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

Ehrnsperger L, Klemm, O

Abstract

Ambient air pollution caused by fine particulate matter (PM) and trace gases is a pressing topicas it affects the vast majority of the world's population, with a particularly heavy influence indensely populated urban environments. Alongside nitrogen oxides (NOx) and PM, ammonia (NH3)is also a relevant air pollutant due to its role as a precursor of particulate ammonium. This is astudy about the short-term temporal dynamics of urban NH3 concentrations in Münster, northwestGermany, the role of road traffic and agriculture as NH3 sources and about the importance of ammonia for secondary particle formation (SPF). The NH3 mixing ratio was rather high (mean: 17 ppb) compared to other urban areas and showed distinct diurnal maxima around 10 a.m. and duringthe night at 9 p.m. The main source for ammonia in Münster was agriculture, but road traffic alsocontributed through local emissions from vehicle catalysts. NH3 from surrounding agriculturalareas accumulated in the nocturnal boundary layer and contributed to SPF in the city center.Modeled emissions of NH3 as estimated by the Handbook for Emission Factors in combinationwith traffic counts were in the same magnitude for NH3. The size-resolved chemical compositionof inorganic ions in PM10 was dominated by NH4+ (8.66 μg m-3), followed by NO3- (3.89 μg m-3),SO42- (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 NH4+ leading to mostly alkaline PM. High ammonia emissions fromsurrounding agricultural areas combined with large amounts of NOx from road traffic play acrucial role for SPF in Münster.