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 CO₂-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.

Black carbon (BC) aerosol emissions in South Asia perturb the regional climate system and significantly degrade the air quality, affecting the health and environment of approximately 1.5 billion people. This study investigates the alterations in BC sources during the societal slowdown during the 2020 COVID-19 pandemic, capitalizing on aerosol samples of the intercepted South Asian outflow at receptor stations of the Maldives Climate Observatory at Hanimaadhoo (MCOH) and the Bangladesh Climate Observatory at Bhola (BCOB). The study used dual-carbon isotopes (Δ¹⁴C and δ¹³C) to understand the impact of societal disruptions on BC levels and sources. The isotope source fingerprinting of BC in the outflow from the Indo-Gangetic Plain (at BCOB) revealed that for the COVID period, a decreasing contribution from fossil fuel (from 49% down to 35%) amidst an increase in the fraction from C₃ biomass burning going from 31% to 55% with C₄ biomass burning remaining as a minor contributor. Similarly, for MCOH, reflecting the outflow from the greater S Asian subcontinent, the contribution from fossil combustion decreased while C₃ combustion correspondingly increased. This likely reflects both decreased transport and an increase in crop residue burning and the use of biomass for heating and cooking. These dual-isotope constraints demonstrate that a decisive shift in emissions of climate-forcing BC aerosols occurred during the pandemic slowdown, suggesting that a societal transformation away from fossil fuel reliance will quickly propagate into the aerosol composition over South Asia. 

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 m² 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.

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