Photochemistry - aqueous phase

In contrast to gas-phase photochemistry, aqueous-phase and solid-phase photochemistry are treated differently. For example, the Beer-Lambert Law for homogenous liquid systems is expressed simply as ... 

It is possible that colloidal photochemistry will provide a new approach to prebiotic syntheses. The work described previously on redox reactions at colloidal ZnS semiconductor particles has been carried on successfully by S. T. Martin and co-workers, who studied reduction of CO2 to formate under UV irradiation in the aqueous phase. ZnS acts as a photocatalyst in the presence of a sulphur hole scavenger oxidation of formate to CO2 occurs in the absence of a hole scavenger. The quantum efficiency for the formate synthesis is 10% at pH 6.3 acetate and propionate were also formed. The authors assume that the primeval ocean contained semiconducting particles, at the surface of which photochemical syntheses could take place (Zhang et al 2007). 

The net enhancement factor for a droplet consisting of pure water can be as much as 1.6 (Madronich, 1987). Calculations by Ruggaber et al. (1997) suggest that the actinic flux inside cloud drops with a typical size distribution and dissolved particulate matter is more than a factor of two greater than in the cloud interstitial air. This effect of enhanced actinic flux inside droplets may be quite important for aqueous-phase photochemistry in fogs and clouds. 

As noted above the polymorphic Fe(III) oxides are potential photochemical catalysts in environmental systems. Many investigations of the photochemical reactivity of these naturally occurring compounds have been carried out over the last 15 years. Hoffmann et al. [137], Waite [138] and Parmon and Zakharenko [ 139] have provided reviews of these processes. In addition, Hoffmann and co-workers [136,140-149] have investigated the chemistry and photochemistry of Fe(III) and Fe(II) compounds and aqueous-phase species in relation to chemical transformations in clouds, fogs, and aquated haze aerosols. 

Nitrite is usually present in the environment at a lower concentration than nitrate, but its higher molar absorptivity and photolysis quantum yield can make it a competitive photoreactant under environmental conditions [13]. Usual concentration values of nitrite are below 2 jiM in seawater [13], below 0.1 mM in surface waters [7] and around 0.1-0.5 xM in the atmospheric aqueous phase in unpolluted areas [9]. Nitrite concentration was, however, found to reach up to 75 xM in fog water from California s Central Valley, and nitrite photochemistry was shown to account for 50-100% ofhydroxyl formation upon irradiation of the collected water samples [14],... 

Nitrate and nitrite photochemistry might also play a role in atmospheric hydrometeors. Nitrite photolysis has been shown to account for the majority of hydroxyl photoformation in irradiated fog water from a polluted site [ 14]. In addition, the generation of mutagenic and carcinogenic compounds from amino acids and amines dissolved in fog water [147] is a process that can be linked with nitrite photochemistry [20,141]. Furthermore, the formation of atmospheric nitrophenols partially takes place in aqueous solution. Reactions in the aqueous phase can account for about 30% of the atmospheric sources of mononitrophenols and for the vast majority of the dinitrophenol ones [ 148], and irradiation of nitrate and nitrite can possibly play a role in the process (see Sect. 3.2). Mono- and dinitrophenols are toxic compounds, and their occurrence in rainwater is thought to be a contributory factor in forest decline [149-151]. 

But of course there can also be homogeneous aqueous phase photochemistry of ozone — that is well known. 

Anastasio C, Allen JM, Faust B (1994) Aqueous-phase photochemical formation of peroxides in authentic cloud waters. In Helz GR, Zepp RG, Crosby DG (eds) Aquatic and surface photochemistry. Lewis, Boca Raton, chap 18 Faust BC, Allen JM (1992) J Geophys Res 97 12, 913 Faust BC, Anastasio C, Allen JM, Arakaki T (1993) Science 260 73 Faust BC, Hoigne J (1990) Atmos Env 24 A 79... 

Surface waters are diverse in nature. They might be near shore or Inland wetland environments or mld-oceanlc ollgotrophlc water. Until recently, sunlight Induced photochemistry was not recognized as an Important pathway for the transformation of natural and anthropogenic chemicals In surface waters. It Is now well established that photochemlcally mediated processes are Important In most, If not all, areas of aqueous phase environmental chemistry. Both direct, primary, and Indirect photoprocesses have been documented In natural waters. 

Clouds are multiphase chemical systems coupling the chemistry in the gas phase to that in the aqueous phase. Small clouds do not greatly ahenuate the intensity of solar radiation and the gas-phase photochemistry continues, but reaction rates change because of the redistribution of gases in the presence of liquid water. 

Tropospheric Aqueous Phase Bulk Photochemistry 4.1 Introduction... 

Iron complex photolysis is mie of the processes that produce reduced iron (Fe(n)) in a highly oxidizing enviromnent like the atmospheric aqueous phase. There are numerous other processes such as reactions with HO species or Cu(I)/ Cu(n) which can reduce or oxidize iron in the troposphere. These reactions can take place simultaneously and cause iron to undergo a so-caUed redox-cycling [167]. Because of the large number of complex interactions in the atmospheric chemistry of the transition metal iron, it is useful to utilize models to assess the impact of the complex iron photochemistry. 

Figure 14 shows simulated concentration time profiles for the Fe(III) ligands pyruvate and oxalate, which have mostly lower concentrations during the daytime, when the photochemistry as described here is active. Thus, it has to be emphasized that Fe(III) complex photolysis reactions can be a major sink for the carboxylate species besides radical reactions, and it is crucial not to neglect these reactions when the fate of carboxylic acids in the atmospheric aqueous phase is considered. 

Volkamer R, Ziemaim PJ, Molina MJ (2009) Secondary organic aerosol formation fi om acetylene (C(2)H(2)) seed effect on SOA yields due to organic photochemistry in the aerosol aqueous phase. Atmos Chem Phys 9(6) 1907-1928... 

A. L. Sobolewski and W. Domcke, Phys. Chem. Chem. Phys., 9,3818-3829 (2007). Computational Studies of Aqueous-Phase Photochemistry and the Hydrated Electron in Finite-Size... 

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