Why is the Indo-Gangetic Plain the region with the largest NH₃ column in the globe during pre-monsoon and monsoon seasons?

Abstract

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 (NOx) 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 NOx emissions. It contrasts to those in the North China Plain, where high SO2 and NOx emissions promote the conversion of gaseous NH3 into particulate ammonium.