Reversing the particulate-phase organosulfate chronology: Direct organosulfur compound synthesis in the gas-phase by SO3 + acid reactions

Organosulfates (OS) are a major constituent of atmospheric secondary organic aerosol (SOA). In lack of apparent gas-phase compounds directly contributing to the particulate bound OS, their synthesis has been thought of taking place by acid-catalyzed reactions in the condensed phase, mainly initiated by H2SO4. The most well-known sulfur bearing molecules are the isoprene epoxydiol derived organosulfates (Riva et al., 2019). In 2015 Mackenzie et al., showed that under very dry condition SO3 can react to form sulfuric anhydrides by carboxylic acid + SO3 reactions (Mackenzie et al., 2015). More recently, several theoretical papers have reported a more general gas-phase source by SO3 reactions with a multitude of atmospheric acids. The potential importance of this newly found chemistry was highlighted by observations of gas-phase SO3 in urban Beijing at concentrations similar to H2SO4 (Yao et al., 2020), strongly implying that SO3 reactions are occurring in urban atmospheres. In the present work we have performed a joint experimental-theoretical characterization of acid + SO3 reactions utilizing flow reactor setups coupled to nitrate (NO3-) chemical ionization mass spectrometry (CIMS) detection combined with supporting quantum chemical computations and master equation simulations. The studied reactions included mono- and dicarboxylic acids, and the strong acids most associated with atmospheric new particle formation events (i.e., H2SO4 and HIO3; Sipilä et al., 2016; Kerminen et al., 2018). Intriguingly, all acids were found to react rapidly with SO3 even with rate coefficients approaching the collision limit and were found to result in analogous acid sulfuric anhydride products. These sulfuric anhydrides provide a path for OS partitioning from gas to particle (i.e., “backwards” in considering the common particulate-phase synthesis route). Furthermore, the subsequent particulate-phase hydrolysis of the formed organic sulfuric anhydrides is a potential source of the small acids into the nanoparticles that would not be expected to partition significantly otherwise. The formed sulfuric anhydrides, especially the disulfuric acid and iodic acid sulfate, are likely to have similar, if not better, properties at initiating NPF as their parent compounds have. References: Kerminen, V.-M. et al., Atmospheric new particle formation and growth: review of field observations, Environ. Res. Lett. 2018, 13, 103003. Mackenzie, R. et al., Gas Phase Observation and Microwave Spectroscopic Characterization of Formic Sulfuric Anhydride, Science 2015, 349, 58−61. Riva, M. et al., Increasing Isoprene Epoxydiol-to-Inorganic Sulfate Aerosol Ratio Results in Extensive Conversion of Inorganic Sulfate to Organosulfur Forms: Implications for Aerosol Physicochemical Properties, Environ. Sci. Technol. 2019, 53, 8682-8694. Sipilä, M. et al., Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3, Nature 2016, 537, 532-534. Yao, L. et al., Unprecedented ambient sulfur trioxide (SO3) detection: Possible formation mechanism and atmospheric implications, Environ. Sci. Technol. Lett. 2020, 7, 809-818.