Hygroscopicity of Inorganic Aerosols: Size and Relative Humidity Effects on the Growth Factor

The hygroscopic properties of inorganic salt particles, including (NH4)(2)SO4, NaCl, Na2SO4 and NaNO3, are investigated using a self-assembled hygroscopic tandem differential mobility analyzer (H-TDMA) system. The iso-GF (growth factor) curves are derived to illustrate the effects of the initial particle size (D-0) and relative humidity (RH) on the GFs. For those salt particles of 100 nm, the GFs measured agreed well with their theoretical Kohler curves. In the size range of 20-200 nm, the GFs of (NH4)(2)SO4, NaCl and Na2SO4 particles all continuously decrease with D-0 increasing below the deliquescence RH (DRH). However, when RH is higher than the DRH, the GFs of those salts aerosols increase with D-0 throughout the investigated size range. Similar increase trend of GFs with D-0 is also observed for NaNO3 aerosols though they do not exhibit the abrupt deliquescence behavior. From iso-GF curves, it can be clearly observed that the GFs of (NH4)(2)SO4, NaCl and Na2SO4 particles all increase with the RH while the values decrease with D-0 below DRH. And above DRH, the GFs are more sensitive to D-0 for particles smaller than 60 nm, while the GFs are more sensitive to RH for particles larger than 80 nm. For NaNO3 aerosols, the iso-GF curves indicate the size-effect becomes more prominent on their hygroscopicity as the RH increases. The iso-GF curves provide a lucid and explicit insight into the hygroscopic growth of salts particles. Through iso-GF curves, we can clearly elucidate the major factor that affects the ultimate particle diameter at ambient atmosphere.