Interpretation of Aura satellite observations of CO and aerosol index related to the December 2006 Australia fires

Enhanced carbon monoxide (CO) in the upper troposphere (UT) is shown by nearly collocated Tropospheric Emission Spectrometer (TES) and Microwave Limb Sounder (MLS) measurements near and down-wind from the known wildfire region of SE Australia from December 12th–19th, 2006. Enhanced ultraviolet (UV) aerosol index (AI) derived from the Ozone Monitoring Instrument (OMI) measurements correlates with these high CO concentrations. The Hybrid Single Particle Langrangian Integrated Trajectory (HYSPLIT) model back trajectories trace selected air parcels, where TES observes enhanced CO in the upper and lower troposphere, to the SE Australia fire region as their initial location. Simultaneously, they show a lack of vertical advection along their tracks. TES retrieved CO vertical profiles in the higher and lower southern latitudes are examined together with the averaging kernels and show that TES CO retrievals are most sensitive at approximately 300–400 hPa. The enhanced CO observed by TES in the upper (215 hPa) and lower (681 hPa) troposphere are, therefore, influenced by mid-tropospheric CO. GEOS-Chem model simulations with an 8-day emission inventory, as the wildfire source over Australia, are sampled to the TES/MLS observation times and locations. These simulations only show CO enhancements in the lower troposphere near and down-wind from the wildfire region of SE Australia with drastic underestimates of UT CO plumes. Although CloudSat along-track ice-water content curtains are examined to see whether possible vertical convection events can explain the high UT CO values, sparse observations of collocated Aura CO and CloudSat along-track ice-water content measurements for the single event precludes any conclusive correlation. Vertical convection that uplifts the fire-induced CO (i.e., most notably referred to as pyro-cumulonimbus (pyroCb)) may provide an explanation for the incongruence between these simulations and the TES/MLS observations of enhanced CO in the UT.