Correlation of DTT activity with different transition metals. Particulate samples are distinguished in two different particle size fractions (PM0.15 and PM2.5). The four panels show the correlation of the DTT activity of two different PM size fractions (PM0.15 and PM2.5) with (a) Mn, (b) Fe, (c) Cu, (d) Zn, respectively. These graphs show that despite the DTT activity of PM samples is not correlated with transition metals in the pooled samples of different size ranges (Table 2) there is a strong correlation with some metals within the two particle size fractions. This may probably be an artifact of the high correlation of metals and PAHs in these samples.