On-Road Ammonia Emissions Characterized by Mobile, Open-Path Measurements

Ammonia (NH₃) is a key precursor species to atmospheric fine particulate matter with strong implications for regional air quality and global climate change. NH₃ from vehicles accounts for a significant fraction of total emissions of NH₃ in urban areas. A mobile platform is developed to measure NH₃, CO, and CO₂ from the top of a passenger car. The mobile platform conducted 87 h of on-road measurements, covering 4500 km in New Jersey and California. The average on-road emission factor (EF) in CA is 0.49 ± 0.06 g NH₃ per kg fuel and agrees with previous studies in CA (0.3–0.8 g/kg). The mean on-road NH₃:CO emission ratio is 0.029 ± 0.005, and there is no systematic difference between NJ and CA. On-road NH₃ EFs increase with road gradient by an enhancement of 53 mg/kg fuel per percentage of gradient. On-road NH₃ EFs show higher values in both stop-and-go driving conditions and freeway speeds with a minimum near 70 km/h. Consistent with prior studies, the on-road emission ratios suggest a highly skewed distribution of NH₃ emitters. Comparisons with existing NJ and CA on-road emission inventories indicate that there may be an underestimation of on-road NH₃ emissions in both NJ and CA. We demonstrate that mobile, open-path measurements provide a unique tool to help quantitatively understand the on-road NH₃ emissions in urban and suburban settings.