Solute Effect

The equilibrium vapor pressure (EVP) is represented (with differences
exaggerated) in the vicinity of three cloud droplets of equal size under
conditions of equal temperatures.  Navy dots represent water (vapor)
molecules.  The top droplet contains hydrophobic impurities.  This
increases the rate of evaporation.  Equilibrium requires a matching rate of
condensation, so a greater density of water vapor molecules (or greater
EVP) is required for equilibrium.  The associated equilibrium relative
humidity (RH_eq) is RH_eq = EVP/SVP > 100%.  The middle droplet is pure water. 
The required number density of water vapor molecules for equilibrium is
fewer than for the droplet with hydrophobic impurities.  The associated
relative humidity, RH_eq = EVP/SVP = 100%.   The bottom droplet contains
hygroscopic impurities.  This decreases the rate of evaporation. 
Equilibrium requires a matching rate of condensation, so a lesser density
of water vapor molecules (or lesser EVP) is required for equilibrium.  The
associated equilibrium relative humidity is RH_eq = EVP/SVP < 100%.

Clearly, less stringent conditions (lower concentrations of water vapor) are
required for cloud droplets containing hygroscopic impurities to survive
and grow.