Humidity effects on photochemical aerosol formation in the SO2-NO-C3H6-air system |
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| Affiliation: | 1. Department of Biosystems Engineering, College of Agriculture and Natural Resources, University of Mohaghegh Ardabili, Ardabil, Iran;2. Faculty of Health, Engineering and Sciences, University of Southern Queensland, Toowoomba, QLD 4350, Australia;1. Silesian University of Technology, Department of Power Engineering and Turbomachinery, Konarskiego 18, 44-100 Gliwice, Poland;2. Silesian University of Technology, Materials Research Laboratory, Konarskiego 18a, 44-100 Gliwice, Poland;1. Department of Mechanical Engineering, Birla Institute of Technology, Mesra, Ranchi, India;2. Department of Mathematics, Birla Institute of Technology, Mesra, Ranchi, India;1. Environmental Science and Technology Group (ESTg), Chemical Engineering Department, School of Engineering, Nazarbayev University, Astana, 010000, Kazakhstan;2. School of Mining and Metallurgical Engineering, National Technical University of Athens, Athens, Greece;3. School of Pharmacy and Biomolecular Sciences, University of Brighton, UK |
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| Abstract: | In order to investigate the effects of humidity on the gas-phase oxidation of SO2 in polluted air and on the subsequent aerosol formation process, photoirradiation experiments were carried out by means of a 4-m3 chamber, in which mixtures containing SO2, NO and C3H6 with concentrations in the ppm range were exposed to simulated solar radiation in different relative humidity (r.h.) conditions. The total amount of oxidized SO2 was quantified from the SO42− yield determined by the chemical analysis of the aerosol product, and a part due to the oxidation by the OH radical was evaluated by estimating the OH concentration from the decay rate of C3H6. The remaining part was assigned to the oxidation by the Criegee intermediate, as it had a good correlation with the progress of the O3 + C3H6 reaction. The contributions of the two oxidizing species to the total conversion and the oxidation rate of SO2 were measured as functions of r.h. As a result, experimental evidence was obtained for the prediction of Calvert and Stockwell's (1983, Envir. Sci. Technol. 17, 428A–443A) simulation that the oxidation due to the Criegee intermediate was retarded by the increase in humidity. The OH contribution, on the other hand, was almost independent of r.h. It was observed consequently that the total oxidized amount of SO2 considerably decreased as r.h. was higher.The humidity effect on the aerosol formation process was found to be more complicated than the effect on the gas-phase chemistry. The maximum rate of increase in the particle number concentration rose linearly with increasing r.h., but the number concentration itself measured at its maximum or at the end of the irradiation reached a ceiling value around r.h. = 30% and went down for higher r.h. The average panicle size in the final stage of the reaction showed a minimum around the same r.h. at which the number concentration was maximum. The H2SO4 concentration in the mist particles, however, decreased monotonically as r.h. got higher. It was suggested that these different responses against the increase in humidity resulted from the cooperation of several processes such as the H2SO4 monomer formation, the H2O condensation, the particle coagulation, etc., which had different dependences on r.h. |
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