Formaldehyde and Acetaldehyde Increase Aqueous-Phase Production of Imidazoles in Methylglyoxal/Amine Mixtures: Quantifying a Secondary Organic Aerosol Formation Mechanism

Formaldehyde and acetaldehyde are commonly found in cloud droplets because of reversible partitioning and hydration reactions. An SOA formation pathway was recently identified in which these common aldehydes are irreversibly incorporated into imidazole derivatives formed by reaction with dicarbonyl species and ammonium salts or amine species. Here we use ultraviolet–visible and nuclear magnetic resonance kinetic measurements to determine the influence of formaldehyde and acetaldehyde on aqueous methylglyoxal chemistry. The presence of formaldehyde increases imidazole product formation rates by factors of 2 and ≥5 in reactions with ammonium sulfate and amines, respectively, and increases imidazole product yields in methylglyoxal + amine reactions by more than an order of magnitude. Acetaldehyde is less likely to be incorporated into imidazole products and increases formation rates and yields only in reactions involving amines. We estimate that aqueous formation of imidazoles could generate as much as 1.05 Tg of C/year SOA from formaldehyde and 3.8 Tg of C/year or 7 Tg/year SOA overall, limited by the availability of aqueous-phase glyoxal and methylglyoxal. While this upper limit represents a negligible formaldehyde sink, it is ∼5% of current estimates of global SOA formation. Formaldehyde's channeling of aqueous dicarbonyl chemistry toward production of imidazoles limits the formation of other oligomer products, including brown carbon species.