Atmos. Chem. Phys. Discuss., 7, 2197-2248, 2007
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Model study of the cross-tropopause transport of biomass burning pollution
B. N. Duncan1,2, S. E. Strahan1,2, and Y. Yoshida1,2
1Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore County, Baltimore, Maryland, USA
2The Atmospheric Chemistry and Dynamics Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Abstract. We present a modeling study of the troposphere-to-stratosphere transport (TST) of pollution from major biomass burning regions to the tropical tropopause layer (TTL) and lower stratosphere (LS). We show that biomass burning pollution regularly and significantly impacts the composition of the TTL/LS. TST occurs through 1) slow ascent in the TTL and 2) quasi-horizontal exchange in the regions of the subtropical jets; we find both pathways to be important. The seasonal oscillation in CO in the TTL/LS (i.e., the CO "tape recorder") is caused largely by seasonal changes in biomass burning. Another contributing factor is the long-range transport of northern hemispheric pollution (e.g., biofuels and fossil fuels) to the northern tropics in boreal winter. Other tropical sources of CO (e.g., methane oxidation) have insignificant seasonal variation, contributing little to the tape recorder. Interannual variation of CO in the TTL/LS is caused by year-to-year variations in biomass burning and the strength, frequency, and locations of deep convection, which lofts pollution to the upper troposphere. During our study period, 1994–1998, we find that the highest concentrations of CO in the TTL/LS occur during the strong 1997/98 El Niño event for two reasons: i.~tropical deep convection was stronger and ii.~emissions were higher. This extreme event can be seen as an upper bound on the impact of biomass burning pollution on the TTL/LS. We estimate that the 1997 Indonesian wildfires increased CO in the entire TTL and tropical LS (<60 mb) by more than 40% and 10%, respectively, for several months. Zonal mean ozone increased and the hydroxyl radical decreased by as much as 20%, increasing the lifetimes and, subsequently TST, of trace gases. Our results indicate that the impact of biomass burning pollution on the TTL/LS is likely greatest during an El Niño event due to favorable dynamics and historically higher burning rates.

Citation: Duncan, B. N., Strahan, S. E., and Yoshida, Y.: Model study of the cross-tropopause transport of biomass burning pollution, Atmos. Chem. Phys. Discuss., 7, 2197-2248, doi:10.5194/acpd-7-2197-2007, 2007.
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