Source Apportionment of Urban Ammonia and its Contribution to Secondary Particle Formation in a Mid-size European City

Ambient air pollution caused by fine particulate matter (PM) and trace gases is a pressing topic as it affects the vast majority of the world's population, with a particularly heavy influence in densely populated urban environments. Alongside nitrogen oxides (NO_x) and PM, ammonia (NH_3) is also a relevant air pollutant due to its role as a precursor of particulate ammonium. This is a study about the short-term temporal dynamics of urban NH_3 concentrations in Münster, northwest Germany, the role of road traffic and agriculture as NH_3 sources and about the importance of ammonia for secondary particle formation (SPF). The NH_3 mixing ratio was rather high (mean: 17 ppb) compared to other urban areas and showed distinct diurnal maxima around 10 a.m. and during the night at 9 p.m. The main source for ammonia in Münster was agriculture, but road traffic also contributed through local emissions from vehicle catalysts. NH_3 from surrounding agricultural areas accumulated in the nocturnal boundary layer and contributed to SPF in the city center. Modeled emissions of NH_3 as estimated by the Handbook for Emission Factors in combination with traffic counts were in the same magnitude for NH_3. The size-resolved chemical composition of inorganic ions in PM_(10) was dominated by NH_4^+ (8.66 μg m^(-3)), followed by NO_3^- (3.89 μg m^(-3)), SO_4^(2-) (1.58 μg m^(-3)) and Cl^- (1.33 μg m^(-3)). Particles in the accumulation range (diameter: 0.1- 1 μm) showed the highest inorganic ion concentrations. The ammonium neutralization index J (111%) indicated an excess of NH_4^+ leading to mostly alkaline PM. High ammonia emissions from surrounding agricultural areas combined with large amounts of NO_x from road traffic play a crucial role for SPF in Münster.