Climate-warming brown carbon aerosols lose warming capacity during atmospheric transport.

Black carbon (BC) aerosols perturb climate and impoverish air quality/human health—affecting ∼1.5 billion people in South Asia. However, the lack of source-diagnostic observations of BC is hindering the evaluation of uncertain bottom-up emission inventories (EIs) and thereby also models/policies. Here, we present dual-isotope-based (Δ 14 C/δ 13 C) fingerprinting of wintertime BC at two receptor sites of the continental outflow. Our results show a remarkable similarity in contributions of biomass and fossil combustion, both from the site capturing the highly populated highly polluted Indo-Gangetic Plain footprint (IGP; Δ 14 C- f biomass = 50 ± 3%) and the second site in the N. Indian Ocean representing a wider South Asian footprint (52 ± 6%). Yet, both sites reflect distinct δ 13 C-fingerprints, indicating a distinguishable contribution of C 4 -biomass burning from peninsular India (PI). Tailored-model-predicted season-averaged BC concentrations (700 ± 440 ng m –3 ) match observations (740 ± 250 ng m –3 ), however, unveiling a systematically increasing model-observation bias (+19% to −53%) through winter. Inclusion of BC from open burning alone does not reconcile predictions ( f biomass = 44 ± 8%) with observations. Direct source-segregated comparison reveals regional offsets in anthropogenic emission fluxes in EIs, overestimated fossil-BC in the IGP, and underestimated biomass-BC in PI, which contributes to the model-observation bias. This ground-truthing pinpoints uncertainties in BC emission sources, which benefit both climate/air-quality modeling and mitigation policies in South Asia.

Sulfate aerosols exert a net cooling effect on the earth-atmosphere system, yet their radiative forcing remains associated with largest of uncertainties in the assessment of climate change. One of the contributing factors is the poor understanding of the sulfate formation pathways, which are thought to be following mostly the mass-dependent fractionation model (i.e., Δ 33 S ~ 0). However, globally, urban sulfate aerosols exhibit significant non-zero Δ 33 S compositions (from -0.6‰ to +0.6‰), resulting in sulfur mass-independent fractionation (S-MIF) processes. The origin(s) of these S-MIF anomalies remain(s) unclear. Here, we conducted dual-isotope (Δ 33 S, δ 34 S) probing of sulfate aerosols from summertime megacity Delhi in South Asia. A shift towards concomitantly high Δ 33 S (from +0.2‰ to +0.5‰) and low δ 34 S (from +5‰ to +1‰) values was observed with the influx of mineral dust. The Fe-to-Al tracer showed significant correlations with sulfate loadings (R 2 =0.84) and Δ 33 S signatures (R 2 =0.77). As such, we postulate that the SO2 photo-oxidation on mineral dust generates S-MIF anomaly ~ +0.35±0.10‰, thereby also explaining the previously observed Δ 33 S values worldwide. Together, the findings help deconvolute S-isotope dynamics in urban regions wherein, contrary to prevailing paradigm, non-anthropogenic factor (i.e., mineral dust) is found to influence the aerosol sulfate-induced pollution affecting air quality/human health. SYNOPSISSulfate-related pollution in urban regions-affecting air quality/human health-could be linked to non-anthropogenic factor i.e., mineral dust.

Changes in the cosmic-ray background of the Earth can impact the ozone layer. High-energy cosmic events (e.g., Supernova, SN) or rapid changes in the Earth's magnetic field (e.g., Geomagnetic Excursion, GE) can lead to a cascade of cosmic rays. Ensuing chemical reactions can then cause thinning/destruction of the ozone layer — leading to enhanced penetration of harmful UV radiation towards the Earth's surface. However, observational evidence for such UV ‘windows’ is still lacking. Here, we conduct a pilot study and investigate this notion during two well-known events: the multiple SN event (≈10 kBP) and the Laschamp GE event (≈41 kBP). We hypothesize that ice-core-Δ33S records—originally used as volcanic fingerprints—can reveal UV-induced background-tropospheric- photochemical imprints during such events. Indeed, we find non-volcanic S-isotopic anomalies (Δ33S≠0 ‰) in background Antarctic-ice-core sulfate during GE/SN periods, thereby confirming our hypothesis. This suggests that ice-core-Δ33S records can serve as a proxy for past ozone-layer-depletion events.

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.

South Asian air is among the most polluted in the world, causing premature death of millions and asserting a strong perturbation of the regional climate. A central component is carbon monoxide (CO), which is a key modulator of the oxidizing capacity of the atmosphere and a potent indirect greenhouse gas. While CO concentrations are declining elsewhere, South Asia exhibits an increasing trend for unresolved reasons. In this paper, we use dual-isotope (δ 13 C and δ 18 O) fingerprinting of CO intercepted in the South Asian outflow to constrain the relative contributions from primary and secondary CO sources. Results show that combustion-derived primary sources dominate the wintertime continental CO fingerprint ( f primary ∼ 79 ± 4%), significantly higher than the global estimate ( f primary ∼ 55 ± 5%). Satellite-based inventory estimates match isotope-constrained f primary -CO, suggesting observational convergence in source characterization and a prospect for model–observation reconciliation. This “ground-truthing” emphasizes the pressing need to mitigate incomplete combustion activities for climate/air quality benefits in South Asia.

Aerosol emissions in South Asia are large. The emitted aerosols can travel significant distances and, during the Asian southwest monsoon especially, are prone to modification through cloud processing and wet scavenging while being transported. The scale of emissions and transport means that the global climate impact of these aerosols are sensitive to modification en route, but the process-level understanding is still largely lacking. In this study, we analyse long-term aerosol data measured at an observatory established in Hanimaadhoo, Republic of Maldives, to investigate the long-term properties of aerosols over the Indian Ocean as well as to understand the effect of precipitation on the aerosol particle size distribution during long-range transport. The observatory location is ideal because it is a receptor site with little local influence, and, depending on the season, receives either polluted air masses coming from the Indian subcontinent or clean marine air masses from the Indian Ocean. We analysed the sub-micron particle number size distribution measured during the years 2004-2008, and 2014-2017, and this is the first inter-seasonal longterm study of the sub-micron aerosol features in the region. The aerosol origin and its relative exposure to wet scavenging during long-range transport were analysed using back-trajectory analysis from HYSPLIT. By comparing aerosol measurements to precipitation along its transport, this study shows that there is a substantial change in particle number size distributions and concentrations depending on the amount of rainfall during transport. During the southwest monsoon season, the aerosol size distribution was notably bimodal and total particle concentrations clearly reduced in comparison with the prevailing aerosol size distribution during the northeast monsoon season. Precipitation during transport usually corresponded with a greater reduction in accumulation mode concentrations than for smaller sizes, and the shape of the median size distribution showed a clear dependence on the trajectory origin and route taken.

The simultaneous monitoring of the triple stable S-isotopes (32S, 33S and 34S) of hydrogen sulfide has been conducted with a VCOF-CRDS set-up (V-shaped cavity for optical feedback coupled to the...