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

The nitrogen dioxide/oxides of nitrogen (NO/NO) ratio is an important surrogate for NO to NO chemistry in dispersion models when estimating NO impacts in a near-road environment. Existing dispersion models use different techniques and assumptions to represent NO to NO conversion and do not fully characterize all of the important atmospheric chemical and mechanical processes. Thus, "real-world" ambient measurements must be analyzed to assess the behavior of NO/NO ratios near roadways. An examination of NO/NO ratio data from a field study conducted in Las Vegas, Nevada (NV), from mid-December, 2008 through mid-December, 2009 provides insights into the appropriateness of assumptions about the NO/NO ratio included in dispersion models. Data analysis indicates multiple factors affect the downwind NO/NO ratio. These include spatial gradient, background ozone (O), source emissions of NO and NO, and background NO/NO ratio. Analysis of the NO/NO ratio spatial gradient indicates that under high O conditions, the change in the ratio is fairly constant once a certain O threshold (≥ 30 ppb) is reached. However, under low O conditions (< 30 ppb), there are differences between weekdays and weekends, most likely due to a decline in O concentrations during the weekday morning hours, reducing the O available to titrate the emitted NO, allowing lower NO/NO ratios. These results suggest that under high O conditions, NO chemistry is driving the NO/NO ratios whereas under low O conditions, atmospheric mixing is the driving factor.