On-road mobile mapping of spatial variations and source contributions of ammonia in Beijing, China

Ammonia (NH₃) measurements were performed with a mobile platform deploying a cavity ring-down spectroscopy NH₃ analyzer in Beijing. The transect and loop sampling strategy revealed that the Beijing urban area is more strongly affected by NH₃ emissions than surrounding areas. Although average enhancements of on-road NH₃ were small compared to background levels, traffic emissions clearly dominated city enhancements of NH₃, carbon dioxide (CO₂), acetaldehyde and acetone. Increments of on-road NH₃ ranged between 5.1 ppb and 11.4 ppb in urban areas, representing an enhancement of 20.6 % to 47.9 % over the urban background. The vehicle NH₃:CO₂ emission ratio was 0.26 ppb/ppm, about a factor of 1.5 higher than the value derived from the available emission inventory. The obtained NH₃ emission factor was approximately 306.9 mg/kg. If the annual gasoline consumption in Beijing is accurate, annual NH₃ emissions from vehicles are estimated at 1.5 Gg. The influx and outflux of NH₃ in Beijing during monitoring periods fluctuated due to variations of wind direction (WD), wind speed (WS), and planetary boundary layer height (PBLH). Net fluxes at the 4^th Ring Road were larger than zero, suggesting that local emissions were important in urban Beijing. Negative net fluxes at the 6^th Ring Road reveal a large amount of NH₃ transported from agricultural regions south of Beijing lost during transport across the city, for example by deposition or particle formation in the city. Our analyses have important implications for regional NH₃ emission estimates and for improving vehicular NH₃ emission inventory allocations.