Smoke-Driven Changes in Photosynthetically Active Radiation During the
U.S. Agricultural Growing Season
Abstract
Wildfire smoke frequently blankets the U.S. throughout the agricultural
growing season, and this 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 2006-2020 using
data from multifilter rotating shadowband radiometers at ten U.S.
Department of Agriculture (USDA) 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 368-870 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 (i.e., plume thickness). Our
analysis provides a foundation for understanding smoke’s impact on PAR,
which carries implications for agricultural crop productivity under a
changing climate.