Exposure of U.S. Mountain West Lakes to Wildfire Smoke as a Source of Reactive Nitrogen Deposition

Deposition of reactive nitrogen (N) to mountain lakes in the Rocky Mountain region is an important source of nutrients to high elevation watersheds. However, excess nutrients alongside increased temperatures have increased algal primary productivity and caused some mountain lakes to change from oligotrophic to eutrophic. Urban, agricultural, and wildfire emissions are the three main sources of reactive N deposited to these ecosystems. Across the mountain west, numerous large animal feeding operations emit substantial quantities of ammonia (NH₃). Meanwhile, wildfire smoke–a major source of reduced N–is becoming increasingly abundant as fires burn larger areas under climate-induced increases in aridity. In this study, we integrate satellite observations of wildfire smoke and NH₃ with lake location data from HydroLAKES to investigate smoke contributions of reactive N to mountain lakes from 2012-2022. We focus on lakes above 1400 m in elevation across the Rocky Mountain region, including parts of Idaho, Colorado, Montana, Wyoming, Utah, and New Mexico. We identify smoke plume location, extent, timing, and concentration using the National Environmental Satellite, Data and Information Service (NESDIS) Hazard Mapping System (HMS) smoke product. The HMS smoke plumes are mapped throughout the day by trained analysts using mainly visible imagery from geostationary satellites. Additional metrics on smoke position and concentration include aerosol optical depth from the Terra+Aqua combined Moderate Resolution Imaging Spectroradiometer (MODIS) Multi-Angle Implementation of Atmospheric Correction (MAIAC) retrieval and carbon monoxide from the Cross Track Infrared Sounder (CrIS). We combine the smoke data with the CrIS daytime (13:30 local overpass) NH₃ retrievals observed from the Suomi National Polar Orbiter Preparatory (SNPP) platform and the Joint Polar Satellite System (JPSS-1). We determine the frequency at which mountain lakes are exposed to wildfire smoke in the atmospheric column (i.e., lake smoke-days) during the warm season (March - October). We use CrIS data to quantify the amount of NH₃ above these mountain lakes and characterize relationships between smoke presence and NH₃ atmospheric concentrations. We contextualize these findings given the proximity of each mountain lake to large agricultural emitters of NH₃. Understanding the source of reactive N deposition in mountain lakes, and particularly the role of wildfire smoke in this process, is crucial for adequate mitigation of NH₃ emissions and assessing the impacts on water quality and ecosystem health.