Aerosols in the atmosphere - Air Pollution Modeling and Its Application - page 381

isothermal dilution (Grieshop et al., 2007; Lipsky and Robinson 2006) and thermal
denuder measurements (An et al., 2007) show that SOA particles shrink when the
driving force of mass transfer favors evaporation, again strongly implying the
subsequent presence of vapors.
These VBS vapors are likely important precursors for additional chemical
production of OA. The vapors produced by SOA experiments will be partially
oxidized compounds. On the other hand, vapors from primary emissions will be
highly reduced. In either case, we can be absolutely certain that these vapors will
react in the atmosphere. They will react in the gas phase with the OH radical in the
very least, and quite rapidly. Most of these compounds have a large number of
CH 2 groups, and such compounds typically have rate constants k OH near 3 × 10 −11
cm 3 molec −1 s −1 (Seinfeld and Pandis, 2006), giving an atmospheric lifetime of
approximately 8 h for 10 6 OH cm −3 . It is also likely that the OH uptake coefficient
to particles is close to unity, which would also result in a condensed-phase lifetime
for organics of a day or so. Shrivastava et al. (2008) have described the chemical
aging of the vapors in the VBS assuming a reaction of the form:
POA n (g) + OH → OPOA n-1 (g)
where POA n (g) is the gas-phase concentration of the components in the n th
volatility bin of the VBS and OPOA n (g) is the gas-phase concentrations of an
oxidized component in the (n − 1) th volatility bin. This reaction (with rate constant
k OH ) assumes that each oxidation step reduces the volatility of the compounds by
one order of magnitude. This formulation with the definition of POA and OPOA
surrogate components allows the simulation of the concentration of both the fresh
POA and its oxidized products and the direct comparison of the results to those of
the AMS: the POA should correspond to the HOA, while the sum of the OPOA
and the traditional SOA should correspond to the OOA. More complicated
reaction schemes are possible including reactions that increase the volatility of the
VBS components (Donahue et al., 2006).
3. Revisiting the Sources of Organic Aerosol
The volatility basis set described above has recently been implemented into the
regional CTM PMCAMx to investigate the effects of partitioning and aging of
primary emissions and multigenerational processing of traditional SOA precursors
on urban and regional OA levels in the Eastern US (Shrivastava et al., 2008; Lane
et al., 2008). Figure 1 shows average predicted ground-level organic aerosol and
vapor concentrations during July 14-28 2001 and January 2002. Details of the
modeling are described in Shrivastava et al. (2008).
This more physically realistic representation implemented reveals that ambient
organic aerosols are a highly-dynamic system dominated by both variable gas-
partitioning and chemical evolution. The majority of the traditionally defined POA
emissions evaporate, substantially reducing the predicted POA concentrations.