| Abstract
| - Secondary organic aerosol (SOA) forms in the atmospherewhen volatile parent compounds are oxidized to form low-volatility products that condense to yield organic particulatematter (PM). Under conditions of intense photochemicalsmog, from 40 to 80% of the particulate organic carbon canbe secondary in origin. Because describing multicomponentcondensation requires a compound-by-compoundidentification and quantification of the condensablecompounds, the complexity of ambient SOA has made itdifficult to test the ability of existing gas/particle (G/P)partitioning theory to predict SOA formation in urban air.This paper examines that ability using G/P data from pastlaboratory chamber experiments carried out with fiveparent hydrocarbons (HCs) (four monoterpenes at 308 Kand cyclohexene at 298 K) in which significant fractions (61−100%) of the total mass of SOA formed from those HCswere identified and quantified by compound. The modelcalculations were based on a matrix representation of themulticomponent, SOA G/P distribution process. Thegoverning equations were solved by an iterative method.Input data for the model included (i) ΔHC (μg m-3), the amountof reacted parent hydrocarbon; (ii) the α values thatgive the total concentration T (gas + particle phase, ngm-3) values for each product i according to Ti = 103 αiΔHC;(iii) estimates of the pure compound liquid vapor pressurevalues (at the reaction temperature) for the products;and (iv) UNIFAC parameters for estimating activity coefficientsin the SOA phase for the products as a function of SOAcomposition. The model predicts the total amount Mo (μgm-3) of organic aerosol that will form from the reactionof ΔHC, the total aerosol yield Y (= Mo/ΔHC), and thecompound-by-compound yield values Yi. An impediment inapplying the model is the lack of literature data onvalues for the compounds of interest or even onvaluesfor other, similarly low-volatility compounds. This wasovercome in part by using the G/P data from the α-pineneand cyclohexene experiments to determinevalues foruse (along with a set of 14 other independent polar compounds)in calculating UNIFAC vapor pressure parameters thatwere, in turn, used to estimate all of the neededvalues.The significant degree of resultant circularity in thecalculations for α-pinene and cyclohexene helped lead tothe good agreement that was found between the Yivalues predicted by the model, and those measuredexperimentally for those two compounds. However, themodel was also able to predict the aerosol yield values fromβ-pinene, sabinene, and Δ3-carene, for which there wassignificatly less circularity in the calculations, therebyproviding evidence supporting the idea that given the correctinput information, SOA formation can in fact be accuratelymodeled as a multicomponent condensation process.
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