Between 25 th April and 29 th April, the average daytime PM0.95 sulfate concentration was ~16.7 µg m 3 , which is extremely elevated compared to the time before the onset of volcanic activity (~ 0.7 µg m 3 ; Figure ). The total concentration of crustal element ions (K + , Na + , Mg 2+ , and Ca 2+ ) was also significantly increased (from ~0.03 µg m 3 to ~ 0.2 µg m 3 ). While this suggests some contribution of volcanic primary ash, the enhancement was much less pronounced than for sulfate. Halogen ion (Br -and Cl -) concentrations were not affected by the onset of volcanic activity. Similarly, the concentration of organic carbon was with ~1 µg m 3 not significantly enhanced compared to the time before, indicating carbon-poor volcanic particles. In contrast to PM0.95, supermicron particle concentration and composition was largely unaffected by the onset of the volcanic episode (see supplementary Figure ), suggesting that particle mass in the volcanic plumes was dominantly in the submicron mode.