963 Influence of Smoke on O3 and PM in Houston, Texas

Wednesday, 9 January 2019
Hall 4 (Phoenix Convention Center - West and North Buildings)
Daniel Jaffe, Univ. of Washington Bothell, Bothell, WA

The Houston metropolitan area has made significant strides in reducing O3 concentrations, but the region still has many days above the current O3 standard. Fires emit O3 precursors and usually result in enhanced O3 downwind. In this work, I used a variety of tools to quantify fire impacts on O3 in the Houston area. First we use Generalized Additive Models (GAMs) to explore the relationship between the maximum daily 8-hour average O3 (MDA8) for Houston for typical, non-smoke, conditions using a variety of meteorological parameters (temp, RH, transport pattern, CAPE, etc). The statistical model can explain up to 80% of the variance in MDA8 for O3 for Houston. The model residual can then be used to tell us about unusual O3 sources.

We use surface PM2.5 data and the NOAA Hazard Mapping System-Fire and Smoke Product (HMS-FSP) to identify days with a high probability of smoke. We define “high confidence smoke days” as those with both overhead HMS smoke and enhanced PM2.5 at the surface (>15.1 ug/m3), defined by the mean of non-smoke days (10.6 ug/m3) plus one S.D. (4.5 ug/m3). For these days, the O3 GAM residuals are elevated by an average of 7 ppb compared to the non-smoke days, but with a high degree of variability. We also find that at similar levels of PM2.5, smoke days show higher O3 MDA8 values compared to non-smoke days. For the high confidence smoke days, the monthly mean GAM residual (extra O3) is between 0-4 ppb (in April through July) and 10-14 ppb (in August through October). This indicates that the impact on O3 from fires in Houston is greatest in late summer-fall. The model residual is correlated with PM2.5 with a slope of 1.2 ppb per μg/m3 for August-October. This provide an approximate measure of the extra MDA8 O3 due to smoke for each μg/m3 of PM2.5.

In addition to smoke, the GAMs can identify “extra O3” from other unusual events. For September 1-3, 2017 Hurricane Harvey disrupted industrial operations in Houston and substantially increased emissions, compared to normal operations. MDA8 O3 exceeded the current air quality standards at numerous sites in Houston during this period. The GAM MDA8 O3 residual for this day was up to 30 ppb and demonstrates the unusual nature of photochemical O3 production during the post-Harvey period.

Finally, we apply the GAM method to a recent case study and show that the method gives reasonable results that are directly applicable to the EPA guidance on “exceptional events”.

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