60 Weinberger et al.: A Review of Spatial Variation of Allergenic Tree Pollen Within Cities Sampling Methodologies Several types of samplers are available to sample the pollen content of the atmosphere. The choice of sampler(s) employed by a study is important as it affects the time scale and unit of the measure- ments. Eight studies conducted pollen monitoring using Hirst-type volumetric samplers (Hirst 1952). Volumetric samplers, which are typically used to take 24-hour samples of atmospheric pollen, draw in air at a constant flow rate and allow pollen to im- pact on a piece of tape secured on a rotating drum. Knowledge of the flow rate allows the calculation of the actual pollen concentration in grains/m3 Three studies used impactor samplers, such as . the Rotorod (Multidata LLC, St. Louis Park, Min- nesota, U.S.). Impactors consist of a set of greased rods or slides attached to a head that rotates such that pollen is impacted on the greased surfaces. As with Hirst-type samplers, impactors allow the calculation of a concentration in grains/m3 . These samplers tend to be less efficient than Hirst-type samplers at retrieving very small pollen grains (less than 10 mm in diameter) (Solomon et al. 1980). The two remaining studies employed gravimetric samplers. These samplers, such as the designs of Tauber (Tauber 1974) and Durham (Durham 1946), depend on gravity to passively sample the pollen content of the atmosphere and are cheaper and simpler than volumetric and impactor samplers. Because the sampling method is passive, pollen counts are reported as influx (i.e., deposition) in grains/cm2 rather than as a concentration. Crispen et al. (2010) have shown that Durham traps, which were used in the two gravimetric studies included in this review, are comparable to Burkard traps when examining the relative frequencies with which pol- len taxa appear (Crispen et al. 2010). Durham traps can be leſt out for varying periods of time, but are typically replaced daily (Singh and Mathur 2012). While all three sampling types allow pollen to be measured on a daily basis (as a concentra- tion for volumetric and impactor samplers, and as influx for gravimetric samplers), most of the included studies examined the total amount of pol- len sampled in a given year by summing the daily values across an entire pollen season or year and comparing this total value across sites. These values are still reported as a concentration or as influx, and are oſten referred to as the “severity” or “mag- ©2015 International Society of Arboriculture nitude” of the pollen season. Some studies report this value only for the aggregate of all tree pollen taxa, while others report taxon-specific values. With the exception of one study employing only two sampling sites (White et al. 2005), all the stud- ies revealed spatial differences in atmospheric tree pollen concentrations or influx within urban areas regardless of sampler type. The following literature review will group the 13 studies by sampling meth- odology and discuss their major findings in detail. A summary of findings is also presented in Appendix I. Evidence from Volumetric Samplers Out of the eight studies using volumetric samplers, the most compelling evidence comes from the four studies in which pollen was measured at a uniform height. Celenk et al. (2010) monitored pollen for a full calendar year at two sites, each 25 m high, in Istanbul, Turkey, one on the European continent and one on the Asian continent. Over the course of a year, the total number of tree pollen grains was more than 35% higher at the European site than at the Asian site (28,227 grains versus 20,894 grains). When the authors examined total annual catches of individual pollen taxa, some were similar between the two sites while others showed large differences. Not all pollen types were more common at the Asian site. For example, total sycamore pollen was six times higher at the European site than the Asian site. Katelaris et al. (2004) measured pollen at three sites in Sydney, Australia, from August to November (Southern Hemisphere spring) of 1999. The total pollen concentrations summed across the whole sampling period were 14,382 grains/m3 grains/m3 , and 9,269 grains/m3 at the three sites (Homebush, Eastern Creek, and Nepean, respec- tively). Thus, the total pollen concentration was approximately 55% higher at Homebush than at Nepean. The authors examined differences in daily pollen concentrations by classifying daily concen- trations of total pollen into high, medium, and low concentrations and evaluating pairs of sites for dis- cordance in the classification of each day. On 8%, 13%, and 17% of days, pollen concentrations were discordant at Eastern Creek versus Nepean, Home- bush versus Nepean, and Homebush versus Eastern Creek, respectively, indicating that spatial variability in pollen exists on a daily as well as on a seasonal timescale. The authors did not examine any pollen , 11,584
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