Aerosols in the atmosphere - Air Pollution Modeling and Its Application - page 378

the need to model the aging of semi-volatile compounds all motivated the
development a new framework for the description of all OA components and their
reactions. This framework blurs the distinction between the traditional primary
and secondary organic aerosol, providing a more realistic picture of the behavior
of atmospheric organic aerosol. Given the increased complexity of the system, the
tens of major OA sources, and the hundreds of SOA precursors the framework
should also result in computationally efficient modules for CTMs, so that the
simulation of OA could still be tractable.
Donahue et al. (2006) proposed using fixed logarithmically-spaced saturation
concentrations bins: the volatility basis set (VBS). The volatility bins are separated
by powers of 10, typically ranging from 0.01 to 10 6 µg m −3 at 298 K, and they
shift with temperature according to the Clausius-Clapeyron equation. The purpose
of this article is not to review the state of organic aerosol science but rather to
view that science through the unifying lens of the VBS. Specifically, we seek to
show how the VBS provides a concise platform to address semi-volatile emissions,
SOA formation, and organic aerosol aging in a single, self-consistent framework.

2.1. OA component partitioning

Following the work of Pankow (1994), the partitioning of a constituent i between
the vapor phase and a condensed phase with mass concentration C OA can be
described by a partitioning coefficient, ξ i . If one assumes that the organic solution
is ideal (or that the activity coefficients are approximately constant in the range of
conditions of interest, pseudo-ideal solution) and the compounds involved have
similar molecular weights then the fraction of this compound in the condensed
phase is given by the following simple saturation curve:

1
*

ξ =

(1)

( )

i

1 + C / C

i OA

where C i * is the effective saturation concentration of the compound (the inverse of
a Pankow type partitioning coefficient). C* is simply a semi-empirical property
that describes partitioning of a complex mixture. Because the C* values are semi-
empirical, they are assumed to effectively include activity coefficients of the
mixture; however, a limitation is that those activity coefficients are assumed to be
roughly constant under atmospheric conditions.
The salient features of Eq. 1 are as follows:
(a). 50% of the material is in each phase when C * = C OA .
(b). Within about one order of magnitude on either side of this equipartition
point the response curve is roughly linear.
(c). Beyond this linear region almost all of the material is in one phase or the
other, mostly in the condensed phase for C * < 0.1 C OA and mostly in the
vapor phase for C * > 10 C OA .