Can ΔPM2.5/ΔCO and ΔNOy/ΔCO Enhancement Ratios Be Used to Characterize the Influence of Wildfire Smoke in Urban Areas?

ABSTRACT In this study we investigate the use of ΔPM2.5/ΔCO and ΔNOy/ΔCO normalized enhancement ratios (NERs) in identifying wildfire (WF) smoke events in urban areas. Nine urban ambient monitoring sites with adequate CO, PM2.5, and/or NOy measurements were selected for this study. We investigated if WF events could be distinguished from general urban emissions by comparing NERs for wildfires with NERs calculated using yearly ambient data, which we call the ambient enhancement ratios (AERs). The PM2.5/CO and NOy/CO AERs represent typical urban concentrations and can provide insight into the dominant emission sources of the city. All 25 WF events were distinguished because they had ΔPM2.5/ΔCO NERs that were significantly greater than the PM2.5/CO AER for each site. The ΔPM2.5/ΔCO NERs for the WF events ranged from 0.057–0.228 µg m–3 ppbv–1. In contrast, we were only able to calculate useful ΔNOy/ΔCO NERs (correlations with R2 > 0.65) for 4 of 17 events (only 17 of 25 events had NOy data). For these 4 events, ΔNOy/ΔCO NERs ranged from 0.044–0.075 ppbv ppbv–1, not all of which were significantly different from the NOy/CO AERs at the site. We conclude that ΔPM2.5/ΔCO NERs are a very useful tool for identifying WF events, but that the high and variable NOy concentrations in urban areas present problems when trying to use ΔNOy/ΔCO NERs.