103
Pesticides in Fog
that occurs in California’s San Joaquin Valley, could capture high concentrations of pesticides—if they could only fgure out a way to sample it. Enter
the Ag Engineers at USDA Beltsville (Lou Liljedahl and others), who came
up with a novel sampling mechanism mounted on the cab of a pickup truck.
With an additional assist from the U.S. Air Force, who few the truck from
Maryland to Fairfeld, California, the researchers now had a way to collect sizeable volumes of fog easily. All they had to do was drive around the
orchards and agricultural felds during fog events.
Their unusual work quickly grabbed attention. Using gas chromatography (GC) analytical techniques with element selective detectors, Seiber,
Glotfelty, Woodrow, McChesney, Lucas, and other researchers at UC, Davis,
and USDA ARS identifed peaks for pesticides, polycyclic aromatic hydrocarbons (PAHs), phthalate esters, organophosphate (OP) esters, herbicides,
and organochlorine (OC) insecticides. The local media highlighted their
work and one headline, titled “Killer Fogs,” fueled interest in the project.
Additional media coverage and publications fueled a multiyear study to
understand contaminants in fog.
Wet deposition, which includes the scavenging of particle-bound pesticides and pesticide vapors into atmospheric moisture (cloud- and fogwater,
rain, and snow), is a potentially major sink for airborne pesticides. The pervasive wintertime tule fogs in California’s Central Valley, studied extensively
in the past 25 years, accumulate organophosphorus, triazine, and other pesticide groups. Concentrations of some pesticides in fogwater can signifcantly
exceed those expected based upon vapor–water distribution coeffcients.
Fogwater deposition has been implicated as a source of inadvertent residues
to nontarget foliage, and of high-risk exposures for wildlife residing in and
around treated areas. The pesticide residue content of fogwater is an example.
Airborne pesticides may exist as vapors or associated with liquid or solid
aerosols (Lewis and Lee, 1976). Vapor–aerosol distribution and partitioning
of airborne pesticides will occur, as it does with all volatile and semivolatile
air contaminants (Bidleman, 1988). Liquid-phase vapor pressure is the controlling physical property. In general, chemicals of vapor pressures less than
about 10 −10 atmospheres favor the particulate phase and those with vapor pressures greater than about 10 −9 atmospheres favor the vapor phase. Pesticides
cover a broad range of vapor pressure, extending from gaseous chemicals
under ambient conditions (e.g., methyl bromide, sulfuryl fuoride, and phosphine) to essentially nonvolatile salts such as paraquat. But the majority of
pesticides are semivolatile organic compounds (SVOCs) with vapor pressure
falling in the range of, approximately, 10 −4 to 10 −10 atmospheres.
Airborne pesticides may be removed from the air by principally three
processes, namely, degradation, wet deposition, and dry deposition.
Degradation may follow oxidative, photooxidative, and/or hydrolytic pathways. Only a few pesticide chemicals have been studied in detail, and they
display the same types of reactions seen for other classes of organics in the
air (see Chapter 3) (Seiber and Woodrow, 1995; Woodrow et al., 1983). In only
Pesticides in Fog
that occurs in California’s San Joaquin Valley, could capture high concentrations of pesticides—if they could only fgure out a way to sample it. Enter
the Ag Engineers at USDA Beltsville (Lou Liljedahl and others), who came
up with a novel sampling mechanism mounted on the cab of a pickup truck.
With an additional assist from the U.S. Air Force, who few the truck from
Maryland to Fairfeld, California, the researchers now had a way to collect sizeable volumes of fog easily. All they had to do was drive around the
orchards and agricultural felds during fog events.
Their unusual work quickly grabbed attention. Using gas chromatography (GC) analytical techniques with element selective detectors, Seiber,
Glotfelty, Woodrow, McChesney, Lucas, and other researchers at UC, Davis,
and USDA ARS identifed peaks for pesticides, polycyclic aromatic hydrocarbons (PAHs), phthalate esters, organophosphate (OP) esters, herbicides,
and organochlorine (OC) insecticides. The local media highlighted their
work and one headline, titled “Killer Fogs,” fueled interest in the project.
Additional media coverage and publications fueled a multiyear study to
understand contaminants in fog.
Wet deposition, which includes the scavenging of particle-bound pesticides and pesticide vapors into atmospheric moisture (cloud- and fogwater,
rain, and snow), is a potentially major sink for airborne pesticides. The pervasive wintertime tule fogs in California’s Central Valley, studied extensively
in the past 25 years, accumulate organophosphorus, triazine, and other pesticide groups. Concentrations of some pesticides in fogwater can signifcantly
exceed those expected based upon vapor–water distribution coeffcients.
Fogwater deposition has been implicated as a source of inadvertent residues
to nontarget foliage, and of high-risk exposures for wildlife residing in and
around treated areas. The pesticide residue content of fogwater is an example.
Airborne pesticides may exist as vapors or associated with liquid or solid
aerosols (Lewis and Lee, 1976). Vapor–aerosol distribution and partitioning
of airborne pesticides will occur, as it does with all volatile and semivolatile
air contaminants (Bidleman, 1988). Liquid-phase vapor pressure is the controlling physical property. In general, chemicals of vapor pressures less than
about 10 −10 atmospheres favor the particulate phase and those with vapor pressures greater than about 10 −9 atmospheres favor the vapor phase. Pesticides
cover a broad range of vapor pressure, extending from gaseous chemicals
under ambient conditions (e.g., methyl bromide, sulfuryl fuoride, and phosphine) to essentially nonvolatile salts such as paraquat. But the majority of
pesticides are semivolatile organic compounds (SVOCs) with vapor pressure
falling in the range of, approximately, 10 −4 to 10 −10 atmospheres.
Airborne pesticides may be removed from the air by principally three
processes, namely, degradation, wet deposition, and dry deposition.
Degradation may follow oxidative, photooxidative, and/or hydrolytic pathways. Only a few pesticide chemicals have been studied in detail, and they
display the same types of reactions seen for other classes of organics in the
air (see Chapter 3) (Seiber and Woodrow, 1995; Woodrow et al., 1983). In only