NO to NO2 conversion rate analysis, and implications for dispersion model chemistry methods using Las Vegas, Nevada near-road field measurements

The nitrogen dioxide/oxides of nitrogen (NO2/NOX) ratio is an important surrogate for NO to NO2 chemistry in dispersion models when estimating NOX 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/NOX ratios near road-ways. An examination of NO2/NOX 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/NOx ratio included in dispersion models. Data analysis indicates multiple factors affect the downwind NO2/NOX ratio. These include spatial gradient, background ozone (O-3), source emissions of NO and NO2, and background NO2/NOX ratio. Analysis of the NO2/NOX ratio spatial gradient indicates that under high O-3 conditions, the change in the ratio is fairly constant once a certain O-3 threshold (>= 30 ppb) is reached. However, under low O-3 conditions (<30 ppb), there are differences between weekdays and weekends, most likely due to a decline in O-3 concentrations during the weekday morning hours, reducing the O-3 available to titrate the emitted NO, allowing lower NO2/NOX ratios. These results suggest that under high O-3 conditions, NOX chemistry is driving the NO2/NOX ratios whereas under low O-3 conditions, atmospheric mixing is the driving factor. Published by Elsevier Ltd.