LAUSR

Since the implementation of the "Air Pollution Prevention and Control Action Plan" (APPCAP) in 2013, the air quality in China has been greatly improved but still much exceeded the WHO guideline limit. Here we employed a novel approach, two-dimensional Si fingerprints, including stable Si isotopic composition (δ30Si) and Si abundance (Si%), to investigate the annual variations in both primary and secondary sources of PM2.5 in Beijing during the APPCAP period (2013-2017). The δ30Si and Si% were used as tracers to reflect the variations in primary and secondary sources, respectively. For primary sources, the mean δ30Si value of PM2.5 in 2015-2017 (> -0.5‰) was significantly more positive than that of 2013 (-1.24‰), indicating a dramatic decline in the contribution of 30Si-depleted sources (i.e., coal burning and industrial emission). For secondary sources, the mean Si% of PM2.5 increased from 1.2% in 2013 to 4.6% in 2017, suggesting a large decrease in the secondary aerosol contribution from 83% to 42%. Noteworthily, we found that the 30Si-depleted sources showed a rebound trend during 2015-2017. This study reveals the responses of anthropogenic emission sources under strong regulation policies and provides a reference for the future policymaking in Beijing and other polluted regions and countries.