Environmental Engineering Reference
In-Depth Information
(d).
Changing the total organic mass concentration C
OA
will cause the
partitioning to change. Consequently, partitioning close to a point source
is very different from partitioning in the remote marine atmosphere.
The phase partitioning of a complex mixture can be accurately described by
lumping material into volatility bins separated by an order of magnitude (at
298 K). This is not only convenient; it is a nearly optimal distribution. We refer
to these bins as the volatility basis set (VBS). Details are in Donahue et al. (2006).
For the majority of this discussion we shall consider a VBS consisting of nine
bins, starting at C* = 0.01 µg m
−3
and ranging up to 10
6
µg m
−3
. Typical
atmospheric C
OA
levels are between 1 and 100 µg m
−3
. To facilitate and
standardize discussion, we propose three sub classes within this range:
Low Volatility Organic Compounds
(LVOCs): C* = {0.01, 0.1} µg m
−3
. These
compounds are mostly in the condensed phase in all but the most remote (and
warm) parts of the atmosphere.
Semi-Volatile Organic Compounds
(SVOCs): C* = {1, 10, 100} µg m
−3
.
Significant fractions of these compounds will be found in both phases under
typical conditions.
Intermediate Volatility Organic Compounds
(IVOCs): C* = {10
3
, 10
4
, 10
5
, 10
6
}
µg m
−3
. These compounds are almost entirely in the gas phase, but they comprise
an enormous number of difficult to measure compounds and (probably) a small
but important fraction of the total atmospheric burden, as we shall see later.
There are two more classes outside the VBS range:
Volatile Organic Compounds
(VOCs): C* > 10
6
µg m
−3
. The vast majority of
emissions and routinely measured organics fall in this traditional category. These
are typically represented explicitly or with lumped compounds in gas-phase
chemistry mechanisms.
Nonvolatile Organic Compounds
(NVOCs): C* < 0.01 µg m
−3
. These compounds
reside always in the particulate phase. They can be placed in the first bin (0.01 µg
m
−3
) of the VBS.
2.2. Emissions
The view of emissions we present here differs markedly from the conventional
view: POA emissions have traditionally been viewed as non-volatile material in
both emission inventories and models, and we now understand that the majority of
these emissions fall in the SVOC and IVOC range of the VBS. POA emissions
have always been modeled as purely non-volatile. This is in part because emissions
measurements have traditionally been limited by the signal-to-noise of filter weight
measurements or chromatographic characterization of individual compounds.
Only very recently have primary emissions measurements been extended down to
ambient mass loadings, and in every case examined to date the majority of material
in the condensed phase under typical sampling conditions has evaporated by the
time the emissions have been diluted to ambient conditions.
