Uncertainties in Modeling Secondary Organic Aerosols:  Three-Dimensional Modeling Studies in Nashville/Western Tennessee

Abstract

The formation of secondary organic aerosols (SOA) is simulated for the Nashville/western Tennessee domain using three recent SOA modules incorporated into the three-dimensional air quality model, CMAQ. The Odum/Griffin et al. and CMU/STI modules represent SOA absorptive partitioning into a mixture of primary and secondary particulate organic compounds (OC), with some differences in the formulation of the absorption process and the selection of SOA species and their precursors. Empirical representations based on measured laboratory SOA yields are used for condensable organic products in both these modules. The AEC module simulates SOA absorption into organic and aqueous particulate phases, and a representation based on an explicit gas-phase mechanism is used in the AEC module. Predicted SOA concentrations can vary by a factor of 10 or more. In general, the gas-phase mechanistic approach predicts a higher yield of SOA than those based on laboratory yields. There exist some differences in the two empirical modules despite their similar basis on experimental data. All three modules predict a dominance of SOA of biogenic origin as compared to SOA of anthropogenic origin. The causes for differences among the three SOA modules include the representation of terpenes, the mechanistic versus empirical representation of SOA-forming reactions, the identities of SOA, and the parameters used in the gas/particle partitioning calculations. Two sensitivity studies show that formation of water-soluble SOA and temperature dependence may be areas of key uncertainties affecting current models.

Modeling secondary organic aerosol formation from isoprene oxidation under dry and humid conditions

“…On one hand, one can treat the atmospheric particles as internal mixtures (all particles of a same size have the same chemical composition) and solve he thermodynamics with possible phase separation (e.g., mostly organic and aqueous phases) as performed by Pun (2008) and Zuend et al (2010). On the other hand, one can treat the atmospheric particles as external mixtures with aqueous (mostly inorganic) particles being distinct from hydrophobic (mostly organic) particles as performed by Pun et al (2002Pun et al ( , 2006. The latter approach is used here.…”

“…The condensation of SOA on the organic phase under high-NO x conditions is then due to the hydrophobic properties of BiNGA (only a small quantity of C 5 -hydroxy-trinitrate is formed under the conditions considered here). The AER/EPRI/Caltech model (AEC) (Pun et al, 2002(Pun et al, , 2003(Pun et al, , 2006) is a model for SOA formation that takes into account the hydrophilic and hydrophobic properties of surrogates. It distinguishes compounds into two types: hydrophilic compounds (which are absorbed only in an aqueous particle) and hydrophobic compounds (which are absorbed only in an organic particle).…”

A computationally-efficient secondary organic aerosol module for three-dimensional air quality models

“…The additional water uptake can occur in the presence of hydrophilic SOA when the ambient relative humidity (RH) is above the DRH of any individual OC in the particulate phase Hildemann, 1996, 1997). The addition of organic ions changes the pH and total water content of aqueous particles, and consequently, affects the partitioning of inorganic compounds, such as nitrate and ammonium (Saxena et al, 1995;Cruz and Pandis, 2000;Pun et al, 2003). The role of SOA in total aerosol water and in transferring of nitrate into the aerosol phase is significant, especially at low RH (<50%) and high SOA mass fractions (>20% of total PM 2.5 ) (Ansari and Pandis, 2000).…”

“…A globallyconvergent Newton/line search method is used to solve the partitioning equations simultaneously. Some simplifications are provided as alternatives to detailed treatments to improve computational efficiency in 3-D applications (Pun et al, 2003). For example, the amount of water associated with hydrophilic OCs is calculated by assuming that the water associated with each hydrophilic compound in a binary solution is additive (i.e., using the ZSR equation).…”

“…The speciation of NMHCs for all the initial CACM organic species except biogenic VOCs (i.e., isoprene (ISOP), and monoterpenes (BIOL+BIOH)) is based on the specific distribution factors of Griffin et al (2002a). Following the approach of Griffin et al (2002a) and Pun et al (2003), the same speciation is applied to the entire simulation domain because of lack of data to distinguish speciation in rural and urban areas. The concentrations of isoprene and monoterpenes in the ambient air are typically in the range of 1 to 10 ppbC (Finlayson-Pitts and Pitts, 1986;NTP, 2005).…”