Why is the Indo-Gangetic Plain the region with the largest NH 3 column in the globe during pre-monsoon and monsoon seasons?
2020
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.
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