Air quality improvement in a megacity: implications from 2015 Beijing Parade Blue pollution control actions

摘要

The implementation of strict emission control measures in Beijing and surrounding regions during the 2015 China Victory Day Parade provided a valuable opportunity to investigate related air quality improvements in a megacity. We measured NH_3, NO_2 and PM_(2.5) at multiple sites in and outside Beijing and summarized concentrations of PM_(2.5), PM_(10), NO_2, SO_2 and CO in 291 cities across China from a national urban air quality monitoring network between August and September 2015. Consistently significant reductions of 12-35% for NH_3 and 33-59% for NO_2 in different areas of Beijing during the emission control period (referred to as the Parade Blue period) were observed compared with measurements in the pre- and post-Parade Blue periods without emission controls. Average NH_3 and NO_2 concentrations at sites near traffic were strongly correlated and showed positive and significant responses to traffic reduction measures, suggesting that traffic is an important source of both NH_3 and NO_x in urban Beijing. Daily concentrations of PM_(2.5) and secondary inorganic aerosol (sulfate, ammonium and nitrate) at the urban and rural sites both decreased during the Parade Blue period. During (after) the emission control period, concentrations of PM_(2.5), PM_(10), NO_2, SO_2 and CO from the national city-monitoring network showed the largest decrease (increase) of 34-72% (50-214 %) in Beijing, a smaller decrease (a moderate increase) of 1-32% (16-44 %) in emission control regions outside Beijing and an increase (decrease) of 6-16% (-2-7 %) in non-emissioncontrol regions of China. Integrated analysis of modelling and monitoring results demonstrated that emission control measures made a major contribution to air quality improve- ment in Beijing compared with a minor contribution from favourable meteorological conditions during the Parade Blue period. These results show that controls of secondary aerosol precursors (NH_3, SO_2 and NO_x ) locally and regionally are key to curb