NO to NO2 Conversion Rate Analysis and Implications for Dispersion Model Chemistry Methods using Las Vegas, Nevada Near-Road Field Measurements

1.1. Emitted NO2/NOX Ratios

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Urban area oxides of nitrogen (NOX) emissions are primarily from fuel combustion sources such as power generation facilities and transportation sources (U.S. EPA, 2010a). In the U.S., for calendar year 2010, NOX emissions from transportation sources comprised 38% of the national inventory. In some urban areas, NOX emissions from transportation sources (e.g., highway vehicles) may contribute substantially more to the area’s total NOX emissions than other sources due to the area’s mix of anthropogenic emission sources. While U.S. NOX emissions from highway vehicles have been declining for many years, a 63% decrease from 2003 to 2016, NOX emissions from highway vehicles continue to be a public health concern (U.S. EPA, 2010b, 2016).

Numerous studies have reported either nitric oxide (NO), nitrogen dioxide (NO2), or NOX concentrations for the urban or near-road environment (Alvarez et al., 2008; Beckerman et al., 2008; Carslaw and Beevers, 2005; Clements et al., 2009; Costantini et al., 2016; Gilbert et al., 2003; McAdam et al., 2011; Yasuyuki et al., 2014). Some studies have also reported NO2/NOX ratios for the urban or near-road environment. For example, Richmond-Bryant et al. (2017) reported the average NO2/NOX ratio for the Las Vegas Near Road Study was 0.39 and Costantini, et al. (2016) reported an average NO2/NOX ratio of 0.36 from the Advanced Collaborative Emissions Study (ACES). Some studies have reported very low NO2/NOX ratios. For example, Wild et al. (2017) reported an average on-road NO2/NOX ratio of 0.053 and Jimenez et al. (2000) reported an average NO2/NOX ratio of 0.078. Several factors may be responsible for the wide range of reported NO2/NOX concentrations, such as differences in measurement instrumentation, distance from roadway, vehicle fleet mix, and meteorology. Carslaw et al. (2005) and Mavroidis and Chaloulakou (2011) have reported an upwards trend in the NO2/NOX ratio that may be due to changes in the vehicle fleet mix and emission control technologies. The implication of these differing NO2/NOX ratios, which span nearly a factor of 10, is that the frequently assumed NO2/NOX ratio of 5% by volume is inappropriate for most freeways (Wang et al., 2011). The NO2/NOX ratio may also be impacted by the local vehicle fleet mix, especially for freeways with a large fraction of heavy-duty trucks and the freeway’s diurnal profile (Wang et al., 2011). Thus, a more robust NO2/NOX emission ratio is needed for dispersion modeling scenarios involving the near road environment.

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