Evidence for the impact of fire activity on daily variations of IASI mid-tropospheric CO₂ anomalies at 8–11 km

Abstract. Biomass burning is a major, highly variable source of atmospheric CO₂, but its impact on the free troposphere remains difficult to quantify because of uncertainties in injection heights and transport. In the tropics, intense fires can trigger pyroconvective plumes that loft combustion products to the mid- and upper troposphere. However, most fire emission inventories and global CO₂ inversions still assume simplified vertical distributions of CO₂ emitted by fires. Weighted columns of CO₂ retrieved from remote sensing instruments that are sensitive to such high-altitude enhancements can inform of such dynamics. Here we combine mid-tropospheric CO₂ (MT-CO₂) retrievals from three IASI instruments with GOES-16 observations of Fire Radiative Energy (FRE) to link daily MT-CO₂ anomalies observed by IASI at 8–11 km altitude to South American fire activity during the 2020 burning season, while accounting for long-range horizontal transport of anomalies. From August–October 2020, about 66 % of the detected anomalies originate from long-range or unknown sources and are discarded. For the remaining anomalies attributed to local fires, 72 h back trajectories do intersect with at least one active fire for 75 % of them. Their daily sum co-varies strongly with FRE, with the ratio between the two depending on the dominant horizontal transport regime. A comparison with CAMS IASI-weighted CO₂ fields shows that model fails to reproduce both the amplitude and structure of the observed anomalies. Overall, our results demonstrate that IASI MT-CO₂ anomalies carry an observational fingerprint of tropical fire activity.