Combustion controls and plume structure of wildfire aerosol optical properties observed by TROPOMI

Abstract. Wildfire smoke influences air quality and climate through the scattering and absorption of solar radiation by aerosol particles, yet the factors controlling aerosol optical properties and their spatial variability within smoke plumes remain uncertain. Here, we use satellite observations of aerosol optical properties and trace gases from the TROPOspheric Monitoring Instrument (TROPOMI) to investigate the influence of combustion conditions on aerosol optical properties and their spatial variability within wildfire smoke plumes during the Australian 2018–2019 and 2019–2020 bushfire seasons.

Substantial differences in aerosol optical depth (AOD) and single-scattering albedo (SSA) are observed among major land cover types. Wildfires characterized by stronger flaming combustion, inferred from nitrogen dioxide (NO₂) and carbon monoxide (CO) column density ratios (ΔNO₂/ΔCO), are associated with more absorbing plume-averaged aerosol populations, whereas smoldering-dominated fires are associated with more scattering smoke and larger AOD. These relationships indicate that combustion conditions exert a first-order control on aerosol absorptivity, while aerosol loading is more strongly influenced by total emissions. Within individual plumes, aerosol loading and trace gas abundance remain concentrated in NO₂-defined plume cores, whereas SSA exhibits much weaker spatial gradients. Independent box-size analyses further show a systematic tendency toward higher SSA away from plume cores and source regions while ΔNO₂/ΔCO decreases, indicating that these spatial changes occur concurrently with atmospheric processing. Although chemical processing, dilution, and mixing cannot be separated, these observations provide constraints for evaluating model representations of wildfire emissions, plume evolution, and smoke radiative effects.