Methods: Volumetric sampling for ragweed pollen was conducted over a 3-year period (2007-2009) using the Burkard Spore Trap. 24-hour slide samples were observed microscopically at 400 X by an NAB certified counter. Ragweed counts were determined at three-hour intervals for 24-hour periods, August through November.
Results: In September, the “peak month” for ragweed pollen levels with 57% of the total annual ragweed pollen load recorded, ragweed levels increased from sunrise into the early afternoon (12:30 pm-3:30pm) before declining. Midday peaks were reproducibly recorded. Overnight levels of ragweed pollen were also evident. During August-October, the maximum concentrations of ragweed occurred in a diurnal pattern. Minimal ragweed levels were observed in November, dissipating through the month.
Conclusions: In Washington, DC, airborne concentrations of ragweed pollen can remain high throughout the day with peak levels occurring midday and sufficient levels recorded overnight into the early morning hours. This diurnal pattern is not as pronounced as that previously observed for area grass pollen. Various environmental factors such as carbon dioxide levels, summer storms and convective patterns, the long distance transport of pollen and numerous meteorological variables may influence this pattern and require further investigation.