Inter-comparison of the Regional Atmospheric Chemistry Mechanism (RACM2) and Master Chemical Mechanism (MCM) on the simulation of acetaldehyde

Acetaldehyde (CH ₃CHO) is a key player of atmospheric chemistry, an important air pollutant, and hence a major target of air quality modeling and management. The Regional Atmospheric Chemistry Mechanism (RACM) is a highly lumped gas-phase chemical mechanism that has been widely applied in atmospheric chemistry modeling studies. A significant update of the latest version of RACM (RACM2) is the addition of CH ₃CHO as an explicit aldehyde species, facilitating the direct simulation of CH ₃CHO. In this study, we compared the performances of RACM2 and Master Chemical Mechanism (MCM; v3.3.1) on the simulation of CH ₃CHO. Zero-dimensional chemical box models based on these two independent mechanisms were prescribed to a polluted scenario to simulate the evolution of ozone (O ₃), hydroxyl radical (OH), C ₂H ₅O ₂ (ETHP) and CH ₃CHO, as well as their detailed chemical budgets. Overall, both mechanisms agreed with the simulation of O ₃ and OH, but the RACM2 model simulated significantly higher levels of ETHP and CH ₃CHO than the MCM model. The difference in the chemical kinetic data in both mechanisms is not the reason for this discrepancy. The oversimplification of the lumped peroxy acyl radicals (RCO ₃) and ≥C3 aldehydes chemistry of RACM2 should be responsible for its higher simulated ETHP and CH ₃CHO. We caution the use of RACM2 or any other highly aggregated chemical mechanism for the simulation of C ₂H ₅O ₂ and CH ₃CHO. Better methods are needed to represent the chemistry of peroxy acyl radicals and ≥C3 aldehydes for aggregated chemical mechanisms. More experiments are required to directly validate and further improve the current chemistry mechanisms.