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 R 2 > 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.

Technical note: Use of PM<sub>2.5</sub> to CO ratio as a tracer of wildfire smoke in urban areas

“…The simulation suggests an optimum Rsmoke value of 140 µg m -3 ppm −1 . So, consistent with the work of Laing et al (2017) and Selimovic et al (2019;, we find that the best-fit NER values at the surface are much lower than emission factors reported for fresh or free tropospheric smoke plumes.…”

“…The data are segregated using (i) the HMS smoke product and (2) a PM2.5/CO ratio greater or less than 30. The value of 30 is chosen based, in part, on the work of Laing et al (2017) and on evaluation of likely smoke influence. We find the slopes and correlations are not strongly influenced by the choice of PM2.5/CO ratio.…”

“…Reported ΔPM2.5/ΔCO NERs in urban areas from several studies are in the range of 21-45 µg m -3 ppm −1 , which would correspond to an emission ratio of 0.018-0.039 g/g, assuming no loss of either species (Dimitriou and Kassomenos, 2014;Patton et al, 2014). Laing et al (2017) looked at the PM2.5 to CO correlation in 8 western U.S. cities for non-smoke periods and found reasonable correlations (R 2 values of 0.32-0.57) and slopes of 21-66 with a median value of 35 µg m -3 ppm −1 (corresponding to emission ratios of 0.018-0.057 g/g). Laing et al (2017) also found that smoke-influenced periods had much higher PM2.5 to CO correlation slopes ranging from 57-228, with a median of 128 µg m -3 ppm −1 (n=25).…”

“…Laing et al (2017) looked at the PM2.5 to CO correlation in 8 western U.S. cities for non-smoke periods and found reasonable correlations (R 2 values of 0.32-0.57) and slopes of 21-66 with a median value of 35 µg m -3 ppm −1 (corresponding to emission ratios of 0.018-0.057 g/g). Laing et al (2017) also found that smoke-influenced periods had much higher PM2.5 to CO correlation slopes ranging from 57-228, with a median of 128 µg m -3 ppm −1 (n=25). Garofolo ( 2019) reported values of 80-400 µg m -3 ppm −1 , with most between 200-300 for the organic aerosol to CO NER for 20 fires during the 2018 WECAN experiment.…”

Relationships between Particulate Matter, Ozone, and Nitrogen Oxides during Urban Smoke Events in the Western US

“…O 3 impacts may instead be assessed observationally using chemical tracers, and several tracers have been used as indicators of smoke, including CO, acetonitrile, , water-soluble potassium (K + ), − levoglucosan, , and PM 2.5 /CO ratios (PM 2.5 = particulate matter <2.5 μm in diameter) . However, fuel type and combustion temperature can modulate the emission of many chemical tracers from wildland fires, − complicating quantitative analyses of smoke influence.…”