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<p>Satellite observations show a global maximum in ammonia (<span class="inline-formula">NH₃</span>) 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 <span class="inline-formula">NH₃</span> 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 <span class="inline-formula">NH₃</span> emission fluxes (0.4&thinsp;t&thinsp;km<span class="inline-formula">⁻²</span>&thinsp;month<span class="inline-formula">⁻¹)</span> due to intensive agricultural activities and high air temperature from June to August. Additionally, low sulfur dioxide (<span class="inline-formula">SO₂</span>) and nitrogen oxides (<span class="inline-formula">NO_<i>x</i></span>) emissions and high air temperature limit the gas-to-particle conversion of <span class="inline-formula">NH₃</span>, particularly for ammonium nitrate formation. Moreover, the barrier effects of the Himalayas in combination with the surface convergence weaken the horizontal diffusion of <span class="inline-formula">NH₃</span>. The high <span class="inline-formula">NH₃</span> loading over the IGP mainly results from the low gas-to-particle partitioning of <span class="inline-formula">NH₃</span> caused by low <span class="inline-formula">SO₂</span> and <span class="inline-formula">NO_<i>x</i></span> emissions. It contrasts to those in the North China Plain, where high <span class="inline-formula">SO₂</span> and <span class="inline-formula">NO_<i>x</i></span> emissions promote the conversion of gaseous <span class="inline-formula">NH₃</span> into particulate ammonium.</p>