Eddy covariance flux measurements of volatile organic compounds from central Beijing

Air pollution is a major public health concern in Beijing, where poor air quality has gained significant public and political attention. Volatile organic compounds (VOCs) contribute significantly to this through the formation of secondary organic aerosol (SOA) and tropospheric ozone. In addition, some VOCs directly affect human health such as the carcinogen benzene. VOCs are emitted into the urban environment from both biogenic and anthropogenic sources. In urban environments, anthropogenic sources are commonly quantified through emissions inventories but biogenic emissions from the city are harder to estimate.

Here we will report mixing ratios and fluxes of biogenic and anthropogenic VOCs recorded in Beijing during two intensive field campaigns: November-December 2016 and May-June 2017. Measurements were made using a PTR-ToF-MS as part of the Sources and Emissions of Air Pollutants in Beijing (AIRPOLL-Beijing) work package within the Air Pollution and Human Health in a Developing Megacity (APHH-Beijing) research programme. Measurements were made from 102m on the Institute of Atmospheric Physics (IAP) meteorological tower, situated between the third and fourth Beijing ring roads, which can be characterised as an urban background site.

In November and December high VOC concentrations were observed (e.g. 1.5 and 1.6 ppbv for benzene and toluene respectively) despite relatively low emissions from the city centre site. VOC concentrations are shown to be controlled by meteorology with concentrations peaking during haze events. In May and June VOC concentrations were lower (0.35 and 0.44 ppbv for benzene and toluene respectively) and less dependent on wind direction with fluxes of aromatic compounds similar to that observed in the winter (0.02 and 0.11 mg m^-2 h^-1 for benzene and toluene respectively). The strong diurnal cycle seen in VOC fluxes is not observed in the concentrations, which are much less temporally variate. VOC flux measurements show that during the daytime biogenic emissions (isoprene and monoterpenes) contribute significantly to the total VOC flux, and that these compounds represent a large daytime source of photochemical ozone creation potential. This demonstrates that biogenic VOC emission makes a significant contribution to the atmospheric chemistry in Beijing.