pH-dependent reaction kinetics between glyoxal and ammonium sulfate in simulated cloud droplets

Aqueous-phase reactions between carbonyls and reduced nitrogen compounds play a considerable role in the formation of secondary organic aerosols and brown carbon in the atmosphere. However, the reported reaction rate constants for these reactions have largely been limited to bulk aqueous-phase simulations, which may not accurately represent the real state of atmospheric cloud droplets. We employed an integration of optical tweezers and Raman spectroscopy to manipulate and analyze simulated cloud droplets (size range 8000-10,000 nm), comprising a mixture of glyoxal and ammonium sulfate. This approach enabled us to delve into the intricate realm of their reaction kinetics at individual droplet level mimicking cloud droplets. Raman spectroscopy provided high temporal resolution (20 s) measurements of the changes in the amount of nitrogen-containing organics (or NOCs as represented by the CN bond) within the droplets. The results indicate that the reaction follows first-order kinetics throughout the monitoring over 80-400 min. The average reaction rate constant for the formation of NOCs within the single droplet was determined to be (6.77 ± 0.98) × 10^-5 s^-1, up to three orders of magnitude higher than those through the bulk aqueous-phase simulations, especially at lower pH levels. Additionally, the reaction rate constant in single droplet increases with increasing pH, consistent with the trend previously reported for the bulk aqueous-phase simulations. The results highlight the difference of the reaction rate constant between bulk aqueous-phase and droplets, which would improve our understanding on the formation and impacts of secondary organic aerosols and brown carbon in atmospheric aqueous phase.