Measurements of biomass burning influences in the troposphere over southeast Australia during the SAFARI 2000 dry season campaign

Abstract

Several studies have observed midtropospheric atmospheric composition anomalies and suggested a link to tropical biomass burning. Such anomalies complicate the use of trace gas profiles in remote regions to infer their surface sources/sinks based on the vertical gradients. The Southern African Regional Science Initiative (SAFARI 2000) campaign in Africa and coordinated downwind measurements in Australia provided an opportunity to confirm this link and elucidate the specific surface and atmospheric processes. Five aircraft missions were conducted by Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atmospheric Research during the campaign. They were scheduled after African outflows of polluted air were observed in satellite images over the Indian Ocean flowing east toward Australia. Air samples collected from near the surface to 7 km were analyzed for a suite of trace gases (12CO2, CH4, CO, H2, N2O, and C2 and C3 hydrocarbons) and one isotopomer (13CO2 to provide vertical composition profiles. Ozone was monitored continuously during flight while a groundbased lidar was employed in the Melbourne region to detect aerosol layers. A preliminary statistical analysis on the Australian data confirms covarying midroposphere enhancements in the biomass burning products. Making rudimentary corrections for photochemical evolution during transit, the trace gas enhancement ratios in affected air samples are comparable to emission ratios in fresh biomass burning plumes. The 13CO2/12CO2 ratios are also consistent with a source from terrestrial plants. Back-trajectory analysis for strongly enhanced samples suggests long-range transport from tropical regions in Africa or South America, the proof of which requires a follow-on analysis with a global chemistry transport model.