Correlation of measured and reconstructed DTT activity for samples in all size modes. (a) Correlation with PAH of molecular weight equal or greater than 252, (b) Correlation with PAH and Cl, (c) Correlation with PAH, Cl, Cr and V. The thin line is the chart diagonal representing the 1:1 line. The correlations demonstrate the strong correlation of DTT activity and PAH which may act as surrogates for other functional groups that have the capacity to reduce oxygen, such as quinones and aromatic nitro-PAHs. The correlation also improves and the intercept decreases when, gradually, Cl, Cr and V are introduced in the regression. The regression parameters that can be used to quantify the effects of each species on the DTT activity are given in Table 4.