Smoke-Driven Changes in Photosynthetically Active Radiation During the US Agricultural Growing Season

Wildfire smoke is frequently present over the U.S. during the agricultural growing season and will likely increase with climate change. Studies of smoke impacts have largely focused on air quality and human health; however, understanding smoke's impact on photosynthetically active radiation (PAR) is essential for predicting how smoke affects plant growth. We compare surface shortwave irradiance and diffuse fraction (DF) on smoke-impacted and smoke-free days from 2006 to 2020 using data from multifilter rotating shadowband radiometers at 10 U.S. Department of Agriculture UV-B Monitoring and Research Program stations and smoke plume locations from operational satellite products. On average, 20% of growing season days are smoke-impacted, but smoke prevalence increases over time (r = 0.60, p < 0.05). Smoke presence peaks in the mid to late growing season (i.e., July, August), particularly over the northern Rocky Mountains, Great Plains, and Midwest. We find an increase in the distribution of PAR DF on smoke-impacted days, with larger increases at lower cloud fractions. On clear-sky days, daily average PAR DF increases by 10 percentage points when smoke is present. Spectral analysis of clear-sky days shows smoke increases DF (average: +45%) and decreases total irradiance (average: -6%) across all six wavelengths measured from 368 to 870 nm. Optical depth measurements from ground and satellite observations both indicate that spectral DF increases and total spectral irradiance decreases with increasing smoke plume optical depth. Our analysis provides a foundation for understanding smoke's impact on PAR, which carries implications for agricultural crop productivity under a changing climate. Plain Language Summary Wildfires in the United States (U.S.) are occurring more often and burning larger areas, and smoke from these fires impacts incoming solar radiation across the country. Sunlight is a necessary ingredient for photosynthesis with the total amount and diffuse fraction (DF) of light affecting plant growth. Smoke particles absorb and scatter light resulting in less total and more diffuse radiation, respectively. Since smoke is present over agricultural regions during the growing season (April-September), understanding how smoke affects sunlight is essential for determining smoke's impact on crops. We use ground-based measurements of solar radiation and satellite-based observations of smoke plume location and thickness to examine how sunlight varies on days with and without smoke at ten agriculturally important locations across the U.S. from 2006 to 2020. We show that smoke is present most often during the mid to late growing season when light characteristics most impact plant growth. One in five growing season days is smoke-impacted and smoke is becoming more frequent over time. The DF is higher on smoke-impacted days with the largest increase occurring when cloud cover is low. While the DF increases with smoke, total irradiance decreases, and these shifts grow stronger as smoke plumes become thicker.

Automated summary

Understanding the impact of wildfire smoke on photosynthetically active radiation (PAR) is essential for predicting plant growth. Our study shows that smoke increases PAR diffuse fraction (DF) and decreases total irradiance, with stronger effects as smoke plume optical depth increases. Smoke is most prevalent during the mid to late growing season, with the largest impact on light characteristics.