Impact of South American biomass burning emissions on elevated South Atlantic upper tropospheric ozone

Abstract. During the SOUTHTRAC mission in autumn 2019 elevated mixing ratios of carbon monoxide (CO), carbon dioxide CO₂, nitrogen oxide (NO) and total reactive nitrogen NO_y were observed during a flight at the beginning of October. The potential plume extended over more than 1000 km (15° latitude) east of the Brasilian coast at altitudes of 13 km in the upper troposphere. In-situ measurements showed elevated ozone in this plume (≈ 100 ppbv), being 20–40 ppbv higher than during a previous flight in early September at exactly the same flight route. For the plume flight positive correlations of ozone and pollutants (CO, NO, NO_y) indicate ozone production in these pollution layers. Lagrangian Analysis shows, that the observed air masses were strongly affected by biomass burning over Amazonia. A combined analysis of chemical Lagrangian box model and a global chemistry climate model (EMAC) revealed that ozone production from biomass burning predominantly caused the ozone enhancements. The effect is eventually intensified by NO_x produced from lightning. Upward transport of the plumes happened ≈ one week before the flight, allowing ozone to be formed and enhanced by 25 % compared to the September flight. Estimates of the climate impact show, that the biomass burning produced ozone has a local effect on the radiative forcing of 50 mWm⁻².