Shear lines facilitate transport of reactive trace gases from East Asia into the deep tropics.

Rapid industrialization and economic development in Eastern Asia (EA) has in recent decades led to increases in anthropogenic pollutants in the regional atmosphere. This pollutant burden is transported from local and regional sources into the global atmosphere and has been shown to be a significant source of intercontinental and cross-border anthropogenic pollution. This presentation focuses on one significant mechanism of such pollutant transport from EA into the deep tropics. Meteorology of the EA is dominated by the monsoonal low-level wind reversal, which in its winter phase facilitates episodic incursions of cold air masses from Siberia into the Philippine and South China Seas and to the Western Pacific (WP). In that phase cold surface temperatures give rise to stable low-level inversions, resulting in the capping of the boundary layer. In this flow regime wet deposition is almost completely absent, allowing regional pollution to build-up in the lower troposphere. As the cold air surges are carried eastward across EA and into the WP, sweeping off low-level pollution in the process, they also advance equatorward. As the cold air advances southward, heating from the ocean surface reduces the thermal gradient across the frontal boundary, and by the time that air reaches tropical latitudes all that remains of the front is a wind shift referred to as the shear line. North of the shear line, an air mass still contains the diluted pollutants picked up over the source regions in EA. Here we present measurements of chemically reactive trace gases collected during the CONTRAST mission, an airborne research study conducted during January-February, 2014, over the tropical WP Ocean. This presentation describes shear line facilitated transport of pollution into the tropics during boreal winter and its effects on chemical composition of the troposphere in the deep tropics. This is the first analysis that links the phenomenon of shear lines to pollution transport out of EA. For this analysis, we use chemical and dynamical data sets obtained during the field campaign and explore relationships between various pollutant tracers to understand the connection between potential origins and chemical signatures of the intercepted air masses, and the resulting transport and photochemical evolution of the chemical species in the tropics.