Abstract
Abstract. Airborne measurements from two Texas Air Quality Study (TexAQS) field campaigns have been used to investigate changes of ozone production in Houston, Texas, from 2000 to 2006, a period of major emission reduction measures for petrochemical and other sources. Simultaneous declines in nitrogen oxides (NOx = NO + NO2) and highly reactive volatile organic compounds (HRVOCs) were observed between the two periods. We simulate HOx (OH and HO2) and organic radicals with a box model, the Dynamically Simple Model of Atmospheric Chemical Complexity, constrained by available airborne observations. Parameters such as total radical production, total OH reactivity of VOCs and ozone production rate (OPR) are computed to characterize the change of ozone production between 2000 and 2006 in the Houston area. The reduction in HRVOCs led to a decline in total radical production by 20–50%. Ozone production rates in the Houston area declined by 40–50% from 2000 to 2006, to which the reduction in NOx and HRVOCs made large contributions. Despite the significant decline in OPR, ozone production efficiency held steady, and VOC-sensitive conditions dominated during times of most rapid ozone formation, while the slow ozone formation continued to be NOx-limited. Our results highlight the importance of a balanced approach of ongoing HRVOC controls with NOx controls to further reduce O3 levels in the Houston area.
Highlights
The Houston metropolitan area has a long history of nonattainment of the US National Ambient Air Quality Standards for ozone (O3), despite substantial emission reduction efforts since the 1970s (Cowling et al, 2007)
Since L(O3) was negligible compared to ozone production rate (OPR) in polluted daytime samples, we focus our attention on OPR rather than net_OPR
Total radagonal line is associated with a constant total OH reactivity of VOCs (TVOC) to NOx ratio
Summary
The Houston metropolitan area has a long history of nonattainment of the US National Ambient Air Quality Standards for ozone (O3), despite substantial emission reduction efforts since the 1970s (Cowling et al, 2007). Springtime ozone exceedences tend to occur under post-frontal passage meteorological conditions, while the summertime events often feature continuous high temperature and stagnant meteorology over southeast Texas (Haman et al, 2012; TCEQ, 2007). Favored by these meteorological conditions, large amounts of co-emitted NOx and VOC from petrochemical facilities in the Houston Ship Channel (HSC) and the nearby urban center lead to high O3 levels (Fig. 1) (Cowling et al, 2007; Parrish et al, 2009; Ryerson et al, 2003). Previous studies have concluded that emission inventories in the HSC underestimated HRVOC emissions by at least one order of magnitude (Cowling et al, 2007; Parrish et al, 2009; Ryerson et al, 2003)
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