Comprehensive Treatment of C₄-C₆ Alkanes and Their Oxidation Products in CAM-chem: The MOZART-T3 Mechanism

Abstract. Most global atmospheric chemistry models represent ≥C₄ alkanes using lumped surrogates, limiting both detailed simulation of their oxygenated products and evaluation against comprehensive observational datasets. We present MOZART-T3, which replaces the lumped BIGALK representation with a more explicit treatment of individual C₄−C₆ alkane species and resolves propane peroxy radical isomers, enabling more mechanistically consistent alkane chemistry in the Community Atmosphere Model with chemistry (CAM-chem). Global simulations demonstrate that T3 maintains similar total alkane burdens compared to previous mechanisms, while substantially altering oxygenated product budgets and distributions. Relative to MOZART-T1, T3 significantly reduces the global burden of methyl ethyl ketone (MEK) primarily through incorporation of more comprehensive n-butane oxidation chemistry, with additional contributions from increased photolysis rates and updated emission speciation. T3 introduces six additional C₅−C₆ ketone species that contribute ∼40% to global ketone sources but only ∼2% to the total burden due to their short lifetimes. The acetaldehyde burden decreases by 8−14% through compound-specific yields that replace the fixed yield in previous mechanisms. The choice of anthropogenic emission inventory drives larger variations in alkane burdens (∼24%) than does mechanism complexity (∼4%), but mechanism choice dominates for oxidation products. T3 enables evaluation of previously unrepresented species including individual alkanes, propanal, and peroxypropionyl nitrate, with generally improved simulation of oxygenated compounds, although the evaluation results vary temporally and spatially. While lumped approaches sufficiently represent global-scale major pollutant concentrations, T3’s detailed treatment enables more comprehensive evaluation and is expected to be more important for urban air quality applications using higher-resolution regional simulations.