Abstract
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.
Highlights
The compilation of spatially and temporally detailed inventories for the emission of anthropogenic volatile organic compounds (VOCs) from urban areas is a necessary requirement for air quality regulatory purposes, effects assessment and research
The higher mixing ratios of methanol compared with the other analytes are typical for urban VOC composition measurements and can be attributed to its relatively low photochemical reactivity (Atkinson, 2000) and the numerous anthropogenic/biogenic sources which contribute to its emissions both in and outside of the city
In the present study we have shown that these techniques can be extended to the urban environment provided a measurement site with suitable elevation above street level can be found
Summary
The compilation of spatially and temporally detailed inventories for the emission of anthropogenic volatile organic compounds (VOCs) from urban areas is a necessary requirement for air quality regulatory purposes, effects assessment and research. Current emission estimates are associated with large degrees of uncertainty (Friedrich and Obermeier, 1999) which may limit their usefulness Much of this uncertainty can be attributed to the large variety of different source categories which contribute to urban VOC emissions, which can be difficult to characterise and validate. The eddy covariance (EC) technique is considered the most direct micrometeorological method available for estimating surface/atmosphere exchange fluxes, as it measures the turbulent flux directly, without reliance on any empirical parameterisations. This approach requires high frequency measurements (typically in the order of 5–20 Hz) of both vertical wind speed and concentration (or mixing ratios) to resolve all eddies that contribute to vertical transport (Lenschow, 1995). This technique is well established for the measurement of some trace gases, such as CO2 and H2O (Aubinet et al, 2001), its application to VOC fluxes has been restricted because of the slow response times of most VOC sensors
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