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 NO_x to CO and CO₂ emission ratios form an important part of characterizing total vehicle NO_x emissions. Vehicle NO_x 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 NO_x 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 NO_x 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 NO_x, and NO_x emission factors relative to CO and CO₂.