| Summary: | Megafires occurred in Australia during the 2019/2020 bushfire season, leading to enhanced concentrations of many tropospheric pollutants. Here, we report on a fire plume with unusually high and persistent nitrous acid (HONO) levels that we could track during one day at free tropospheric levels over the Tasman Sea on 4 January 2020 using IASI and CrIS satellite observations. HONO concentrations up to about 25 ppb were retrieved during nighttime. Persistent HONO concentrations (>10 ppb) were still observed at sunrise, likely due to large aerosol concentrations within the plume, preventing HONO photodissociation. In addition, comparison with carbon monoxide (CO) measurements suggest a secondary production during the plume transport. Model simulations confirm that the plume is transported in the lower free troposphere with concentrations as high as 30 ppb at about 4 km. However, many uncertainties and unknowns remain in the plume aerosol load and in the chemical processes which may explain the model inability to reproduce elevated HONO concentrations at sunrise.
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