Qualitative and quantitative analysis of terpene-derived esters in laboratory-generated and ambient secondary organic aerosol (SOA)

Oligomeric esters, including ”dimer esters” (molecular weight 300-450 Da), are one of the components of secondary organic aerosol (SOA) and account for around 0.5-1.5 % of ambient SOA mass. Esters are low-volatile (LVOCs) or extremely low-volatile organic compounds (ELVOCs), built from derivatives of terpenoic acids. They are primarily concentrated in the particle phase.^{[1, 2]} Oxidation via ozonolysis of monoterpenes (C₁₀H₁₆) at night is known as the primary source of esters in the atmosphere.^[3] Additionally, esters play a crucial role in Cloud Condensation Nuclei (CCN) and have a significant impact on particle formation and growth. On the other hand, many compounds are not fully characterised or unreported, and an accurate surrogate standard has yet to be proposed.

Our research is focused on ester formation via ozonolysis and OH*-oxidation of α- and β-pinene in laboratory and ambient SOA. The aim is to identify and describe dimer esters in both analysed environments, with an emphasis on previously unknown substances. Also, the C₁₉H₃₀O₆ ester will be studied as a surrogate standard. Laboratory-derived and ambient aerosol samples were collected from the aerosol chamber at the Leibniz Institute for Tropospheric Research in Leipzig (TROPOS) and during field campaigns at two forest sites in Poland: Kampinos National Park and Borecka Forest. Water was used as the solvent for the extraction of the aerosol sample. All analyses were performed using an ultra-high-performance liquid chromatograph coupled with a mass spectrometer (UHPLC-MS), with an ElectroSpray Ionisation (ESI) source and Quadrupole Time-of-Flight (QToF) detector.

Overall, during qualitative analysis, 39 esters were identified in laboratory-derived aerosol samples. For all compounds, chemical formulas were matched or established. For some compounds, new structures were predicted. Despite the complex nature of environmental aerosol, quantitative analysis (performed on 15 substances) reveals traces of esters in ambient SOA, where ester C₁₉H₂₈O₇ was the most abundant, with a concentration of approximately 0.5 ng×m^-3. However, larger amounts of different esters were detected in laboratory-derived SOA. Experiments in a laboratory environment have shown that C₁₆H₃₀O₁₁, C₁₅H₃₀O₁₀ and C₁₉H₂₈O₇ are the most common, with amounts reaching 0.7 µg×m^-3. Concentrations were established by using a calibration curve based on C₁₉H₃₀O₆, providing results that matched well with the expected ester quantities in the SOA.

 

References:

[1] K. Kristensen et al., Environ. Sci. Technol. Lett., 2016, 3, 280−285

[2] C.M. Kenseth et al., Environ. Sci. Technol., 202054, 12829−12839

[3] C.M. Kenseth et al., Science, 2023, 382, 787-792