Technical note: Use of PM_2.5 to CO ratio as an indicator of wildfire smoke in urban areas

Abstract

Abstract. Wildfires and their resulting smoke are an increasing problem in many regions of the world. However, identifying the contribution of smoke to pollutant loadings in urban regions can be challenging at low concentrations due to the presence of the usual array of anthropogenic pollutants. Here we propose a method using the difference in PM2.5 to CO emission ratios between smoke and typical urban pollution. For temperate wildfires, the mean emission ratio of PM2.5 to CO is in the range of 0.14–0.18 g PM2.5 g CO−1, whereas typical urban emissions have a PM2.5 to CO emissions ratio that is lower by a factor of 2–20. This gives rise to the possibility of using this ratio as an indicator of wildfire smoke. We use observations at a regulatory surface monitoring site in Sparks, NV, for the period of May–September 2018–2021. There were many smoke-influenced periods from numerous California wildfires that burned during this period. Using a PM2.5 / CO threshold of 30.0 µgm-3ppm-1, we can split the observations into smoke-influenced and no-smoke periods. We then develop a Monte Carlo simulation, tuned to local conditions, to derive a set of PM2.5 / CO values that can be used to identify smoke influence in urban areas. From the simulation, we find that a smoke enhancement ratio of 140 µgm-3ppm-1 best fits the observations, which is significantly lower than the ratio observed in fresh smoke plumes (e.g., 200–300 µgm-3ppm-1). The most likely explanation for this difference is loss of PM2.5 during dilution and transport to warmer surface layers. We find that the PM2.5 / CO ratio in urban areas is an excellent indicator of smoke and should prove to be useful to identify biomass burning influence on the policy-relevant concentrations of both PM2.5 and O3. Using the results of our Monte Carlo simulation, this ratio can also quantify the influence of smoke on urban PM2.5.

Optical properties of biomass burning aerosol during the 2021 Oregon fire season: comparison between wild and prescribed fires

“…Ambient temperature may affect the ΔPM 1 /ΔCO NER given that warmer temperatures can slow OA condensation and enhance evaporation. 114 As discussed by Selimovic et al 112 and Jaffe et al , 115 ΔPM/ΔCO NERs derived from ground-based measurements of smoke are typically lower than those from smoke sampled at higher altitudes from aircraft or mountain-top sites like MBO. The 4 PF events analyzed in our study were sampled in April–May 2021, when the average ambient temperature at MBO was lower (3.7 ± 6.6 °C during PF events) compared to summertime WF events (11 ± 4.0 °C).…”

Real‐World Vehicular Source Indicators for Exhaust and Non‐Exhaust Contribution to PM2.5 During Peak and Off‐Peak Hours Using Tunnel Measurement

“…To preliminarily explore the occurrence of exhaust and non‐exhaust in tunnel, we introduced a metric, which is PM 2.5 /CO. CO is chemically stable in ambient air at the ground level and is an ideal indicator of the accumulation extent (Jaffe et al., 2022; Shen et al., 2022; Xiu et al., 2022). It is produced via combustion processes, whereas in tunnel environment, it can be necessarily derived from vehicle engine combustion, produced simultaneously with exhaust PM 2.5 .…”

A Multi-site Passive Approach for Studying the Emissions and Evolution of Smoke from Prescribed Fires

“…ERs based on NEMRs are widely used (Liu et al, 2017b;Collier et al, 2016;Burling et al, 2011;Gkatzelis et al, 2024). They are compiled in reviews and emission inventories for ambient (Andreae, 2019;Prichard et al, 2020) and laboratory fire studies (Yokelson et al, 2013), and for evaluating or making model predictions (Xiu et al, 2022;Jaffe et al, 2022).…”