New Tools to Identify Wildfire Influence on O3 in Houston

While the Houston metropolitan area has made significant strides in reducing O₃ concentrations, the region still has many days above the U.S. air quality standard. Wildfires emit O₃ precursors and usually result in enhanced O₃ downwind. In this work, I used a variety of tools to quantify wildfire impacts on O₃ in the Houston area. This is important because if a state can show that a daily O₃ exceedance occurred due to an uncontrollable source, such as a wildfire, it may be allowed to exclude this data from regulatory consideration. But there are large variations from fire to fire in emissions, plume heights and photochemical processing which make it very challenging to model O₃ production due to wildfires using Eulerian models. In this work, we describe a statistical approach (Generalized Additive Modeling) to characterize the maximum daily 8-hour average O₃ (MDA8) for Houston for typical, non-fire, conditions using a variety of meteorological parameters (temp, RH, transport pattern, etc). The statistical model can explain approximately 78% of the variance in MDA8 for Houston. We then examine the residual from the model under conditions with elevated particulate matter (PM) and satellite observed smoke (“smoke days”). For these days the residuals are elevated by an average of 7 ppb compared to the non-smoke days, but with a high degree of variability. We also show that a published method that does not correct for transport pattern gives rise to large over-estimates in the amount of O3 from fires, particularly for coastal cities which have large gradients between marine and land emission sources. Finally, we apply this method to several case studies in 2015 and 2016 and show that the method gives reasonable results that are directly applicable to the EPA guidance on excluding data due to an uncontrollable source.