Regional Influence of Wildfires on Atmospheric Aerosol in the Western US and Insights into Emission and Aging of Biomass Burning Organic Aerosol

ANALYTICAL & ENVIRONMENTAL CHEMISTRY DIVISION and
ATMOSPHERIC CHEMISTRY PROGRAM SEMINAR
Prof. Qi Zhang
UC Davis
Jointly sponsored by the Department of Chemistry and Biochemistry, CIRES, and the Environmental Program
(Pizza will be provided)
"Biomass burning (BB) is one of the most important contributors to atmospheric aerosols on a global scale and a large source of emissions that impact regional air quality and global climate. In this study, aerosols in wildfire emissions in the Pacific Northwest region of the United States were studied from the Mt. Bachelor Observatory (MBO, ~ 2700 m a.s.l.) in Central Oregon and from the G-1 aircraft in summer 2013 during the DOE Biomass Burning Observation Project (BBOP) field campaign. Episodes of high aerosol concentrations were observed at MBO due to the impacts of plumes transported from large wildfire clusters upwind of the site. Organic aerosols (OA) were a dominant component of particles in these plumes and the regional enhancements of BBOA concentration normalized by amount of fuel burned were found to be mainly driven by the modified combustion efficiency (MCE), an index of the combustion processes of a fire. The ratio of the enhancement of OA mass to the enhancement of CO above their respective backgrounds (ΔOA/ΔCO) in fire plumes appeared to be constant independent of plume aging although BBOA composition was significantly influenced by atmospheric aging. Three types of BBOA were identified during this study, including a semivolatile BBOA-1 (~ 20% of OA mass) which appeared to represent BB POA and two more oxidized BBOAs (BBOA-2 and BBOA-3) that appeared to represent BB SOA. BBOA-3 was highly oxidized (O/C = 1.06; 31% of OA mass), contained no levoglucosan, showed very low volatility with only ~ 40% mass loss at 200°C, and had a similar mass spectrum as low-volatility oxygenated OA (LV-OOA) commonly observed in regional airmass. The chemical evolution of BBOA was examined for an episode when fire plumes originated from a single fire source were sampled continuously for 36 hours. Longer solar radiation evidently led to a higher mass fraction of the chemically aged BBOA-2 and BBOA-3 and substantially more oxidized OA although negligible net amount of OA production was observed in plumes photochemically aged compared to plumes transported primarily during night. These results suggest that SOA formation was balanced by BBOA volatilization, leading to almost no net amount of OA mass added during aging in wildfire plumes."