The hygroscopic tandem differential mobility analyzer (HTDMA) has been frequently used to measure the hygroscopic properties of atmospheric aerosols at a fixed high relative humidity (RH) of about 90%. To evaluate if such measurements could be used to determine the hygroscopicity of aerosols at lower RH, simultaneous hygroscopic growth factor (GF) and size-resolved composition measurements were made with an HTDMA and a high-resolution aerosol mass spectrometer (HR-AMS), respectively, at a coastal site in Hong Kong from January to June and in August 2012. A total of 58 cycles of dehydration (decreasing RH) and hydration (increasing RH) of 100nm and 200nm particles with organic-to-inorganic mass ratio ranging from 0.19 to 1.97 were measured at RH = 10-93%. The Kappa () equation developed by Petters and Kreidenweis in the year 2007 was used to determine (i) at individual RHs ((RH)) and (ii) best-fit covering the range of RHs measured ((f)) for the more-hygroscopic (MH) mode, which describes more than 80% of the particles in each cycle, during dehydration. Overall, at 90% RH or above ((>90)) fell between 0.18 and 0.48, and was within 15% of (f) in 83% of the datasets. Regression analysis between (>90) or (f) and AMS mass fractions showed that was positively correlated with sulfate bu