Atmospheric Processes of Aromatic Hydrocarbons in the Presence of Mineral Dust Particles in an Urban Environment

The impact of Gobi Desert dust (GDD) particles on the secondary organic aerosol (SOA) formation was studied to understand the photooxidation of volatile organic compounds, namely, toluene, 1,3,5-trimethylbenzene, and gasoline vapor, in the presence of NOx using a large outdoor photochemical smog chamber under natural sunlight. SOA yields of all three precursors were found to significantly increase in the presence of GDD particles. The mechanistic role of mineral dust in SOA growth changed with the dust-phase water content. In the presence of dry dust, the formation of carboxylate salts increased SOA growth, but it was limited by the dust's buffering capacity associated with the quantity of alkaline carbonates that react with carboxylic acids. In the presence of wet GDD particles, the pathway via aqueous reactions of oxygenated organic products enhanced SOA growth. Nitric acid absorbed on wet GDD particles suppressed gas-dust partitioning of weakly acidic carboxylic acids. The formation of alkaline carboxylates was also supported by Fourier transform infrared spectroscopy of the SOA formed in the presence of dry GDD. The partitioned SOA products can be further oxidized by dust-phase OH radicals generated by photoactivation of semiconducting metal oxides (i.e., TiO2). This heterogeneous oxidation of SOA products was observed by studying the degradation of a surrogate compound, benzoate, in the presence of GDD under ambient sunlight. In typical urban atmospheres (high NOx levels) with relative humidity higher than 40%, authentic dust can be easily nitrated, elevating dust's hygroscopicity and significantly increasing SOA growth potential.