Re-examine the APEC blue in Beijing 2014

APEC blue was coined to describe the impact of short-term curbs on air pollution during the Asian-Pacific Economic Cooperation (APEC) Summit organized in Beijing in November 2014 and has been a hot topic among both general public and scientific sector in China. The consensus that gaseous agents NO₂ and SO₂ are greatly reduced in response to the control and restriction strategies implemented during the Summit period is shared by earlier literature. However, the re-examination of APEC blue conducted in the present study comes to a more contrasted conclusion. The remarkable drop in NO₂ abundances is confirmed in terms of both surface concentration and vertical column, whereas corresponding SO₂ changes are found to be marginal and not statistically significant, indicating that the decline of SO₂ was more tied to natural or random variability rather than externally forced. To explain the contrasted responses of NO₂ and SO₂ during the APEC summit, short-term variations of these species are further placed in the context of a longer term perspective, which reveals a striking contrast in the pathways of the secular tendency in NO₂ and SO₂ emissions and corresponding measured abundances of both pollutants. On the one hand, NO₂ emissions exhibit a sharp rise by 30–50% from 2006-2010 to 2011–2014; on the other hand, SO₂ emissions have undergone a gradual decrease in the last decade and have currently returned to their pre-2000 level. Therefore, short-term control measures are expected to be more effective in reducing the levels of NO₂ than SO₂.