Seasonal and Diurnal Variation in Atmospheric Ammonia in an Urban Environment Measured Using a Quantum Cascade Laser Absorption Spectrometer

Abstract

Extensive measurements were made using an Aerodyne quantum cascade laser absorption spectrometer (QCLAS) to study the diurnal and seasonal cycles of NH3 concentrations in Manchester city centre. Measurements made at rooftop levels showed traffic to be a significant source of NH3 concentrations in the winter. This was illustrated by a bimodal diurnal cycle of NH3 concentrations that was synchronized with traffic, and also by a correlation with NOx, a traffic related pollutant. These patterns were not observed during the summer, suggesting other sources become more important. Measurements were also made at street level during winter and summer, close to the traffic source. This time the contribution from traffic was also observed in the summer, albeit weaker. Enhanced NH3 concentrations were often seen in winds from the southwest that could not be related to local sources, suggesting that ambient concentrations in the city are strongly influenced by sources outside the city. It is estimated that the total NH3 emission from the city centre is between 0.7 and 2.3 t km−2 year−1.