Regional background O3 and NOx in the Houston-Galveston- Brazoria (TX) region: A decadal-scale perspective

Other literature type English OPEN
Suciu, Loredana G. ; Griffin, Robert J. ; Masiello, Caroline A. (2016)

Ozone (O<sub>3</sub>) in the lower troposphere is harmful to people and plants, particularly during summer, when photochemistry is the most active and higher temperatures favor local chemistry. Because of its dependence on the volatile organic compounds (VOCs) to nitrogen oxides (NO<sub>x</sub>) ratio, ground-level O<sub>3</sub> is difficult to control locally, where many sources of these precursors contribute to its mixing ratio. In addition to local emissions, chemistry and transport, larger-scale factors also contribute to local O<sub>3</sub> and NO<sub>x</sub>. These additional contributions (often referred to as "regional background") are not well quantified within the Houston-Galveston-Brazoria (HGB) region, impeding more efficient controls on precursor emissions to achieve compliance with the National Ambient Air Quality Standards for O<sub>3</sub>. In this study, we estimate regional background O<sub>3</sub> and NO<sub>x</sub> in the HGB region and quantify their decadal-scale trends. <br><br> We use four different approaches based on principal component analysis (PCA) to quantify background O<sub>3</sub> and NO<sub>x</sub>. Three of these approaches consist of independent PCA on both O<sub>3</sub> and NO<sub>x</sub> for both 1-h and 8-h levels to compare our results with previous studies and to highlight the effect of both temporal and spatial scales. In the fourth approach, we co-varied O<sub>3</sub>, NO<sub>x</sub> and meteorology. <br><br> Our results show that the estimation of regional background O<sub>3</sub> has less inherent uncertainty when it was constrained by NO<sub>x</sub> and meteorology, yielding a statistically significant temporal trend of &minus;0.69 ± 0.27&thinsp;ppb&thinsp;y<sup>&minus;1</sup>. Likewise, the estimation of regional background NO<sub>x</sub> trend constrained by O<sub>3</sub> and meteorology was &minus;0.04 ± 0.02&thinsp;ppb&thinsp;y<sup>&minus;1</sup>. Our best estimates of 17-y average of season-scale background O<sub>3</sub> and NO<sub>x</sub> were 46.72 ± 2.08&thinsp;ppb and 6.80 ± 0.13&thinsp;ppb, respectively. <br><br> Regional background O<sub>3</sub> and NO<sub>x</sub> both have declined over time in the HGB region. This decline is likely caused by a combination of state of Texas controls on precursor emissions since 2007 and the increase in frequency of flow from the Gulf of Mexico over the same time period.
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