To ensure good air quality for the 2014 Asia-Pacific Economic Cooperation (APEC) summit, stringent emission controls were implemented in Beijing and its surrounding regions, leading to a significant reduction in PM 2.5 loadings. To investigate the impact of the emission controls on aerosol chemistry, high-volume PM 2.5 samples were collected in Beijing from 8 October to 24 November 2014 and determined for secondary inorganic aerosols (SIA, i.e., SO 4 2− , NO 3 − , and NH 4 + ), dicarboxylic acids, keto-carboxylic acid, and α -dicarbonyls, as well as stable carbon isotope composition of oxalic acid (C 2 ). Our results showed that SIA, C 2 , and related secondary organic aerosols in PM 2.5 during APEC were 2–4 times lower than those before APEC, which is firstly ascribed to the strict emission control measures and secondly attributed to the relatively colder and drier conditions during the event that are unfavorable for secondary aerosol production. C 2 in the polluted air masses, which mostly occurred before APEC, are abundant and enriched in 13 C. On the contrary, C 2 in the clean air masses, which mostly occurred during APEC, is much less abundant but still enriched in 13 C. In the mixed type of clean and polluted air masses, which mostly occurred after APEC, C 2 is lower than that before APEC but higher than that during APEC and enriched in lighter 12 C. A comparison on chemical composition of fine particles and δ 13 C values of C 2 in two events that are characterized by high loadings of PM 2.5 further showed that after APEC SIA and the total detected organic compounds (TDOC) are much less abundant and fine aerosols are enriched with primary organics and relatively fresh, compared with those before APEC.