Significance Wildfire emissions in the western United States have had increasingly larger impacts on air quality, health, and climate forcing in recent decades. However, our understanding of how wildfire plume composition evolves remains incomplete. Particularly, the evolution of carbonaceous material, including fine particle mass (PM _2.5 ) and light-absorbing brown carbon, has remained elusive because of limited knowledge about the interplay between primary emissions and subsequent chemical and physical transformations that convert material between vapors and particles. Using a comprehensive analysis of in situ wildfire smoke measurements in combination with simulation chamber experiments, we quantitatively assess primary versus secondary organic particulate matter and brown carbon light absorption in authentic wildfires plumes. These results improve our fundamental knowledge of wildfire plume composition and evolution.