Micrometeorological flux measurements of pollutant emissions above Beijing

Air pollution is estimated to cause 1.6 million premature deaths in China every year and in the winter 2016/17 Beijing had to issue health alerts and put in place ad hoc limitations on industrial and vehicular activity. Much of this pollution is attributed to emissions from industrial processes and in particular coal combustion. By contrast, the diffuse pollutant sources within the city are less well understood. This includes, e.g., emissions from the Beijing traffic fleet, the sewage system, food preparation, solid fuel combustion in the streets and small industrial processes. Within the framework of a major UK-Chinese collaboration to study air pollution and its impact on human health in Beijing, we therefore measured fluxes of a large range of pollutants from a height of 102 m on the meteorological tower at the Institute of Atmospheric Physics during two contrasting periods (Nov/Dec 2016 and May/Jun 2017). We mounted some instruments on the tower and housed others in a measurement container at its base. Fluxes were quantified by eddy-covariance for the gases CO2, CO, NOx, O3, a large number of VOCs and, for the first time above an urban area, NH3. In addition, we measured aerosol fluxes in terms of total particle number, size-segregated particle number (0.06 to 20 µm) and chemically resolved, using an eddy-covariance flux system based on an Aerosol Mass Spectrometer (non-refractory PM1 NH4+, NO3-, SO42-, Cl- organic aerosol) and trialling a single particle soot spectrometer (SP2, DMT) for black carbon. Fluxes were similar in magnitude to Western city centre locations, but with important differences: (a) aerosol emissions included SO2- and Cl- not previously observed; (b) NH3 fluxes were larger than estimated for, e.g., UK cities; (c) NOx fluxes were lower than predicted by the emission inventories, whilst the opposite has been found in the UK and Austria.