Science Inventory

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

Citation:

Kimbrough, Sue, C. Owen, M. Snyder, AND J. Richmond-Bryant. NO to NO2 conversion rate analysis and implications for dispersion model chemistry methods using Las Vegas, Nevada near-road field measurements. ATMOSPHERIC ENVIRONMENT. Elsevier Science Ltd, New York, NY, 165:23-24, (2017). https://doi.org/10.1016/j.atmosenv.2017.06.027

Impact/Purpose:

Dispersion models are typically used to simulate real-world atmospheric and chemical processes such as physical characteristics of source emissions, atmospheric transport, photochemical reactions, deposition, and meteorological interactions. 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. This analysis examined NO2/NOX ratio spatial gradients near a major freeway in Las Vegas, NV and the results indicate multiple factors drive the NO2/NOX ratio. Existing dispersion models do not employ all of the important atmospheric chemical and mechanical processes that are important to estimating the evolution of the NO2/NOX ratio. Thus, a single assumption about the NO2 chemistry cannot account for all of these processes. These results highlight the usefulness of long-duration gradient data in assessing the behavior of NO2/NOX ratios to inform air quality modeling strategies in the near road environment. These results also support the need for future enhancements to existing models to incorporate ambient background NO2/NOX ratio, emissions NO2/NOX ratio, and O3 concentration into a single NOX chemistry scheme within a dispersion model. These results are important to dispersion modelers across the spectrum of environmental agencies that may have a need to perform dispersion modeling for air quality management applications.

Description:

Nitrogen dioxide/oxides of nitrogen (NO2/NOX) ratios are an important surrogate for nitric oxide (NO) NO-to-NO2 chemistry in dispersion models when estimating NOX emissions 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 on-going and competing atmospheric chemical and mechanical processes. These models are important tools when assessing air quality strategies in the near-road environment and need to more accurately model these processes. An examination of NO2/NOX ratio data from a field study conducted in Las Vegas, Nevada (NV) from mid-December, 2008 thru mid-December, 2009 provides insights into the appropriateness of the NO2/NOX ratio assumptions included in dispersion models. Data analysis indicates multiple important factors including spatial gradient, background ozone, source emissions of NO and NO2, background NO2/NOX ratio. Analysis of the NO2/NOX ratio spatial gradient is important and indicates that under high ozone conditions, the change in the ratio is fairly constant once a certain ozone threshold (≥ 30 ppb) is reached. However, in low ozone conditions (< 30 ppb) there is a significant difference between weekdays and weekends most likely due to a severe dip in the ozone in the morning hours, which means that in the morning the available ozone to titrate the emitted NO is very low, allowing for lower NO2/NOX ratios. These results suggest that under high ozone conditions NOX chemistry is driving the NO2/NOX ratios whereas under low ozone conditions atmospheric mixing is the more important factor. These results highlight the usefulness of long-duration gradient data in assessing the behavior of NO2/NOX ratios near roadways and support the need for future enhancements to the NOX chemistry in existing dispersion models.