The mid-values of annual TSP distributions, when plotted on lognormal coordinates, frequently isplay patterns—referred to as Patterns 1, 2 and 3 of this paper. These patterns, as well as what is called Pattern 4 (which seldom occurs in measured TSP data), can all result from composite distributions made up from two separate log-normal distributions with different values of the mean and standard deviation. They can also result from random sampling from truly log-normal distributions. The frequency of occurrence of the various patterns in actual air quality data indicates that most of the observed cases of all but Pattern 1 can be due to random sampling from populations which are well represented by log-normal distributions. Pattern 1 occurs with sufficient frequency and strength to suggest that many of its occurrences represent situations where the observed distribution is a composite of two distributions; the distribution with the lower mean has the higher standard deviation.