Relative Humidity-Dependent HTDMA Measurements of Ambient Aerosols at the HKUST Supersite in Hong Kong, China

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 100 nm and 200 nm 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 (R_H) 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 but negatively correlated with organic and nitrate. In most cases, R_H increased as RH decreased and the average increase in was 45% from 90% RH to 40% RH, but these differences yielded insignificant changes in the GF-RH curves. The Zdanovskii-Stokes-Robinson (ZSR) estimated were mostly within 20% of > ₉₀ and _f. GF predictions using the empirical correlation of with AMS mass fractions or the ZSR estimated were within 10% of additional measurements and hence > ₉₀ is useful for predicting GF at lower RHs.