资源环境科技发展态势分析平台

Satellite observations show a global maximum in ammonia (NH₃) over the Indo-Gangetic Plain (IGP), with a peak from June to August. However, it has never been explained explicitly. In this study, we investigated the causes of high NH₃ loading over the IGP during the pre-monsoon and monsoon seasons using WRF-Chem (Weather Research and Forecasting model coupled to chemistry). The IGP has relatively high NH₃ emission fluxes (0.4 t km⁻² month⁻¹) due to intensive agricultural activities and high air temperature from June to August. Additionally, low sulfur dioxide (SO₂) and nitrogen oxides (NO_x) emissions and high air temperature limit the gas-to-particle conversion of NH₃, particularly for ammonium nitrate formation. Moreover, the barrier effects of the Himalayas in combination with the surface convergence weaken the horizontal diffusion of NH₃. The high NH₃ loading over the IGP mainly results from the low gas-to-particle partitioning of NH₃ caused by low SO₂ and NO_x emissions. It contrasts to those in the North China Plain, where high SO₂ and NO_x emissions promote the conversion of gaseous NH₃ into particulate ammonium.