Inventorying national direct emissions of ammonia, nitrogen oxides and nitrous oxide from managed soils using process modelling-based Tier 3 methodology

Within the methodology framework of the Intergovernmental Panel on Climate Change on national emission inventories, process-oriented modelling is referred to as Tier 3 approach. This framework covers the climatically/environmentally important nitrogen (N) gases, including ammonia (NH 3 ), nitrogen oxides (NO x ) and nitrous oxide (N 2 O), from managed soils. Dynamical inventories of these gases with fine resolutions are urgently needed to well elucidate the responses of coupled air pollution and climate change to human activities, e.g., heavy use of N fertilizers. Here we report a case study dynamically and synchronically inventorying the national direct emissions of NH 3 , NO x and N 2 O from croplands of China mainland, using the CNMM-DNDC which is a high-resolution, process-oriented hydro-biogeochemical model, to meet Tier 3 needs. The model performed robustly in validation against observations at 17 field sites across diverse climatic zones, showing normalized root mean square errors of 46%, 23% and 31% and Nash-Sutcliffe Index values of 0.73, 0.97 and 0.95 for the three gases, respectively. Summing up the 3-hour and 30-second simulations, the national annual direct emissions due to application of synthetic nitrogen fertilizers in 2015 were estimated at 4.66 ± 1.49 (NH 3 ), 0.152 ± 0.041 (NO x ) and 0.188 ± 0.047 (N 2 O) in Tg N (1 Tg = 10 12 g). Logically, the simulated NH 3 emissions hot-spotted in the North China Plain and peaked in May and October while NO x and N 2 O emissions in subtropical provinces such as Hunan and Hubei in June and October. The background emissions from croplands were equivalent to about 4%, 19% and 25% of the national direct NH 3 , NO x and N 2 O emissions, respectively. The model simulations also resulted in aggregated annual direct emission factors at logical levels for the national widely applied synthetic N fertilizers in 2015, which were on average 15.67% for NH 3 , 0.56% for NO x and 0.84% for N 2 O. This study implicates that the applied model not only acts as a reliable and robust Tier 3 approach to generate dynamic national direct emission inventories of the three N gases with fine resolutions, but also possesses the profoundly distinguished capacity in predicting direct emission factors of the gaseous species for complex/extensive conditions where field observation is impracticable.