Evidence from field measurements suggests that organosulfates contribute substantially to ambient secondary organic aerosol (SOA) and might dominate a considerable fraction of total sulfur in tropospheric particles. While alcohols and epoxides are suggested to be most likely precursors for organosulfates in SOA, their reactivity in acidic particles and their potential for organosulfate formation are still unclear. In the present study, a series of aerosol chamber experiments was performed to investigate the formation of organosulfates from reactive uptake of monoterpene oxides (α-pinene oxide and β-pinene oxide) and acid catalysed isomerisation compounds of α-pinene oxide (campholenic aldehyde and carveol) on neutral and acidic sulfate particles. Organosulfate formation was observed only under acidic conditions for both monoterpene oxides and, to a lesser extent, campholenic aldehyde, indicating that epoxides most likely serve as precursors for some of the organosulfates reported from both ambient and laboratory SOA samples. Structures of organosulfates were elucidated by comparing the tandem mass spectrometric, accurate mass and ion mobility data obtained for both the synthesised reference compounds and aerosol chamber-generated organosulfates. In the experiment performed using β-pinene oxide and acidic sulfate seed particles, an organosulfate with a sulfate group at a tertiary carbon atom accounts for 64% of the detected organosulfates. In contrast, an organosulfate with a sulfate group at a secondary carbon atom accounts for 80% of the detected organosulfates in the sample from α-pinene oxide/acidic sulfate particle experiment. The concentration of β-pinene-derived organosulfates was higher than known α-pinene oxidation products such as pinic acid and pinonic acid in an ambient aerosol sample collected at a Norwegian spruce forest site during the summer time, ranging up to 23 ng m⁻³. Furthermore, α-pinene oxide is found to isomerise readily on the wet seed particle surface, forming campholenic aldehyde. It is likely that other epoxides also play an important role for the formation of organosulfates under atmospheric conditions, and the isomerisation of epoxides may be an important route for the formation of some SOA constituents whose structures do not resemble precursor volatile organic compounds (VOCs).

Stable high-molecular-weight esters are present in α-pinene ozonolysis secondary organic aerosol (SOA) with the two most abundant ones corresponding to a hydroxypinonyl ester of cis-pinic acid with a molecular weight (MW) of 368 (C₁₉H₂₈O₇) and a diaterpenylic ester of cis-pinic acid with a MW of 358 (C₁₇H₂₆O₈). However, their molecular structures are not completely elucidated and their relationship with highly oxygenated molecules (HOMs) in the gas phase is still unclear. In this study, liquid chromatography in combination with positive ion electrospray ionization mass spectrometry has been performed on high-molecular-weight esters present in α-pinene ozonolysis SOA with and without derivatization into methyl esters. Unambiguous evidence could be obtained for the molecular structure of the MW 368 ester in that it corresponds to an ester of cis-pinic acid where the carboxyl substituent of the dimethylcyclobutane ring and not the methylcarboxyl substituent is esterified with 7-hydroxypinonic acid. The same linkage was already proposed in previous work for the MW 358 ester (Yasmeen et al., 2010), but could be supported in the present study. Guided by the molecular structures of these stable esters, we propose a formation mechanism from gas-phase HOMs that takes into account the formation of an unstable C₁₉H₂₈O₁₁ product, which is detected as a major species in α-pinene ozonolysis experiments as well as in the pristine forest atmosphere by chemical ionization–atmospheric pressure ionization–time-of-flight mass spectrometry with nitrate clustering (Ehn et al., 2012, 2014). It is suggested that an acyl peroxy radical related to cis-pinic acid (RO₂⚫) and an alkoxy radical related to 7- or 5-hydroxypinonic acid (R′O⚫) serve as key gas-phase radicals and combine according to a RO₂ + R′O⚫ → RO₃R′ radical termination reaction. Subsequently, the unstable C₁₉H₂₈O₁₁ HOM species decompose through the loss of oxygen or ketene from the inner part containing a labile trioxide function and the conversion of the unstable acyl hydroperoxide groups to carboxyl groups, resulting in stable esters with a molecular composition of C₁₉H₂₈O₇ (MW 368) and C₁₇H₂₆O₈ (MW 358), respectively. The proposed mechanism is supported by several observations reported in the literature. On the basis of the indirect evidence presented in this study, we hypothesize that RO₂ + R′O⚫ → RO₃R′ chemistry is at the underlying molecular basis of high-molecular-weight ester formation upon α-pinene ozonolysis and may thus be of importance for new particle formation and growth in pristine forested environments.