The Measured Impact of Wildfires on Ozone in Western Canada From 2001 to 2019

The impacts on atmospheric ozone (O-3) due to wildfires are difficult to characterize due to the many factors that affect O-3's formation rate and the episodic nature of fire events. This study uses a very large set of air quality data (518,987 6-hr data points) collected in Western Canada from 2001 to 2019 to determine the prevalence and severity of fire-driven increases to measured O3 values. Wildfire events are identified using the automated Trajectory-Fire Interception Method (TFIM), looking for interceptions between HYSPLIT back-trajectories and wildfire hotspots. As with other studies, which have used more restricted sets of measurements, the results from this large-scale, data-driven approach indicate increases in the O-3 mixing ratio with wildfire impact, on average similar to 2 ppbv across all wildfire time periods. To understand the factors which lead to the largest increases, and to better compare to other studies looking at individual fire events, wildfire events are classified using their distance from the air quality measurement location, time of measurement, and corresponding PM2.5 value. Increases to O-3 are largest during the daytime, when fires occur close to the air quality measurement, and with corresponding measurements of PM2.5 > 25 mu g/m(3). When an upper-limit correction for the bias in UV photometric detection of ozone with MnCl2 scrubbers is applied, the analysis still yields a persistent increase in O-3 during wildfires except for the highest PM2.5 levels. However, a more accurate correction to the potential bias is needed to fully understand the magnitude of the impact of wildfires on O-3.