Temporal and spatial variability of atmospheric ammonia in the Lombardy region (Northern Italy)

This work investigates the spatial and temporal variability of atmospheric ammonia concentrations in the Lombardy region in Northern Italy, where its continuous measurement at hourly resolution began in 2007 at monitoring sites representative of three different land use areas (urban, rural, and mountain locations). Ammonia concentration data have been jointly analyzed with wind direction and speed to highlight the association between the origin of the air masses and the concentration levels observed at the monitoring sites far from the primary sources, essentially consisting of farming activities and cattle and pigs breeding located in the southeastern part of the Lombardy region. The annual average concentrations of ammonia observed at urban (4.4–13.4 μg m⁻³ range) and rural (17–35 μg m⁻³ range) monitoring sites are in substantial agreement with literature data, which are however limited and strongly influenced by the measurement techniques used. The lowest concentration levels (1.1–5.8 μg m⁻³ range) are observed at the monitoring sites in the mountain areas. Both the seasonal and daily time patterns of the concentrations appear strongly related to some features of the measurement sites, namely with regard to their exposure to agricultural activities, whose seasonal practices arise in emissions responsible for strong variations in the ammonia atmospheric levels. Conversely, in the mountain areas in the north of region, local conditions affecting the atmospheric circulation seem to play a more important role than local sources, with the highest concentrations occurring when the breezes transport NH₃-rich air masses from the southern part of the region.