Ice core δ¹⁵N of Ammonium Reveals Changing Atmospheric Processing of NH_x during European industrialisation

Abstract. Ammonia (NH₃) is the main alkaline gas in the atmosphere and a key regulator of aerosol formation, nitrogen deposition, and air quality. Yet the relationship between NH₃ emissions and deposited ammonium (NH₄⁺) in remote archives, remains poorly constrained. Here, we present the first ice-core record of δ¹⁵N(NH₄⁺), covering 1000 years of European atmosphere. Ice-core δ¹⁵N(NH₄⁺) is systematically enriched relative to inventory-based δ¹⁵N(NH₃), consistent with atmospheric isotopic fractionation between emitted NH₃ and deposited NH₄⁺. The 10-year seasonally resolved record shows lower δ¹⁵N(NH₄⁺) in winter than in summer, which is consistent with NH₃–NH₄⁺ partitioning integrated during transport, although no unique pathway can be identified as other mechanisms remain insufficiently constrained. Between 1900 and 1970, lower δ¹⁵N(NH₄⁺) broadly coincides with higher acidity and lower neutralisation factor, consistent with increasing proportion of NH₄⁺ through NH₃–NH₄⁺ partitioning, although cloud processing and other atmospheric processes cannot be separated. After 1980, this relationship breaks down, with a divergence between ice-core NH₄⁺ and NH₃ emission inventories, the isotope–concentration relationship disappears, and a decrease in apparent isotopic fractionation. Boundary layer observations suggest that a large underestimation of NH₃ emissions is unlikely. This shift therefore indicates a change in the source-to-deposition atmospheric processes, although the responsible pathway remains unresolved. Ice-core δ¹⁵N(NH₄⁺) therefore provides a long-term view of the source-to-deposition regime shifts, a perspective increasingly needed as Europe moves toward a less acidic, more NH₃-rich atmosphere.