In this study, we investigated the reaction mechanism and kinetics of ozone with trans -2-pentenal using density functional theory (DFT) and conventional transition state theory (CTST). At 298 K and 1 atm, the gas-phase reaction mechanisms and kinetic parameters were calculated at the level of CCSD(T)/6-311+G(d,p)//M06-2X/6-311+G(d,p). Both C C and C O bond cycloaddition as well as hydrogen abstraction were found. The calculations indicated that the main reaction path is 1,3-dipole cycloaddition reactions of ozone with C C bond with the relatively lower syn -energy-barrier of 3.35 kcal mol−1 to form primary ozonide which decomposed to produce a carbonyl oxide called a Criegee intermediate (CI) and an aldehyde. The subsequent reactions of CIs were analysed in detail. It is found that the reaction pathways of the novelty CIs containing an aldehyde group are extremely similar with general CIs when they react with NO, NO 2 , SO 2 , H 2 O, CH 2 O and O 2. The condensed Fukui function were calculated to identify the active site of the chosen molecules. At 298 K and 1 atm, the reaction rate coefficient was 9.13 × 10−18 cm3 molecule−1 s−1 with atmospheric lifetime of 1.3 days. The calculated rate constant is in general agreement with the available experimental data. The branching ratios indicated that syn -addition pathways are prior to anti -addition. The atmospheric ratios for CIs formation and the bimolecular reaction rate constants for the Criegee intermediates with the variety of partners were calculated. Our theoretical results are of importance in atmospheric chemistry of unsaturated aldehyde oxidation by ozone. [Display omitted] • The primary ozonides (POZs) are dominant for ozone with pentenal reactions. • The final products are Criegee intermediates by POZs decomposition. • Further reactions of Criegee intermediates could lead to secondary organic aerosols. • The calculated rate constants are consistent with experimental data. [ABSTRACT FROM AUTHOR]