Hourly concentrations of ammonia during the winter in Manchester, UK, related to traffic and background sources

Concentrations of atmospheric ammonia have been measured using a high resolution Aerodyne Quantum Cascade Tunable Diode Laser Absorption Spectrometer from a rooftop laboratory centrally located in a large urban conurbation, Manchester, UK, overlooking a heavily trafficked street canyon. Continuous mean hourly concentrations measured over one month in winter 2004, are compared and related to background and local anthropogenic sources including traffic emissions. Mean concentrations were 2.3 ppb, (median 1.8 ppb, range 0.4 - 10.6 ppb). The highest ammonia concentrations were observed when the wind was from the southwest and during low wind conditions. Potential sources in the southwest have been identified, but require further investigation. Ammonia concentrations were related to concentrations of NO_x and CO, suggesting that traffic is a major urban source of ammonia. The contribution by traffic was also indicated by a diurnal cycle with peaks corresponding to those in traffic rates, although other urban sources could be as significant. Comparison with unpublished ammonia data from 1997 shows an increase in the slope of the relationship between ammonia and both NO_x and CO by approximately 40% since that time. In the same period, the proportion of petrol-engine cars fitted with catalytic converters has approximately doubled, thus supporting previous findings that link higher traffic emissions of ammonia to these vehicles.