Canadian Wildfire Smoke Impacts on Reduced Nitrogen in the Upper Midwest: Insights From the 2023 Fire Season

Wildfires are the largest terrestrial source of atmospheric ammonia (NH 3 ), yet their impacts on NH 3 concentrations and ammonium (NH 4 + ) deposition remain poorly quantified. In this study, we evaluate the effects of the record‐breaking 2023 Canadian wildfire season on NH 3 concentration and NH 4 + deposition across the Upper Midwest. This study integrates satellite observations, ground‐based data, and in situ aircraft measurements. In May–June 2023, NH 3 concentrations increased at 83% of ground sites, and NH 4 + deposition flux rose at 100% of ground sites in the Upper Midwest. Satellite data showed significantly higher column‐averaged NH 3 in 47% of grid cells in the Upper Midwest. On 1 August, a smoke plume over the Midwest corresponded with an AEROMMA flight observing enhanced NH 3 , NH 4 + , carbon monoxide, and acetonitrile. These findings highlight the substantial impact of wildfire smoke on NH 3 and NH 4 + at regional scales, with implications for nitrogen cycling, air quality, and atmospheric modeling.