186
Pesticides, Organic Contaminants, and Pathogens in Air
volatilization. What is not known is the stability of the diglycolamine salt of
dicamba. The amine salt can be viewed as a conjugate acid. So, the question
is: what is the pKa of the salt? For example, the pKa for the conjugate acid
of ethanolamine (and the amine itself) is 9.5, characteristic of a strong base.
The diglycolamine salt of dicamba is probably of a similar order of magnitude, implying that the salt should be very stable (pKa data are not available).
However, at acid pH (tank mix/soil), protonated diglycolamine may be more
stable than the salt, depending on the pKa of its conjugate acid. Perhaps, this
is where the effect of the temperature factor also comes into play, in addition
to promoting increased volatilization of the already formed dicamba acid;
volatility increases up to about 30°C. Another factor that might affect the
stability of the salt is treated leaf surface area. Apparently, as the water-based
tank mix dries on the leaf surfaces, the dicamba salt, as it interacts with
the leaf cuticle, becomes less stable, leading to volatile dicamba. For larger
treated surface areas, more of the volatile component is available to damage
nontarget, susceptible crops. An additional consideration is the exposure of
target leaf residues to sunlight. However, partly due to a low quantum yield
(<0.02), photolysis is a relatively minor dissipation process compared to the
timescale of dicamba volatilization and atmospheric dispersion (Aguer et al.,
2000; Waite et al., 2005).
10.5 Conclusions
Lessons learned from chemical residue behavior in the air can be applied in
responding to airborne exposures to other toxicants including metals such
as mercury (that volatilizes as methyl mercury from the ocean particularly
in near-shore ocean spray and surf) and cadmium, allergens, and infective
viruses. Modeling frameworks developed for pesticide monitoring could also
be used to address emerging concerns (that thus far have sparse proof) of
transport and deposition of pathogens such as Salmonella and E. coli 0157H7
to nearby almonds or leafy greens, thought to travel through air from such
sources as nearby feedlots for dairy cows and other farm animals.
The issue of exposure of people to pesticides in air or those deposited has
been raised in several communities where pesticides are used in or around
orchards or in crop land. Of particular interest are in ongoing community,
agency, and grower discussions in Lompoc, McFarland, and other communities in California, Oregon, and Washington. In some cases, county
or municipalities have enacted ordinances to ban or restrict application of
conventional/synthetic pesticides, leaving only the options of using safer
chemicals like biopesticides or alternative pest management strategies (see
Chapter 12 for more on biopesticides).
Pesticides, Organic Contaminants, and Pathogens in Air
volatilization. What is not known is the stability of the diglycolamine salt of
dicamba. The amine salt can be viewed as a conjugate acid. So, the question
is: what is the pKa of the salt? For example, the pKa for the conjugate acid
of ethanolamine (and the amine itself) is 9.5, characteristic of a strong base.
The diglycolamine salt of dicamba is probably of a similar order of magnitude, implying that the salt should be very stable (pKa data are not available).
However, at acid pH (tank mix/soil), protonated diglycolamine may be more
stable than the salt, depending on the pKa of its conjugate acid. Perhaps, this
is where the effect of the temperature factor also comes into play, in addition
to promoting increased volatilization of the already formed dicamba acid;
volatility increases up to about 30°C. Another factor that might affect the
stability of the salt is treated leaf surface area. Apparently, as the water-based
tank mix dries on the leaf surfaces, the dicamba salt, as it interacts with
the leaf cuticle, becomes less stable, leading to volatile dicamba. For larger
treated surface areas, more of the volatile component is available to damage
nontarget, susceptible crops. An additional consideration is the exposure of
target leaf residues to sunlight. However, partly due to a low quantum yield
(<0.02), photolysis is a relatively minor dissipation process compared to the
timescale of dicamba volatilization and atmospheric dispersion (Aguer et al.,
2000; Waite et al., 2005).
10.5 Conclusions
Lessons learned from chemical residue behavior in the air can be applied in
responding to airborne exposures to other toxicants including metals such
as mercury (that volatilizes as methyl mercury from the ocean particularly
in near-shore ocean spray and surf) and cadmium, allergens, and infective
viruses. Modeling frameworks developed for pesticide monitoring could also
be used to address emerging concerns (that thus far have sparse proof) of
transport and deposition of pathogens such as Salmonella and E. coli 0157H7
to nearby almonds or leafy greens, thought to travel through air from such
sources as nearby feedlots for dairy cows and other farm animals.
The issue of exposure of people to pesticides in air or those deposited has
been raised in several communities where pesticides are used in or around
orchards or in crop land. Of particular interest are in ongoing community,
agency, and grower discussions in Lompoc, McFarland, and other communities in California, Oregon, and Washington. In some cases, county
or municipalities have enacted ordinances to ban or restrict application of
conventional/synthetic pesticides, leaving only the options of using safer
chemicals like biopesticides or alternative pest management strategies (see
Chapter 12 for more on biopesticides).