10 Mineral dust particles have photochemical properties that can promote heterogeneous reactions on 11 their surfaces and therefore alter atmospheric composition. Even though dust photocatalytic nature 12 has received significant attention recently, most studies have focused on inorganic trace gases. Here 13 we investigated how light changes the chemical interactions between butanol and Arizona test dust, 14 a proxy for mineral dust, under atmospheric conditions. Butanol uptake kinetics were measured, 15 exploring the effects of UV light irradiation intensity (0-1.4 mW/cm), relative humidity (0-10%), 16 temperature (283-298 K), and butanol initial concentration (20-55 ppb). The composition of the 17 gas phase was monitored by a high-resolution proton-transfer-reaction mass spectrometer (PTR-ToF-18 MS) operating in H3O + mode. Water was observed to play a significant role, initially reducing 19 heterogeneous processing of butanol, but enhancing reaction rates once it evaporated. Gas phase 20 products were identified, showing that surface reactions of adsorbed butanol led to the emission of a 21 variety of carbonyl containing compounds. Under actinic light these compounds will photolyze and 22 produce hydroxyl radicals, changing dust processing from a sink of VOC into a source of reactive 23 compounds. 24 25