Fires to Farms: How Wildfire Smoke Impacts Surface-Level Photosynthetically Active Radiation

Wildfire smoke blankets much of the U.S., including major agricultural regions far downwind of active fires, throughout the growing season (April-September), particularly in severe fire years like 2018 and 2020. For example, the Midwest experienced smoke aloft 30-60% of days in August from 2006-2020 . Aerosols in smoke plumes change the amount of solar radiation reaching the surface and the fraction of the radiation that is diffuse. Yet, the ultimate impact of wildfire smoke on agricultural productivity remains uncertain as decreasing photosynthetically active radiation (PAR; 400-700 nm) can harm plant growth, but increasing the diffuse fraction (DF) can improve productivity as more light reaches the lower canopy. Thus, quantifying the impacts of smoke on total irradiance and DF across the PAR spectrum is required to accurately assess the impact of climate change on U.S. food systems. This study provides a longitudinal assessment of wildfire smoke impacts on total surface PAR and PAR DF across various U.S. regions from 2006-2020. We examine smoke-induced changes in PAR using ground-based spectrally-resolved measurements from the USDA UV-B Monitoring and Research Program (UVMRP). We identify smoke-impacted days using the NOAA/NESDIS Hazard Mapping System smoke product and estimate plume thickness using aerosol optical depth (AOD) from the satellite-based Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. Interactions with clouds are addressed using cloud fraction and combined cloud-AOD data from MODIS and UVMRP, respectively. Preliminary results show median PAR DF increased on average 6.5 percentage points on smoke-impacted days compared to smoke-free days for cloud fractions up to 0.5. Analysis of clear-sky irradiance at six PAR wavelengths indicates DF is higher and total irradiance is lower at all wavelengths on smoke-impacted days. The increase in DF is greatest at shorter wavelengths and the decrease in total irradiance is greatest near peak solar irradiance (500 nm). On smoke-impacted clear-sky days, higher AOD (i.e., thicker smoke) corresponds to higher DF and lower total irradiance than days with lower AODs (i.e., thinner smoke). Future work will merge these results with USDA agricultural production data to determine the average impact of wildfire smoke on U.S. crop productivity.