“…The SO 3 • H 2 O complex can form readily, however early investigations of the hydrolysis of SO 3 to form H 2 SO 4 found the uncatalysed rate to be too low to explain the concentration of atmospheric H 2 SO 4 (Wang et al, 1988;Hofmann & Von Ragué Schleyer, 1994;Loerting & Liedl, 2000). Further studies revealed that the formation of H 2 SO 4 is catalysed by collisions with other atmospheric molecules (Akhmatskaya et al, 1997;Hazra & Sinha, 2011;Long et al, 2012;Torrent-Sucarrat, Francisco & Anglada, 2012;Bandyopadhyay, Kumar & Biswas, 2017;Sarkar, Oram & Bandyopadhyay, 2019), where the common denominator is the facilitation of a double proton transfer via the catalysing molecule (Kumar, Sinha & Francisco, 2016). The relative importance of collisions with different molecules is determined by both the atmospheric concentration of the colliding molecule and the species-dependent reaction rate.…”