50
Pesticides, Organic Contaminants, and Pathogens in Air
3.4.2.3 Biota and Vapor
An approach analogous to the managed wetlands can be taken to clean the
air of contaminants from around factories, feedlots, freeways, etc. Aston and
Seiber (1997) followed the fate of pesticides in air from the large agricultural
area in the San Joaquin Valley as the residues moved downwind and up gradient to the more pristine Sierra Nevada Mountains. They postulated that
the heavily forested slope leading to the higher elevations would scrub pesticides from the air. This means that a volume of air downwind from the
agricultural area would have less contaminant than what might be expected
if only dilution were occurring. That is, some airborne chemical could be
partitioned or absorbed by the pine needles. A cross section of a pine needle
shows a waxy cuticle not unlike the coating on a solid-phase microextraction (SPME) device commonly used to sorb contaminants from air and water
(see Chapter 6). The net result is that the air is “cleaned” following passage
through evergreen forests. Forest buffers around factories and in urban parks
(e.g., New York’s Central Park) may play important roles in purifying the air
(e.g., capturing excess carbon dioxide from combustion sources). People have
increasingly come to recognize the value of planting trees or allowing some
agricultural felds to be located around cities and transportation corridors.
3.5 Reactivity
In the summer months, when sunlight is its most intense and skies are relatively cloudless, pesticide application is heaviest. This is signifcant because
light is the main factor driving pesticide transformations in air (Woodrow
et al., 2018). Theoretically, this makes sense (Woodrow et al., 1983): The bond
energies in volatile organic chemicals are in the range of energies of wavelengths of sunlight. Direct reactions with air occur at wavelengths from 290
to 400 nm. Considering the energy at each wavelength:
E = Lhc/˜
(3.36)
where L is Avogadro’s number (6.022 × 10 23 molecules/mol), h is Planck’s
constant (6.626 × 10 –27 erg/sec), and c is the speed of light (3 × 10 10 cm/sec). E,
energy (kcal/mol), of the C–C bond in ethane is 88 kcal/mol and for a C–H
bond in the same molecule is 98 kcal/mol, while the energies of sunlight in
the lower atmosphere are from 96 to 72 kcal/mol at wavelengths (λ) from 300
to 400 nm. In the stratosphere, lower wavelengths are present that can catalyze activation of even stronger bonds (see Chapter 9). Many pesticides absorb
sunlight from 290 to 400 nm and readily undergo photocatalyzed reactions
in air. Indirect reactions also occur in air in which singlet and triplet oxygen,
Pesticides, Organic Contaminants, and Pathogens in Air
3.4.2.3 Biota and Vapor
An approach analogous to the managed wetlands can be taken to clean the
air of contaminants from around factories, feedlots, freeways, etc. Aston and
Seiber (1997) followed the fate of pesticides in air from the large agricultural
area in the San Joaquin Valley as the residues moved downwind and up gradient to the more pristine Sierra Nevada Mountains. They postulated that
the heavily forested slope leading to the higher elevations would scrub pesticides from the air. This means that a volume of air downwind from the
agricultural area would have less contaminant than what might be expected
if only dilution were occurring. That is, some airborne chemical could be
partitioned or absorbed by the pine needles. A cross section of a pine needle
shows a waxy cuticle not unlike the coating on a solid-phase microextraction (SPME) device commonly used to sorb contaminants from air and water
(see Chapter 6). The net result is that the air is “cleaned” following passage
through evergreen forests. Forest buffers around factories and in urban parks
(e.g., New York’s Central Park) may play important roles in purifying the air
(e.g., capturing excess carbon dioxide from combustion sources). People have
increasingly come to recognize the value of planting trees or allowing some
agricultural felds to be located around cities and transportation corridors.
3.5 Reactivity
In the summer months, when sunlight is its most intense and skies are relatively cloudless, pesticide application is heaviest. This is signifcant because
light is the main factor driving pesticide transformations in air (Woodrow
et al., 2018). Theoretically, this makes sense (Woodrow et al., 1983): The bond
energies in volatile organic chemicals are in the range of energies of wavelengths of sunlight. Direct reactions with air occur at wavelengths from 290
to 400 nm. Considering the energy at each wavelength:
E = Lhc/˜
(3.36)
where L is Avogadro’s number (6.022 × 10 23 molecules/mol), h is Planck’s
constant (6.626 × 10 –27 erg/sec), and c is the speed of light (3 × 10 10 cm/sec). E,
energy (kcal/mol), of the C–C bond in ethane is 88 kcal/mol and for a C–H
bond in the same molecule is 98 kcal/mol, while the energies of sunlight in
the lower atmosphere are from 96 to 72 kcal/mol at wavelengths (λ) from 300
to 400 nm. In the stratosphere, lower wavelengths are present that can catalyze activation of even stronger bonds (see Chapter 9). Many pesticides absorb
sunlight from 290 to 400 nm and readily undergo photocatalyzed reactions
in air. Indirect reactions also occur in air in which singlet and triplet oxygen,