Elucidation of exhaust gas recirculation deposit formation mechanism based on chemical analysis of components

•Formation mechanism of lacquer-like hard deposits and sticky and wet soft deposits.•Mechanism elucidated using quantitative and molecular-level analyses.•The hard and soft deposits mainly comprised polycyclic aromatic hydrocarbons (PAHs)•PAHs in the exhaust gas condense when the gas temperature reaches their dew points.•Deposits contain polymerized compounds from oxygenated and nitrogenated PAHs.