Impact of wildfire smoke on ozone concentrations using a Generalized Additive model in Salt Lake City, Utah, USA, 2006-2022.

Abstract

We investigated the impact of wildfires on maximum daily 8-hr average ozone concentrations (MDA8 O₃) at four sites in Salt Lake City (SLC), Utah for May to September for 2006-2022. Smoke days, which were identified by a combination of overhead satellite smoke detection and surface PM_2.5 data and accounted for approximately 9% of the total number of days, exhibited O₃ levels 6.8 to 8.9 ppb higher than no-smoke days and were predominantly characterized by high daily maximum temperatures and low relative humidity. A Generalized Additive Model (GAM) was developed to quantify the impact of wildfire contributions to O₃. The GAM, which provides smooth functions that make the interpretation of relationships more intuitive, employed 17 predictors and demonstrated reliable performance in various evaluation metrics. The mean of the residuals for all sites was approximately zero for the training and cross-validation data and 5.1 ppb for smoke days. We developed three approaches to estimate the contribution of smoke to O₃ from the model residuals. These generate a minimum and maximum contribution for each smoke day. The average of the minimum and maximum wildfire contributions to O₃ for the SLC sites was 5.1 and 8.5 ppb, respectively. Between 2006 and 2022, an increasing trend in the wildfire contributions to O₃ was observed in SLC. Moreover, trends of the fourth-highest MDA8 O₃ before and after removing the wildfire contributions to O₃ at the SLC Hawthorne site in 2006-2022 were quite different. Whereas the unadjusted data do not meet the current O₃ standard, after removing the contributions from wildfires the SLC region is close to achieving levels that are consistent with meeting the O₃ standard. We also found that the wildfire contribution during smoke days was particularly high under conditions of high temperature, high PM_2.5 concentration, and low cloud fraction.Implications: In this study, we quantified the impact of wildfires on maximum daily 8-hr average ozone concentrations (MDA8 O₃) in Salt Lake City, Utah, using a Generalized Additive Model (GAM). The GAM results demonstrate the importance of wildfires as contributors to O₃ air pollution. Our results suggest that states could use the GAM approach to assist in quantifying the wildfire contribution to MDA8 O₃ under the U.S. EPA exceptional events rule. These findings also highlight the need for strategies to manage wildfires and their subsequent impacts on air quality in an era of climate warming.