IntroductionMany extensive measurements exist on urban hydrocarbon concentrations. In the UK, the operation of the Automated Urban Network (AUN) has provided a wealth of data.1However, much less is known about long-term measurements in UK remote/rural locations, although intensive campaigns have been undertaken at locations such as Mace Head2(Ireland), and Weybourne3(Norfolk). Long-term rural hydrocarbon measurements would have a variety of uses, for example providing information on the air quality arriving at urban locations, assessing the impact of city plumes and outflow on rural areas, and on the effect of European pollution events on the UK. The sole monitoring station in the AUN network classified as a rural monitoring station is sited at Harwell (Oxfordshire). Its location, within the highly urbanized southeast corner of the UK, makes it unrepresentative of rural concentrations within the UK as a whole.The University of Leeds field station at Haverah Park, located on open moorland, approximately 20 km to the north of Leeds, offers the opportunity to study a number of the above issues, dependent on the origin of air mass. Samples originating from the north and northwest will be relatively clean, having encountered few anthropogenic sources within the UK. Conversely, air masses originating from the south-southwest will bring relatively high concentrations of hydrocarbons from the Leeds–Bradford urban conurbation. During anti-cyclonic conditions, easterly airflows will bring in continental European air, without significant enhancement from major UK sources.The relative concentrations of various hydrocarbons have been used to assess the chemical history of the incoming air mass. A variety of quantitative approaches can be used4,5but they have a common qualitative approach,i.e.that as the air mass ages it is relatively enhanced in the more unreactive species. In most cases, OH is the most important species for hydrocarbon oxidation, but the influence of other initiating species such as Cl, Br, NO3and O3can be detected from such studies.6–8During the winter, Haverah Park is mainly subject to northwesterly or southwesterly airflows, bringing in relatively clean and dirty air respectively and thus we would expect to see a significant difference in relative hydrocarbon concentrations if chemical processing has been taking place. The C6–C8aromatic hydrocarbons considered in the study are ideal for this work as they are primarily attributed to a single source, automotive exhaust, and with a relatively constant initial distribution.9This paper reports the concentrations of C6–C8hydrocarbons, taken at approximately hourly intervals in a two-week period during January 1999 to assess the suitability of Haverah Park as a long-term monitoring station. Although the dataset is comparatively small, the weather patterns encountered during this period are representative of the entire winter and, therefore, we expect the observed concentrations to bare comparison with longer-term measurements from elsewhere in the UK. Variations in concentration levels can be rationalized from the trajectories of the parent air masses with both the short and long-term origin of the air mass contributing to the observed concentration levels. Ratios of hydrocarbon concentrations have been analysed in order to investigate the degree of chemical processing that has taken place during the air mass trajectory.