Control of fine particulate nitrate during severe winter haze in "2+26" cities

The "2+26" cities, suffering the most severe winter haze pollution, have been the key region for air quality improvement in China. Increasing prominent nitrate pollution is one of the most challenging environmental issues in this region, necessitating development of an effective control strategy. Herein, we use observations, and state-of-the-art model simulations with scenario analysis and process analysis to quantify the effectiveness of the future SO₂-NO_X-VOC-NH₃ emission control on nitrate pollution mitigation in "2+26" cities. Focusing on a serious winter haze episode, we find that limited NO_X emission reduction alone in the short-term period is a less effective choice than VOC or NH₃ emission reduction alone to decrease nitrate concentrations, due to the accelerated NO_X-HNO₃ conversion by atmospheric oxidants and the enhanced HNO₃ to NO₃^- partition by ammonia, although deep NO_X emission reduction is essential in the long-term period. The synergistic NH₃ and VOC emission control is strongly recommended, which can counteract the adverse effects of nonlinear photochemistry and aerosol chemical feedback to decrease nitrate more. Such extra benefits will be reduced if the synergistic NH₃ and VOC reduction is delayed, and thus reducing emission of multiple precursors is urgently required for the effective control of increasingly severe winter nitrate pollution in "2+26" cities.