Can d;PM2.5/d;CO and d;NOy/d;CO Enhancement Ratios Be Used to Characterize the Influence of Wildfire Smoke in Urban Areas?

ABSTRACT

 In this study we investigate the use of ΔPM_2.5/ΔCO and ΔNO_y/ΔCO normalized enhancement ratios (NERs) in identifying wildfire (WF) smoke events in urban areas. Nine urban ambient monitoring sites with adequate CO, PM_2.5, and/or NO_y 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 PM_2.5/CO and NO_y/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 ΔPM_2.5/ΔCO NERs that were significantly greater than the PM_2.5/CO AER for each site. The ΔPM_2.5/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 ΔNO_y/ΔCO NERs (correlations with R² > 0.65) for 4 of 17 events (only 17 of 25 events had NO_y data). For these 4 events, ΔNO_y/ΔCO NERs ranged from 0.044–0.075 ppbv ppbv^–1, not all of which were significantly different from the NO_y/CO AERs at the site. We conclude that ΔPM_2.5/ΔCO NERs are a very useful tool for identifying WF events, but that the high and variable NO_y concentrations in urban areas present problems when trying to use ΔNO_y/ΔCO NERs.