Complexity in the evolution, composition, and spectroscopy of brown carbon in aircraft measurements of wildfire plumes

Biomass burning is a major source of light-absorbing organic aerosol (brown carbon), but its composition, chemical evolution, and lifetime are not well known. We measured water-soluble brown carbon absorption from 310 to 500 nm on the National Oceanic and Atmospheric Administration Twin Otter aircraft during flights downwind of western United States wildfires in summer 2019. The sampling strategy was near-Lagrangian and the plume ages spanned 0-5 hr. Trends in brown carbon mass absorption coefficient with plume age varied between flights, and did not show an exponential decay over these short time scales. The measured absorption spectra were smoothly varying, without identifiable contributions from individual chromophores with structured absorption. Using aerosol tracer ions and reference absorption spectra, the calculated contribution of 4-nitrocatechol to total absorption was <22 +/- 9% and <11 +/- 5%, although spectral fitting showed that it may be as low as <1.1% and <0.6% at 365 and 405 nm, respectively.