On the evolution of pollution from South and Southeast Asia during the winter‐spring monsoon

Abstract

The NOAA Geophysical Fluid Dynamics Laboratory three‐dimensional Global Chemical Transport Model (GFDL GCTM) is used to examine the winter‐spring evolution of pollution (fossil fuel combustion and biomass burning) from South and Southeast Asia with special focus on the Indian Ocean region. We find that during the monsoonal winter‐spring outflow, pollution over the Indian Ocean north of the ITCZ is concentrated in the maritime boundary layer and originates from both regions. South Asian emissions dominate over the Arabian Sea and the Western Indian Ocean, while the Southeast Asian emissions have the greatest impact over the Bay of Bengal and Eastern Indian Ocean. Over these oceanic regions, CO pollution in both source regions, most of which is from biomass burning, accounts for 30–50% of the boundary layer CO. It is transported equatorward from South and Southeast Asian source regions and episodically lofted into the upper troposphere by tropical convection events. This transport path has a noticable impact (10–20%) on total CO at 300 mb and produces a maximum in a tropical belt over and north of the ITCZ. Another free troposphere transport path, primarily open to Southeast Asian emissions, carries CO from that region out over the North Pacific and around the Northern Hemisphere. O3 production is driven by NOx, which, unlike CO, comes almost equally from biomass burning and fossil fuel combustion in this region and has a chemical lifetime of a few days or less. The resulting NOx distributions, while qualitatively similar to CO, have much steeper gradients, are transported much less widely, have a much lower background, and over the Indian and Pacific Oceans, are strongly dominated by pollution. O3 resulting from these anthropogenic sources generally exhibits patterns similar to those found for CO and NOx. Pollution accounts for 20–50% of the near‐surface O3 and 5–10% of the O3 in the upper troposphere. South and Southeast Asian emissions only produce 25% of the boundary layer O3 in the continental source regions. The maximum impact of the emissions occurs over the Indian Ocean (25–40%) with comparable contributions from O3 produced in the continental emission regions and O3 produced over the ocean by transported precursors. Convective lifting of the transported pollution O3 supplies ∼10% of the O3 in the tropical upper troposphere. While both emission regions have modest impacts on O3 (5–10%) outside of the Indian Ocean region, Southeast Asian pollution impacts free troposphere O3 in a midlatitude belt across the North Pacific, similar to NOx.