Estimating NH3 emissions from agricultural fertilizer application in China using the bi-directional CMAQ model coupled to an agro-ecosystem model

Atmospheric ammonia (NH3) plays an important role in atmospheric chemistry. China is one of the largest NH3 emitting countries with the majority of NH3 emissions coming from the agricultural practices, such as fertilizer application and livestock. The current NH3 emission estimates in China are mainly based on pre-defined emission factors that lack the temporal or spatial details, which are needed to accurately predict NH3 emissions. In this study, we estimate, for the first time, the NH3 emission from the agricultural fertilizer application in China online using an agricultural fertilizer modeling system coupling a regional air quality model (the Community Multi-Scale Air Quality model, CMAQ) and an agro-ecosystem model (the Environmental Policy Integrated Climate model, EPIC), which improves the spatial and temporal resolution of NH3 emission from this sector. Cropland area data of 14 crops from 2710 counties and the Moderate Resolution Imaging Spectroradiometer (MODIS) land use data are combined to determine the crop distribution. The fertilizer application rate and method for different crop are collected at provincial or agriculture-regional level. The EPIC outputs of daily fertilizer application and soil characteristics are inputed into the CMAQ model and the hourly NH3 emission are calculated online with CMAQ running. The estimated agricultural fertilizer NH3 emission in this study is about 3 Tg in 2011. The regions with the highest modeled emission rates are located in the North China Plain. Seasonally, the peak ammonia emissions occur from April to July.Compared with previous researches, this method considers more influencing factors, such as meteorological fields, soil and the fertilizer application, and provides improved NH3 emission with higher spatial and temporal resolution.