Hygroscopic growth factor (GF) distributions of 13, 25, 50, 100, 200, and 400 nm particles measured with a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) from 2009 to 2012 at the Southern Great Plains (SGP) site in Oklahoma, U.S. were used to describe time of day- and annually-averaged hygroscopicity parameters (κ). A diel pattern was often observed with an average daytime κ that was higher than that at other times, especially on days with new particle formation (NPF) events. The average hygroscopicity of the smaller and larger particles at the tails of the measured size range was higher than that in between, with the minimum for each of the 4 years at 50 nm. This pattern is thought to result in part from addition of soluble inorganic and organic compounds formed through gas phase and aqueous phase reactions for the smaller and larger particles, respectively. The size dependence is reflected in the averaged κ and in the frequency with which GF distributions possessed modes categorized as nearly-hydrophobic, less hygroscopic, and hygroscopic. A hygroscopicity-based mixing state parameter, MShyg, defined as the ratio of the standard deviation (SD) of a measured GF distribution to the size specific threshold SD roughly separating internal and external mixtures, was used to study the diel and seasonal variation in particle mixing state. Internal mixtures were found to be more common during the daytime and during the summer, likely reflecting more rapid photochemical processing and growth at those times.