Organic aerosol growth by acid-catalyzed heterogeneous reactions of octanal in a flow reactor

Abstract Octanal was chosen as a model carbonyl compound to study aerosol growth by the heterogeneous acid-catalyzed reactions. The heterogeneous reactions of octanal in the presence/absence of acidified seed aerosols were conducted in a 2 m flow reactor (2.5 cm ID) under darkness in the presence of background seed aerosols. To facilitate hemiacetal/acetal formation via the acid-catalyzed heterogeneous reaction of octanal, 1-nonanol was also co-injected with inorganic seed aerosols into the flow reactor system. The aerosol population was measured from a series of sampling ports down the flow reactor as a function of distance using a scanning mobility particle sizer (SMPS). These results were used to predict the apparent rate constants for heterogeneous reactions of octanal, optimizing experimentally observed aerosol growth from condensation and heterogeneous reactions. The aldehyde heterogeneous reactions were accelerated in the presence of an acid catalyst (H 2 SO 4 ), and led to higher aerosol yields, than when H 2 SO 4 was not present in the seed aerosol. Our results showed that the log of the organic aerosol yield is inversely related to humidity in the presence of an acid-catalyst ( R 2 =0.98). When aerosols were not catalyzed with an acid, aerosol growth %yields were neither sensitive nor linear with %RH ( R 2 =0.18). These results may be explained by the fact that acid-catalyzed heterogeneous reactions of aldehydes are significantly accelerated as the acidity increases. The SMPS was also operated in two different aerosol-sampling flow rates to characterize particle off-gassing while particles were inside the SMPS. The off-gassing of organic compounds from the particle phase was smaller with an acid catalyst than without an acid catalyst. Fourier transform infrared spectrometry (FTIR) was employed to demonstrate the direct transformation of chemical functional groups by acid-catalyzing the reactions of octanal with 1-nonanol as a thin liquid layer on a zinc selenide (ZnSe) FTIR disk. It was concluded that carbonyls, which are produced by atmospheric photochemical oxidation reactions, can significantly contribute to secondary organic aerosol formation through acid-catalyzed heterogeneous reactions.

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