Aircraft\nMeasurements from a U.S. Western Wildfire\nDemonstrating Day and Night Differences in the Chemical Composition\nand Optical Properties of Biomass Burning Aerosols and Their Atmospheric\nEvolution

The\ncomposition and transformations of biomass burning aerosols\n(BBA) have been measured onboard the NOAA Twin Otter research aircraft\nduring the Fire Influence on Regional to Global Environments and Air\nQuality field study. We analyze real-time aerosol mass spectrometry\nmeasurements across three flights during the afternoon, late afternoon,\nand night of August 28, 2019, for one midsized wildfire. Analysis\nof several metrics showed that the aerosol composition and optical\nproperties varied depending on the burning conditions at the fire\nzone and the time of day the BBA was emitted, with substantial variations\nin the available sunlight. The total aerosol mass loadings were dominated\nby organic components with a much smaller contribution from inorganic\nspecies. A gradual buildup of organic material was observed during\nthe afternoon as the plume aged, indicating the condensation of photochemically\nformed low-volatility oxidized organic compounds. Highly hygroscopic\nammonium nitrate was the main inorganic component, suggesting potential\nwater content in BBA particles and the likelihood of their aqueous-phase\nreactivity. Depletions of particle-phase NO<sub>3</sub><sup>–</sup> and Cl<sup>–</sup> relative to carbon monoxide were observed\nin the late afternoon and nighttime plumes, respectively, aligning\nwith known gas-particle partitioning thermodynamics and the heterogeneous\nchemistry of dissolved nitrate and chloride. The wavelength-dependent\nlight absorption by aerosol species was higher for the plume sampled\nat night and showed no significant changes with plume age, despite\nobserved trends in composition and mass downwind. These differences\nin particle composition and optical properties demonstrate that the\nprocesses involved in BBA aging are not uniform for the same wildfire\nover the course of the day and depend highly on when the BBA was emitted,\nas well as the burning phase at the emissions source.