Pyrogenic HONO seen from space: insights from global IASI observations

Nitrous acid (inline-formulaHONO) is a key atmospheric component, acting as a major source of the hydroxyl radical (inline-formulaOH), the primary oxidant in the Earth's atmosphere. However, understanding its spatial and temporal variability remains a significant challenge. Recent TROPOspheric Monitoring Instrument (TROPOMI)/Sentinel-5 Precursor (S5P) ultraviolet–visible (UV–Vis) measurements of fresh fire plumes shed light on the impact of global pyrogenic inline-formulaHONO emissions. Here, we leverage Infrared Atmospheric Sounding Interferometer (IASI)/MetOp's global infrared satellite measurements, complementing midday TROPOMI observations with morning and evening overpasses, to detect and retrieve pyrogenic HONO in 2007–2023. Employing a sensitive detection method, we identify inline-formulaHONO enhancements within concentrated fire plumes worldwide. Most detections are in the Northern Hemisphere (NH) mid- and high latitudes, where intense wildfires and high injection heights favour inline-formulaHONO detection. IASI's nighttime measurements yield 10-fold more inline-formulaHONO detections than daytime measurements, emphasizing inline-formulaHONO's extended lifetime in the absence of photolysis during the night. The annual detection count increases by at least 3–4 times throughout the IASI time series, mirroring the recent surge in intense wildfires at these latitudes. Additionally, we employ a neural-network-based algorithm for retrieving pyrogenic inline-formulaHONO total columns from IASI and compare them with TROPOMI in the same fire plumes. The results demonstrate TROPOMI's efficacy in capturing inline-formulaHONO enhancements in smaller fire plumes and in proximity to fire sources, while IASI's morning and evening overpasses enable inline-formulaHONO measurements further downwind, highlighting the survival of inline-formulaHONO or its secondary formation along long-range transport in smoke plumes.