Aging of Organic Gases and Particles in the 2017 Northern California Wildfire Plumes

Abstract:

Biomass Burning is the dominant source of carbonaceous aerosols and the second largest source of non-methane organic gases in Earth’s atmosphere. The size and frequency of wildfires in the western United States have been increasing over the past 20 years. Recent lab fire experiments have updated the emission profiles of biomass burning organics. However, the emissions from wildfires and their transformation in the atmosphere especially at night are not well-understood.

The 2017 Northern California Wildfires caused the PM_2.5 concentration to exceed 100 μg m^-3 throughout the San Francisco Bay Area. During the fire periods, we measured particle and gas phase organic compounds at UC Berkeley campus, an urban site ~60 km downwind of the fire sites, using offline GCxGC-VUV/EI-HRTOFMS and online PTR-ToF-MS, respectively. Daytime aging produced maleic anhydride, PAN and nitrophenol (mainly in gas phase), and multifunctional oxygenated compounds (semi-volatile). The formation of (methyl)-nitrocatechols was stronger at night. The fast formation of (methyl)-nitrocatechols made their temporal variability extremely similar to primary biomass burning tracers. The net growth of OC was negligible when plumes became aged, which suggests the evaporation of particle phase organic compounds and the condensation of gas phase organic compounds occurred at similar rates.