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On-Road Chemical Transformation as an Important Mechanism of NO2 Formation
Yang, B., M. Zhang, R. Snow, R. Baldauf, P. Deshmukh, X. Wu, W. Xu, S. Zhang, S. Batterman, Y. Wu, Q. Zhang, AND Z. Li. On-Road Chemical Transformation as an Important Mechanism of NO2 Formation. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 52(8):4574-4582, (2017). https://doi.org/10.1021/acs.est.7b05648
Nitrogen dioxide (NO2) not only is linked to adverse effects on the respiratory system but also contributes to the formation of ground-level ozone (O3) and fine particulate matter (PM2.5). Our curbside monitoring data analysis in Detroit, MI, and Atlanta, GA, strongly suggests that a large fraction of NO2 is produced during the “tailpipe-to-road” stage. To substantiate this finding, we designed and carried out a field campaign to measure the same exhaust plumes at the tailpipe-level by a portable emissions measurement system (PEMS) and at the on-road level by an electric vehicle-based mobile platform. Furthermore, we employed a turbulent reacting flow model, CTAG, to simulate the on-road chemistry behind a single vehicle. We found that a three-reaction (NO–NO2–O3) system can largely capture the rapid NO to NO2 conversion (with time scale ≈ seconds) observed in the field studies. To distinguish the contributions from different mechanisms to near-road NO2, we clearly defined a set of NO2/NOx ratios at different plume evolution stages, namely tailpipe, on-road, curbside, near-road, and ambient background. Our findings from curbside monitoring, on-road experiments, and simulations imply the on-road oxidation of NO by ambient O3 is a significant, but so far ignored, contributor to curbside and near-road NO2. This study would be of interest to environmental regulatory agencies and scientific researchers and model developers in the fields of air quality, transportation management, and traffic-related pollutant health effects.
Nitrogen dioxide (NO2) not only is linked with a number of adverse effects on the respiratory system, but also contributes to the formation of ground-level ozone (O3) and fine particulate matter (PM2.5) pollution. NO2 levels near major roads have been monitored as part of the one-hour and annual NO2 standard in the revised National Ambient Air Quality Standards (NAAQS). Our analysis of near-road monitoring data in Detroit, MI and Atlanta, GA strongly suggests that a large fraction of NO2 is actually produced through chemical reactions with O3 during the “tailpipe-to-road” stage, even with a relatively short residence time. To further substantiate this finding, we designed a field campaign to compare tailpipe-level and on-road NO2 concentrations normalized by CO2 concentrations. This comparison was accomplished through measuring the same exhaust plumes at the tailpipe-level NO2 and CO2 concentrations by a Portable Emission Measurement System (PEMS) and at the on-road level by an Electric Vehicle-based mobile platform. The results showed that CO2-normalized NO2 concentrations, taking into account the effect of dilution, were significantly higher at the on-road level than those at the tailpipe level. Furthermore, we employed a turbulent reacting flow model, CTAG, to simulate the coupled on-road turbulence and chemistry behind a single vehicle, and found that the simplified chemical mechanism using a three-reaction (NO-NO2-O3) system can largely capture the rapid NO to NO2 conversion (with timescale ~ seconds) observed in the field studies. In summary, results from near-road monitoring, on-road experiments and numerical simulations all support the importance of on-road NOx chemical transformation. Our findings provide insights into developing future near-road NO2 mitigation and monitoring strategies that would benefit the vehicle industry and government agencies.
On-Road Chemical Transformation as an Important Mechanism of NO2 Formation Exit