Meteorology-soil nitrogen emission coupled modelling in China: development and evaluation

Volatilization of reactive nitrogen (Nr) gases like HONO, NH3 and NOx from fertilizer application and soil is an important pathway of nitrogen losses in agricultural ecosystems and deteriorate air pollution by contributing to ozone and PM2.5. The volatilization of Nr gases highly depends on environmental and meteorological conditions, however, this phenomenon is poorly described in current emission inventory and atmospheric models. Here, we develop a dynamic soil nitrogen emission model capable of calculating NH3 and HONO emission rate interactively with time- and spatial-varying meteorological and soil conditions. The NH3 flux parameterization relies on several meteorological factors and anthropogenic activity including fertilizer application, livestock waste, traffic, residential and industrial sectors.  HONO emission scheme considers soil temperature and moisture as well as the type of underlying surface. The model is then embedded into a regional WRF-Chem model and is evaluated against field measurements of Nr emission flux and ambient concentration. The evaluation shows a substantial improvement in the model performance of NH3 flux and ambient HONO concentration in China.  Compared with normal simulations using fixed emission inventory input, this model features superior capability in simulating NH3 emission flux and concentration during planting seasons and drastic weather changes like frontal activities and precipitation. Such advances in emission quantification also improve the model performance of secondary inorganic aerosol on synoptic scales. While more laboratory and field measurements are still needed for better parameterization of soil nitrogen volatilization, the seamless coupling of soil emission with meteorology provides a better understanding of NH3 and HONO emission evolution and its contribution to atmospheric chemistry.