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On-Road Chemical Transformation as an Important Mechanism of NO2 Formation
Citation:
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
Impact/Purpose:
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.
Description:
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.
URLs/Downloads:
DOI: On-Road Chemical Transformation as an Important Mechanism of NO2 Formation
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