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

In this study we investigate the use of Delta PM2.5/Delta CO and Delta NOy/Delta CO 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.PM2.5/Delta CO NERs that were significantly greater than the PM2.5/CO AER for each site. The Delta PM2.5/Delta CO NERs for the WF events ranged from 0.057-0.228 mu g m(-3) ppbv(-1). In contrast, we were only able to calculate useful.NOy/Delta CO NERs (correlations with R-2 > 0.65) for 4 of 17 events (only 17 of 25 events had NOy data). For these 4 events, Delta NOy/Delta CO 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.PM2.5/Delta CO 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 Delta NOy/Delta CO NERs.