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A chemical reaction chamber system has been developed for the processing of\udoleic acid aerosol particles with ozone under two relative humidity\udconditions: dry and humidified to 65 %. The apparatus consists of an\udaerosol flow tube, in which the ozonolysis occurs, coupled to a scanning\udmobility particle sizer (SMPS) and an aerosol time-of-flight mass\udspectrometer (ATOFMS) which measure the evolving particle size and\udcomposition. Under both relative humidity conditions, ozonolysis results in\uda significant decrease in particle size and mass which is consistent with\udthe formation of volatile products that partition from the particle to the\udgas phase. Mass spectra derived from the ATOFMS reveal the presence of the\udtypically observed reaction products: azelaic acid, nonanal, oxononanoic\udacid and nonanoic acid, as well as a range of higher molecular weight\udproducts deriving from the reactions of reaction intermediates with oleic\udacid and its oxidation products. These include octanoic acid and 9- and\ud10-oxooctadecanoic acid, as well as products of considerably higher\udmolecular weight. Quantitative evaluation of product yields with the ATOFMS\udshows a marked dependence upon both particle size association (from 0.3 to\ud2.1 µm diameter) and relative humidity. Under both relative humidity\udconditions, the percentage residual of oleic acid increases with increasing\udparticle size and the main lower molecular weight products are nonanal and\udoxononanoic acid. Under dry conditions, the percentage of higher molecular\udweight products increases with increasing particle size due to the poorer\udinternal mixing of the larger particles. Under humidified conditions, the\udpercentage of unreacted oleic acid is greater, except in the smallest\udparticle fraction, with little formation of high molecular weight products\udrelative to the dry particles. It is postulated that water reacts with\udreactive intermediates, competing with the processes which produce high\udmolecular weight products. Whilst the oleic acid model aerosol system is of\udlimited relevance to complex internally mixed atmospheric aerosol, the\udgeneric findings presented in this paper give useful insights into the\udnature of heterogeneous chemical processes

Size-dependent chemical ageing of oleic acid aerosol under dry and humidified conditions