Environmental Engineering Reference
In-Depth Information
2.2
Model of Long Range Transport and Characteristic
Travel Distance
Estimation of mass loss by transformation and deposition . As a parcel of air con-
taining 100 ng CPY m −3 is conveyed downwind, the total mass and concentrations
of CPY decrease. The mass decreases as a result of transformation processes, pri-
marilyreactionwith•OHradicalsandnetdeposition.Oxidationprimarilyresults
in the formation of CPYO. The rate of the overall process can be represented ( 2 )
as follows:
VCk
××
or 0 693
.
××
V Ct
(2)
R
Where: V is volume, C is concentration and k R and t are the first-order rate constant
(0.23 h −1 ) and half-life (3 h), respectively. There is also loss of mass of CPY by trans-
port from air to the ground, specifically due to deposition in rain or snow, sorption to
aerosol particles that subsequently are deposited by wet and dry deposition, and
direct sorption to terrestrial and aquatic surfaces as shown by Aston and Seiber
( 1997 ), LeNoir et al. ( 1999 ) and Bradford et al. ( 2013 ). Estimates of these process
rates can be made and the overall results can be compared to measured concentrations
of CPY. Rates of these processes can be combined into a chemical-specific net mass
transfer coefficient or velocity k M m h −1 . The rate of deposition is described in ( 3 ):
Ck M
´
gm 21
--
(3)
and the loss of mass is described by ( 4 ):
ACk M
´´
gh 1
-
(4)
Where: A is the area and is equivalent to the volume (V) divided by the parcel height
(H, expressed in m). Thus, the rate of loss of mass of CPY is described ( 5 ) as:
VCk
H
××
M
g h
1
(5)
The parameter k
H
M
can be regarded as a rate constant. The TAPL3 model, which
is discussed later, suggests that this rate constant is approximately 0.0016 h −1 for an
atmospheric height H of 1,000 m, which is a factor of 144 slower than transforma-
tion. For deposition from a lesser atmospheric height such as 100 m, the rate con-
stant is correspondingly greater by a factor of 10, thus there will be greater deposition
from a near-ground level plume of higher concentration. The total rate of loss of
mass by reaction and deposition is then described ( 6 ) as:
´´ + ´´
VCk
H
gh 1
-
VCk
M
(6)
R
 
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