《Table 1–Airflow distribution under different pollution periods from 2013 to 2016 in Beijing (%) .》下

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《A framework for investigating the air quality variation characteristics based on the monitoring data: Case study for Beijing during 2013–2016》

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The contribution rates of pollutant（i.e.，PM2.5，NO2，SO2and CO）concentrations to the annual average concentrations under different airflow directions were calculated based on the method proposed in Section 2.5 from 2013 to 2016，and displayed in Table 2.It could be found that the contribution rates of typical pollutant（i.e.，PM2.5，NO2，SO2and CO）concentrations to the annual mean concentrations for west airflow and south airflow had the most significant changes during the period of 2013 to 2016，followed by north airflow，and the lower value appeared in east airflow direction.However，the differences were that the contribution rates of south airflow decreased with 23.8%，18.8%，16.9%and 20.1%for PM2.5，SO2，NO2and CO from 2013 to 2016，but those of west airflow increased with 29.1%，28.3%，24.6%and 26.8%，respectively.The contribution rates of north airflow were with fluctuation change.And the contribution rates of east airflow were always the lowest among four directions，with values of4.2%–16.1%，4.6%–16.3%，2.5%–16.1%and 5.3%–16.2%for PM2.5，SO2，NO2and CO，respectively.