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Ozone (O3) in the lower troposphere is harmful topeople and plants, particularly during summer, when photochemistry is mostactive and higher temperatures favor local chemistry. Local precursoremissions, such as those of volatile organic compounds (VOCs) and nitrogenoxides (NOx), together with their chemistry contribute to the O3and NOx mixing ratios in the Houston–Galveston–Brazoria (HGB) region.In addition to local emissions, chemistry and transport, larger-scalefactors also contribute to local O3 and NOx. These additionalcontributions (often referred to as <q>regional background</q>) are not wellquantified within the HGB region, impeding more efficient controls onprecursor emissions to achieve compliance with the National Ambient AirQuality Standards for O3. In this study, we estimate ground-levelregional background O3 and NOx in the HGB region and quantifytheir decadal-scale trends. We use four different approaches based on principal component analysis (PCA)to quantify background O3 and NOx. Three of these approachesconsist of independent PCA on both O3 and NOx for both 1 and 8 hlevels to compare our results with previous studies and to highlight theeffect of both temporal and spatial scales. In the fourth approach, weco-varied O3, NOx and meteorology.Our results show that the estimation of regional background O3 has lessinherent 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 trendconstrained 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 bestestimates of the 17-year average of season-scale background O3 and NOxwere 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 isconsistent with previous studies and between the approaches used in thisstudy, although the approaches based on 8 h averages likely overestimatebackground O3 compared to the hourly median approach by 7–9 ppb.Similarly, the upper bound of average background NOx is consistentbetween approaches in this study (A–C) but overestimated compared to thehourly approach by 1 ppb, on average. We likely overestimate the upper-boundbackground NOx due to instrument overdetection of NOx and the 8 haveraging of NOx and meteorology coinciding with MDA8 O3. Regional background O3 and NOx in the HGB region both havedeclined over the past 2 decades. This decline became steadier after 2007,overlapping with the effects of controlling precursor emissions and aprevailing southeasterly–southerly flow

atmospheric chemistry and physics

Regional background O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; in the Houston–Galveston–Brazoria (TX) region: a decadal-scale perspective