Work

Ice core δ 15 N of Ammonium Reveals Changing Atmospheric Processing of NH x during European industrialisation

Year: 2026

Type: preprint

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.

DOI: 10.5194/egusphere-2026-5825