Evaluation of isoprene photooxidation using smog chamber data and chemical mechanisms

Chemical mechanisms are used in air quality models to simulate chemical reactions occurring in the atmosphere. Volatile organic compounds (VOCs), emitted from both anthropogenic and biogenic sources, may form photochemical smog as well as forming or partitioning onto existing organic aerosols. The chemical mechanisms used to simulate these processes are being constantly updated as new information becomes available such as reaction rate or reaction product yields. These mechanisms also need to be validated against experimental data to ensure that the mechanisms are able to accurately simulate important atmospheric events. We evaluate the ability of two different chemical mechanisms to simulate the formation and reaction of smog-producing species from isoprene. Isoprene is of particular importance as it is the most abundant non-methane hydrocarbon emitted into the atmosphere. Experiments were conducted using various initial conditions with isoprene and NOx (nitrogen oxides) in the CSIRO environmental chamber. Two chemical mechanisms were evaluated, the SAPRC mechanism (SAPRC-99) and the Master Chemical Mechanism (MCMv3.1). Both chemical mechanisms were able to predict the oxidation of isoprene and the formation of ozone accurately during the initial stages of the experiments. Other important organic species such as formaldehyde, methacrolein, methyl vinyl ketone and peroxyacetyl nitrate were observed. However the final Δ(O₃-NO) concentration was under-predicted by 15% by both mechanisms. Possible reasons for the under-prediction in final ozone by the chemical mechanisms were investigated, and high peroxy acyl nitrate formation was determined to be the most likely reason for this under-prediction.