Context <p>Polycyclic aromatic hydrocarbons (PAHs) have a substantial impact on the decomposition process of the atmosphere and the hydrosphere and also pose health risks due to toxicity and long-range transport capabilities. Despite extensive experimental and field studies, the detailed molecular-level understanding of their chemical reactivity, transformation pathways and toxicity after ozonolysis remains limited. This study investigates the structural, electronic and toxicological changes occurring to PAH (pyrene and benzo[e]pyrene) upon ozonolysis using a quantum chemical approach. Ozonolysis products of pyrene and benzo[e]pyrene exhibit altered stability and reactivity, which suggests that they are unlikely to participate in subsequent atmospheric reactions. Importantly, the toxicity assessment of these compounds indicates that products are less toxic than parental compounds, except pyrene mono-ozonide and benzo[e]pyrene dialdehyde. By integrating these insights, environmental simulations can better predict the behaviour of PAHs in the atmosphere and their role in ozone decomposition, contributing to the development of a more accurate atmospheric model.</p> Methods <p>Gaussian 09 software is used to carry out Density Functional Theory calculations for investigating electronic and structural properties of PAHs and their ozonolysis products. The B3LYP, M06-2X, ωB97XD, PW6B95 and PBE0 functionals with basis sets 6–311++G(d,p), def2-TZVP and cc-pVTZ are employed to determine the suitable method. Among all, B3LYP/6–311++G(d,p) demonstrates better alignment with experimental data for pyrene. Therefore, B3LYP/6–311++G(d,p) level of theory is used to analyse the HOMO–LUMO, electrostatic potential and frequency of reactants and product molecules. The toxicity profile of these compounds is assessed using the ecological structure activity relationships (ECOSARV2.2) software, which estimates their environmental exposure risk.</p>