Regional background O₃ and NO_x in the Houston–Galveston–Brazoria (TX) region: a decadal-scale perspective

Abstract. Ozone (O₃) in the lower troposphere is harmful to people and plants, particularly during summer, when photochemistry is most active and higher temperatures favor local chemistry. Local precursor emissions, such as those of volatile organic compounds (VOCs) and nitrogen oxides (NO_x), together with their chemistry contribute to the O₃ and NO_x mixing ratios in the Houston–Galveston–Brazoria (HGB) region. In addition to local emissions, chemistry and transport, larger-scale factors also contribute to local O₃ and NO_x. These additional contributions (often referred to as regional background) are not well quantified within the HGB region, impeding more efficient controls on precursor emissions to achieve compliance with the National Ambient Air Quality Standards for O₃. In this study, we estimate ground-level regional background O₃ and NO_x in the HGB region and quantify their decadal-scale trends.

We use four different approaches based on principal component analysis (PCA) to quantify background O₃ and NO_x. Three of these approaches consist of independent PCA on both O₃ and NO_x for both 1 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 O₃, NO_x and meteorology.

Our results show that the estimation of regional background O₃ has less inherent uncertainty when it was constrained by NO_x and meteorology, yielding a statistically significant temporal trend of −0.68 ± 0.27 ppb yr⁻¹. Likewise, the estimation of regional background NO_x trend constrained by O₃ and meteorology was −0.04 ± 0.02 ppb yr⁻¹ (upper bound) and −0.03 ± 0.01 ppb yr⁻¹ (lower bound). Our best estimates of the 17-year average of season-scale background O₃ and NO_x were 46.72 ± 2.08 ppb and 6.80 ± 0.13 ppb (upper bound) or 4.45 ± 0.08 ppb (lower bound), respectively. Average background O₃ is consistent with previous studies and between the approaches used in this study, although the approaches based on 8 h averages likely overestimate background O₃ compared to the hourly median approach by 7–9 ppb. Similarly, the upper bound of average background NO_x is consistent between approaches in this study (A–C) but overestimated compared to the hourly approach by 1 ppb, on average. We likely overestimate the upper-bound background NO_x due to instrument overdetection of NO_x and the 8 h averaging of NO_x and meteorology coinciding with MDA8 O₃.

Regional background O₃ and NO_x in the HGB region both have declined over the past 2 decades. This decline became steadier after 2007, overlapping with the effects of controlling precursor emissions and a prevailing southeasterly–southerly flow.