04 Mar 2022
04 Mar 2022
Status: this preprint is currently under review for the journal ACP.

Technical note: Use of PM2.5 to CO ratio as a tracer of wildfire smoke in urban areas

Daniel Jaffe1,2, Brendan Schnieder3, and Daniel Inouye3 Daniel Jaffe et al.
  • 1School of STEM, University of Washington, Bothell, WA, 98011, USA
  • 2Department of Atmospheric Sciences, University of Washington, Seattle, WA, 98195, USA
  • 3Washoe County Health District, Air Quality Management Division, Reno, NV, USA

Abstract. Wildfires, and the 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 lower concentrations due to the presence of the usual array of anthropogenic pollutants. Here we propose a method using the difference in PM to CO emission ratios between smoke and typical urban pollution. For smoke, emission ratios of PM2.5 to CO are between 200–300 µg m3 ppb−1, whereas typical urban sources have an emission ratio that is lower by a factor of 4–10. This gives rise to the possibility of using this ratio as an indicator of smoke extent. We use observations a regulatory surface monitoring sites in Sparks, NV, for the period of May–September 2018–2021. During this time, there were many smoke-influenced periods from numerous California wildfires that burned during this period. Using a PM / CO ratio of 30, we can split the data 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 µg m−3 ppb−1 best fits the observations, which is significantly lower than the ratio observed in fresh smoke plumes. The most likely explanation for this difference is greater 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.

Daniel Jaffe et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-138', Farren L. Herron-Thorpe, 05 Apr 2022
  • RC2: 'Comment on acp-2022-138', Anonymous Referee #2, 06 Apr 2022

Daniel Jaffe et al.

Daniel Jaffe et al.


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Short summary
In this manuscript we use commonly measured pollutants (PM2.5 and carbon monoxide) to develop a Monte Carlo simulation of the mixing of urban pollution with smoke. The simulations compare well with observations from a heavily impacted smoke site and show that we can use standard regulatory measurements to quantify the amount of smoke in urban areas.