Pronounced increases in nitrogen emissions and deposition due to the historic 2020 wildfires in the western US

Wildfire outbreaks can lead to extreme biomass burning (BB) emissions of both oxidized (e.g., nitrogen oxides; NOx = NO+NO2) and reduced form (e.g., ammonia; NH3) nitrogen (N) compounds. High N emissions are major concerns for air quality, atmospheric deposition, and consequential human and ecosystem health impacts. In this study, we use both satellite-based observations and modeling results to quantify the contribution of BB to the total emissions, and approximate the impact on total N deposition in the western U.S. Our results show that during the 2020 wildfire season of August-October, BB contributes significantly to the total emissions, with a satellite-derived fraction of NH3 to the total reactiveN emissions (median similar to 40%) in the range of aircraft observations. During the peak of the western August Complex Fires in September, BB contributed to similar to 55%(for the contiguous U.S.) and similar to 83%(for thewestern U.S.) of the monthly total NOx and NH3 emissions. Overall, there is good model performance of the George Mason University-Wildfire Forecasting System(GMU-WFS) used in this work. The extreme BB emissions lead to significant contributions to the total N deposition for different ecosystems in California, with an average August - October 2020 relative increase of similar to 78%(from 7.1 to 12.6 kg ha(-1) year(-1)) in deposition rate tomajor vegetation types (mixed forests+ grasslands/shrublands/savanna) compared to the GMU-WFS simulations without BB emissions. For mixed forest types only, the average N deposition rate increases (from 6.2 to 16.9 kg ha(-1) year(-1)) are even larger at similar to 173%. Such large N deposition due to extreme BB emissions are much (similar to 6-12 times) larger than low-end critical load thresholds for major vegetation types (e.g., forests at 1.5-3 kg ha(-1) year(-1)), and thus may result in adverse N deposition effects across larger areas of lichen communities found in California's mixed conifer forests.