Observed and Modeled Changes in Boundary-Layer and Surface-Level Actinic Flux Due To Wildfire Smoke Plumes in the California Central Valley in Summer 2018

Wildfire smoke is increasingly degrading U.S. air quality via the emission and transport of pollutants. Smoke's direct role as a pollutant is well-documented; however, smoke also affects pollutant concentration indirectly by changing the shortwave actinic flux necessary for photochemical reactions. We compute smoke-driven changes in surface-level and boundary-layer downwelling actinic flux (F↓) at 550 and 380 nm (NO₂ photolysis peak) along a 2018 Western wildfire Experiment for Cloud chemistry, Aerosol absorption, and Nitrogen (WE-CAN) research flight through the California Central Valley. The onboard HIAPER Airborne Radiation Package (HARP)–Actinic Flux instrument measured F↓. To assess changes in F↓ relative to smoke-free conditions and at altitudes not sampled by the aircraft, we calculate F↓ under assumed background and observed smoke conditions using the U.S. National Science Foundation (NSF) National Center for Atmospheric Research (NCAR) Tropospheric Ultraviolet and Visible (TUV) radiation model. Under smoke-impacted conditions, modeled F↓ minorly underestimates HARP observations; the average modeled-to-measured ratio is 0.93 at 550 nm and 0.89 at 380 nm. Relative to modeled background conditions, observed (modeled) smoke-impacted F↓ at 380 nm decreased by 24% (38%), 15% (24%), and 8% (18%) at 0–0.5 km, 0.5–1 km, and 1–1.5 km, respectively. At the ground, smoke decreased modeled F↓ at 380 nm by 43%—likely an upper bound, as the modeled values slightly underestimate observations. As climate change drives more severe wildfire seasons, understanding smoke's impact on actinic flux is essential for constraining future air quality, and recent extreme seasons like 2018 offer opportunities for such analyses.