Monthly atmospheric ammonia gas concentration data from Northern Ireland ALPHA® Ammonia Monitoring Network, 2019-2020 - Tang, Y.S.; Tomlinson, S.J.; Carnell, E.J.; Williams, M.; Simmons, I.; Twigg, M.; Stephens, A.C.M.; Iwanicka, A.; Leaver, D.; Braban, C.F.; Mcllroy, J.; Pereira, M.G.; et al. (NERC EDS Environmental Information Data Centre, 2022)