Gas-particle partitioning of toluene oxidation products: an experimental and modeling study

Abstract

Abstract. Aromatic hydrocarbons represent a large fraction of anthropogenic volatile organic compounds and significantly contribute to tropospheric ozone and secondary organic aerosol (SOA) formation. Toluene photooxidation experiments were carried out in an oxidation flow reactor (OFR). We identified and quantified the gaseous and particulate reaction products at 280, 285 and 295 K using a proton-transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) coupled to a CHemical Analysis of aeRosol ONline (CHARON) inlet. The reaction products accounted for both ring-retaining compounds such as cresols, benzaldehyde, nitrophenols, nitrotoluene, bicyclic intermediate compounds, as well as ring-scission products such as dicarbonyls, cyclic anhydrides, small aldehydes and acids. The chemical system exhibited a volatility distribution mostly in the semi-volatile (SVOCs – semi-volatile organic compounds) regime. The saturation concentration (Ci*) values of the identified compounds were mapped onto the two-dimensional volatility basis set (2D-VBS). Temperature decrease caused a shift of Ci* towards lower values while there was no clear relationship between Ci* and oxidation state. The CHARON PTR-ToF-MS instrument identified and quantified approximately 70–80 % of the total organic mass measured by an aerosol mass spectrometer (AMS). The experiments were reproduced by simulating SOA formation with the SSH-aerosol box model. A semi-detailed mechanism for toluene gaseous oxidation was developed. It is based on the MCM and GECKO-A deterministic mechanisms modified following the literature in particular to update cresols and ring-scission chemistry. The new mechanism improved secondary species representation with an increment of the major identified species (+35 % in number). Light compounds formation (i.e. m/z < 100) is enhanced and accumulation of heavy compounds (i.e. m/z ≥ 100) is reduced, especially in the gas phase. Additional tests on (i) partitioning processes such as condensation into aqueous phase, (ii) interactions of organic compounds between themselves and with inorganics and (iii) wall losses were also performed. When all these processes were taken into account the simulated SOA mass concentration showed a much better agreement with the experimental results. Finally, an irreversible partitioning pathway for methylglyoxal was introduced and considerably improved the model results, opening a way to further developments of partitioning in models. Our results underline that the volatility itself is not sufficient to explain the partitioning between gas and particle phase: the organic and the aqueous phases need to be taken into account as well as interactions between compounds in the particle phase.

Simulation of Regional Secondary Organic Aerosol Formation From Monocyclic Aromatic Hydrocarbons Using a Near‐Explicit Chemical Mechanism Constrained by Chamber Experiments

“…We then grouped these products into extremely low volatility (saturation concentration (C*) ≤ 3 × 10 −4 μg m −3 ), low volatility (3 × 10 −4 < C* ≤ 0.3 μg m −3 ), semivolatile (0.3 < C* ≤ 300 μg m −3 ), intermediate volatility (300 < C* ≤ 3 × 10 6 μg m −3 ), and volatile (C* > 3 × 10 6 μg m −3 ) categories as shown in Figure S6 of Supporting Information . Based on the volatility distribution of toluene SOA, we found that semivolatile organic compounds (SVOCs) contributed about 49.6% of total SOA mass concentration (TS2‐3), which is slightly lower than observed fractions (62%–71%) for toluene SOA produced in an oxidation flow reactor (OFR) with a comparable initial toluene condition but lower temperature (Lannuque et al., 2023). This difference might be attributed to differences in initial toluene concentrations, experimental temperature, and speciation variation among toluene oxidation products generated under different reaction durations between OFR and chambers.…”

“…Lannuque et al. (2023) found that MCM overestimated gaseous products with high carbon numbers (C 7 ) and their oxidation state compared to gas‐ and particle‐phase products detected in toluene SOA experiments. Consequently, the contribution of C 7 products to toluene SOA was overpredicted.…”

“…Lastly, most simulated chamber experiments have primarily focused on dry conditions with RH levels below 10%, rarely observed within the planetary boundary layer. However, a recent study has highlighted the significant role of gas‐aqueous mass transfer in accurately simulating toluene SOA formation within an OFR under RH levels ranging from 24% to 41% (Lannuque et al., 2023). Nonideal interactions between organic species and inorganic ions are crucial under such conditions; neglecting them may lead to overestimating SOA formation within models.…”

Secondary organic aerosol formed by EURO 5 gasoline vehicle emissions: chemical composition and gas-to-particle phase partitioning

“…The last decade, online high-resolution instrumentation has been developed for the identification of the OA species at a molecular level (Williams et al, 2006(Williams et al, , 2014Kreisberg et al, 2009;Hohaus et al, 2010;Zhang et al, 2014) or for the identification of the chemical formula using soft-ionization mass spectrometry (Lopez-Hilfiker et al, 2014;Isaacman-VanWertz et al, 2017;Stark et al, 2017;Gkatzelis et al, 2018;Lannuque et al, 2023). Volatility measurements are performed by alternative measurements between the gas and the particle phase of the same ion (Hohaus et al, 2015;Lopez-Hilfiker et al, 2016;Isaacman-VanWertz et al, 2016;Stark et al, 2017;Gkatzelis et al, 2018;Lannuque et al, 2023). To our knowledge these new techniques are still limited.…”

“…To our knowledge these new techniques are still limited. They have been applied to biogenic OA systems in the ambient (Lopez-Hilfiker et al, 2016;Isaacman-VanWertz et al, 2016;Stark et al, 2017) or in laboratory produced biogenic SOA (Hohaus et al, 2015;Gkatzelis et al, 2018), and just recently on toluene SOA (Lannuque et al, 2023).…”