Aerosol fluxes above Beijing

As part of the UK‐China collaborative research programme “Air Pollution and Human Health in a Chinese Megacity”, aerosol fluxes were measured during two contrasting periods in Nov/Dec 2016 and May/June 2017, using the micrometeorological eddy‐covariance (EC) flux measurement technique. A fast response ultrasonic anemometer and a total of three particle counters were operated at a height of 102 m on the 325 m meteorological tower at the Institute for Atmospheric Physics (IAP) of the Chinese Academy of Sciences. These included a water‐based condensation particle counter (CPC3785, TSI Inc.), and a combination of an Ultra High Sensitive Aerosol Spectrometer (UHSAS, DMT) and Aerodynamic Particle Sizer (APS3321, TSI Inc), together providing size‐segregated particle fluxes over the size range 60 nm to 20 μm. Moreover, a sample inlet brought air to a measurement container at the foot of the tower, where an Aerosol Mass Spectrometer (HR‐ToF‐AMS; Aerodyne Research Inc) and Single Particle Soot Photometer (SP2, DMT) provided a fast response measurement of aerosol chemical composition, suitable for flux calculations of non‐refractory NH4 +, NO3 ‐, SO4 2‐, Cl‐ and organic aerosol in PM1, as well as black carbon. The AMS total organic aerosol flux was further decomposed by application of Positive Matrix Factorisation to virtual eddy‐accumulation data, which was derived from the EC data. Chemically speciated fluxes were dominated by organic carbon. During winter, both SO4 2‐ and Cl‐ showed emission, most likely from coal combustion, which was absent in summer. NO3 ‐ and NH4 + were emitted in winter, but downward fluxes were observed during summer, which reflects evaporation of NH4NO3 in the urban surface layer, below the measurement height. As a consequence the size‐segregated particle fluxes were bi‐directional during summer daytime conditions, downward fluxes were observed in the accumulation mode (> 0.22 μm) at the same time as the smaller particles were emitted. The PMF results indicated that inner urban organic aerosol emissions were very much dominated by cooking organic aerosol, with further contributions from fossil fuel combustion, whilst secondary organic aerosol components showed little flux. This is in stark contrast to the composition of the concentration, which was dominated by secondary organic aerosol components that must have originated from more regional emissions. Compared with cooking aerosol emissions, local fossil fuel emissions were quite small, in particular in summer, possibly reflecting the fact that diesel‐powered HGV are banned from Beijing during the day. This would suggest that controlling sources such as cooking may have more potential in reducing urban exposure than further control of tail pipe emissions. Copyright © 2018 by the International Aerosol Conference (IAC).