This study addressed the scarcity of NH3 measurements in urban Europe and the diverse monitoring protocols,
hindering direct data comparison. Sixty-nine datasets from Finland, France, Italy, Spain, and the UK across
various site types, including industrial (IND, 8), traffic (TR, 12), urban (UB, 22), suburban (SUB, 12), and
regional background (RB, 15), are analyzed to this study. Among these, 26 sites provided 5, or more, years of
data for time series analysis. Despite varied protocols, necessitating future harmonization, the average NH3
concentration across sites reached 8.0 ± 8.9 μg/m3. Excluding farming/agricultural hotspots (FAHs), IND and TR
sites had the highest concentrations (4.7 ± 3.2 and 4.5 ± 1.0 μg/m3), followed by UB, SUB, and RB sites (3.3 ±
1.5, 2.7 ± 1.3, and 1.0 ± 0.3 μg/m3, respectively) indicating that industrial, traffic, and other urban sources were
primary contributors to NH3 outside FAH regions. When referring exclusively to the FAHs, concentrations ranged
from 10.0 ± 2.3 to 15.6 ± 17.2 μg/m3, with the highest concentrations being reached in RB sites close to the
farming and agricultural sources, and that, on average for FAHs there is a decreasing NH3 concentration gradient
towards the city. Time trends showed that over half of the sites (18/26) observed statistically significant trends.
Approximately 50 % of UB and TR sites showed a decreasing trend, while 30 % an increasing one. Meta-analysis
revealed a small insignificant decreasing trend for non-FAH RB sites. In FAHs, there was a significant upward
trend at a rate of 3.51[0.45,6.57]%/yr. Seasonal patterns of NH3 concentrations varied, with urban areas
experiencing fluctuations influenced by surrounding emissions, particularly in FAHs. Diel variation showed
differing patterns at urban monitoring sites, all with higher daytime concentrations, but with variations in peak