Increased contributions of climate-driven wildfires to nitrogen deposition in the United States.

Climate change and increases in the frequency and severity of climate-driven wildfires, particularly in the western United States, has serious ramifications for enhanced downwind reactive nitrogen (Nr) emissions, deposition, and critical load exceedances. Here we present a multi-decadal (2002 - 2021), harmonized model-data-driven study using the George Mason University North American Chemical Reanalysis (NACR) system and simulations including both "with-fire" and "without-fire" conditions to quantify the change in trends of fire activity and source contributions to total Nr emissions and deposition over the U.S. Our results show that fire activity has increased substantially in the western U.S., especially in the west-northwest U.S. for wildfires, and that this increase is associated with positive annual near-surface temperature and vapor pressure deficit anomalies compared to the period average. Major results and implications of this work are increasing trends in the contribution of climate-driven wildfires to higher Nr emissions, deposition, and critical load exceedances of up to 20-40% due to fires in the western U.S. There are also smaller increases (<5 %) in Nr deposition trends for the eastern U.S., which are related to greater occurrence and reporting of agricultural and prescribed burns.