Modeling the formation of secondary organic aerosol. 1. Application of theoretical principles to measurements obtained in the alpha-pinene/, beta- pinene/, sabinene/, Delta(3)-carene/, and cyclohexene/ozone systems
Jf. Pankow et al., Modeling the formation of secondary organic aerosol. 1. Application of theoretical principles to measurements obtained in the alpha-pinene/, beta- pinene/, sabinene/, Delta(3)-carene/, and cyclohexene/ozone systems, ENV SCI TEC, 35(6), 2001, pp. 1164-1172
Citations number
47
Categorie Soggetti
Environment/Ecology,"Environmental Engineering & Energy
Secondary-organic aerosol (SOA) forms in the atmosphere when volatile paren
t compounds are oxidized to form low-volatility products that condense to y
ield organic particulate matter (PM). Under conditions of intense photochem
ical smog, from 40 to 80% of the particulate organic carbon can be secondar
y in origin. Because describing multicomponent condensation requires a comp
ound-by-compound identification and quantification of the condensable compo
unds, the complexity of ambient SOA has made it difficult to test the abili
ty of existing gas/particle (G/P) partitioning theory to predict SOA format
ion in urban air. This paper examines that ability using G/P data from past
laboratory chamber experiments carried out with five parent hydrocarbons (
HCs) (four monoterpenes at 308 K and cyclohexene at 298 K) in which signifi
cant fractions (61-100%) of the total mass of SOA formed from those HCs wer
e identified and quantified by compound. The model calculations were based
on a matrix representation of the multicomponent, SOA G/P distribution proc
ess. The governing equations were solved by an iterative method. Input data
for the model included (i) Delta HC (mug m(-3)), the amount of reacted par
ent hydrocarbon; (ii) the ci values that give the total concentration T (ga
s + partical phase, ng m(-3)) values for each product i according to T-i =
10(3) alpha (i)Delta HC; (iii) estimates of the pure compound liquid vapor
pressure p degrees (L) values (at the reaction temperature) for the product
s; and (iv) UNIFAC parameters for estimating activity coefficients in the S
OA phase for the products as a function of SOA composition. The model predi
cts the total amount M-0(mug m-3) of organic aerosol that will form from th
e reaction of Delta HC, the total aerosol yield Y(= M-0/Delta HC), and the
compound-by-compound yield values Y-i. An impediment in applying the model
is the lack of literature data on p(L)degrees values for the compounds of i
nterest or even on p(L)degrees values for other, similarly low-volatility c
ompounds. This was overcome in part by using the G/P data from the alpha -p
inene and cyclohexene experiments to determine p(L)degrees values for use (
along with a set of 14 other independent polar compounds) in calculating UN
IFAC vapor pressure parameters that were, in turn, used to estimate all of
the needed pe values. The significant degree of resultant circularity in th
e calculations for alpha -pinene and cyclohexene helped lead to the good ag
reement that was found between the Y-i values predicted by the model, and t
hose measured experimentally for those two compounds. However, the model wa
s also able to predict the aerosol yield values from beta -pinene, sabinene
, and Delta (3)-carene, for which there was significantly less circularity
in the calculations, thereby providing evidence supporting the idea that gi
ven the correct input information, SOA formation can in fact be accurately
modeled as a multicomponent condensation process.