Real-time monitoring of atmospheric ammonia during a pollution episode in Madrid (Spain)

Ammonia emissions play an important role in the formation of secondary inorganic aerosols. Road traffic is considered to be one of the main sources of ammonia in urban environments, although other sources have been identified. An experimental study was carried out in a suburban area of Madrid (Spain), in which high time resolution measurements of ammonia concentrations were obtained using Cavity-Enhanced Laser Absorption Spectroscopy. Mean hourly ammonia concentration during the study period of December 2014 to January 2015 was 2.17 ± 1.70 μg m−3, with 9.3 μg m−3 being the maximum recorded value for a period of 10 min. Stagnant conditions dominated the measurement period and influenced the observed concentrations. Other gaseous and particulate matter parameters, including size-segregated mass concentration, particle number size distribution and aerosol chemical composition (non-refractory species) were monitored at the same site in order to support the data interpretation. No relationships with traffic-related pollutants were found, however, a nearby wastewater treatment plant was identified as the main source affecting the observed ambient concentrations of ammonia and aerosols. Ammonia emissions from wastewater treatment plants may be significant annual sources of ammonia in urban environments. High-resolution observations of air chemistry help to resolve emission inventories in urban environments, and improve the effectiveness of policy to reduce secondary inorganic aerosols.