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 air quality across the U.S. 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 nm
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 wildfire seasons grow more severe with
climate change, understanding smoke’s combined impact on actinic flux and
concentrations of VOCs and nitrogen species is essential for constraining
future air quality.