Seasonality of Pesticides in Air and Rain

Locally high concentrations of pesticides in rain and air are very seasonal and are correlated to local use. The highest concentrations in air and rain usually occur in the spring and summer months (see Figure 7), coinciding with application times and warmer temperatures. However, insecticide concentrations in air, rain, and fog can also be high during autumn and winter in some areas, if there is high use at that time. This occurs in California where stone-fruit orchards are sprayed for insects when trees are dormant.

Figure 7. Average concentration of atrazine and alachlor in rain sampled in the vicinity of Wye River, Maryland (Glotfelty and others, 1990).

Pesticides have also been detected at low levels during periods before and substantially after use, but determining the sources has proven difficult. These off-season occurrences could be due to volatilization and wind erosion of previously applied material, or the result of long-range transport from areas where the growing season started earlier.