On-road measurements of vehicle NO2/NOx emission ratios in Denver, Colorado, USA

Nitrogen oxides (NOx = NO + NO2) emitted by on-road combustion engines are important contributors to tropospheric ozone production. The NOx fraction emitted as nitrogen dioxide (NO2) is usually presumed to be small but can affect ozone production and distribution, and this fraction is generally not reported in emissions inventories. We have developed an accurate method for determination of this primary NO2 emission and demonstrated it during measurement of on-road vehicle emission plumes from a mobile laboratory during July and August 2014 in the region between Denver and Greeley in Colorado. During a total of approximately 90 h of sampling from an instrumented mobile laboratory, we identified 1867 vehicle emission plumes, which were extracted using an algorithm that looks for rapid and large increases in measured NOx. We find a distribution of NO2/NOx emissions similar to a log-normal profile, with an average emission ratio of 0.053 ± 0.002 per sampled NOx plume. The average is not weighted by the total NOx emissions from sampled vehicles, which is not measured here, and so may not represent the NO2/NOx ratio of the total NOx emission if this ratio is a function of NOx itself. Although our current data set does not distinguish between different engine types (e.g., gasoline, light duty diesel and heavy duty diesel), the ratio is on the low end of recent reports of vehicle fleet NO2 to NOx emission ratios in Europe.