Large Enhancements in Southern Hemisphere Satellite-Observed Carbon Monoxide and Methanol due to 2019/2020 Australian Wildfires

Abstract:

The 2019/2020 Australian wildfires (leading to the so-called “black summer”) were some of the largest fires in that region in recent decades and emitted large quantities of aerosol and trace gas pollutants. Peak fire activity occurred on eucalyptus forest vegetation in New South Wales (south eastern Australia) during December and January. Using state-of-the-art near-real-time (NRT) satellite retrievals of tropospheric composition, we present a detailed analysis of several emitted trace gases and their long-range transport, and compare to the previous (2018/2019) fire season.

Observations of carbon monoxide (CO) by the Infrared Atmospheric Sounding Interferometer (IASI) on MetOp-B show that fire emissions were so intense that the distinct Australian fire plume managed to zonally circumnavigate the Southern Hemisphere (SH) within a few weeks, with eastward propagation over the South Pacific, South America, the South Atlantic, Africa and the Indian Ocean. Elevated atmospheric methane levels were also detected by IASI in January 2020 in fire plumes over the South Pacific by using CO as a reference tracer, even though geographical sampling was restricted by aerosols and clouds. Furthermore, IASI retrievals show significant enhancements of methanol (CH₃OH) from the fires, evidenced by CH₃OH:CO enhancement ratios. These enhancement ratios increase downwind within the aged plume over the South Pacific, indicative of secondary in-plume CH₃OH formation. As far as we are aware, this is the first study to detect increasing CH₃OH:CO enhancement ratios in such fire plumes from space. Despite its short atmospheric lifetime of up to a few days, regional enhancements in nitrogen dioxide (NO₂) were also detected by TropOMI in close proximity to urban regions such as Sydney. By sub-sampling the NO₂ pixels in fire-detected hotspots, we find a substantial increase in fire-induced NO₂ in 2019/2020 compared to the previous year.

With future climate and land-use change it is expected that wildfires will become more common and intense. Therefore, Earth observation (EO) will become an increasingly important method for monitoring the atmospheric impacts of these fires especially as advanced instruments are launched on satellites in polar and geostationary orbits.