Key factors influencing the relative humidity dependence of aerosol light scattering

We have measured the relative humidity dependence of aerosol light extinction ( f σ( ep ) (80% RH, Dry )) at 532 nm for non‐absorbing surrogate atmospheric aerosols to determine the influence of particle size, composition (inorganic vs. organic), and mixing state (internal vs. external) on aerosol light scattering. We present results for mixtures of NaCl and (NH 4 ) 2 SO 4 with a few dicarboxylic acids. For atmospheric conditions the variability in the RH dependence of aerosol light scattering ( f σ( sp ) ( RH, Dry )) is most sensitive to aerosol composition and size. The influence of the mixing state on f σ( sp ) ( RH, Dry ) is small. These laboratory results imply that f σ( sp ) ( RH, Dry ) can be reasonably estimated from the aerosol size distribution and composition (inorganic/organic) using the mass‐weighted average of f σ( sp ) ( RH, Dry ) for the individual components.