On-road Emissions of Reactive Nitrogen through In-situ, Mobile Measurements

Abstract:

Ammonia (NH₃) is a key precursor to atmospheric fine particulate matter (PM_2.5), with strong implications for regional air quality and global climate change. Existing atmospheric measurements suggest that urban traffic may provide significant amount of NH₃. NH₃ emissions in urban areas may cause greater impact on air quality and human health, because other aerosol tracers, like NO and NO₂, are emitted by similar on-road sources. However, the on-road NH₃ emission inventories are subject to significant uncertainties. A mobile platform is developed by mounting multiple portable (total power ~ 100 W, weight ~ 25 kg), high-resolution (10 Hz), open-path sensors on top of a passenger car. On-road NH₃ emissions are quantified in the real-world driving conditions by synchronized NH₃, CO, and CO₂ measurements. The mobile platform has covered over 16,000 km in the US and China since 2013. The total on-road sampling time is over 300 hours. Major US metropolitan areas that have been sampled include LA, SF, Houston, Philadelphia, and Denver. Three Chinese megacities (Beijing, Baoding, and Shijiazhuang) were sampled in both 2013 and 2014. The average emission factors (grams of NH₃ emitted per kilogram of fuel) range from 0.3 to 0.5 g/kg. Different methodologies were compared, including on-road emission ratios, tunnel measurements, and city-scale gradient measurements. These methodologies yielded the same emission factor for Houston (0.4±0.05 g/kg) within the sampling uncertainties and showed that multiple approaches are consistent with one another. The observed NH₃ emission ratios indicate that National Emisison Inventory (NEI) underestimates on-road NH₃ emissions by up to 50% in some major urban areas. On-road NH₃ emission factors show higher values in both stop-and-go driving conditions and freeway speeds with a minimum near 70 km/h. This is consistent with another observation that the emission factors in urban traffic are generally larger than suburban traffic. Road gradient is found to influence emission factors substantially, with emission factors doubling at a 7% grade compared to a flat surface. An open-path NO sensor is now being incorporated to better quantify the contributions from on-road sources to secondary aerosol formation, and initial results will be presented.