Cluster analysis of variations in the diurnal pattern of grass pollen concentrations in Northern Europe (Copenhagen) and Southern Europe (Cordoba)

Abstract

From an allergological point of view, Poaceae pollen is one of the most important type of pollen that the population is exposed to in the ambient environment. There are several studies on intra-diurnal patterns in grass pollen concentrations, and agreement on the high variability. However, the method for analysing the different patterns is not yet well established. The aim of the present study is therefore to examine the method of pattern analysis by statistical clustering, as well as relating the proposed patterns to time of season and meteorological variables at two highly different biogeographical locations; Córdoba, Spain and Copenhagen, Denmark. Airborne pollen is collected by Hirst type volumetric spore traps and counted using an optical microscope at both sites. The counts were converted to bi-hourly concentrations and a new method based on cluster analysis was applied with the aim of determining the most frequent diurnal patterns in pollen concentrations and their dependencies of site, season and meteorological variables. Three different well defined diurnal patterns were identified at both locations. The most frequent pattern in Copenhagen was associated with days having peak pollen concentrations in the evening (maximum between18h-20h), whereas the most frequent pattern at Córdoba was associated with days having peak pollen concentrations in the afternoon (maximum between 14h-16h). These three patterns account for 70% of days with no rain and pollen concentrations above 20 grains m-3. The most frequent pattern accounts for 40% and 57% of the days in Córdoba and Copenhagen respectively. The analysis clearly shows the great variation in pollen concentration pattern, albeit a dominating pattern can be found. It was not possible to explain all the differences in the patterns by the meteorological variables when examined individual. Clustering method is estimated to be an appropriate methodology for studying aerobiological phenomena with high variability.