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Ozone (O-3) 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 (NOx), together with their chemistry contribute to the O-3 and NOx mixing ratios in the Houston-Galveston-Brazoria (HGB) region. In addition to local emissions, chemistry and transport, larger-scale factors also contribute to local O-3 and NOx. 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 O-3. In this study, we estimate ground-level regional background O-3 and NOx in the HGB region and quantify their decadal-scale trends.

We use four different approaches based on principal component analysis (PCA) to quantify background O-3 and NOx. Three of these approaches consist of independent PCA on both O-3 and NOx 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 O-3, NOx and meteorology.

Our results show that the estimation of regional background O-3 has less inherent uncertainty when it was constrained by NOx and meteorology, yielding a statistically significant temporal trend of 0.68 +/- 0.27 ppb yr(-1). Likewise, the estimation of regional background NOx trend constrained by O-3 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 O-3 and NOx 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 O-3 is consistent with previous studies and between the approaches used in this study, although the approaches based on 8 h averages likely overestimate background O-3 compared to the hourly median approach by 79 ppb. Similarly, the upper bound of average background NOx 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 NOx due to instrument overdetection of NOx and the 8 h averaging of NOx and meteorology coinciding with MDA8 O-3.

Regional background O-3 and NOx 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.