In Situ Monitoring of the Aging of Mineral Particles by Methylglyoxal Using Infrared Spectroscopy. Part II: Unravelling the Impact of Humidity on the Uptake Mechanism

Abstract

Mineral dust is a major atmospheric aerosol component and an efficient reactive surface for trace gases, yet the mechanisms governing volatile organic compound uptake under realistic humidity conditions remain poorly constrained. Here, the first mechanistic investigation of methylglyoxal (MGL) uptake is presented on natural mineral dust under humid atmospheres. Using in situ diffuse reflectance infrared Fourier transform spectroscopy, we examine MGL interactions with Saharan (M’Bour) and Gobi dusts, as well as CaCO₃, at relative humidity (RH) from 20 to 80% under atmospheric pressure and room temperature. Results show that MGL uptake remains efficient at atmospherically relevant RH but decreases with increasing humidity due to competition with water for surface sites. Beyond physical adsorption, MGL undergoes reactive processing on dust surfaces, forming enolic and diketone species. The balance between reversible monomer uptake and reactive conversion is strongly controlled by RH and dust mineralogy. While M’Bour dust exhibits a Langmuir–Hinshelwood-type two-step mechanism, Gobi dust displays more complex behavior involving multiple surface sites and partial irreversibility. Importantly, MGL uptake induces permanent depletion of surface hydroxyl groups, suggesting long-term modifications of dust hygroscopicity. These findings demonstrate that mineral dust acts as an active, humidity-dependent sink for MGL and provide mechanistic constraints essential for improving representations of dicarbonyl heterogeneous chemistry and secondary organic aerosol formation in atmospheric models.