Observation of Wildfire Smoke Particles That Accumulated Secondary Aerosol during Long-Range Transport

Wildfires impact air quality, climate, and health near and far from the emission source. Here, real-time size and chemical composition of 2,419,048 individual particles were measured with an aerosol time-of-flight mass spectrometer (ATOFMS) in Ann Arbor, MI, between September 8 and 18, 2018. Of these mainly submicron particles, 97%, by number, were identified as aged biomass burning (smoke) particles, characterized by potassium, organic carbon, and elemental carbon. These aged biomass burning particles also contained secondary ammonium, nitrate, sulfate, nitric acid, sulfuric acid, and organic aerosol, including dicarboxylic acids, amines, and organosulfates. The Navy Aerosol Analysis and Prediction System (NAAPS) global aerosol model tracked smoke from wildfires in western North America in an air mass traveling northeast that then mixed with anthropogenic and biogenic aerosol from southern Canada and northern U.S. before reaching southeastern Michigan. Together, the observational and model results indicate that smoke particles accumulated secondary aerosol, including from biogenic and anthropogenic sources, during long-range transport. While the measurements and model show similar concentrations of submicron aerosol mass present, the ATOFMS measurements show that the smoke and secondary aerosol were mixed within the same, rather than separate, individual particles. In fact, most models that evaluate aerosol impacts on air quality and climate treat smoke and secondary aerosol as separate externally mixed species, which has implications for cloud formation, optical properties, reactivity, and possibly health impacts. This study highlights the participation of wildfire smoke in secondary aerosol formation, with biomass burning aerosols serving as substrates for heterogeneous reactions, condensation seeds for gas-particle partitioning, and cloud nuclei for subsequent aqueous-phase reactions during long-range transport of primary wildfire emissions.