Mixing ratios and eddy covariance flux measurements of volatile organic compounds from an urban canopy (Manchester, UK) B. Langford1, B. Davison1, E. Nemitz2, and C. N. Hewitt1 1Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK 2Centre for Ecology & Hydrology (CEH) Edinburgh, Bush Estate, Penicuik, EH26 0QB, UK
Abstract. Mixing ratios and fluxes of six selected volatile organic compounds (VOCs)
were measured above the city of Manchester (UK) during the summer of 2006. A
proton transfer reaction-mass spectrometer was used for the measurement of
mixing ratios, and fluxes were calculated from these using both the disjunct
and the virtual disjunct eddy covariance techniques. The two flux systems,
which operated in alternate half hours, showed good agreement, with R2
values ranging between 0.74 and 0.9 for the individual analytes. On average,
fluxes measured in the disjunct mode were approximately 20% lower than those
measured in the virtual mode. This difference is due to both the dampening of
the VOC signal by the disjunct flux sampler and carry over from one sample to
the next. Correcting for these effects reduced the difference to less than
7%. Observed fluxes are thought to be largely controlled by anthropogenic
sources, with vehicle emissions the major contributor. However, both
evaporative and biogenic emissions may account for some of the VOCs present.
Concentrations and fluxes of the oxygenated compounds were highest on
average, ranging between 0.15 to 1 mg m−2 h−1; the fluxes of
aromatic compounds were lower, between 0.12 to 0.28 mg m−2 h−1.
The observed fluxes were up-scaled to give city wide emission estimates for
each compound and the results compared to estimates made by the National
Atmospheric Emission Inventory (NAEI) for the same flux footprint. Fluxes of
toluene and benzene compared most closely differing by approximately 50%,
while in contrast the oxygenated fluxes were found to be between 3.6–6.3
times larger than the annual average predicted by the NAEI.
Citation: Langford, B., Davison, B., Nemitz, E., and Hewitt, C. N.: Mixing ratios and eddy covariance flux measurements of volatile organic compounds from an urban canopy (Manchester, UK), Atmos. Chem. Phys., 9, 1971-1987, doi:10.5194/acp-9-1971-2009, 2009.