On-Road Measurements of NO2 /NOx and NOx / CO Vehicle Emission Ratios in Colorado Summer Traffic
Abstract:Emissions of nitrogen oxides (NO and NO2) from vehicles are important precursors to ozone (O3) formation, and thus contribute to environmental and health issues. Both carbon dioxide (CO2) and carbon monoxide (CO) are large emissions from combustion, with the former a marker for total fuel consumption and the latter a commonly used combustion tracer. Thus, measurements of NOx to CO and CO2 emission ratios form an important part of characterizing total vehicle NOx emissions. Vehicle NOx emissions are primarily in the form of NO, although the NO2 content of these emissions may be changing as a result of control technology implementation, particularly for Diesel engines. Characterization of direct NO2 emission factors (i.e., NO2 to NOx ratio) is important because NO2 produces ozone upon its photolysis, while NO emission titrates ozone near sources.
NO + O3 → NO2 + O2
NO2 + hν (+ O2) → NO + O3
Higher NO2 to NO emission ratios will likely result in higher ozone levels per unit NOx emitted. Higher NO2 emission ratios also lead to ozone production in closer proximity to the emission sources. There is a substantial lack of top-down measurements to assess emission inventories. Here we present an analysis of on-road measurements of vehicular emissions. We simultaneously measured NO, NO2 and O3 with a shared inlet at 20 pptv/s precision using a custom-built cavity ring-down measurement system in an instrumented van, together with high time resolution measurements of CO and CO2. Measurements were performed while driving throughout the Colorado Front Range urban area. We present a statistical analysis of vehicle plumes and their corresponding emissions ratios that can be used to constrain the direct emission ratio of NO2 in NOx, and NOx emission factors relative to CO and CO2.