Pirimiphos-methyl (PMM) and pirimicarb (PM) are two typical N,N-dialkyl substituted pyrimidine pesticides. The heterogeneous reactions of suspended PMM and PM particles with NO3 radicals are investigated using an online aerosol time-of-flight mass spectrometer and a real-time atmospheric gas analysis mass spectrometer. Three products for PMM and five products for PM are observed and assigned with the aid of GC/MS. Phosphoric acid 2-diethylamino-6-methyl-4-pyrimidirtyl dimethyl ester and 2-(dimethylamino)5,6-dimethyl-4-hydroxy-pyrimidine are the main reaction products observed for PMM and PM, respectively. The effective rate constants for the reactions of PMM and PM particles with NO3 radicals are (9.9 +/- 0.3) x 10(-12) and (7.5 +/- 0.3) x 10(-3) cm(3) molecule(-1) s(-1), respectively, obtained using a mixed-phase relative rate method. Geometries and energies of transition states (TS) and intermediates (IM) are obtained by DFT calculation to elucidate the detailed mechanism of the P=S group oxidation into the P=O group for PMM. The theoretical studies present the reasonable intermediates including the S-oxide and the diradical (IM1(a) and IM2(a)). The mechanism explanation may provide useful information for understanding the degradation mechanism of organothionophosphorus compounds in the environment.