Biomass burning from wildfires in the Western US is a major source of organic aerosols, which have a large influence on air quality, health, and climate. For example, fine particulate organic aerosols (PM2.5) are dangerous to human health and brown carbon aerosols absorb solar radiation and thus act to warm the atmosphere. We know these aerosols exist and change within wildfire plumes. What is still up for debate is how many come from primary sources directly emitted by burning biomass and how many from secondary sources formed from gas-to-particle conversions following the oxidation of emitted gases. Researchers, funded in part by CPO’s Atmospheric Chemistry, Carbon Cycle, & Climate (AC4) program, collected airborne measurements from fresh wildfire smoke over the western US in 2018 in order to quantify the primary vs. secondary sources of biomass-burning organic particulate matter and brown carbon in wildfire plumes. Published in PNAS, their results show at least half of the secondary sources are explained by formation from evaporated primary organic aerosols in the first 3 to 6 hours of wildfire plume evolution. In particular, these oxidation products are substantial contributors to total brown carbon in the observed wildfire plumes. Overall the study concludes that the oxidation of primary organic aerosols plays a dominant role in determining aerosol properties of wildfire plumes, though more study should be done to understand nighttime plume evolution as the current study only collected measurements during the day.