On the measurement uncertainty of Hirst-type volumetric pollen and spore samplers

Hirst-type volumetric spore traps are used across the globe and are the current standard instrument for monitoring pollen. While they suffer from various issues related to sampling, measurement, and human error, they are, nevertheless, relatively cost-efficient and robust instruments that have been in use for decades. They are also the only reference against which newer instruments can be directly evaluated and it is thus important to understand and quantify all errors to make fair comparisons. Here, we investigate the variability across three Hirst-type traps run in parallel for three months during the main pollen season in Payerne, Switzerland. A variety of temporal resolutions is studied. Overall, daily average values show median relative differences of 16% for total pollen (inter-quartile range (IQR) = 8–32%) and between 23 and 67% for the top three taxa considered. The values are identical for total pollen when only concentrations > 10 pollen grains/m 3 are considered, but decrease notably for the individual taxa investigated, with median relative differences ranging 14–30%. At the 2-hourly resolution, there is considerably more variability between the three samplers, with median relative differences of 42% for total pollen (IQR = 20–88%) and 62–120% for the top three taxa, respectively. Again, when the lowest concentrations are not included, these differences decrease somewhat to 40% for total pollen (IQR = 19–78%) and 41–56% for the top three taxa, respectively. Observations of low concentrations below 10 pollen grains/m 3 therefore have a large impact on the measurement uncertainty: for daily average total pollen concentrations there are differences of over 100% between the three samplers, and for individual taxa differences reach up to 200%, the maximum value possible in terms of pairwise comparisons.