Tracing the biomass burning emissions over the Arabian Sea in winter season: Implications from the molecular distributions and relative abundances of sugar compounds.

The widespread haze pollution over South Asia typically occurs in winter, affecting the abundance of organic aerosols (OA) over the Arabian Sea due to prevailing meteorology. We determined the concentrations of biomass burning (BB) derived anhydrosugars (levoglucosan: Lev, galactosan: Gal, and mannosan: Man), sugars (glucose, fructose, sucrose, and trehalose) and sugar alcohols (arabitol, mannitol, erythritol, and inositol) over the Arabian Sea during a winter cruise (6-24 December 2018). Molecular distributions revealed predominance of levoglucosan or sucrose. Besides, levoglucosan strongly correlated with mannosan, galactosan, sugar alcohols and elemental carbon, emphasizing their BB-origin. Backward air mass trajectories intercepted by the satellite-based fire counts over the Indo-Gangetic Plain together with relationship between stable carbon isotopic composition of TC (δ13CTC) and levoglucosan-C to organic carbon (%), confirmed the impact of BB-derived OA. A comparison of Lev/Man (av. 16.2) and Lev/K+ (av. 0.27) ratios over the Arabian Sea with the source-emissions revealed their origin from crop-residue burning. Rather high concentrations of Lev over the Arabian Sea compared to those documented over the Bay of Bengal, East China Sea, Sea of Japan and the western North Pacific further suggests a stronger impact of BB in the continental outflow over this marine basin. Besides, Lev/K+ ratios in marine aerosols exponentially decreased with an apparent increase in ambient relative humidity and temperature over the Arabian Sea during the South Asian outflow, implying a photochemical oxidation of BBOA. Such field-based relationship of Lev with the meteorological parameters can be useful for modelling the impact of BBOA on the surface Ocean. Besides, the aeolian input of sugar-C and water-soluble organic carbon (WSOC) accounted for 83 % and 92 % of that riverine supply to the Arabian Sea, respectively. This means atmospheric dry-deposition of sugars is an important external source of dissolved organic compounds to the surface water.

Unraveling the sources of atmospheric organic aerosols over the Arabian Sea: Insights from the stable carbon and nitrogen isotopic composition.

The isotopic composition of stable carbon (δ13C) and nitrogen (δ15N) in marine aerosols influenced by the continental outflows are useful proxies for understanding the aging and secondary formation processes. Every winter, the haze pollutants transported from South Asia significantly affect the chemical composition of marine atmospheric boundary layer of the Arabian Sea. Here, we assessed the δ13C of total carbon (TC) and δ15N of total nitrogen (TN) in marine aerosols collected over the Arabian Sea during a winter cruise (6-24 December 2018). TC (2.1-13.4 µg m-3) is strongly correlated with TN (0.9-5.0 µg m-3), likely because of their common source-emissions, biomass burning and fossil-fuel combustion in the Indo-Gangetic Plain and South Asia (corroborated by backward-air mass trajectories and satellite fire counts). Besides, the linear relationship between the mass ratios of water-soluble organic carbon (WSOC) to TC (0.04-0.65) and δ13CTC (-25.1‰ to -22.9‰) underscores the importance of aging process. This means oxidation of organic aerosols during transport not only influences the WSOC levels but also affects their δ13CTC. Likewise, the prevalent inverse linear relationship between the equivalent mass ratio of (NH4+/non-sea-salt- or nss-SO42-) and δ15NTN (+15.3‰ to +25.1‰) emphasizes the overall significance of neutralization reactions between major acidic ([nss-SO42-] ≫ [NO3-]) and alkaline species (NH4+) in aerosols. Higher δ15NTN values in winter than the spring inter-monsoon clearly emphasizes the significance of the anthropogenic combustion sources (i.e., biomass burning) in the South Asian outflow. A comparison of δ13CTC and δ15NTN with the source emissions revealed that crop-residue burning emissions followed by the coal fired power plants mostly dictate the atmospheric abundance of organic aerosols in the wider South Asian outflow.

Evidence for brown carbon absorption over the Bay of Bengal during the southwest monsoon season: a possible oceanic source.

The near UV-visible light-absorbing organic carbon (OC) of ambient aerosols, referred to here as brown carbon (BrC), significantly influences the atmospheric radiative forcing on both regional and global scales. Here, we documented BrC absorption in the aqueous and methanol extracts of marine aerosols collected over the Bay of Bengal (BoB: September-October 2017) and a city, Visakhapatnam (May-June 2018), in southern India during the southwest monsoon (i.e., a transition period with weak continental impact). The absorption spectra of BrC over the BoB showed several peaks around 300-400 nm and differ from those observed over Visakhapatnam. The absorption coefficient of BrC over the BoB, unlike Visakhapatnam data, does not seem to covary with other chemical proxies of biomass burning (non-sea-salt or nss-K+) and coal combustion (nss-SO42-) in the continental outflows, suggesting a different source of BrC over the BoB. Besides, we observed higher proportions of water-insoluble organic carbon (WIOC/OC: 0.89 ± 0.02) and significant enrichment of Mg2+ over Na+ (i.e., relative to seawater) in BoB aerosols. This result and the backward air mass trajectories both hinted their major source of OC from marine-derived organic matter. In contrast, the absorption spectra of BrC over Visakhapatnam are like those from biomass burning emissions in the Indo-Gangetic Plain. This observation is further supported by the satellite-based fire counts and backward air mass trajectories. Therefore, our study underscores the BrC aerosols from the oceanic sources and southern India, hitherto unknown, and can improve our understanding of the regional climate effects of carbonaceous aerosols if included in models.