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.