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 (τBC) 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 ΔBC/ΔCO 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 τBC of 8 ± 0.5 days was higher than global-mean τBC of 5.5 days employed in climate models and suggests greater regional longevity of wintertime BC. The ΔBC/ΔCO inversion-estimated ‘top-down’ BC emission flux of ∼200 Gg/month was in fact higher by a factor of ∼1.5 than wintertime monthly BC emission flux from scaled ‘bottom-up’ emission inventory (∼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.

A combination of high-resolution mass spectrometry and computational molecular characterization techniques can structurally annotate up to 17% of organic compounds in fine particulate matter in highly polluted air sampled in the Maldives. 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 (δ¹³C and δ¹⁸O) 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. 

The highly populated South Asian region is facing rapid economic growth and urbanization. Here, both climate- and health-affecting atmospheric agents such as light-absorbing aerosols black carbon (BC) and brown carbon (BrC), trace gas carbon monoxide (CO), are often found in relatively high levels compared to in other regions. However, atmospheric chemistry-transport/climate models are unable to fully capture the extent of the abundance of BC, CO, and BrC in the regional atmosphere during winter. The Thesis aims to address potentially important uncertainties that may be contributing to the model-observation offset — uncertainties in the ambient optical properties of BrC, uncertainties in the relative source contributions of BC (biomass burning vs. fossil fuel combustion) and CO (direct emission-derived vs. atmospheric chemical oxidation-derived), uncertainties in the regional lifetime and absolute emission fluxes of BC. For the Thesis work, field sampling was conducted at three sites, megacity Delhi (key source region), the Bangladesh Climate Observatory–Bhola Island (BCOB; receptor site for the highly-polluted Indo-Gangetic Plain) and the Maldives Climate Observatory–Hanimaadhoo Island (MCOH; receptor site for wider South Asia).

The light-absorptivity of water-soluble BrC is found to decrease by ~84% during transport of haze from source-to-receptor regions i.e., Delhi-to-BCOB-to-MCOH — much greater than estimated in chamber studies and accounted in models. Atmospheric photochemical oxidation is found to be a likely driver for the loss of water-soluble BrC light-absorption in the S Asian outflow (with an estimated bleaching rate of 0.20±0.05 day⁻¹) (Paper I). Radiocarbon (Δ¹⁴C)-based source apportionment of BC aerosols shows a stark similarity in the relative contributions of fossil (~50%) and biomass sources (~50%) at BCOB as well as at MCOH, suggesting a regional homogeneity in BC source contributions. However, a distinct stable isotopic fingerprint (δ¹³C) of BC in the N Indian Ocean is found to be arising from a small yet significant contribution (upto 10%) from C₄-biomass burning in peninsular India (region south of 23.4°N) (Paper II). Comparison of source-segregated observed and emission inventory-driven modeled BC concentrations indicates regional offsets in the anthropogenic emission fluxes of BC in emission inventories—overestimated fossil-BC in the Indo-Gangetic Plain and underestimated biomass-BC in peninsular India (Paper II). Dual-isotope (δ¹³C, δ¹⁸O)-based source apportionment of CO shows a significantly large contribution (~80%) from direct emissions of primary sources (biomass burning and fossil fuel combustion) in South Asia, in contrast to modeled CO budget (Paper III). The BC-to-CO ratio in South Asia is found to be higher, by a factor of 2-3, than in other polluted regions such as in East Asia during winter. The regional lifetime and emission flux of BC are estimated to be 8±0.5 days (higher than values used in models) and ~2.4±1 Tg/yr (significantly higher than estimated in current emission inventories), respectively (Paper IV).

Taken together, for convergence between models and observations in wintertime South Asia, i) the ‘dynamic’ nature of BrC light-absorption should be considered in models, ii) improvements in emission information of BC and CO are needed for better-simulating concentrations. Controls on activities such as open burning (such as agricultural crop residue burning, domestic burning of wood and dung as fuel) in South Asia could enable a reduction in BC, CO, and BrC, thereby leading to improved air quality and paving the way for achieving some of the key sustainable development goals outlined by the United Nations.

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 long-term 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.

Black carbon (BC) aerosols perturb climate and impoverish air quality/ human health-affecting similar to 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 (Delta C-14/delta C-13) 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; Delta C-14-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 delta C-13-fingerprints, indicating a distinguishable contribution of C-4-biomass burning from peninsular India (PI). Tailored-model-predicted seasonaveraged 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.

Light-absorbing organic aerosols, known as brown carbon (BrC), counteract the overall cooling effect of aerosols on Earth's climate. The spatial and temporal dynamics of their light-absorbing properties are poorly constrained and unaccounted for in climate models, because of limited ambient observations. We combine carbon isotope forensics (delta C-13) with measurements of light absorption in a conceptual aging model to constrain the loss of light absorptivity (i.e., bleaching) of water-soluble BrC (WS-BrC) aerosols in one of the world's largest BrC emission regions-South Asia. On this regional scale, we find that atmospheric photochemical oxidation reduces the light absorption of WS-BrC by similar to 84% during transport over 6000 km in the Indo-Gangetic Plain, with an ambient first-order bleaching rate of 0.20 +/- 0.05 day(-1) during over-ocean transit across Bay of Bengal to an Indian Ocean receptor site. This study facilitates dynamic parameterization of WS-BrC absorption properties, thereby constraining BrC climate impact over South Asia.

Black carbon (BC) aerosols cause climate perturbation and affect air quality/human health. In densely populated South Asia—where the warming effect of BC is estimated to be ~ orders of magnitude higher than the global average—model simulations underestimate the wintertime atmospheric abundance of BC relative to surface observations. Two possible explanations for this model-observation offset are ill-constrained regional emission fluxes and atmospheric BC lifetime  (τ_BC). Here, we combine hourly-resolved BC and carbon monoxide (CO) measurements for three successive winters from a South Asian receptor site—the Maldives Climate Observatory at Hanimaadhoo (MCOH)—in inverse frameworks, to assess the impact of emissions fluxes and lifetimes on ambient BC concentrations. The average ΔBC/ΔCO ratio (background corrected) at MCOH of 14±5 ng m^-3 ppb^-1 is found to be 2-3 times higher than in the East Asian outflow (range: 2 to 8 ng m^-3 ppb^-1). A BC transport efficiency of ~86% suggests low influence of wet scavenging processes during the dry South Asian winter period. Using statistical time series analysis, the τ_BC for dry wintertime South Asia is estimated to be 8±0.5 days, which is higher than commonly used in models. By coupling air mass back trajectories, lifetimes, and the ΔBC/ΔCO time-series data within an inverse modelling framework, we provide an observation-based “top-down” BC emission flux for South Asia estimated to be ~2.4±1 Tg/year. This is significantly higher than estimates from current “bottom-up” emission inventories (EIs). Taken together, this study suggests  that to reconcile long-standing BC model-observation offsets in South Asia, existing emission estimates may need to be more than doubled in magnitude.