Seasonal variation of trans-Pacific transport of carbon monoxide (CO) in the upper troposphere: MLS observations and GEOS-Chem and GEM-AQ simulations

Abstract. Multi-year Microwave Limb Sounder (MLS) carbon monoxide (CO) measurements at 215 hPa are employed to present a climatological view of seasonal variation of upper tropospheric trans-Pacific transport of Asian air pollution. The measurements show that the transport peaks in late boreal spring and early boreal summer. Although the strongest Asian air pollution outflow occurs in boreal summer, the "transport pathway" over the northeast Pacific is narrower in summer than in spring. Results from two tropospheric chemistry models GEOS-Chem and GEM-AQ are compared to MLS observations. Both models reproduce the strong trans-Pacific transport in boreal spring and summer well, but show different morphologies over Southeast Asia in winter and fall. A tagged CO simulation using GEOS-Chem indicates that Asian fossil fuel is the biggest source of upper tropospheric CO over the north Pacific in all seasons, excepting methane (CH₄) and non-methane hydrocarbons, although there are large fires in Southeast Asia in boreal spring and fall. A sensitivity test indicates that deep convection has a large effect on upper tropospheric CO abundances, increasing the abundances by more than 40%, over the north Pacific in boreal spring. In boreal summer, however, the increase is not significant over the north Pacific although it is large over continental Asia.