Ammonia Air‐Sea Exchange in Southwest UK

Abstract Ammonia (NH 3 ) measurements are challenging to make, and air‐sea flux estimates are sparse, with coastal observations complicated by overlapping marine and terrestrial signals. Here we present high‐resolution NH 3 air‐sea flux estimates based on two years (2022–2023) of atmospheric and seawater measurements at the Penlee Point Atmospheric Observatory (PPAO), on the southwest UK coast. NH 3 concentrations peak in spring and summer and were strongly influenced by terrestrial air masses from the UK and Continental Europe. Net NH 3 deposition occurred across all seasons, with seasonal median deposition ranging from 20.5 to 34.5 μmol m −2 d −1 or 1.0 to 1.8 kg NH 3 ‐N ha −1 yr −1 . The flux direction is primarily controlled by atmospheric NH 3 , while seasonal changes in seawater biogeochemistry have a secondary role. These findings highlight the role of anthropogenic NH 3 in coastal nitrogen budgets and underscore the need for high‐resolution flux estimates.