Light absorption characteristics of brown carbon over the Bay of Bengal: impact of Indo-Pacific trade route

Abstract Atmospheric aerosols play a vital role in offsetting warming caused by greenhouse gases. However, the light-absorbing fractions of carbonaceous aerosols, such as black carbon (BC) and brown carbon (BrC), introduce significant uncertainties into climate models. A significant reason for this uncertainty is that the properties of carbonaceous aerosols are poorly constrained by existing observations. This lack of clarity needs to be addressed to understand their accurate and precise impact on climate and air quality. This study investigated the mass and absorption characteristics of various carbonaceous aerosol species using in-situ observations with a multi-wavelength Aethalometer during a cruise expedition across the Bay of Bengal transect from 2° N to 18° N in April–May 2024. The results showed increased BC mass at higher (northern) latitudes. Air mass back trajectory analysis indicated that the elevated BC levels in the northern latitudes were largely due to long-range transport of carbonaceous aerosols from the Indian subcontinent. Interestingly, the contribution of BrC was relatively large (17%) at southern latitudes compared to the northern latitudes (10%), where air masses originated from remote oceanic regions. Evidence showed a strong positive correlation between NO 2 emissions and BrC absorption at lower (southern) latitudes in proximity to the Indo-Pacific trade route, suggesting that NO x emissions from ship traffic may enhance the production of nitrogen-containing BrC chromophores, which are more light-absorbing.