Kinetics and Mechanisms of Reactive Uptake of Methylglyoxal on Deliquesced Ammonium Salts: Acid Catalysis and Sulfate Inhibition

Reactive uptake of methylglyoxal (Mgly) on aerosol particles is an important source of secondary organic aerosol (SOA), yet its significance remains highly uncertain due to the poorly constrained uptake coefficients (γ_Mgly). Here, we quantified γ_Mgly on deliquesced pH-buffered ammonium nitrate (AN) and sulfate/nitrate/ammonium (SNA) aerosols via flow tube experiments by directly measuring SOA formation under variable relative humidity (RH, 75-92%) and pH (3.1-4.4). For AN aerosols, γ_Mgly ranged from 1.92 × 10^-4 to 7.29 × 10^-4, increasing with enhanced RH due to salting-out effects. Moreover, γ_Mgly decreased by a factor of 2 to 10 as pH rose from 3.15 to 4.4 with NH₃ addition, suggesting that acid-catalyzed reactions dominate the Mgly uptake. The pH dependence was captured by a first-order reaction rate constant (<i>k</i>^<i>I</i> = 10^{2.44-0.85·<i>pH</i>}, <i>R</i>² = 0.93). This kinetic parameter, together with effective Henry's law constants, can be implemented to update the γ_Mgly parametrization. Addition of sulfate aerosols was found to strongly suppress γ_Mgly, reducing <i>k</i>^<i>I</i> to 12-38% of the estimation on AN-alone aerosols at a similar pH. Our findings underscore the critical role of aerosol pH and composition in Mgly uptake and provide kinetic parameters to atmospheric models to improve predictions of Mgly SOA.