This dataset is from the Northern Ireland (NI) ALPHA® Ammonia Monitoring network, and consists of monthly concentrations of atmospheric ammonia (NH3) gas measured with the UKCEH ALPHA® method at 25 sites across NI for the years 2019 to 2020. NH3 data were also collected with a monthly measurement frequency with the UKCEH DELTA® method at a further 4 sites, as part of the NI DELTA® Gas and Aerosol Monitoring Network. At one site, ALPHA® and DELTA® methods were co-located to inter-compare NH3 concentrations. Monthly NH3 data (2019 – 2020) from the NI DELTA® network are reported separately in a companion submission. The network is operated jointly by Agri-Food and Biosciences Institute (AFBI) and the UK Centre for Ecology & Hydrology (UKCEH), and funded by the Department of Agriculture, Environment and Rural Affairs (DAERA), Northern Ireland.