Implementation and Evaluation of NOx to NO2 Chemistry in R-LINE – a Line source Dispersion model for Traffic-related Sources

R-LINE - a new research line-source dispersion model for traffic-related sources includes advanced dispersion algorithms, but does not include any treatment of chemical processes. Specifically, since R-LINE is designed to model traffic-related sources, NO2 is a key pollutant of interest. Most NOx from combustion sources (about 90-95%) are emitted as NO, which is then readily converted to NO2 in the ambient air, while the remainder is emitted largely as NO2. Thus, bulk of ambient NO2 is formed due to secondary production in the atmosphere, which R-LINE cannot predict in its current form. EPA's 1-hour form of the National Ambient Air Quality Standard (NAAQS) for NO2 promulgated in 2010 (set at 100 ppb for a 98th percentile value, averaged over 3 years) is designed to address adverse exposure due to high short-term peaks in the vicinity of the near-road environment. EPA and states are in the process of deploying an expanded network of NO2 monitors. NO2 concentrations near major roads are appreciably higher than those measured at monitors in existing networks. Thus, there is a need to incorporate at least a reduced-form chemical mechanism in R-LINE to account for NO2 formation. We will present an updated version of R-LINE, which includes the treatment of a near-source chemistry algorithm, specifically for the NOx to NO2 conversion. We will present the performance of the new R-LINE chemistry algorithm against near road monitoring data (such as Detroit, Las Vegas, Raleigh). Our results will include comparison of the performance of R-LINE with chemistry against that of AERMOD's existing NO2 algorithm, which uses three different methods - the Plume Volume Molar Ratio Method (PVMRM), the Ozone Limiting Method (OLM) and the Ambient Ratio Method (ARM), and algorithms in the ADMS-Roads model.