NO to NO₂ Conversion Rate Analysis and Implications for Dispersion Model Chemistry Methods using Las Vegas, Nevada...

The nitrogen dioxide/oxides of nitrogen (NO₂/NO_X) ratio is an important surrogate for NO to NO₂ chemistry in dispersion models when estimating NO_X impacts in a near-road environment. Existing dispersion models use different techniques and assumptions to represent NO to NO₂ 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 NO₂/NO_X ratios near roadways. An examination of NO₂/NO_X 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 NO₂/NO_X ratio included in dispersion models. Data analysis indicates multiple factors affect the downwind NO₂/NO_X ratio. These include spatial gradient, background ozone (O₃), source emissions of NO and NO₂, and background NO₂/NO_X ratio. Analysis of the NO₂/NO_X ratio spatial gradient indicates that under high O₃ conditions, the change in the ratio is fairly constant once a certain O₃ threshold (≥ 30 ppb) is reached. However, under low O₃ conditions (< 30 ppb), there are differences between weekdays and weekends, most likely due to a decline in O₃ concentrations during the weekday morning hours, reducing the O₃ available to titrate the emitted NO, allowing lower NO₂/NO_X ratios. These results suggest that under high O₃ conditions, NO_X chemistry is driving the NO₂/NO_X ratios whereas under low O₃ conditions, atmospheric mixing is the driving factor.