Model study of the cross-tropopause transport of biomass burning pollution

[摘要] We present a modeling study of the troposphere-to-stratosphere transport(TST) of pollution from major biomass burning regions to the tropical uppertroposphere and lower stratosphere (UT/LS). TST occurs predominately through1) slow ascent in the tropical tropopause layer (TTL) to the LS and 2)quasi-horizontal exchange to the lowermost stratosphere (LMS). We show thatbiomass burning pollution regularly and significantly impacts thecomposition of the TTL, LS, and LMS. Carbon monoxide (CO) in the LS in oursimulation and data from the Aura Microwave Limb Sounder (MLS) shows anannual oscillation in its composition that results from the interaction ofan annual oscillation in slow ascent from the TTL to the LS and seasonalvariations in sources, including a semi-annual oscillation in CO frombiomass burning. The impacts of CO sources that peak when ascent isseasonally low are damped (e.g. Southern Hemisphere biomass burning) andvice-versa for sources that peak when ascent is seasonally high (e.g.extra-tropical fossil fuels). Interannual variation of CO in the UT/LS iscaused primarily by year-to-year variations in biomass burning and thelocations of deep convection. During our study period, 1994–1998, we findthat the highest concentrations of CO in the UT/LS occurred during thestrong 1997–1998 El Niño event for two reasons: i. tropical deep convectionshifted to the eastern Pacific Ocean, closer to South American and AfricanCO sources, and ii. emissions from Indonesian biomass burning were higher.This extreme event can be seen as an upper bound on the impact of biomassburning pollution on the UT/LS. We estimate that the 1997 Indonesianwildfires increased CO in the entire TTL and tropical LS (>60 mb) by morethan 40% and 10%, respectively, for several months. Zonal mean ozoneincreased and the hydroxyl radical decreased by as much as 20%,increasing the lifetimes and, subsequently TST, of trace gases. Our resultsindicate that the impact of biomass burning pollution on the UT/LS is likelygreatest during an El Niño event due to favorable dynamics andhistorically higher burning rates.

[学科分类] 大气科学