105
Pesticides in Fog
of wintertime fogs. The Central Valley comprises a land mass of nearly
50,000 km 2 residing between California’s coastal mountain range on the
west, the Sierra Nevada Mountain range foothills on the east, and extending roughly from the cities of Redding (north) to south of Bakersfeld. The
northern portion, the Sacramento Valley, is dominated by rice and wheat as
major feld crops, and almond and other fruit and nut orchards. The southern portion, the San Joaquin Valley, produces cotton, wheat, corn, and a
variety of vegetables (carrots, broccoli, cabbage, melons) as major feld crops
while grape, citrus, almond, walnut, nectarine, peach, plum, and apricot are
among the major vineyard and orchard crops. Pesticide use in California is
extensive (see Table 2.1 for the leading pesticides used in California in 2018).
Soil fumigants (methyl bromide, metam sodium, chloropicrin), OP and chlorinated hydrocarbon insecticides-fungicides, triazine and thiocarbamate
herbicides, cotton defoliants such as sodium chlorate, paraquat, and DEF
(S,S,S-tributyl phosphorotrithioate), and many others are included. Fog episodes in the Central Valley occur predominately in the November–February
period. This period coincides with the dormant spraying of fruit and nut
orchards. These sprays include typically sulfur, mineral oil, OP insecticides,
and some other chemicals (Steinheimer et al., 2000).
7.4 Fogwater Sampling Methodology
Seiber and Glotfelty began their fog research using the Caltech Rotating Arm
Collector (RAC) (Figure 7.2), which featured a 1.5-HP motor to drive a 63 cm
long stainless steel rod at 1,700 rpm through the air (Jacob et al., 1984). Each
end of the rod had a slot milled into its leading edge to collect impacting
fogwater droplets. High-density polyethylene bottles mounted on each end
of the rod catch the fogwater samples as they are accelerated from each slot.
Collection rates up to 2 mL/min were obtained in the feld (Collett et al., 1990).
They soon transitioned to a new design—a high-volume collector developed
at USDA-ARS laboratory in Beltsville, MD (Glotfelty et al., 1986). The innovation was a rotating screen mechanism, 50 cm in diameter, in which four
layers of stainless steel screen material were rotated around a central axis at
720 rpm. Fogwater obtained from droplets impacting on the screen was centrifuged to the periphery, collected in a slotted aluminum tube, and drained
into a collection vessel. A large fan pulled air through the device at a sampling rate of ~160 m 3 /min. The sampling rate typically allowed collections of
approximately 1 L/hr of fogwater depending on the suspended water content of the fog. This device was employed mounted on a pickup truck, which
moved slowly (5–15 km/hr) through the foggy area to be sampled. Vehicle
and gas-powered generator fumes were thus continually swept toward the
rear and away from the fog sampler intake.
Pesticides in Fog
of wintertime fogs. The Central Valley comprises a land mass of nearly
50,000 km 2 residing between California’s coastal mountain range on the
west, the Sierra Nevada Mountain range foothills on the east, and extending roughly from the cities of Redding (north) to south of Bakersfeld. The
northern portion, the Sacramento Valley, is dominated by rice and wheat as
major feld crops, and almond and other fruit and nut orchards. The southern portion, the San Joaquin Valley, produces cotton, wheat, corn, and a
variety of vegetables (carrots, broccoli, cabbage, melons) as major feld crops
while grape, citrus, almond, walnut, nectarine, peach, plum, and apricot are
among the major vineyard and orchard crops. Pesticide use in California is
extensive (see Table 2.1 for the leading pesticides used in California in 2018).
Soil fumigants (methyl bromide, metam sodium, chloropicrin), OP and chlorinated hydrocarbon insecticides-fungicides, triazine and thiocarbamate
herbicides, cotton defoliants such as sodium chlorate, paraquat, and DEF
(S,S,S-tributyl phosphorotrithioate), and many others are included. Fog episodes in the Central Valley occur predominately in the November–February
period. This period coincides with the dormant spraying of fruit and nut
orchards. These sprays include typically sulfur, mineral oil, OP insecticides,
and some other chemicals (Steinheimer et al., 2000).
7.4 Fogwater Sampling Methodology
Seiber and Glotfelty began their fog research using the Caltech Rotating Arm
Collector (RAC) (Figure 7.2), which featured a 1.5-HP motor to drive a 63 cm
long stainless steel rod at 1,700 rpm through the air (Jacob et al., 1984). Each
end of the rod had a slot milled into its leading edge to collect impacting
fogwater droplets. High-density polyethylene bottles mounted on each end
of the rod catch the fogwater samples as they are accelerated from each slot.
Collection rates up to 2 mL/min were obtained in the feld (Collett et al., 1990).
They soon transitioned to a new design—a high-volume collector developed
at USDA-ARS laboratory in Beltsville, MD (Glotfelty et al., 1986). The innovation was a rotating screen mechanism, 50 cm in diameter, in which four
layers of stainless steel screen material were rotated around a central axis at
720 rpm. Fogwater obtained from droplets impacting on the screen was centrifuged to the periphery, collected in a slotted aluminum tube, and drained
into a collection vessel. A large fan pulled air through the device at a sampling rate of ~160 m 3 /min. The sampling rate typically allowed collections of
approximately 1 L/hr of fogwater depending on the suspended water content of the fog. This device was employed mounted on a pickup truck, which
moved slowly (5–15 km/hr) through the foggy area to be sampled. Vehicle
and gas-powered generator fumes were thus continually swept toward the
rear and away from the fog sampler intake.