Atmospheric accretion reactions of octanal with sulfuric acid as a catalyst were investigated in bulk liquid−liquid experiments and gas-particle experiments. In bulk studies, trioxane, α,β-unsaturated aldehyde, and trialkyl benzene were identified by gas chromatography−mass spectrometry as major reaction products with increasing sulfuric acid concentrations (0−86 wt%). Cyclotrimerization and one or multiple steps of aldol condensation are proposed as possible accretion reaction pathways. High molecular weight (up to 700 Da) oligomers were also observed by electrospray ionization-mass spectrometry in reactions under extremely high acid concentration conditions (86 wt%). Gas-particle experiments using a reaction cell were carried out using both high (∼20 ppmv) and low (∼900 ppbv) gas-phase octanal concentrations under a wide range of relative humidity (RH, from <1% to 50%, corresponding to >80 wt% to 43 wt% H₂SO₄) and long reaction durations (24 h). One or multiple steps of aldol condensation occurred under low RH (<1% and 10%, >80 wt% and 64 wt% H₂SO₄, respectively) and high octanal concentration (∼20 ppmv) conditions. No cyclotrimerization was observed in the gas-particle experiments even under RH conditions corresponding to similar sulfuric acid concentration conditions that favor cyclotrimerization in bulk studies. No accretion reaction product was found in the low octanal concentration (∼900 ppbv) experiments, which indicates that the accretion reactions are not significant as expected when the gas-phase octanal concentration is low. A kinetic analysis of the first-step aldol condensation product was performed to understand the discrepancies between the bulk and gas-particle experiments and between the high and low octanal concentrations in the gas-particle experiments. The comparisons between experimental results and kinetic estimations suggest that caution should be exercised in the extrapolation of laboratory experiment results to ambient conditions.