Assessing the Mitigation Potential of Winter Cover Crops for Ammonia Reduction: Implications for Air-Quality Policy in Piedmont, Italy

Ammonia (NH3) emissions from intensive livestock farming are a major contributor to the formation of secondary particulate matter (PM2.5) in northern Italy, particularly during winter, when high humidity and low temperatures promote ammonium nitrate formation. This phenomenon is further exacerbated by the extensive use of maize monoculture, which leaves the soil bare for most of the winter season, thereby reducing aerodynamic roughness and limiting dry deposition processes. This study evaluates the potential of winter cover crops to enhance atmospheric ammonia removal by increasing dry deposition velocities. Drawing on established deposition models and regional air-quality data, we estimate that replacing bare soil with autumn–winter vegetation can increase deposition velocities by up to an order of magnitude (from <0.1 to ≈1 cm s−1). In hotspot areas of the Cuneo–Turin plain, where winter NH3 concentrations regularly exceed 30 µg m−3, cover crops can generate additional deposition fluxes of approximately 30–70 kg NH3 ha−1 yr−1. Applied to the 90,000 ha of intensive monoculture identified in the study area, this corresponds to a potential removal of ~2800 t NH3 yr−1, equivalent to 8–9% of total agricultural emissions in Piedmont. These reductions would also decrease the availability of precursors for secondary PM2.5 formation. Existing regional rural-development measures provide a suitable policy framework, but they are currently underutilized, indicating substantial potential for large-scale implementation.