Hygroscopicity of the SOA particles collected on the Teflon filter was measured using a Quartz Crystal Microbalance (QCM). After collection, a piece of SOA-laden filter was pressed on the surface of a clean QCM sensor (Novaetech, AT5-14-12-AU). The filter was then peeled off, leaving a thin film of SOA remaining on the sensor surface. After preparation, the QCM crystals were mounted in a temperature-controlled flow module (Q-sense, QFM401) and purged with zero air at a total flow rate of 30 cm 3 min -1 . The RH in the flow cell was switched between dry (< 1% RH) and humidified (10-95% RH) conditions by changing the mixing ratio of dry and humidified pure air flows using two mass flow controllers. An RH sensor (Rotronic, HC2A-S) continuously monitored the RH in the cell. The QCM (Q-sense Explorer) continuously monitored the mass of SOA film, as described elsewhere. The SOA mass at the dry condition (< 1% RH) was measured before and after the measurement at an elevated RH, and possible evaporation of dry organic material was accounted for (e.g. SI, Fig. ). The mass sensitivity of the QCM was < 1 ng cm -2 , corresponding to one-tenth of a single molecule-layer of water. This sensitivity was sufficient for accurately detecting water uptake at RH values > 10%. The adsorption of water vapor on a blank sensor surface was determined and subtracted from the hygroscopic growth calculation (SI, Fig. ). From the QCM measurements, we determined the mass-based hygroscopic growth factor, which is the ratio of the particle mass at a specific RH to the particle mass under dry condition. In addition, we determined the massbased hygroscopicity parameter (κ m ) using the following equation: 72