Characterization of oligomers from methylglyoxal under dark conditions: a pathway to produce secondary organic aerosol through cloud processing during nighttime

摘要

Aqueous-phase oligomer formation from methylglyoxal, a major atmosphericphotooxidation product, has been investigated in a simulated cloud matrixunder dark conditions. The aim of this study was to explore an additionalpathway producing secondary organic aerosol (SOA) through cloud processeswithout participation of photochemistry during nighttime. Indeed,atmospheric models still underestimate SOA formation, as field measurementshave revealed more SOA than predicted. Soluble oligomers (n = 1–8) formed inthe course of acid-catalyzed aldol condensation and acid-catalyzed hydrationfollowed by acetal formation have been detected and characterized bypositive and negative ion electrospray ionization mass spectrometry. Aldolcondensation proved to be a favorable mechanism under simulated cloudconditions, while hydration/acetal formation was found to strongly depend onthe pH of the system and only occurred at a pH<3.5. No evidence was foundfor formation of organosulfates. The aldol oligomer series starts with aβ-hydroxy ketone via aldol condensation, where oligomers are formedby multiple additions of C₃H₄O₂ units (72 Da) to the parentβ-hydroxy ketone. Ion trap mass spectrometry experiments wereperformed to structurally characterize the major oligomer species. Amechanistic pathway for the growth of oligomers under cloud conditions andin the absence of UV-light and OH radicals, which could substantiallyenhance in-cloud SOA yields, is proposed here for the first time.