Molecular distributions and stable carbon isotope compositions of oxalic acid and related SOA in Beijing before, during and after the 2014 APEC

To ensure the good air quality for the 2014 APEC, stringent emission controls were implemented in Beijing and its surrounding regions, leading to a significant reduction in PM2.5 loadings. To investigate the impacts of the emission controls on aerosol composition and formation, high-volume PM2.5 samples were collected in Beijing from 08/10/2014 to 24/11/2014 and determined for secondary inorganic ions (SIA, i.e., SO42−, NO3− and NH4+), dicarboxylic acids, keto-carboxylic acid and α-dicarbonyls, as well as stable carbon isotope composition of oxalic acid (C2). Our results showed that SIA in PM2.5 are 52 ± 47, 18 ± 13 and 33 ± 29 μg m−3 before-, during- and after-APEC, accounting for 29 %, 18 % and 20 % of PM2.5, respectively. As the leading dicarboxylic acid, C2 in PM2.5 during the three phases are 502 ± 564, 101 ± 69 and 166 ± 157 ng m−3, accounting for 46 %, 31 % and 34 % of total detected organic compounds (TDOC, i.e., the sum of dicarboxylic acids, keto-carboxylic acids and α-dicarbonyls). The higher values of concentrations and relative abundances of SIA and C2 before-APEC suggest that PM2.5 aerosols during this period are more enriched with secondary products, mainly due to an enhanced photochemical oxidation under the higher temperature and more humid conditions. SIA, C2 and related SOA in PM2.5 during-APEC were 2–4 times lower than those before-APEC. C2 in the regional air masses, which mostly occurred before-APEC, are abundant and enriched in 13C. On the contrary, C2 in the long-range transport air masses, which mostly occurred during-APEC, is much less abundant but still enriched in 13C. In the local air masses, which mostly occurred after-APEC, C2 concentration is lower than that before-APEC but higher than that during-APEC and enriched in lighter 12C. A comparison on chemical composition of PM2.5 and δ13C values of C2 in two events that are characterized by the highest PM2.5 levels before- and after-APEC, respectively, further showed that after-APEC SIA and TDOC are much less abundant and fine aerosols are enriched with primary organics and relatively fresh, compared with those before-APEC. Such reduction in secondary aerosols after-APEC, along with a similar reduction during-APEC, is largely due to the decreasing temperatures. Our results indicate that the significant reduction in PM2.5 during-APEC is mainly due to the efficient emission controls, but the effect of the decreasing temperatures, which suppressed secondary aerosol production, may also take an important role.