Abstract
Abstract. The Houston-Galveston Airshed (HGA) is one of the major metropolitan areas in the US that is classified as a nonattainment area of federal ozone standards. Formaldehyde (HCHO) is a key species in understanding ozone related air pollution; some of the highest HCHO concentrations in North America have been reported for the HGA. We report on HCHO measurements in the HGA from summer 2006. Among several sites, maximum HCHO mixing ratios were observed in the Houston Ship Channel (HSC), a region with a very high density of industrial/petrochemical operations. HCHO levels at the Moody Tower (MT) site close to downtown were dependent on the wind direction: southerly maritime winds brought in background levels (0.5–1 ppbv) while trajectories originating in the HSC resulted in high HCHO (up to 31.5 ppbv). Based on the best multiparametric linear regression model fit, the HCHO levels at the MT site can be accounted for as follows: 38.5±12.3% from primary vehicular emissions (using CO as an index of vehicular emission), 24.1±17.7% formed photochemically (using peroxyacetic nitric anhydride (PAN) as an index of photochemical activity) and 8.9±11.2% from industrial emissions (using SO2 as an index of industrial emissions). The balance 28.5±12.7% constituted the residual which cannot be easily ascribed to the above categories and/or which is transported into the HGA. The CO related HCHO fraction is dominant during the morning rush hour (06:00–09:00 h, all times are given in CDT); on a carbon basis, HCHO emissions are up to 0.7% of the CO emissions. The SO2 related HCHO fraction is significant between 09:00–12:00 h. After 12:00 h HCHO is largely formed through secondary processes. The HCHO/PAN ratios are dependent on the SO2 levels. The SO2 related HCHO fraction at the downtown site originates in the ship channel. Aside from traffic-related primary HCHO emissions, HCHO of industrial origin serves as an appreciable source for OH in the morning.
Highlights
The Houston Galveston Airshed (HGA) continues to be a non-attainment region under United States federal ozone standards
In contrast to the Houston’s Ship Channel (HSC) sites, HCHO levels at Moody Tower (MT) show a peak in the median values during the morning hours (06:00–10:00 h) likely indicating rush hour impact
The MT(HSC) diurnal maximum occurs between the two MT(urban) HCHO peaks
Summary
The Houston Galveston Airshed (HGA) continues to be a non-attainment region under United States federal ozone standards. Rapid ozone (O3) formation processes are associated with releases of highly reactive volatile organic compounds (HRVOCs) from industrial facilities predominantly located in Houston’s Ship Channel (HSC), which comprises large agglomerations of petrochemical industries (Kleinman et al, 2002; Daum et al, 2003, 2004; Ryerson et al, 2003; Berkowitz et al, 2005); the impact of non methane hydrocarbons on downwind non-industrialized areas has been extensively studied (Leuchner and Rappengluck, 2010). Among HRVOCs compounds that serve as radical sources are of particular concern. While HCHO may be emitted primarily from incomplete combustion in either mobile or stationary sources (Zweidinger et al, 1988; Altshuller, 1993; Chen et al, 2004; Dasgupta et al, 2005), it can be formed from ozonolysis of terminal olefins, a pathway that does not consume OH
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