| Summary: | Based on the data of the PM<sub>2.5</sub> mass concentrations from the monitoring point at the US embassy in Beijing from 2013 to 2017,the characteristics of PM<sub>2.5</sub> mass concentration and its correlation with the mass concentrations of NO<sub>2</sub> and SO<sub>2</sub> in Beijing in the past five years are analyzed.The emphasis is focused on the variation characteristics of PM<sub>2.5</sub> after the action on comprehensive control of air pollution implemented in the autumn and winter of 2017,which is aimed at the emission reduction from banning of uncontrolled coal combustion.The results show that the annual average of PM<sub>2.5</sub> mass concentration in Beijing has decreased yearly since 2013,and the decreasing range of the annual average in 2017 has reached 50%.Analysis of data in different seasons shows that the PM<sub>2.5</sub> mass concentration in winter is the highest,followed by autumn,spring,summer in descending order in 2013 to 2016.However,after the action on comprehensive control of air pollution,it is noticed that the mass concentration of PM<sub>2.5</sub> in winter 2017 was lower than that in autumn and spring,approaching the level of summer.In addition,compared with the past years,the mass concentrations of NO<sub>2</sub> and SO<sub>2</sub> in the air also decreased significantly in winter 2017,in which the decrease of SO<sub>2</sub> mass concentration is more obvious.These results show that the emission from burning coals has been effectively controlled after the action on comprehensive control of air pollution.Finally,the correlation analysis shows that the mass concentrations of PM<sub>2.5</sub> has a significant positive correlation with those of SO<sub>2</sub>,indicating that countermeasures for the banning of uncontrolled coal combustion such as "coals replaced by gas" and "coals replaced by electricity" have an obvious effect on reducing the PM<sub>2.5</sub> emission.
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