The Role of Hydrocarbon and Halocarbon Species in the Polluted Urban Atmosphere of Bristol, England.

The urban environment is a complex mixture of chemicals, however, due to the high levels of NOx that are generally present, ozone formation is VOC (volatile organic compound) limited. Therefore, it is of great importance to determine the type of VOC that is present in the urban environment, its concentration and how this varies both spatially and temporally. The results of a field campaign carried out from early spring through to the late summer of 2000, in Bristol, England, are presented. Continuous measurements of over 40 hydrocarbons have been made at an urban background site, located at Bristol University, for approximately nine months using a GC-FID system and for a selection of halocarbons for approximately one month using a GC-ECD system. Measurements of a smaller set of hydrocarbons were made simultaneously at a roadside site in the centre of Bristol, as part of the U.K. national monitoring network. In this paper the form of the halocarbon time-series is investigated by comparison with the hydrocarbon time-series, air-mass back trajectories and also local weather conditions. The variability of hydrocarbon concentrations within the urban environment are also investigated and reasons for discrepancies are discussed. Using principal component analysis sources for these hydrocarbons have been apportioned. In addition, ozone levels recorded in Bristol have been compared with hydrocarbon levels and in conjunction with trajectory modelling the role played by certain VOCs in the formation of ozone and radicals such as OH is assessed. A simple approximation of radical fluxes is also presented based on the variations of the measured hydrocarbons and the role of biogenically produced compounds such as isoprene in the urban environment is also considered. This study has investigated both local and remote effects on levels of pollutants in the Bristol conurbation. Like any other town or city, Bristol has its own distinctive atmospheric characteristics. A detailed understanding of the different influences on local air quality is important to inform relevant policy decisions such as transport planning and healthcare provision and this type of research provides such information.