Secondary Organic Material Produced by the Dark Ozonolysis of α-Pinene Minimally Affects the Deliquescence and Efflorescence of Ammonium Sulfate

The hygroscopic phase transitions and growth factors of mixed particles having as components ammonium sulfate and secondary organic material (SOM) were measured. The SOM was generated by the dark ozonolysis of α-pinene, and organic particle mass concentrations of 1.63 and 12.2 μg m−3 were studied. The hygroscopic properties were investigated using a 1×3 tandem differential mobility analyzer (1×3-TDMA). The 1×3-TDMA takes advantage of the hysteresis between solid-to-aqueous and aqueous-to-solid phase transitions to determine the efflorescence and deliquescence relative humidities (ERH and DRH, respectively) of materials. Overall, the influence of the SOM produced by the dark ozonolysis of α-pinene on the ERH and DRH of ammonium sulfate was small, shifting for example the DRH from 80% for pure ammonium sulfate to 77% for organic volume fractions of 0.96. The ERH likewise shifted by only a small amount across this composition range, specifically from 31 to 29%. The SOM produced at the lower organic particle mass concentrations shifted ERH and DRH even less, indicating an influence of SOM chemical composition on phase transitions. The hygroscopic growth factors of the mixed particles were adequately modeled across the range of studied RH (50 to 83%) using volume-averaged growth factors of the pure materials. The results for ERH, DRH, and the growth factors were all consistent with a model of phase separation between the inorganic and organic phases in individual particles, at least for the studied RH values (<83%) and for SOM prepared by α-pinene ozonolysis.