EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Air pollution in an urban street canyon: Novel insights from highly resolved traffic information and meteorology

Laura Ehrnsperger and Otto Klemm
Laura Ehrnsperger and Otto Klemm
  • University of Münster, Institute for Landscape Ecology, Climatology Research Group, Münster, Germany (

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, especially in densely populated urban environments. The main sources of ambient air pollution in cities are road traffic, industries and domestic heating. Alongside nitrogen oxides (NOx) and PM, ammonia (NH3) is also a relevant air pollutant due to its role as a precursor of particulate ammonium (NH4+). To examine the temporal patterns and sources of air pollutants, this study used fast-response air quality measurements in combination with highly resolved traffic information in Münster, NW Germany. The temporal dynamics of NOx and the particle number concentration (PN10) were similar to the diurnal and weekly courses of the traffic density. On very short timescales, the real-world peak ratios of NOx and PM ≤ 10 µm diameter (PM10) exceeded the predicted pollutant emission ratios of the Handbook for Emission Factors for Road Transport (HBEFA) by a factor of 6.4 and 2.0, respectively. A relative importance model revealed that light-duty vehicles (LDVs) are the major relative contributor to PN10 (38 %) despite their low abundance (4 %) in the local vehicle fleet.  Diesel and gasoline vehicles contributed similarly to the concentrations of PM10 and PN10, while the impact of gasoline vehicles on the PM1 concentration was greater than that of diesel vehicles by a factor of 4.4. The most recent emission class Euro 6 had the highest influence on PM10. Meteorological parameters explained a large portion of the variations in PM10 and PM1, while meteorology had only a minor influence on PN10. We also studied the short-term temporal dynamics of urban NH3 concentrations, the role of road traffic and agriculture as NH3 sources and 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 9 p.m. The main source for ammonia in Münster was agriculture, but road traffic also contributed through local emissions from vehicle catalysts. NH3 from surrounding agricultural areas accumulated in the nocturnal boundary layer and contributed to SPF in the city center. The size-resolved chemical composition of inorganic ions in PM10 was dominated by NH4+ (8.7 µg m-3), followed by NO3- (3.9 µg m-3), SO42- (1.6 µg m-3) and Cl- (1.3 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 from surrounding agricultural areas combined with large amounts of NOx from road traffic play a crucial role for SPF in Münster. Our results further indicate that replacing fossil-fuelled LDVs with electrical vehicles would greatly reduce the PN10 concentrations at this urban site.

How to cite: Ehrnsperger, L. and Klemm, O.: Air pollution in an urban street canyon: Novel insights from highly resolved traffic information and meteorology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15830,, 2021.


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