Measurements of the peroxycarboxylic nitric anhydrides (PANs = peroxyacetic nitric anhydride, PAN, peroxypropionic anhydride, PPN, and peroxymethacrylic nitric anhydride, MPAN) and aldehydes (acetaldehyde, propanal, and methacrolein) were made at Nashville, TN in 1999 and Houston, TX in 2000. The data were interpreted with a sequential reaction model that included reaction of aldehydes with hydroxyl radical and formation or loss of PANs mediated by peroxyacyl radicals {RC(O)OO}. The comparison of the measured ratios with those predicted by the model showed a disagreement for PAN/acetaldehyde and PPN/propanal in Nashville, but agreement in Houston. The Nashville data set also showed a disagreement in the case of PAN/acetaldehyde and MPAN/methacrolein, but agreement for PPN/propanal and MPAN/methacrolein. These features were consistent with the relative importance of isoprene to PAN formation at each site. The ratios were found to be highly correlated with ozone mixing ratios at each site. ¿ 2001 American Geophysical Union