Roles of water-soluble aerosol coatings for the enhanced radiative absorption of black carbon over south asia and the northern indian ocean

H R C R Nair; Krishnakant Budhavant; M R Manoj; Elena N Kirillova; S K Satheesh; Örjan Gustafsson

doi:10.1016/j.scitotenv.2024.171721

Roles of water-soluble aerosol coatings for the enhanced radiative absorption of black carbon over south asia and the northern indian ocean

Sci Total Environ. 2024 May 20:926:171721. doi: 10.1016/j.scitotenv.2024.171721. Epub 2024 Mar 16.

Authors

H R C R Nair¹, Krishnakant Budhavant², M R Manoj¹, Elena N Kirillova³, S K Satheesh⁴, Örjan Gustafsson⁵

Affiliations

¹ Department of Environmental Science and the Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden.
² Maldives Climate Observatory at Hanimaadhoo, H. Dh. Hanimaadhoo, Maldives; Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India.
³ Department of Environmental Science and the Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden; Institute of Medicine, Ecology and Physical Education, Ulyanovsk State University, Ulyanovsk, Russia.
⁴ Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India; Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India; DST-Centre of Excellence in Climate Change, Indian Institute of Science, Bangalore, India.
⁵ Department of Environmental Science and the Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden. Electronic address: orjan.gustafsson@aces.su.se.

PMID: 38494028
DOI: 10.1016/j.scitotenv.2024.171721

Abstract

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 SO₄^2-/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.

Keywords: Absorption Enhancement; Atmospheric Aerosols; Black carbon; Climate warming; Mass Absorption Enhancement; Water Soluble Organic Carbon.