Photochemical Formation of Water-Aerosol/Droplet in Air: Optical Manipulation and Reaction Mechanism

Photo-induced nucleation of water vapor in air was reported many years ago [1], but did not draw much attentions by the atmospheric scientists. We report a photochemical formation of water droplet/aerosol in wet air at different conditions, its optical manipulation, and we propose a comprehensive reaction mechanism by observing reaction intermediates in combination with theoretical simulations. Since aerosol/droplet formation is induced by photons, it can be manipulated by changing of wavelengths, duration of irradiations, light intensities, pulse repetition rates, etc. The wavelengths of light applied were 185 nm (Hg lamp), 193 nm (ArF laser), and 248 nm (KrF laser). Water aerosol/droplets were produced in air-filled 1 atm-reaction vessels with different temperature and relative humidity (RH) conditions and were observed by light scattering, mass spectrometry, and photographic recording. Particle sizes were heavily dependent on humidity from 50nm to ca.0.1mm. At the sub-micron levels the number of particles produced increased significantly (two orders of magnitude) by manipulating durations of irradiation-intermission cycles of Hg lamp. The growth of particle sizes was also observed by sequential irradiations. The mechanism of the reaction is proposed with experimental confirmation and theoretical simulations as follows. Photo-dissociation of oxygen produces ozone and the latter is photo-dissociated by a second photon to an active singlet-oxygen (1D). It reacts with a water molecule and produces OH radicals and further dark reactions give a final stable molecule of hydrogen peroxide (H2O2). This works as a seeding nucleus since the vapor pressure of hydrogen peroxide is ca. two orders of magnitude smaller than water. The experimental observation of HO2 as a reaction intermediate was made in situ by chemical amplification method combined with laser-induced fluorescence [2]. The experimental observation was successfully simulated by solving differential equations with 30 related elementary reactions [3]. Interesting dependence of laser intensity and repetition rates was simulated on the amounts of intermediates and the final product. Particle formation and their dynamics were recognized almost at any temperature and humidity conditions. For example, even under very hot and dry conditions, like 50 degree centigrade and RH 10%, a large amount (>1,000,000/ml) of particles were formed by a KrF laser. This laser could be important due to its high transmittance in the air. Reference [1] C.T.B. Wilson, Proc. Camb. Phil. Soc. 9, 392 (1987). More recently, F.C. Wen, T. McLaughlin, J.J. Katz, Science, 200, 769 (1978). [2] Y. Sadanaga, J. Matsumoto, K. Sakurai, R. Isozaki, S. Kato, T. Nomaguchi, H. Bandow, Y. Kajii, Rev. Sci. Instr., 75, 864 (2004). [3] K. Yoshihara, Y. Takatori, K. Miyazaki, Y. Kajii, Proc. Jpn. Acad, B83, 320 (2007).