Some of the atmospheric processes leading to aerosol formation and growth, that are described in the foregoing discussion, require a photo-excitation in order to be initiated. For instance, the photolysis of ozone or water produces the highly reactive OH-radical, which, via the oxidation of sulphates and VOCs, generate sulfuric and organic acids. As discussed in Sec. 2.3.5 and 2.4.3, these species are known to strongly assist the formation of aerosols in the atmosphere. This photo-chemical trigger is of course provided by the UV component of solar radiation, whose photons are the most energetic of solar spectrum. Such photo-excitation features however a low efficiency, due to the fact that, in this spectral region, the solar radiation is also absorbed by oxygen and ozone (see Sec. 1.3.1).
As soon as intense light sources such as gas-discharge lamps and, later, lasers, were available, the ability of light to induce gas-to-particle conversion has been investigated in laboratory experiments. This research field has been pioneered by Tyndall [107] in 1869, who observed the formation of visible particles out of a mixture of air and alkyl nitrites vapor, under the action of light emitted by an arc lamp. Unfortunately, he was not able to observe any particle production from a gas mixture more typical of the natural atmosphere, such as pure air and water vapor.
This result was achieved by Wilson [39], about 30 years later, who pursued Tyn-dall’s studies focusing on the nucleation of water vapor by illumination of several
2.6 Photo-induced nucleation
energetic radiations such as X-rays, α particles and also UV light emitted by a spark between zinc terminals. Wilson itself pointed out that its method was simi-lar to Tyndall’s one, the only difference being "the fact that", in his experiments,
"the rays from the arc lamp were allowed to traverse no material such as glass, which is opaque to the ultra-violet rays, before entering the tube containing the moist air".
Wilson developed an apparatus for the study of photo-induced nucleation, con-sisting in a sealed glass tube with quartz windows at its ends, filled with a non-condensible carrier gas mixed with water vapor. This gas mixture could be driven into supersaturation by adiabatic expansion; a window allowed for the visual in-spection of the generated fog13.
The classical theory for homogeneous nucleation predicts that the threshold for nucleation of water vapor lies betweenS=3 andS=4, as it has been shown in Sec. 2.3. By illuminating the gas with increasing UV irradiation dose, Wilson re-ported of a significant reduction of the supersaturation needed for the appearance of droplets in the cloud chamber. Surprisingly, Wilson observed the formation of fog for saturation ratio exceeding unity only by few percent; moreover, the generated droplets were stable, as they still persisted few hours after ceasing the illumination.
Further experiments conducted with the same type of apparatus lead Wilson to conclude that "pure oxygen and water vapor alone were sufficient to enable a cloud to be produced under the influence of ultra-violet light". From the unex-pected stability of the generated droplets, far below the limit for homogeneous nucleation for water, he argued that the vapor pressure of water droplets was low-ered due to the dissolution of a photo-chemical product of the species present in the chamber, i.e. water and oxygen. Therefore, he suggested that this compound could be hydrogen peroxide.
These observations on the production of condensation nuclei in gases under the action of UV rays opened a new line of research, pursued by many researchers that repeated and refined its experiments. Wilson conclusions on the crucial role played by oxygen in the nucleation of new droplets and the neutrality of generated nuclei were further supported by Farley [108], and by Wen and co-workers14[14].
In this latter work, the observation of photo-induced clouds at saturation ratio as low as 1.00042, i.e. practically just saturated, is reported. A UV power density of
13This original version of his apparatus was an ancestor of the cloud chamber for which he was awarded the Nobel Prize in 1911.
14In the cited paper they concluded that oxygen did not participate to the photo-nucleation process, as they did not detect any change in the nucleation rate when 5 torr of O2were added to the chamber. However, in a later publication [109], they admitted that the chamber could have been contaminated with an unidentified water soluble impurity, that might have also been molecular oxygen.
photo-induced processes 0.2 W/m2 was required in order to induce the nucleation of water vapor at such low supersaturation. The delay time for the start of nucleation was a function of light intensity (cf Fig.3 in Ref [14]); while at the highest level the nucleation started after 2 s, no droplet formation could be detected beyond ∼20 s for any irradiation level. The same time scales were observed for the ceasing of nucle-ation after ceasing illuminnucle-ation. These results were in agreement with Farley and Wilson observations.
Wen and co-authors suggested a mechanism relying on the clustering of photo-excited water molecules to explain the formation of condensation nuclei, summa-rized as follows:
This pathway is initiated by the excitation into a higher energy state of few H2O molecules upon irradiation with UV light, that may be subsequently embedded into clusters together with un undefined number of neutral molecules. The clusters that aggregate a critical numbercof excited molecules, can serve as condensation nuclei.
In Wen’s experiment, light with wavelength between 200 and 320 nm was used to trigger the photo-nucleation; in this spectral range, no known absorption of H2O had been reported at that time. This left the important open question of what ac-tually does absorb light, in order to produce the excited water molecule.
Several years later, Byers Brown [110] suggested that this unknown absorber could be a complex of water and oxygen. He re-considered the photo-nucleation spectra obtained by Wen et al. in the light of experimental measurements, by Heidt and co-authors, of the absorption of oxygenated liquid water in the same spectral range (200−260 nm). These two quantities are plotted together in Fig. 2.17, showing a similar wavelength dependence. Byers Brown’s new idea was to as-sume that the hydrated oxygen molecule also exists in the gas phase, featuring a similar absorption spectrum. He proposed the hypothesis that "UV is absorbed by collision (van der Waals) complex of water and oxygen, H2O·O2, to form a charge-transfer complex H2O+O2− " [110]. Such complex, in the vapor phase, is expected to immediately attract water molecules thus giving rise to clusters that he referred to as Wilson clusters. The subsequent chemistry of said clusters is not investigated by Byers Brown, although he suggested that they might lead to the production of condensation nuclei of hydrogen peroxide, as it was originally argued by Wilson.
2.6 Photo-induced nucleation
Figure 2.17: Left hand axis: UV light intensity required to induce a nucleation rate of 1 drop per cm3 per s, as a function of the wavelength, for water vapor in a cloud chamber atT =297 K and saturation ratioS=3.059. Experimental data obtained by Wen et al. [14]. Right hand axis: Absorption spectrum of oxygenated liquid water atT =297 K, measured by Heidt et al. [15]. Reprint from [16].
While the mechanism proposed by Byers Brown could explain the crucial role of oxygen in the photo-nucleation of water vapor as observed in cloud chamber experiments, extending the validity of this pathway to the atmospheric formation of aerosols, as the author suggests in another published work [16], might be ques-tioned.
The role of hydrogen peroxide as photo-induced condensation nucleus has been also proposed in a very recent work by Yoshihara et al. [111], which reports of photo-induced water aerosol formation at 248 nm, upon irradiation with a KrF laser. Owing to the technology of aerosol detectors nowadays available, they were able to count and size particles down to a diameter of 10 nm. By illuminating the moist air in a reaction vessel, they observed aerosol formation for temperatures be-tween 0 and 50◦C and for relative humidities even much lower than 100%. They suggest a reaction mechanism started by photo-dissociation of oxygen, involv-ing the subsequent formation of ozone and its dissociation to OH-radicals, which form H2O2as a final stable product with a vapor pressure two orders of magnitude smaller than that of water. It might thus be expected to stabilize condensing
wa-photo-induced processes ter molecules and thus contribute to aerosol formation, although it appears, from simulated reaction kinetics, that its concentrations are too low to prevent the evap-oration of freshly formed clusters.