Flux Measurements of Volatile Organic Compounds from an Urban Tower Platform in Houston, Texas: Trends and Tracers



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Energy and trace gas fluxes were measured from anthropogenic and biogenic emission sources in the Houston urban surface layer. Air sampling from a tall tower platform began in 2008 and continued through April 2013. A relaxed eddy accumulation (REA) system combined with a dual channel GC-FID was used to measure the flux of 19 different volatile organic compounds (VOCs) from C4 through C8 species. We discuss a time series comparison of local concentrations and fluxes of benzene, toluene, ethylbenzene, and xylenes (BTEX) from winter 2009 compared to spring 2013. Median concentrations over the four-year period fell between 20 to 34 percent, comparable to long-term VOC reduction trends observed in other major metropolitan areas. Emissions of these species fell accordingly with median flux reductions of 25 to 54 percent. For emissions inventory validation purposes, traffic counts were taken along major commuter roads surrounding the tower. We observed a strong correlation between selected vehicle exhaust VOC fluxes and traffic counts except during variable working hours.

To assign measured fluxes to local sources, we tested a bulk flux footprint model (Kormann and Meixner model) designed for uniform emission surface areas in this urban, heterogeneous landscape. Tracer releases of known amounts of acetone and methyl-ethyl-ketone (MEK) were performed within the footprint region to validate the model. Four out of six tracer releases matched the fluxes measured at the GC within the given levels of uncertainty for the footprint model and the REA GC-FID system. There were reasonable causes of error for the two releases that did not match. The footprint model was also tested using a known n-pentane emissions source approximately one mile SSE of the tower. Using modeled footprints under wind directions directly from this source, we calculated that the facility emitted an average of 6.35 ? 3.63 (1 sd) kg of n-pentane per hour. These rates fell within the facility?s TCEQ permitted hourly emissions allowing 10.5 kg of VOC per hour. However, if occurring daily, the calculated emission would be above the permit?s yearly emission rate of 4.67 kg of VOC per hour at a 68% likelihood based on a normal distribution.