A method includes measuring a leakage current through a particulate matter sample in a humidity chamber, the leakage current is measured as a relative humidity in the humidity chamber is incrementally increased, and plotting a logarithm of the measured leakage current as a function of the relative humidity. The plot of the logarithm of the measured leakage current as a function of the relative humidity has an inversion region, a low relative humidity region, and a high relative humidity region.