Evaluating present-day and future impacts of agricultural ammonia emissions on atmospheric chemistry and climate

Abstract. Agricultural practices are responsible for a major source of ammonia (NH3) to the atmosphere which has implications for air quality, climate and ecosystems. Due to the intensification of food and feed production, ammonia emissions are expected to increase significantly by 2100 and would therefore affect atmospheric composition such as nitrate (NO−3 ) or sulfate (SO2−4 ) particle formation and surface deposition feedback. Chemistry-climate models which integrate the key atmospheric physicochemical processes along with the ammonia cycle represent a useful tool to investigate present-day and also future ammonia pathways and their impact on the global scale. Ammonia sources are, however, challenging to quantify because of their dependencies on environmental variables and agricultural practices and represent a crucial input for chemistry-climate models. In this study, we use the chemistry-climate model LMDZ-INCA with agricultural and natural soil ammonia emissions from a global land surface model (ORCHIDEE with the integrated CAMEO module) for the present-day and 2090–2100 period under different socio-economic pathways. We show that this new set of emissions improves the spatial and temporal atmospheric ammonia representations in Africa, Latin America, and the US compared to the static reference inventory (CEDS). Higher ammonia emissions in Africa, as simulated by CAMEO compared to other studies, reflect enhanced present-day reduced nitrogen (NHx) deposition flux. This partially contributes to the 20 % higher NHx deposition in our results compared to other modeling studies at the global scale. Future CAMEO emissions lead to an overall increase of the global NH3 burden ranging from 37 % to 70 % while NO−3 burden increases by 38 %–50 % depending on the scenario even when global NOx emissions decrease. When considering the most divergent scenarios (SSP5-8.5 and SSP4-3.4) for agricultural ammonia emissions the direct radiative forcing resulting from secondary inorganic aerosol changes ranges from -114 to -160 mW.m−2. By combining a high level of NH3 emissions with decreased or contrasted future sulfate and nitrate emissions, the nitrate radiative effect can either overshoot (net total sulfate and nitrate effect of -200 mW.m−2) or be offset by the sulfate effect (net total sulfate and nitrate effect of +180 mW.m−2). We also show that future oxidation of NH3 could lead to an increase in N2O emissions of 0.43 to 2.10 Tg(N2O)yr−1 compared to the present-day levels. Our results suggest that accounting for nitrate aerosol precursor emission levels but also for the ammonia oxidation pathway in future studies is particularly important to understand how ammonia will affect climate, air quality, and nitrogen deposition.