51
Properties of Pesticides and Contaminants
hydroxyl radicals, ozone, and nitrogen oxides transfer the energies needed
to break bonds in atmospheric pesticides and toxics (Atkinson et al., 1999;
Woodrow et al., 2018). The frequencies of these indirect reactions are dependent on air quality. Particulate load and moisture in the air also infuence
reactions (Epstein and Nizkorodov, 2012; Socorro et al., 2017). The reactions
in air affect the atmospheric lifetime of a pesticide and thus its long-range
transfer to remote regions, its impacts on the atmosphere, and its toxicity.
Parathion applied to orchards in the midday in summertime is oxidized
to its more toxic counterpart paraoxon in minutes (Woodrow et al., 1977).
Higher ratios of paraoxon to parathion downwind of the application site confrmed the chemical conversion of parathion to paraoxon in air is a major
transformation route. Eventually, parathion is converted to the breakdown
product, p-nitrophenol. Conversion of parathion to paraoxon is accelerated
by sunlight, but slower conversion does occur at night (half-life during day
is 2 minutes, but at night it is 131 minutes) (Woodrow et al., 1978). Similar
experiments were used to measure transformation of chlorpyrifos to its more
stable reactant product chlorpyrifos oxon (Aston and Seiber, 1997). As the
elevation increased, so did the levels of chlorpyrifos oxon compared to the
parent, indicating that atmospheric oxidation is a dominant pathway.
Chambers in the laboratory can also be used to study photoinitiated reactions using lamps that simulate sunlight. Trifuralin photodecomposed to
multiple products in the lab in a vapor phase reactor equipped with an air
sampler collecting vapors on a coated solid sorbent with limit of detection of
less than 1 ng/m 3 (Soderquist et al., 1975). The half-life of trifuralin was 47
minutes in the lab and between 21 and 193 min in the feld, showing good
agreement. Photolysis occurring at the soil surface followed by volatilization
was the dominant occurrences. Trifuralin is stable in the dark (Woodrow
et al., 1978).
Sulfur is applied at the largest volume rate as a pesticide worldwide including in California (see Table 2.2) (Griffth et al., 2015). The dissipation routes of
sulfur from foliage include wind erosion, washing off, microbial activity, oxidation, and photooxidation. At 39°C, sulfur has a vapor pressure of 1.33 × 10 –3
Pa, corresponding to a loss of 449 µg/m 2 ·hr assuming little or no interaction
with plant surfaces soon after application (Woodrow et al., 2001). There are
no direct studies of the atmospheric reactions of agricultural sulfur; however,
it is expected it would undergo the same oxidation transformation reactions
as other sulfur species.
Volatile organic compounds partition to vapor phase (fog and clouds)
where they undergo aqueous phase photolysis reactions and reactions with
hydroxyl radicals (Kaur and Anastasio (2017). Epstein and Nizkorodov (2012)
used complex modeling to evaluate photolysis reactions for several classes of
organics and point out that this is another signifcant pathway for pesticides
in air. The importance of aqueous-phase photolysis can be assessed by the
propensity of the chemical to partition to the fog or cloud droplets from air
(see Chapter 7).
Properties of Pesticides and Contaminants
hydroxyl radicals, ozone, and nitrogen oxides transfer the energies needed
to break bonds in atmospheric pesticides and toxics (Atkinson et al., 1999;
Woodrow et al., 2018). The frequencies of these indirect reactions are dependent on air quality. Particulate load and moisture in the air also infuence
reactions (Epstein and Nizkorodov, 2012; Socorro et al., 2017). The reactions
in air affect the atmospheric lifetime of a pesticide and thus its long-range
transfer to remote regions, its impacts on the atmosphere, and its toxicity.
Parathion applied to orchards in the midday in summertime is oxidized
to its more toxic counterpart paraoxon in minutes (Woodrow et al., 1977).
Higher ratios of paraoxon to parathion downwind of the application site confrmed the chemical conversion of parathion to paraoxon in air is a major
transformation route. Eventually, parathion is converted to the breakdown
product, p-nitrophenol. Conversion of parathion to paraoxon is accelerated
by sunlight, but slower conversion does occur at night (half-life during day
is 2 minutes, but at night it is 131 minutes) (Woodrow et al., 1978). Similar
experiments were used to measure transformation of chlorpyrifos to its more
stable reactant product chlorpyrifos oxon (Aston and Seiber, 1997). As the
elevation increased, so did the levels of chlorpyrifos oxon compared to the
parent, indicating that atmospheric oxidation is a dominant pathway.
Chambers in the laboratory can also be used to study photoinitiated reactions using lamps that simulate sunlight. Trifuralin photodecomposed to
multiple products in the lab in a vapor phase reactor equipped with an air
sampler collecting vapors on a coated solid sorbent with limit of detection of
less than 1 ng/m 3 (Soderquist et al., 1975). The half-life of trifuralin was 47
minutes in the lab and between 21 and 193 min in the feld, showing good
agreement. Photolysis occurring at the soil surface followed by volatilization
was the dominant occurrences. Trifuralin is stable in the dark (Woodrow
et al., 1978).
Sulfur is applied at the largest volume rate as a pesticide worldwide including in California (see Table 2.2) (Griffth et al., 2015). The dissipation routes of
sulfur from foliage include wind erosion, washing off, microbial activity, oxidation, and photooxidation. At 39°C, sulfur has a vapor pressure of 1.33 × 10 –3
Pa, corresponding to a loss of 449 µg/m 2 ·hr assuming little or no interaction
with plant surfaces soon after application (Woodrow et al., 2001). There are
no direct studies of the atmospheric reactions of agricultural sulfur; however,
it is expected it would undergo the same oxidation transformation reactions
as other sulfur species.
Volatile organic compounds partition to vapor phase (fog and clouds)
where they undergo aqueous phase photolysis reactions and reactions with
hydroxyl radicals (Kaur and Anastasio (2017). Epstein and Nizkorodov (2012)
used complex modeling to evaluate photolysis reactions for several classes of
organics and point out that this is another signifcant pathway for pesticides
in air. The importance of aqueous-phase photolysis can be assessed by the
propensity of the chemical to partition to the fog or cloud droplets from air
(see Chapter 7).