri, during the 29th Indian Antarctic Expedition carried out during Dec., 2009 to March 2010. The main objective of this study was to characterize the chemical composition of fresh and surface snow at coastal Antarctica and to determine the scavenging ratios using composition of snow and aerosol samples. The pH of surface and fresh snow were 6.03 and 5.64 respectively. The surface snow samples were collected along a 127-km transect from the seaward edge of the ice shelf to the Antarctic plateau and analyzed for the presence of the major inorganic components SO42-, NO3-, Cl-, NH4+, Na+, K+, Ca2+ and Mg2+. It was observed that Na+ and Cl- were the most abundantly occurring ions at Antarctica. Considerable amount of SO42- was also found in the both fresh and surface snow which may be attributed to the long range transport from Northern Hemisphere as well as to the oxidation of DMS produced by marine phytoplankton. A higher percentage of the ions in fresh snow may be because of trapping of the particulate matter in it. The sea-salt components i.e., Na+, Cl- and Mg2+ decreased with increasing distance from the coast. The acidic components were neutralized mainly by NH4+ and Ca2+. The scavenging ratio was maximum for Na+ and minimum for NO3-, indicating that the scavenging efficiency was higher for coarse size particles and lower for fine size particles. In addition, we have attempted to find out the possible sources of the observed chemical species in snow-water.

Atmospheric aerosols strongly influence the global climate through their light absorption properties (e.g., black carbon (BC) and brown carbon (BrC)) and scattering properties (e.g., sulfate). This study presents simultaneous measurements of ambient-aerosol light absorption properties and chemical composition obtained at three large-footprint southern Asian receptor sites during the South Asian Pollution Experiment (SAPOEX) from December 2017 to March 2018. The BC mass absorption cross section (BC-MAC678) values increased from 3.5 +/- 1.3 at the Bangladesh Climate Observatory at Bhola (BCOB), located at the exit outflow of the Indo-Gangetic Plain, to 6.4 +/- 1.3 at two regional receptor observatories, the Maldives Climate Observatory at Hanimaadhoo (MCOH) and the Maldives Climate Observatory at Gan (MCOG), representing an increase of 80 %. This likely reflects a scavenging fractionation, resulting in a population of finer BC with higher MAC678 that has greater longevity. At the same time, BrC-MAC365 decreased by a factor of 3 from the Indo-Gangetic Plain (IGP) exit to the equatorial Indian Ocean, likely due to photochemical bleaching of organic chromophores. The high chlorine-to-sodium ratio at the BCOB, located near the source region, suggests a significant contribution of chorine from anthropogenic activities. Particulate Cl- has the potential to be converted into Cl radicals, which can affect the oxidation capacity of polluted air. Moreover, Cl- is shown to be nearly fully consumed during long-range transport. The results of this synoptic study, conducted on a large southern Asian scale, provide rare observational constraints on the optical properties of ambient BC (and BrC) aerosols over regional scales, away from emission sources. They also contribute significantly to understanding the aging effect of the optical and chemical properties of aerosols as pollution from the Indo-Gangetic Plain disperses over the tropical ocean.

Anthropogenic aerosols mask the climate warming caused by greenhouse gases (GHGs). In the absence of observational constraints, large uncertainties plague the estimates of this masking effect. Here we used the abrupt reduction in anthropogenic emissions observed during the COVID-19 societal slow-down to characterize the aerosol masking effect over South Asia. During this period, the aerosol loading decreased substantially and our observations reveal that the magnitude of this aerosol demasking corresponds to nearly three-fourths of the CO 2 -induced radiative forcing over South Asia. Concurrent measurements over the northern Indian Ocean unveiled a ~7% increase in the earth’s surface-reaching solar radiation (surface brightening). Aerosol-induced atmospheric solar heating decreased by ~0.4 K d −1 . Our results reveal that under clear sky conditions, anthropogenic emissions over South Asia lead to nearly 1.4 W m −2 heating at the top of the atmosphere during the period March–May. A complete phase-out of today’s fossil fuel combustion to zero-emission renewables would result in rapid aerosol demasking, while the GHGs linger on.

Effects of aerosols such as black carbon (BC) on climate and buildup of the monsoon over the Indian Ocean are insufficiently quantified. Uncertain contributions from various natural and anthropogenic sources impede our understanding. Here, we use observations over 5 y of BC and its isotopes at a remote island observatory in northern Indian Ocean to constrain loadings and sources during little-studied monsoon season. Carbon-14 data show a highly variable yet largely fossil (65 ± 15%) source mixture. Combining carbon-14 with carbon-13 reveals the impact of African savanna burning, which occasionally approach 50% (48 ± 9%) of the total BC loadings. The BC mass-absorption cross-section for this regime is 7.6 ± 2.6 m2/g, with higher values during savanna fire input. Taken together, the combustion sources, longevity, and optical properties of BC aerosols over summertime Indian Ocean are different than the more-studied winter aerosol, with implications for chemical transport and climate model simulations of the Indian monsoon.

The MODerate Resolution Imaging Spectroradiometer (MODIS) retrieved (2009–2015) aerosol and cloud products over South-Central India, including adjoining coastal areas, were quantitatively analyzed to explore aerosol–cloud interaction and to estimate aerosol indirect effect (AIE). The spatial distribution of aerosol optical depth (AOD) showed high AOD550 nm (~ 0.7) over the northern India and is attributed to the long- and short-range transport of desert dust aerosols, dense population, and industrialization. The mean seasonal AOD550 nm over region decreases from (0.55 ± 0.11) to (0.45 ± 0.05) and to (0.39 ± 0.04) for monsoon, post-monsoon, and winter seasons, respectively. The western Indian regions, particularly Pune and Jaipur, showed higher mean AOD500 nm as compared to other cities. The positive correlation of aerosol indirect effect (AIE) with AOD for Pune and Pondicherry through pre- as well post- monsoon seasons for all cities except Pondicherry is ascribed to hygroscopic aerosol particulate growth. The aerosol size spectra undergo significant transformation from dominant high accumulation-mode during September–February months [with Angström exponent (AE) > 1.0] to dominant augmented coarse-mode (AE < 1.0) through March to June–July period. The AOD500 nm-to-cloud fraction (CF) correlation coefficients range between 0.05 and 0.46 for coastal region, while for Jaipur, Pune, and overall study region, the CFs are 0.53, 0.65, and 0.75, respectively. The pre-monsoon months recorded lower (0.4–0.6 µm) cloud effective radii values than monsoon season (0.8–1.3 µm). The AOD-to-cloud liquid water path correlations for Goa and Pune (0.30–0.48) are higher than other cities (0.01–0.19). The AIE average values for metropolitan, semi-arid, coastal, high-altitude sites, and overall study region were found to be − 0.168, − 0.025, − 0.104, − 0.101, and − 0.128, respectively. There occurs a noticeable negative AIE for three categories (viz., Metropolitan, coastal, and high-altitude station), while for semi-arid category, there exists a prominent Twomey (positive AIE) effect.

Fine particulate-matter is an important component of air pollution that impacts health and climate, and which delivers anthropogenic contaminants to remote global regions. The complex composition of organic molecules in atmospheric particulates is poorly constrained, but has important implications for understanding pollutant sources, climate-aerosol interactions, and health risks of air pollution exposure. Here, comprehensive nontarget high-resolution mass spectrometry was combined with in silico structural prediction to achieve greater molecular-level insight for fine particulate samples ( n = 40) collected at a remote receptor site in the Maldives during January to April 2018. Spectral database matching identified 0.5% of 60,030 molecular features observed, while a conservative computational workflow enabled structural annotation of 17% of organic structures among the remaining molecular dark matter. Compared to clean air from the southern Indian Ocean, molecular structures from highly-polluted regions were dominated by organic nitrogen compounds, many with computed physicochemical properties of high toxicological and climate relevance. We conclude that combining nontarget analysis with computational mass spectrometry can advance molecular-level understanding of the sources and impacts of polluted air.