Environmental Engineering Reference
In-Depth Information
Source emission models have also been developed that are based on
mass transfer processes. Emission rates for wet sources (paint coatings, etc.)
can be described by the simplified equation:
ER = K
g
(C
s
- C)
(9.7)
where K
g
= mass transfer coefficient (m/hr)
C
s
= vapor concentration (mg/m
3
) just above the emitting surface
C
= chamber/room concentration (mg/m
3
)
The vapor concentration (C
s
) is the vapor pressure expressed as a con-
centration in equilibrium with the source. It is linearly associated with the
mass of source VOCs. The mass transfer coefficient (K
g
) is determined from
the vapor diffusivity, air velocity above the source, and source geometry.
The calculation of the emission rate (ER) over time requires the use of a
modeling equation of the form
ER = [C
v
K
g
/(r
1
- r
2
)][(r
2
+ N)e
-r1t
- (r
2
+ N)e
-r2t
]
(9.8)
where coefficients r
1
and r
2
must be solved using another equation.
Emissions from source materials depend on a variety of factors. These
include source moisture content and age, air exchange or ventilation rate,
temperature, humidity (in some cases), loading rate, and sink effects. Wet
sources, such as freshly applied coatings, have higher emission rates than
dry products. Older sources have lower emission rates than new sources.
Source materials subject to higher ventilation or air exchange conditions have
higher emission rates and more rapid emission decay. Emission rates increase
with temperature and, in the case of HCHO, this increase is exponential.
Formaldehyde emissions increase linearly with increases in relative humid-
ity. Humidity also appears to affect emissions of some VOCs.
The loading rate is a major determinant of emission rates, as can be seen
in Equation 9.4. Since emission rate is inversely related to loading rate, high
loading rates result in high chamber or space concentrations, which result
in reduced source emissions.
The emission characteristics of many sources depend on sink effects.
Sinks are indoor surfaces and materials that sorb or desorb contaminants
generated by source materials. These sink effects result in lower initial air
concentrations from which emission rate data are determined and, as a result
of desorption, increase the emission decay period. In many cases, sinks are
a source of contaminant emissions even after the primary emission source
has been removed.
C. Full-scale studies
Full-scale or whole-house studies are occasionally used to validate chamber
data. They provide the opportunity to evaluate the effect of variable air
