110
Pesticides, Organic Contaminants, and Pathogens in Air
use in California’s Central Valley. The OP insecticides frequently exceeded
10 µg/L in fog, which is 2–3 orders of magnitude greater than for these and
similar compounds in rain from other locations, and in the Central Valley
itself (Seiber et al., 1993). Because these California samples were collected
during wintertime dormant spraying in the Central Valley, the elevated
levels of diazinon, methidathion, and chlorpyrifos were ascribed to this
usage.
The California samples contained a number of OAs of OP pesticides, with
parathion OA (paraoxon) being the most abundant. The source of parathion
and paraoxon compounds is unknown. OAs form in the gas phase, by reaction of parent thions with atmospheric oxidants (Woodrow et al., 1983), and
also on surfaces (Spear et al., 1975; Woodrow et al., 1977). OAs are potent
inhibitors of cholinesterase and are generally responsible for most of the
toxic effects of OPs (Henderson et al., 1994). The fog collected in Lodi, which
had the highest measured concentration of paraoxon (184 µg/L), yielded a
lab-measured cholinesterase inhibition which matched that of equivalent
concentrations of pure paraoxon standard.
The Henry’s law constant, which generally describes the distribution of
low solubility organics between the vapor- and aqueous phase in equilibrium (Suntio et al., 1988), was not a good quantitative indicator of the distribution of pesticides between the vapor- and liquid phase in these fog
samples. The measured air–water distribution coeffcient (D) was very much
less than H (Table 7.1). D < H implies an aqueous-phase enrichment; more
pesticide is dissolved in the aqueous phase than would be expected in an
ideal solution at equilibrium. The extents of the aqueous phase enrichment
were given as enrichment factors (EF = H/D), which were quite large, even
several thousand-fold, for chemicals such as pendimethalin and diazinon.
The possible underlying reasons for enrichment, and its consequences, are
discussed below.
The distribution of pesticides in foggy Central Valley atmospheres was
studied in more depth (Glotfelty et al., 1990b). This study was conducted at
one Central Valley sampling site (Kearney Agricultural Research Center,
University of California, Parlier, CA) and included one foothill site about
50 km east of Parlier, at approximately 500 m elevation. The distribution of
four OP insecticides (diazinon, parathion, chlorpyrifos, and methidathion)
and their oxons between the droplet and air phases was studied during six
fog events. Up to 50 µg/L was found for the total of the four OPs and up to
75 µg/L for the total of their oxons in the fogwater. Nearly all of the compounds exhibited aqueous phase enrichment, ranging from a mean of 1.4
(methidathion) to 58 (diazinon) (Table 7.2).
The oxon to thion ratios in the fogwater were generally less than 1, with the
exception of two sampling sites and dates. Atmospheric oxidation, especially
during daylight hours, followed by uptake in the fogwater was implicated.
Even though there were high concentrations and high EF for the water
phase, the very low volume of suspended water in fog leads to the highest
Pesticides, Organic Contaminants, and Pathogens in Air
use in California’s Central Valley. The OP insecticides frequently exceeded
10 µg/L in fog, which is 2–3 orders of magnitude greater than for these and
similar compounds in rain from other locations, and in the Central Valley
itself (Seiber et al., 1993). Because these California samples were collected
during wintertime dormant spraying in the Central Valley, the elevated
levels of diazinon, methidathion, and chlorpyrifos were ascribed to this
usage.
The California samples contained a number of OAs of OP pesticides, with
parathion OA (paraoxon) being the most abundant. The source of parathion
and paraoxon compounds is unknown. OAs form in the gas phase, by reaction of parent thions with atmospheric oxidants (Woodrow et al., 1983), and
also on surfaces (Spear et al., 1975; Woodrow et al., 1977). OAs are potent
inhibitors of cholinesterase and are generally responsible for most of the
toxic effects of OPs (Henderson et al., 1994). The fog collected in Lodi, which
had the highest measured concentration of paraoxon (184 µg/L), yielded a
lab-measured cholinesterase inhibition which matched that of equivalent
concentrations of pure paraoxon standard.
The Henry’s law constant, which generally describes the distribution of
low solubility organics between the vapor- and aqueous phase in equilibrium (Suntio et al., 1988), was not a good quantitative indicator of the distribution of pesticides between the vapor- and liquid phase in these fog
samples. The measured air–water distribution coeffcient (D) was very much
less than H (Table 7.1). D < H implies an aqueous-phase enrichment; more
pesticide is dissolved in the aqueous phase than would be expected in an
ideal solution at equilibrium. The extents of the aqueous phase enrichment
were given as enrichment factors (EF = H/D), which were quite large, even
several thousand-fold, for chemicals such as pendimethalin and diazinon.
The possible underlying reasons for enrichment, and its consequences, are
discussed below.
The distribution of pesticides in foggy Central Valley atmospheres was
studied in more depth (Glotfelty et al., 1990b). This study was conducted at
one Central Valley sampling site (Kearney Agricultural Research Center,
University of California, Parlier, CA) and included one foothill site about
50 km east of Parlier, at approximately 500 m elevation. The distribution of
four OP insecticides (diazinon, parathion, chlorpyrifos, and methidathion)
and their oxons between the droplet and air phases was studied during six
fog events. Up to 50 µg/L was found for the total of the four OPs and up to
75 µg/L for the total of their oxons in the fogwater. Nearly all of the compounds exhibited aqueous phase enrichment, ranging from a mean of 1.4
(methidathion) to 58 (diazinon) (Table 7.2).
The oxon to thion ratios in the fogwater were generally less than 1, with the
exception of two sampling sites and dates. Atmospheric oxidation, especially
during daylight hours, followed by uptake in the fogwater was implicated.
Even though there were high concentrations and high EF for the water
phase, the very low volume of suspended water in fog leads to the highest