Fast nitrate formation aloft supported by typical synoptic weather patterns and complex terrain in the North China Plain

Abstract

North China Plain has been suffering from the increasing prominent nitrate pollution. In addition to intensive anthropogenic emissions, typical synoptic weather patterns and complex terrain in this region potentially contribute to fast nitrate formation, which has not been well understood. Focusing on a winter haze episode, we investigate their combined effects on nitrate formation by combining regional observations, weather situation analysis, and model simulations. In addition to the commonly proposed accumulating pollutants, our results indicate that synoptic weather patterns and complex terrain can uplift the precursors from the ground level and transport O3 from the free troposphere, enhancing the nitrate formation in the residual layer, with a faster formation than near the surface. The downward transport of nitrate aloft aggravates the nitrate pollution at the ground level. Our study demonstrates the notable effects of synoptic weather patterns and complex terrain on shaping nitrate production vertically, and highlights the importance of nitrate formation aloft to nitrate pollution at the ground level.