Characteristics And Sources Of Submicron Aerosols Above The Urban Canopy (260m) In Beijing During The 2014 APEC Summit

As the economic, political and cultural center of China, Beijing has experienced frequent severe fine particle pollution during the last decade. Although the sources and formation mechanisms of aerosol particles in Beijing have been extensively investigated on the basis of ground measurements, real-time characterization of aerosol particle composition and sources above the urban canopy in Beijing is rare. In this study, we presented a detailed characterization of submicron aerosol composition and sources above the urban canopy (260m) in Beijing before and during the 2014 Asia-Pacific Economic Cooperation (APEC) summit (from 10 October to 30 November 2014). To ensure the air quality, many strict emission controls were implemented during the 2014 APEC summit. Real-time data of non-refractory submicron aerosol (NR-PM1) composition including organics, sulfate, nitrate, ammonim, and chloride were synchronously measured by using an aerosol chemical speciation monitor (ACSM) and a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) at 260m and ground, respectively. In addition, the vertical distribution of atmospheric SO2 and its impact on the formation of sulfate (SO42-) were also analyzed. The main conclusions are as follows:(1) The NR-PMi composition above the urban canopy (260m) in Beijing was dominated by organics (48%), followed by nitrate (27%), sulfate (13%), ammonium (8%) and chloride (6%) during the entire study periods, nitrate has become more important. Organic aerosols (OA) were mainly composed of secondary OA (SOA) before and during the heating period. The contributions of SOA were 61% and 67%, respectively. Different from the measurements at the ground site, primary OA (POA) correlated moderately with SOA at 260 m, likely suggesting a high contribution from regional transport above the urban canopy.(2) During this study, the average mass concentration of NR-PMi above the urban canopy in Beijing were 65.8 μg m-3,24.8 μg m-3, and 74.3 μg m-3 before, during and after APEC. All aerosol species were shown to have significant decreases of 40-80% during APEC from those measured before APEC, suggesting that emission controls over regional scales substantially reduced PM levels. However, the bulk aerosol composition was relatively similar before and during APEC, organics and nitrate were the main components. After APEC, the concentration of pollutants were increased substantially along with higher contribution of sulfate and chloride due to coal combustion. Comparatively, the contribution of nitrate was decreased in the heating period. In addition to emission controls, meteorological conditions especially the routine circulations of mountain-valley breezes also played an important role in alleviating PM levels and achieving the "APEC blue" effect. The analysis of backward trajectory showed that regional transport (especially the air mass via the south-west of Beijing) and local coal emissions were the main pollution sources of pollutants above the urban canopy in Beijing. Our results clearly imply that synergetic controls of the emissions of precursors over a regional scale are efficient for mitigating air pollution in north China.(3) All submicron species showed overall similar variations at the two different heights, but some diffterences were also observed especially during severe haze episode. The main compositional difference was secondary inorganic aerosol (SIA:SO42-, NO3- and NH4+) before APEC, whereas the compositional difference was dominated by organics during APEC, accounting for 68%. Comparatively, there were no significant differences between the two heights after APEC. The vertical developments of pollution condition were lagerly influenced by the physical and chemical characteristics of boundary layer.(4) By analyzing the vertical distribution of atmospheric SO2 and its impact on the formation of secondary sulfate (SO42-0), we found that the ground concentration of SO2 and SO42- were low before the heating period, and pollutant concentrations were higher at 260m because they were affected by regional transport significantly, the change trends of pollutants at the two heights were obviously different; after the APEC summit, the concentration of SO2 and SO42- were largely increased, the differences between two layers were reduced, pollutants sampled at the two heights had similar trends. The higher humidity (RH) at ground facilitated the aqueous-phase reaction of SO2 to SO42-, leading to higher sulfur oxidation ratio (SOR) at ground than 260m. The analysis of correlation and backward trajectory showed that regional transport (especially the air mass via the south-west of Beijing) and local coal emissions were the main pollution sources in the periods before and during heating season. During the heating season, there were a large amount of emissions at ground which can be convected to the urban canopy, increasing the importance of local emissions at a higher height in the megacity.