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. O 3 production is driven by NO x , 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 NO x 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. O 3 resulting from these anthropogenic sources generally exhibits patterns similar to those found for CO and NO x . Pollution accounts for 20–50% of the near‐surface O 3 and 5–10% of the O 3 in the upper troposphere. South and Southeast Asian emissions only produce 25% of the boundary layer O 3 in the continental source regions. The maximum impact of the emissions occurs over the Indian Ocean (25–40%) with comparable contributions from O 3 produced in the continental emission regions and O 3 produced over the ocean by transported precursors. Convective lifting of the transported pollution O 3 supplies ∼10% of the O 3 in the tropical upper troposphere. While both emission regions have modest impacts on O 3 (5–10%) outside of the Indian Ocean region, Southeast Asian pollution impacts free troposphere O 3 in a midlatitude belt across the North Pacific, similar to NO x .