Chemical Characteristics, Size Distributions, Molecular Composition, and Brown Carbon in South Asian Outflow to the Indian Ocean

Abstract

The transport of South Asian (SA) anthropogenic haze to the northern Indian Ocean (NIO) is often observed during winter, however, its chemical and molecular constituents are not well explored. To better address this problem, shipborne measurements of size-resolved aerosol sampling were carried out over the northern and equatorial Indian Ocean (EIO) for the quantification of aerosol composition (organic and inorganic) and molecular markers during the winter of 2018. The most striking outcome was the dominance of anthropogenic aerosols derived from biomass-burning and combustion-related emissions over the NIO during the SA outflow. Low-level concentrations were noticed over the EIO where oceanic air mass was dominated. Organic carbon (OC), sea-salts (Na+ and Cl−), calcium (Ca2+), and nitrates (NO3−) were enriched in the coarse mode (>2.0 µm) while elemental carbon (EC), water-soluble OC, sulfates (SO42−), and potassium (K+) were abundant in fine mode (<2.0 µm) aerosols. Water-soluble nitrogen aerosols showed a bimodal size distribution with a significant peak in fine mode. The molecular composition of organic aerosols (OAs) in PM1.1 was characterized by high abundances of sugars and lignin/resin products followed by hydroxy/polyacids and plasticizers. These results underscore that biomass-burning emissions (BBEs), plastic combustions, and secondary anthropogenic aerosols are the leading sources of OAs in the SA outflow as further corroborated by the linear-regression analysis between organic and inorganic marker compounds. The mass absorption efficiency (MAE) of organic (brown carbon; BrC) aerosols (MAEOC-BrC-365nm) was two times higher in SA outflow and largely contributed from BBEs as confirmed by the positive relationship with levoglucosan.