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The Sun Shines on Water and Atmospheric Aerosols

Angela-Guiovana Rincon
W. M. Keck Laboratory, California Institute of TechnologyThe "free" energy carried by sunlight photons can drive accretion processes, such as photosynthesis, and degradative ones, such as the breakdown of air and water components. There is a growing interest in the development of new processes for water disinfection since the traditional processes, such as chlorination, generate toxic by-products. Sunlight is a promising alternative for water disinfection and Photocatalytic treatments based on innocuous titanium dioxide (TiO2) as ck&sensitizer vastly improve the disinfection efficiency of conventional solar water treatment. Photoexcited TiO2 induces the formation of highly reactive species, such as hydroxyl radicals, which universally inactivate bacteria and degrade organic pollutants.Key parameters influencing process efficiency involve physicochemical, biological and engineering aspects which are illustrated by laboratory and field scale experiments on solar disinfection. The sensitivity of bacteria to solar disinfection, in the absence or presence of TiO2, depend on microorganism species, strain, growth stage, cultivation medium and initial bacterial load. Disinfection results are affected by plating media used for bacterial cultivation and counting. Physicochemical parameters and reactor design also influence the process. A residual disinfection effect in the dark after solar treatment could only be observed in the presence of TiO2. Compliance with disinfection system standards require establishing the duration of the irradiation period, or effective disinfection time (EDT), that achieves full (> 99%) disinfection under specific conditions. EDT is strongly dependent on light intensity.The Earth's radiative balance is largely controlled by scattering and absorption of incoming sunlight by tropospheric aerosols. Their optical properties in the e>300nm range are essentially determined by their chemically complex "black" and "brown" natural and anthropogenic organic fractions. This phenomenon has moved to the forefront of climate change discussions, particularly after the failure of the Copenhagen summit, because it may ultimately provide an emergency geoengineering tool to control global warming via sulfate aerosol seeding.Atmospheric aerosols are exposed half of the time to intense sunlight. The key role of sunlight in their transformations is presented here using model organic aerosol. Photolysis of aqueous pyruvic acid (a surrogate for aerosol v-dicarbonyls absorbing at e>300nm) generates mixtures of identifiable aliphatic polyfunctional oligomers that develop absorptions in the visible upon standing in the dark. These absorptions can be repeatedly bleached and retrieved without carbon loss or changes in their electrospray ionization mass spectra. These observations, in conjunction with the evidence of supramolecular interactions among components of the mixture, together the non-linearity of Beer's law plots at e>350nm strongly suggest that full speciation is insufficient and possibly unnecessary for understanding the optical properties of the organic aerosol.Further studies in the presence of ammonium bisulphate indicate that the optical absorptivity of tropospheric aerosol particles is not an intrinsic property but a function of time that will respond to changes in insolation, ambient temperature and relative humidity. These phenomena and the time scales are consistent with the daily cycles of aerosol scattering and absorption observed over Mexico City.

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Contact: Evangeline Reyes