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Pesticides, Organic Contaminants, and Pathogens in Air
the valley foor (Figure 8.9, dashed arrows) (Honaganahalli and Seiber, 2000).
These calmer nighttime winds resulted in the highest measured concentrations of MeBr to be around the city of Salinas at night (8.98 and 8.37 μg/m 3 at
SS and SN). In contrast, daytime levels of MeBr did not exceed (0.881 and 1.85
μg/m 3 at SS and SN). Similar average concentrations during the day (0.435
μg/m 3 ) and at night (0.346 μg/m 3 ) at KC suggest a declining diurnal infuence
over the southern part of the valley.
On leeward (east-facing) slopes of the Sierra de Salinas range, both during
daytime and nighttime consistently, lower concentrations were recorded at
higher elevations (e.g., MT), and higher concentrations at lower levels (e.g., BF).
This could be due to the funneling of the air mass containing MeBr toward the
southern end of the valley and not vertically. At MB, the presumed upwind
site, concentrations were observed mostly when the site was under the infuence of a land breeze at night. MeBr, which was transported over the ocean
at night, may have returned the following day with the sea breeze. This may
explain MeBr presence at the MB receptor during one daytime session.
8.3.2.2 Flux from Multiple Fields: ISCST3 Model Performance
The EPA ISCST dispersion model is a general model that can be used for fat
or rolling terrain. It is built on a straight line, steady-state Gaussian plume
equation. It is best used for transport distances less than 50 km and works
for a variety of averaging times. It was adapted to agricultural felds similar
to the aforementioned bare strawberry feld fumigated with MeBr (Seiber
et al., 1996).
The major input terms are (1) MeBr applications at the source, (2) source
fux emission rates, and (3) meteorological data (wind speed, direction, surface air temperature, stability class [fuctuations in wind], and urban/rural
mixing height). The output is average MeBr concentrations for each location
during the period indicated.
MeBr sources (felds treated with MeBr) within the Salinas Valley were
identifed from the Monterey County Agricultural Commissioner’s Annual
Pesticide Use Report database (MCAPUR) (Honaganahalli and Seiber, 2000).
Flux was not measured in individual felds; this source term was adapted
from two estimates from feld studies obtained under similar but not identical conditions (Seiber et al., 1996; Yates et al., 1996).
ISCST3 simulations used meteorological data from the closest California
Irrigation Management Information Systems (CIMIS) station (Honaganahalli
and Seiber, 2000). “The ISCST3 model requires meteorology that is representative over the entire modeling domain. Because Salinas Valley has
such complex wind patterns, meteorological data obtained from any single
weather station in the valley will not represent the meteorology prevailing
over the entire valley.” In the north, near the receptors, nighttime breezes
were from the mountain slopes toward the Pacifc Ocean. But in the middle
and southern regions of the valley, southerly winds prevailed. The problem
Pesticides, Organic Contaminants, and Pathogens in Air
the valley foor (Figure 8.9, dashed arrows) (Honaganahalli and Seiber, 2000).
These calmer nighttime winds resulted in the highest measured concentrations of MeBr to be around the city of Salinas at night (8.98 and 8.37 μg/m 3 at
SS and SN). In contrast, daytime levels of MeBr did not exceed (0.881 and 1.85
μg/m 3 at SS and SN). Similar average concentrations during the day (0.435
μg/m 3 ) and at night (0.346 μg/m 3 ) at KC suggest a declining diurnal infuence
over the southern part of the valley.
On leeward (east-facing) slopes of the Sierra de Salinas range, both during
daytime and nighttime consistently, lower concentrations were recorded at
higher elevations (e.g., MT), and higher concentrations at lower levels (e.g., BF).
This could be due to the funneling of the air mass containing MeBr toward the
southern end of the valley and not vertically. At MB, the presumed upwind
site, concentrations were observed mostly when the site was under the infuence of a land breeze at night. MeBr, which was transported over the ocean
at night, may have returned the following day with the sea breeze. This may
explain MeBr presence at the MB receptor during one daytime session.
8.3.2.2 Flux from Multiple Fields: ISCST3 Model Performance
The EPA ISCST dispersion model is a general model that can be used for fat
or rolling terrain. It is built on a straight line, steady-state Gaussian plume
equation. It is best used for transport distances less than 50 km and works
for a variety of averaging times. It was adapted to agricultural felds similar
to the aforementioned bare strawberry feld fumigated with MeBr (Seiber
et al., 1996).
The major input terms are (1) MeBr applications at the source, (2) source
fux emission rates, and (3) meteorological data (wind speed, direction, surface air temperature, stability class [fuctuations in wind], and urban/rural
mixing height). The output is average MeBr concentrations for each location
during the period indicated.
MeBr sources (felds treated with MeBr) within the Salinas Valley were
identifed from the Monterey County Agricultural Commissioner’s Annual
Pesticide Use Report database (MCAPUR) (Honaganahalli and Seiber, 2000).
Flux was not measured in individual felds; this source term was adapted
from two estimates from feld studies obtained under similar but not identical conditions (Seiber et al., 1996; Yates et al., 1996).
ISCST3 simulations used meteorological data from the closest California
Irrigation Management Information Systems (CIMIS) station (Honaganahalli
and Seiber, 2000). “The ISCST3 model requires meteorology that is representative over the entire modeling domain. Because Salinas Valley has
such complex wind patterns, meteorological data obtained from any single
weather station in the valley will not represent the meteorology prevailing
over the entire valley.” In the north, near the receptors, nighttime breezes
were from the mountain slopes toward the Pacifc Ocean. But in the middle
and southern regions of the valley, southerly winds prevailed. The problem