Environmental Engineering Reference
In-Depth Information
This new understanding is based on multiple, independent lines of evidence,
including: measured changes in partitioning of primary particulate emissions from
diesel and woodsmoke upon isothermal dilution (Lipsky and Robinson, 2006;
Shrivastava et al., 2006); volatility-based chromatography of primary emissions
samples (Hildemann et al., 1991); similar volatility-based measurements of urban
samples (Fraser et al., 1997, 1998). The bottom line is that all emissions with
C* > 1 µg m
−3
, corresponding roughly to a saturation mixing ratio greater than
0.1 ppbv, will be found, at least partially, in the gas phase under typical ambient
conditions. These constitute 50-90% of the emissions that have traditionally been
modeled as non-volatile POA.
2.3. Chemical production of organic aerosol
Chemical production involves both reactions of VOCs that generate lower-
volatility products in the ('traditional' SOA formation), and reactions within the
VBS. Traditional SOA precursors such as α-pinene and toluene have very high C*
(10
7
or 10
8
μg m
−3
). The oxidation of these precursors result in a set of products,
which can be schematically represented as
VOC + oxidant → a
1
P
1
+a
2
P
2
+ … a
9
P
9
where
a
i
are the set mass yields for products distributed over the VBS. This reaction
is almost never an elementary reaction, but rather the left-hand-side represents the
(initial) rate-limiting step of a reaction sequence, where subsequent reactions
leading to the VBS products will be some combination of rapid gas-phase radical
reactions and rapid condensed-phase reactions. As a consequence, the yields
a
i
may be functions of NO
x
levels, temperature, relative humidity, etc. The current
state of SOA research is summarized in an excellent recent review by Kroll and
Seinfeld (2008). The VBS can help modelers simply address several facets of
SOA chemistry, including mass balance, dependence on ambient conditions (NO
x
,
RH, UV, T), and ongoing aging.
It is clear that the VBS includes thousands of organic compounds in both the
vapor and condensed phases, and that these compounds will continue to react while
they reside in the atmosphere (Robinson et al., 2006). These reactions constitute
“chemical aging”, or reactions within the VBS. These reactions will almost certainly
make products with altered volatility. Mechanisms based on explicit product
representation or even multiple surrogates, develop a profusion of products when
multiple generation reactions are treated. However, then volatility alone is considered,
reactions within the VBS simply redistribute material from one bin to another.
This greatly simplifies aging parameterizations, provided that the appropriate aging
parameters can be constrained.
Most of the material in the VBS, both in the atmosphere and in most experi-
ments, exists in the vapor phase. Partitioning theory demands this, and experiments
on both growth and evaporation confirm it. The vast majority of SOA experiments
show increasing mass fractions with increasing aerosol mass concentrations, and
as predicted by partitioning theory (Pankow, 1994) in Eq. 1. Likewise, both
