Climate relevant processing of mineral dust by volatile organic compounds: first results on composition of complex dust/organic systems from the uptake of glyoxal

Mineral dust aerosols, which account for about 40% of global annual aerosol emissions, contribute to the persistent and large uncertainties on the global radiative budget and the oxidative capacity of the atmosphere.

Indeed, the uptake of atmospheric volatile organic compounds (VOCs) on mineral dust particles can contribute to the formation of secondary organic aerosols (SOA), with consequent modification of the chemical and optical properties of the dust.

Glyoxal, one of the most important VOC in the atmosphere, is a precursor of SOA, capable of interacting with mineral dust and forming SOA as a consequence of the interaction.

In this experimental study we investigate the formation of SOA by the uptake of glyoxal on mineral dust particles. We present the results of the heterogeneous interaction obtained in the CESAM atmospheric simulation chamber (Chambre Expérimentale de Simulation Atmosphérique Multiphasique), used to conduct aging experiments in various controlled conditions in terms of relative humidity, irradiation, and gas phase composition. Prior entering the chamber, particles from a real soil sample (Gobi Desert) are size-selected using an aerodynamic aerosol classifier (AAC) in order to obtain a monodispersed size distribution centered at 300 nm in mobility diameter, narrow enough to be able to appreciate a dimensional variation from glyoxal condensation.

In experiments conducted in humid conditions (RH=80%), a rapid uptake of glyoxal was observed on sub micrometric dust particles. 15 minutes after the injection of 1 ppm of glyoxal into the chamber, the mass of the particles increased by about 10%, with a variation of the modal diameter of the size distribution. As a consequence of glyoxal uptake in humid conditions, an increase of the aerosol organic mass concentration occurred immediately after the interaction, which was not observed in dry conditions. At the same time, the aerosol chemical speciation monitor (ACSM) mass spectra of the organic fraction show the increase in intensity of the glyoxal marker signals at m/z 58 and m/z 29. It is interesting to note also the drop of O/C ratio of the dust organic fraction after the injection of the glyoxal from 1.5 (the one of the dust itself) to a value close to 1, that is the one of the glyoxal.

Hence, the first results of the study suggest the presence of a fast glyoxal uptake on submicronic mineral dust particles in high relative humidity conditions. This process modifies the mass and the size distribution of the aerosol, as well as the chemical composition of its organic fraction.