Impact of Wildfires on Ozone Exceptional Events in the Western U.S.

Abstract

Wildfires generate substantial emissions of nitrogen oxides (NO_<i>x</i>) and volatile organic compounds (VOCs). As such, wildfires contribute to elevated ozone (O₃) in the atmosphere. However, there is a large amount of variability in the emissions of O₃ precursors and the amount of O₃ produced between fires. There is also significant interannual variability as seen in median O₃, organic carbon and satellite derived carbon monoxide mixing ratios in the western U.S. To better understand O₃ produced from wildfires, we developed a statistical model that estimates the maximum daily 8 h average (MDA8) O₃ as a function of several meteorological and temporal variables for three urban areas in the western U.S.: Salt Lake City, UT; Boise, ID; and Reno, NV. The model is developed using data from June-September 2000–2012. For these three locations, the statistical model can explain 60, 52, and 27% of the variability in daily MDA8. The Statistical Model Residual (SMR) can give information on additional sources of O₃ that are not explained by the usual meteorological pattern. Several possible O₃ sources can explain high SMR values on any given day. We examine several cases with high SMR that are due to wildfire influence. The first case considered is for Reno in June 2008 when the MDA8 reached 82 ppbv. The wildfire influence for this episode is supported by PM concentrations, the known location of wildfires at the time and simulations with the Weather and Research Forecasting Model with Chemistry (WRF-Chem) which indicates transport to Reno from large fires burning in California. The contribution to the MDA8 in Reno from the California wildfires is estimated to be 26 ppbv, based on the SMR, and 60 ppbv, based on WRF-Chem. The WRF-Chem model also indicates an important role for peroxyacetyl nitrate (PAN) in producing O₃ during transport from the California wildfires. We hypothesize that enhancements in PAN due to wildfire emissions may lead to regional enhancements in O₃ during high fire years. The second case is for the Salt Lake City (SLC) region for August 2012. During this period the MDA8 reached 83 ppbv and the SMR suggests a wildfire contribution of 19 ppbv to the MDA8. The wildfire influence is supported by PM_2.5 data, the known location of wildfires at the time, HYSPLIT dispersion modeling that indicates transport from fires in Idaho, and results from the CMAQ model that confirm the fire impacts. Concentrations of PM_2.5 and O₃ are enhanced during this period, but overall there is a poor relationship between them, which is consistent with the complexities in the secondary production of O₃. A third case looks at high MDA8 in Boise, ID, during July 2012 and reaches similar conclusions. These results support the use of statistical modeling as a tool to quantify the influence from wildfires on urban O₃ concentrations