Does Smoke Aloft Impact Surface Air Quality?

Increasing wildfire activity is leading to more smoke-impacted days across the U.S. with well-documented air quality impacts from surface-level smoke. However, smoke aerosols throughout the column can affect atmospheric chemistry by altering incoming shortwave solar radiation necessary for chemical reactions. Via absorption and scattering, smoke affects actinic flux and changes the photolysis rates that dictate, for example, the formation of ozone (O₃) and the lifetimes of other trace gases. The overlap between O₃ and fire season motivates a need to better understand how smoke affects surface-level photolysis rates across CONUS and in major urban areas. In this study, we leverage aircraft and satellite measurements during smoke events to estimate smoke-driven changes in surface-level photolysis frequencies across the western U.S. We rely on data from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) aircraft campaign, which sampled smoke plumes across the western U.S. in 2018. Specifically, we use actinic flux measurements from the HIAPER Airborne Radiation Package (HARP) instrument and calculated photolysis frequencies (j-values) from the National Center for Atmospheric Research’s (NCAR) Tropospheric Ultraviolet and Visible (TUV) radiative transfer model to determine smoke-driven changes along the flight path. We map these changes in photolysis rates by plume age, optical thickness, and altitude, and we use TUV to estimate the corresponding actinic flux and photolysis rates at the surface. Finally, we extrapolate these photolysis relationships with plume age and aerosol properties beyond the WE-CAN flight track. We integrate these relationships with satellite observations of smoke location and optical depths as well as TUV to estimate photolysis rates more broadly. We compare the aerosol-free TUV configuration output to the smoke-impacted output to determine impacts on surface-level photolysis rates, which drive surface air quality. As smoke becomes more abundant, particularly in cities across the U.S., understanding how smoke aloft changes surface photochemistry is an essential component for constraining future air quality conditions.