Comparison of wintertime CO to NOx ratios to MOVES and MOBILE6.2 on-road emissions inventories

Abstract The CO-to-NO x molar emission ratios from the US EPA vehicle emissions models MOVES and MOBILE6.2 were compared to urban wintertime measurements of CO and NO x . Measurements of CO, NO x , and volatile organic compounds were made at a regional air monitoring site in Boise, Idaho for 2 months from December 2008 to January 2009. The site is impacted by roadway emissions from a nearby busy urban arterial roads and highway. The measured CO-to-NO x ratio for morning rush hour periods was 4.2 ± 0.6. The average CO-to-NO x ratio during weekdays between the hours of 08:00 and 18:00 when vehicle miles travelled were highest was 5.2 ± 0.5. For this time period, MOVES yields an average hourly CO-to-NO x ratio of 9.1 compared to 20.2 for MOBILE6.2. Off-network emissions are a significant fraction of the CO and NO x emissions in MOVES, accounting for 65% of total CO emissions, and significantly increase the CO-to-NO x molar ratio. Observed ratios were more similar to the average hourly running emissions for urban roads determined by MOVES to be 4.3.