The diurnal variation of the ejection duration fraction on 17
July (Fig. 14b) shows a similar pattern as discussed above,
already emerging at the lowest threshold level. At this
threshold, juwj ¼ 0:01m2 s2 (open symbols), the ejection
duration fraction starts to rise sharply just after 0715 EDT
and peaks during the first sampling period that has a
significant pollen concentration (0900–1000 EDT). The duration
fraction is then reduced again to a minimum coinciding with
the sampling period of minimum pollen concentration (1000–
1100 EDT) after which the duration fraction as well as the
pollen concentration increases again.