Abstract
Abstract. Secondary organic aerosol (SOA) formation from a volatile organic compound (VOC) involves multiple generations of oxidation that include functionalization and fragmentation of the parent carbon backbone and likely particle-phase oxidation and/or accretion reactions. Despite the typical complexity of the detailed molecular mechanism of SOA formation and aging, a relatively small number of functional groups characterize the oxidized molecules that constitute SOA. Given the carbon number and set of functional groups, the volatility of the molecule can be estimated. We present here a functional group oxidation model (FGOM) that represents the process of SOA formation and aging. The FGOM contains a set of parameters that are to be determined by fitting of the model to laboratory chamber data: total organic aerosol concentration, and O : C and H : C atomic ratios. The sensitivity of the model prediction to variation of the adjustable parameters allows one to assess the relative importance of various pathways involved in SOA formation. An analysis of SOA formation from the high- and low-NOx photooxidation of four C12 alkanes (n-dodecane, 2-methylundecane, hexylcyclohexane, and cyclododecane) using the FGOM is presented, and comparison with the statistical oxidation model (SOM) of Cappa et al. (2013) is discussed.
Highlights
M ganic aerosol concentration, and O : C and H : C atomic ra- in the mechanism
As in the earlier generation of secondary organic aerosol (SOA) models, predictions can be adjusted to fit laboratory chamber data by determination of the optimal values of a number of parameters in the model. (This is in contrast to the fully explicit chemical model, for which rate constants and branching ratios are generally specified a priori based, for example, on structure–activity relationships.) We present here a new variation of the SOA model that is based on explicit chemical information in terms of the types of functional groups that result from the oxidation of a parent volatile organic compound (VOC)
The partibtionntonuthme pbaerrticalne dphvasoel.atility for which a molecule can appreciably type and number of functional groups added to the parpartition to the particle phase
Summary
The essential nature of functionalization is the addition, for a particular parent VOC, of a certain number and type of functional groups at each generation of reaction. The addition of various combinations of these four groups via photochemical oxidation accounts for a majority of the gas-phase reactions involving semivolatile product aging This assumption is based on observations pertinent to the photooxidation of VOCs and IVOCs (alkanes, alkenes, terpenes, and aromatics). Progressive gas-phase oxidation of VOC leads to the formation of oxygenated products that include alcohol, ketone, aldehyde, carboxylic acid, alkyl nitrate, hydroperoxide, and/or peroxyacyl nitrate functional groups (Atkinson, 2000). Additional functionalities, such as ether and ester groups, are found in smaller amounts. Since particle size dependence of the kinetic parameters is not considered in the model, size dependence of rp is not accounted for
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