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 4th Ring Road were larger than zero, suggesting that local emissions were important in urban Beijing. Negative net fluxes at the 6th 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.