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China has experienced severe particulate matter(PM)events in recent years.Stringent emission controls have then been implemented through the Five-Year Plan and the Action Plan on Prevention and Control of Air Pollution that mainly control primary PM,SO2 and Nox emissions,while ammonia(NH3)emissions have not been regulated.Here we use the regional WRF-Chem model to quantify the impact and effectiveness of NH3 emission reductions on fine PM concentrations over North China.Our NH3 emissions over North China(110°-120°E,35°-43°N)are estimated to be 0.12 Tg/month in January and 0.29 Tg/month in July.Our results show that in January NH3 levels over North China are excessive for secondary inorganic aerosol formation,and that a 20-40%reduction in total NH3 emissions has only small impact on PM2.5 concentrations.Only when the total emission reduction reaches 60%or even higher,the impacts on ammonium and nitrate aerosols become much larger.When NH3 emissions over North China are reduced by 20-40%,the Beijing-Tianjin-Hebei(BTH)population-weighted PM2.5 concentration(PWC)can be reduced by 1.42-3.81μg/m3.When NH3 emissions are reduced by 60-80%,the PM2.5 PWC would be reduced by 8.05-15.76μg/m3.Our results also show that the effectiveness of NH3 emission reductions is significantly higher in July than in January;a 20%NH3 emission reduction in July would lead to 5-10%reduced PM2.5 concentrations over BTH.We further find the effective control of SO2 and Nox emissions from 2015 to 2017 over North China might curtail the efficiency of NH3 emission reductions.