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

Climate change continues to alter patterns and trends in weather, landscapes, and composition across the U.S. Recent increases in the frequency and severity of climate-driven wildfires, particularly in the west, has serious ramifications for enhanced downwind reactive nitrogen (Nr) emissions, deposition, and critical load (CL) exceedances. Until now, however, these impacts have only been studied on a relatively short-term, case-by-case basis, with little to no understanding on the impacts of the long-term trends in contributions of climate-driven wildfires to Nr deposition and CLs across terrestrial landscapes such as in the U.S. To address this knowledge gap, 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 with 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 northwest U.S., 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 CL exceedances of up to 20-40% due to fires in the western U.S.