A cluster analysis of long range air transport pathways and associated pollutant concentrations within the UK

A cluster analysis of four-day back trajectories for January 1998 to December 2001 arriving mid-afternoon in Birmingham, UK at three different within boundary layer arrival heights has been performed in order for a better understanding of the pollution meteorology influencing this region. The time period was purposely chosen to encompass the Pollution in the Urban Midlands Atmosphere field campaign. Six natural synoptic scale transport patterns were identified with three, strong-westerly, westerly and slow-easterly, showing seasonal variation in frequency. Significant differences in air pollutant concentrations and behaviour were found between air mass cluster types when they were analysed with measurements taken from an urban background site in Birmingham and a rural site in Harwell. Highest concentrations of primary pollutants were associated with a slow-easterly air mass from mainland Europe, while lowest concentrations were associated with south-westerly and strong-westerly air masses passing over the Atlantic Ocean. The polluted slow-easterly air mass was associated with highest ozone concentration for the warm season and lowest ozone concentration for the cool season. This could be explained by photochemical ozone production during the warm season and NOx titration of background ozone during the cool season, when photochemically “inactive” conditions prevailed. A wealth of information was determinable, including influences of short and long-range transport and photochemical productivity.