40
Pesticides, Organic Contaminants, and Pathogens in Air
on the volatility of the chemical, the outside air intake port may need to be
fltered to avoid biasing fux determinations within the chamber.
However, for open feld measurements of fux (soil, plant canopy), different approaches are required. While several different methods have been
developed for chemical fux determination in the open feld, three that are
commonly used are the aerodynamic gradient (AG), theoretical profle shape
(TPS), and integrated horizontal fux (IHF) methods. These micrometeorological methods give essentially equivalent results (Majewski et al., 1990).
Choosing one method over another depends on preference, feld confguration, availability of equipment, and budgetary considerations.
AG chemical fuxes (F AG ) can be estimated using a modifed form of the
Thornthwaite–Holzman equation (Thornthwaite and Holzman, 1939) corrected for atmospheric stability conditions (Majewski et al., 1995):
2
˘
2
F = k c
˛ ˛u / ˝ ˝ ˙ ln (z /z )
AG
(
) ( m p ˇ 2 1 )
(3.20)
ˆ
where k is the von Kármán constant (~0.41), Δc and Δu are the average chemical concentration and wind speed differences, respectively, between heights
z 1 and z 2 above the treated surface, and φ m and φ p are atmospheric stability
functions. To use the AG method, it is necessary to determine vertical gradients of wind speed and chemical concentrations. Also, a fetch of about 100:1
needs to be established. The fetch is the ratio of the upwind edge distance of
the feld to the height of the sampling tower, usually located at the center of
the feld (Figure 3.8).
In an earlier study, the AG method was used to estimate the post-application
methyl bromide (MeBr) volatilization loss rates from two different application
FIGURE 3.8
AG equipment setup for monitoring the volatilization of MeBr from a treated and tarped feld.
Michael Majewski is standing near right center of photo.
Pesticides, Organic Contaminants, and Pathogens in Air
on the volatility of the chemical, the outside air intake port may need to be
fltered to avoid biasing fux determinations within the chamber.
However, for open feld measurements of fux (soil, plant canopy), different approaches are required. While several different methods have been
developed for chemical fux determination in the open feld, three that are
commonly used are the aerodynamic gradient (AG), theoretical profle shape
(TPS), and integrated horizontal fux (IHF) methods. These micrometeorological methods give essentially equivalent results (Majewski et al., 1990).
Choosing one method over another depends on preference, feld confguration, availability of equipment, and budgetary considerations.
AG chemical fuxes (F AG ) can be estimated using a modifed form of the
Thornthwaite–Holzman equation (Thornthwaite and Holzman, 1939) corrected for atmospheric stability conditions (Majewski et al., 1995):
2
˘
2
F = k c
˛ ˛u / ˝ ˝ ˙ ln (z /z )
AG
(
) ( m p ˇ 2 1 )
(3.20)
ˆ
where k is the von Kármán constant (~0.41), Δc and Δu are the average chemical concentration and wind speed differences, respectively, between heights
z 1 and z 2 above the treated surface, and φ m and φ p are atmospheric stability
functions. To use the AG method, it is necessary to determine vertical gradients of wind speed and chemical concentrations. Also, a fetch of about 100:1
needs to be established. The fetch is the ratio of the upwind edge distance of
the feld to the height of the sampling tower, usually located at the center of
the feld (Figure 3.8).
In an earlier study, the AG method was used to estimate the post-application
methyl bromide (MeBr) volatilization loss rates from two different application
FIGURE 3.8
AG equipment setup for monitoring the volatilization of MeBr from a treated and tarped feld.
Michael Majewski is standing near right center of photo.