Size-segregated chemistry of particulate dicarboxylic acids in the Arctic atmosphere

Gas–particle interactions of low-molecular-weight dicarboxylic acids were studied at a coastal Arctic site during the summer. Size segregated measurements with a Berner low-pressure impactor displayed up to four modes for ionic compounds: an Aitken mode, an accumulation mode, and two supermicron modes. The lower supermicron mode was ascribed to sea-salt, whereas the upper mode consisted mostly of species associated with continental particles. All four modes could be identified for oxalic acid, with the lower supermicron mode being the dominant. Malonic acid displayed a supermicron mode but was not found in the submicron size range. Succinic acid had an accumulation mode and, in a few samples, a supermicron mode. Glutaric acid displayed sometimes and accumulation mode, sometimes a supermicron mode, and occasionally both. The most probable formation pathway for submicron oxalic and glutaric acid was condensation from the gas phase, even though production in cloud droplets cannot be ruled out either. A slightly different formation pathway may have been important for submicron succinic acid production. Supermicron oxalic acid was probably formed by condensation from the gas-phase, by heterogeneous reactions occurring on the surface of pre-existing sea-salt and continental particles, or in cloud droplets. A larger mass median diameter for supermicron malonic and glutaric acid might be indicative of liquid-phase production in aqueous sea-salt particles. Evidence on possibly substantial sampling artifacts related to measuring dicarboxylic acids using filters were also obtained.