Why is the Indo-Gangetic Plain the region with the largest NH 3 column in the globe during pre-monsoon and monsoon seasons?

Wang, Tiantian; Song, Yu; Xu, Zhenying; Liu, Mingxu; Xu, Tingting; Liao, Wenling; Yin, Lifei; Cai, Xuhui; Kang, Ling; Zhang, Hongsheng; Zhu, Tong

Satellite observations show a global maximum in ammonia (inline-formulaNH3) 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 inline-formulaNH3 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 inline-formulaNH3 emission fluxes (0.4 t kminline-formula−2 monthinline-formula−1) due to intensive agricultural activities and high air temperature from June to August. Additionally, low sulfur dioxide (inline-formulaSO2) and nitrogen oxides (inline-formulaNOx) emissions and high air temperature limit the gas-to-particle conversion of inline-formulaNH3, particularly for ammonium nitrate formation. Moreover, the barrier effects of the Himalayas in combination with the surface convergence weaken the horizontal diffusion of inline-formulaNH3. The high inline-formulaNH3 loading over the IGP mainly results from the low gas-to-particle partitioning of inline-formulaNH3 caused by low inline-formulaSO2 and inline-formulaNOx emissions. It contrasts to those in the North China Plain, where high inline-formulaSO2 and inline-formulaNOx emissions promote the conversion of gaseous inline-formulaNH3 into particulate ammonium.

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Wang, Tiantian / Song, Yu / Xu, Zhenying / et al: Why is the Indo-Gangetic Plain the region with the largest NH3 column in the globe during pre-monsoon and monsoon seasons?. 2020. Copernicus Publications.

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