Parameterising secondary organic aerosol from α-pinene using a detailed oxidation and aerosol formation model
A new parameter model for α-pinene secondary organic aerosol (SOA)\nis presented, based on simulations with the detailed model BOREAM\n(Biogenic hydrocarbon Oxidation and Related Aerosol formation Model).\nThe parameterisation takes into account the influence of temperature,\ntype of oxidant, NOx-regime, photochemical ageing and water uptake,\nand is suitable for use in global chemistry transport models. BOREAM\nis validated against recent photooxidation smog chamber experiments,\nfor which it reproduces SOA yields to within a factor of 2 in most\ncases. In the simple chemical mechanism of the parameter model, oxidation\nof α-pinene generates peroxy radicals, which, upon reaction with\nNO or HO2, yield products corresponding to high or low-NOx conditions,\nrespectively. The model parameters – i.e. the temperature-dependent\nstoichiometric coefficients and partitioning coefficients of 10 semi-volatile\nproducts – are obtained from simulations with BOREAM, including a\nprescribed diurnal cycle for the radiation, oxidant and emission\nlevels, as well as a deposition sink for the particulate and gaseous\nproducts. The effects of photooxidative ageing are implicitly included\nin the parameterisation, since it is based on near-equilibrium SOA\nconcentrations, obtained through simulations of a two-week period.\nIn order to mimic the full BOREAM model results both during SOA build-up\nand when SOA has reached an equilibrium concentration, the revolatilisation\nof condensable products due to photochemical processes is taken into\naccount through a fitted pseudo-photolysis reaction of the lumped\nsemi-volatile products. Modelled SOA mass yields are about ten times\nhigher in low-NOx than in high-NOx conditions, with yields of more\nthan 50% in the low-NOx OH-initiated oxidation of α-pinene, considerably\nmore than in previous parameterisations based on smog chamber experiments.\nSensitivity calculations indicate that discrepancies between the\nfull model and the parameterisation due to variations in assumed\noxidant levels are limited, but that changes in the radiation levels\ncan lead to larger deviations. Photolysis of species in the particulate\nphase is found to strongly reduce SOA yields in the full model. Simulations\nof ambient conditions at 17 different sites (using oxidant, radiation\nand meteorological data from a global chemistry-transport model)\nshow that overall, the parameterisation displays only little bias\n(2%) compared with the full model, whereas averaged relative deviations\namount to about 11%. Water uptake is parameterised using fitted activity\ncoefficients, resulting in a good agreement with the full model.