Photochemical Evolution of Primary and Secondary Organic Aerosol in Large Wildfire Plumes.

Abstract:

Wildfires are the largest combustion-related source of primary organic aerosol (POA) and volatile organic compounds (VOCs) to the atmosphere. However, the VOC oxidation leading to secondary organic aerosol (SOA) formation and the combined evolution of the concentrations, composition, and properties of POA and SOA in wildfire plumes is largely uncertain. Moreover, there appears to be a disconnect between laboratory studies that show an enhancement in OA mass and field observations where the OA mass remains relatively constant. In this work, we use a state-of-the-science model (SOM-TOMAS) to investigate the evolution of OA in wildfire plumes characterized during the WE-CAN (Western wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen) aircraft campaign. The SOM-TOMAS (Statistical Oxidation Model-TwO Moment Aerosol Sectional) model simulates the multigenerational gas- and particle-phase chemistry, thermodynamics properties, and phase state-influenced kinetic gas/particle partitioning of OA. A plume version of the SOM-TOMAS model is currently being used to study the OA evolution in several large wildfires sampled using a pseudo-Lagrangian strategy. The model is being used to (i) determine important precursors and oxidants contributing to SOA formation, (ii) examine the role of heterogeneous chemistry and particle phase state on OA evolution, (iii) test if dilution-driven evaporation of POA is proportionately balanced by simultaneous SOA formation, and (iv) study differences in the dynamics of OA evolution between the core versus the edge of the plume. The insights from the plume model will be useful in accurately representing OA in chemical transport models.