Model-measurement comparison of functional group abundance in α-pinene and 1,3,5-trimethylbenzene secondary organic aerosol formation

Secondary organic aerosol (SOA) formed by alpha-pinene and 1,3,5-trimethylbenzene photooxidation under different NOx regimes is simulated using the Master Chemical Mechanism v3.2 (MCM) coupled with an absorptive gas-particle partitioning module. Vapor pressures for individual compounds are estimated with the SIMPOL. 1 group contribution model for determining apportionment of reaction products to each phase. We apply chemoinformatic tools to harvest functional group (FG) composition from the simulations and estimate their contributions to the overall oxygen to carbon ratio. Furthermore, we compare FG abundances in simulated SOA to measurements of FGs reported in previous chamber studies using Fourier transform infrared spectroscopy. These simulations qualitatively capture the dynamics of FG composition of SOA formed from both alpha-pinene and 1,3,5-trimethylbenzene in low-NOx conditions, especially in the first hours after start of photooxidation. Higher discrepancies are found after several hours of simulation; the nature of these discrepancies indicates sources of uncertainty or types of reactions in the condensed or gas phase missing from current model implementation. Higher discrepancies are found in the case of alpha-pinene photooxidation under different NOx concentration regimes, which are reasoned through the domination by a few polyfunctional compounds that disproportionately impact the simulated FG abundance in the aerosol phase. This manuscript illustrates the usefulness of FG analysis to complement existing methods for model-measurement evaluation.

Functional group distribution of secondary organic aerosol generated using the Master Chemical Mechanism: similarities and discrepancies with environmental controlled chamber studies

Fabian Alexander Bernhard, Giulia Ruggeri, Sarah Elizabeth Shipley, Satoshi Takahama

In this work, composition of organic aerosol simulated by the Master Chemical Mechanism coupled with a dynamic absorptive partitioning model is compared against infrared spectroscopy measurements of functional group abundance in chamber experiments. The Master Chemical Mechanism (MCM, Jenkin et al., 1997; Saunders et al., 2003; Jenkin et al., 2003; Bloss et al., 2005) is a near-explicit gas phase chemistry model that describes the degradation of various emitted volatile organic compounds. SIMPOL.1, a group contribution model (Pankow and Asher, 2008), was used to calculate pure components vapour pressures of the degradation products. Fourier transform infrared spectroscopy (FTIR) is able to quantify the vibrational absorption of molecular bonds (Maria et al., 2002) and is used to study aerosol functional group composition to have information on the sources and processes involved in aerosol formation and aging (Russell et al., 2011). MCM has been used to study aerosol atomic composition in terms of O:C and H:C ratios (Chen et al., 2011), but in this work we provide further information on the yields of various oxygenated functional groups, such as carbonyl and carboxylic CO, alcohol COH and organonitrate ONO2. In this work the organic aerosol functional group distribution predicted using MCM model is compared to chamber studies that use FTIR as the analytical technique in order to test its prediction capabilities of the functional group distribution of organic aerosol. Carbonyl and carboxylic CO, alcohol OH, alkane CH and organonitrate CONO2 relative fractions of the OA (Organic Aerosol) simulated were compared with the results of photooxidation and ozonolysis of α-pinene and trimethyl benzene in environmental controlled chamber experiments found in literature (Sax et al., 2005; Holes et al., 2007; Chhabra et al., 2011). The alkane CH, carbonyl CO and alcohol OH fraction are found to be well predicted by MCM for α- pinene, suggesting that the chemical pathways responsible for the addition of these functional groups to the parent VOC may be occurring predominantly in the gas phase. The nitrate and carboxylic acid fractions are less well predicted for the organic aerosol derived by α- pinene photooxidation. This may suggest that the condensed phase chemistry plays an important role in the formation or depletion of these functional groups from the organic mixture. This study contributes to understand how well the MCM model, widely used by the community, can predict aerosol phase functional group abundances in different scenarios and which kind of chemical reaction possibly occurring in the aerosol phase should be included to diminish the gap.

2015

Functional group distribution of secondary organic aerosol generated using the Master Chemical Mechanism: similarities and discrepancies with environmental controlled chamber studies

Fabian Alexander Bernhard, Giulia Ruggeri, Sarah Elizabeth Shipley, Satoshi Takahama

2015