Vehicle emissions of radical precursors and related species observed in the 2009 SHARP campaign

<div><p>The 2009 Study of Houston Atmospheric Radical Precursors (SHARP) field campaign had several components that yielded information on the primary vehicular emissions of formaldehyde (HCHO) and nitrous acid (HONO), in addition to many other species. Analysis of HONO measurements at the Moody Tower site in Houston, TX, yielded emission ratios of HONO to the vehicle exhaust tracer species NO_x and CO of 14 pptv/ppbv and 2.3 pptv/ppbv, somewhat smaller than recently published results from the Galleria site, although evidence is presented that the Moody Tower values should be upper limits to the true ratios of directly emitted HONO, and are consistent with ratios used in current standard emissions models. Several other Moody Tower emission ratios are presented, in particular a value for HCHO/CO of 2.4 pptv/ppbv. Considering only estimates of random errors, this would be significantly lower than a previous value, though the small sample size and possible systematic differences should be taken into account. Emission factors for CO, NO_x, and HCHO, as well as various volatile organic compounds (VOCs), were derived from mobile laboratory measurements both in the Washburn Tunnel and in on-road exhaust plume observations. These two sets of results and others reported in the literature all agree well, and are substantially larger than the CO, NO_x, and HCHO emission factors derived from the emission ratios reported from the Galleria site.</p><p>Implications: <i>Emission factors for the species measured in the various components of the 2009 SHARP campaign in Houston, TX, including HCHO, HONO, CO, CO₂, nitrogen oxides, and VOCs, are needed to support regional air quality monitoring. Components of the SHARP campaign measured these species in several different ways, each with their own potential for systematic errors and differences in vehicle fleets sampled. Comparisons between data sets suggest that differences in sampling place and time may result in quite different emission factors, while also showing that different vehicle mixes can yield surprisingly similar emission factors.</i></p>