During cloud formation, water vapor condenses onto aerosol particles, resulting in water content and acidity changes that have the potential to drive reactions that alter the speciation of organic compounds in aerosols, which in turn may alter aerosol climate-forcing properties. Specifically, glyoxal, methylglyoxal and their polymers, which are common in tropospheric aerosols, can undergo hydration, hydrolysis and/or polymerization reactions upon dilution. Glyoxal and methylglyoxal are likely to be highly polymerized in aerosols before cloud formation, but some polymers may decompose upon dilution. The effects of cloud formation on glyoxal and methylglyoxal speciation were simulated by rapid dilution of approximately 40 wt.% glyoxal or methylglyoxal in water. The chemical composition of the solution was monitored as a function of time since dilution by high resolution quadrupole mass spectrometry and nuclear magnetic resonance spectroscopy. Results show that the kinetics are slow compared to typical cloud droplet lifetimes since speciation changes continue hours after dilution. Most glyoxal species attain equilibrium by 100 minutes, while a few species continue changing for approximately 150 minutes. Methylglyoxal speciation is generally even slower with some polymers still changing concentration after four hours. Since the observed chemical composition changes during cloud processing are on the order of minutes to hours, any impact on climate-forcing properties of aerosols will depend on cloud droplet lifetimes.

Kinetics of glyoxal and mthylglyoxal polymer speciation in aerosols during cloud formation: Implications for climate

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