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 (γMgly). Here, we quantified γMgly 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, γMgly ranged from 1.92 × 10-4 to 7.29 × 10-4, increasing with enhanced RH due to salting-out effects. Moreover, γMgly 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 (kI = 102.44-0.85·pH, R2 = 0.93). This kinetic parameter, together with effective Henry's law constants, can be implemented to update the γMgly parametrization. Addition of sulfate aerosols was found to strongly suppress γMgly, reducing kI 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.
Liu et al. (Mon,) studied this question.