Ammonia variability and trends from globally distributed FTIR measurements and model simulations

Abstract. Ammonia (NH₃) is an important constituent in the global nitrogen cycle, present in both urban and remote environments. It is a source of reactive nitrogen and a precursor for particulate matter, thereby affecting atmospheric chemistry and radiative forcing. This work presents the seasonal and diurnal variability, along with long-term trends, of atmospheric NH₃ total columns retrieved from Fourier transform infrared (FTIR) spectroscopic solar absorption measurements at 22 ground-based sites, globally distributed from 45° S to 80° N. Comparisons are made with simulations from the GEOS-Chem High Performance (GCHP) chemical transport model and the Tropospheric Chemistry Reanalysis (TCR-2) NH₃ product. The mean NH₃ total columns from the FTIR time series ranged from 0.12×10¹⁵ to 19.20×10¹⁵ molecules cm⁻², with the smallest columns found at the Arctic and high-altitude sites, and the largest in urban areas. Significant enhancements were attributed to biomass burning, and NH₃ emissions from volcanic eruptions were detected at the Izana site. The seasonal patterns are similar across most sites, with maxima mainly related to the volatilization of NH₃ due to higher temperatures. The diurnal variability differs significantly and depends on the characteristics of each site and local sources. Most sites have positive trends in the total column, with a mean value (and 95 % confidence interval) for all sites of 3.82 (3.29–4.35) % for the FTIR measurements, 3.66 (3.35–3.97) % for GCHP, and 6.49 (2.00–10.98) % for TCR-2. GCHP exhibited a better general agreement with the FTIR observations than TCR-2; potential reasons for this are explored, including a sensitivity test on the emissions used.