Abstract
In epidemiological studies, outdoor exposure to pollen is typically estimated using rooftop monitoring station data, whilst exposure overwhelmingly occurs at street level. In this study the relationship between street level and roof level grass pollen concentrations was investigated for city centre street canyon environments in Aarhus, Denmark, and London, UK, during the grass pollen seasons of 2010 and 2011 respectively. For the period mid-day to late evening, street level concentrations in both cities tended to be lower than roof-level concentrations, though this difference was found to be statistically significant only in London. The ratio of street/roof level concentrations was compared with temperature, relative humidity, wind speed and direction, and solar radiation. Results indicated that the concentration ratio responds to wind direction with respect to relative canyon orientation and local source distribution. In the London study, an increase in relative humidity was linked to a significant decrease in street/roof level concentration ratio, and a possible causative mechanism involving moisture mediated pollen grain buoyancy is proposed. Relationships with the other weather variables were not found to be significant in either location. These results suggest a tendency for monitoring station data to overestimate exposure in the canyon environment.
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Notes
The precise height above ground level of a monitoring station will depend upon local urban topography. Ideally a station should be situated above any surrounding physical structures such as trees or buildings that may interfere with local airflow patterns (Lacey and Venette 1995), pp 424–425).
The briefer sampling period in London was due to the collection of additional data sets not presented in this study.
A single rod samples air at 21.7 l min−1, over twice the rate of the 7-Day sampler (10 l min−1).
The H/W ratio of the Upper Street canyon depends on wind direction because the height of the buildings on either side differ. For winds from the East side of the street the ratio is 0.69. The exact height of the buildings on the West side is not known; however, it is slightly taller, thus the H/W ratio will be slightly greater.
In the context of a street canyon, this is equivalent to the leeward side of the street.
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Acknowledgements
The first author would like to extend special thanks to Stine Rødjajn for help with sample collection, to Janne Sommer at Astma-Allergi Danmark for providing access to data and facilities, and to Dr. Harry Morrow Brown for generously lending equipment. The Danish Air Quality Monitoring Programme, and in particular Thomas Ellermann, are also thanked for contributing meteorological data.
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Peel, R.G., Kennedy, R., Smith, M. et al. Do urban canyons influence street level grass pollen concentrations?. Int J Biometeorol 58, 1317–1325 (2014). https://doi.org/10.1007/s00484-013-0728-x
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DOI: https://doi.org/10.1007/s00484-013-0728-x