JANUARY 2002: Monsoonal injection of Asian pollution to the upper troposphere

A summertime ozone pollution event in the upper troposphere (UT) over Hong Kong (22^oN,114^oE) on August 21, 1996. The black line shows ozonesonde observations and the red solid line the GEOS-CHEM model results. Also shown as a vertical profile are simulated ozone concentrations when Asian fossil fuel emissions are suppressed (red dotted line). The map shows ozone concentration enhancements from Asian pollution at 160 hPa (13 km), as determined from a sensitivity simulation with Asian anthropogenic emissions shut off. Arrows are wind vectors and the white dot shows the location of Hong Kong. The prevailing low-altitude monsoonal flow in summer transports Asian pollution along the Pacific Rim towards the northeast where it is then subject to frequent lifting to the upper troposphere by deep convection. While part of this pollution in the upper troposphere is transported eastward to the North Pacific, most circulates southward and then westward around the Tibetan anticylone (South Asia high), as is evident in the case shown here. The Tibetan anticylone is a climatological feature of the UT circulation over eastern Asia in summer and is characterized by southward transport along the eastern Asia coast and westward transport at low latitudes as part of the tropical easterly jet. The resulting circulation has important implications for the direction of Asian outflow in summer. This work was led by Hongyu Liu and a full account is given in Liu et al. [2002].