Clearing the atmosphere from gaseous pollution and aerosols occurs due to the water cycle. As a rule, water in the atmosphere is represented in three aggregation states: gas, liquid and solid. Recently, increased attention is focused on clusters which are formed due to hydrogen bonding. Clusters, especially aggregates of small size, can be considered as a special state of substance. Water vapor as the main representative of water in the atmosphere contributes significantly to its clearing. Clusters are formed from water vapor and subsequently they form water drops or snowflakes which fall as precipitation. At their forming stage and during their subsequent presence in the atmosphere, both clusters and larger formations absorb molecules of pollution substances. Because of a high number of molecules on the surface compared to the number of bulk molecules, clusters are the most active absorbents of molecules. Cluster’s reaction to external electromagnetic radiation is defined by the frequency of vibrations of the total dipole moment of the cluster. For clusters of smaller sizes (up to 100 molecules) this frequency hardly changes with the change of cluster size. Hence, all clusters have energetically close responses, and the bigger the cluster the lower activity per molecule to radiation it has. As a result, at cluster formation from molecules the ability to absorb and disperse radiation reduces. Hence, as a whole, the clusterization should be accompanied by a cooling effect. Aerosols also contribute to clearing of the atmosphere: they absorb gaseous pollution, and then together with the deposits which they have absorbed, come back to the Earth.

Influence of water clusters on formation of IR spectra and their participation in creation of a greenhouse effect has been discussed for a long time [1-4]. Population of the atmosphere by water dimers is supposed at modeling of water recondensation [5]. Calculations specify, that water dimers can exist in significant concentration ((~ 10¹⁶ сm^-3 at 313 K and 100 % relative humidity) and influence physical and chemical processes in the atmosphere [6]. The spectra of air received with the use of ionic spectrometer, show the presence of ionic clusters of the size about 1 nm with almost constant concentration [7]. Until recently the existence of atmospheric clusters not carrying electric charge (neutral) was represented as an open question. It is caused by the difficulty of their direct detection. Experimental confirmation of the presence of neutral clusters in the atmosphere demands usage of unique counters for condensation particles [8]. The usage of equipment for cross chemical ionization in experiments on nucleation in ternal system already has revealed a big number of neutral clusters [9]. Water clusters that absorbed SO₃ molecules prove themselves by catalytic effect at formation of sulfuric acid [10] and by formation of liquid aerosols in the atmosphere.

References

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2.       Gebbie H.A. Observations of anomalous absorption in the atmosphere. in Atmospheric water vapor, A. Deepak, T.D. Wilkerson and L.H. Ruhnke, Ed. New York: Academic Press, 1980, pp. 133–141.

3.       Low G.R., Kjaergaard H.G. Calculation of OH-stretching band intensities of the water dimer and trimer // J. Chem. Phys. 1999. V. 110. pp. 9104–9115.

4.       Goss L.M., Sharpe S.W., Blake T.A., Vaida V., Brault J.W. Direct absorption spectroscopy of water clusters // J. Phys. Chem. A 1999. V. 103. pp. 8620–8624.

5.       Slanina Z., Crifo J.F. A refined evaluation of the gas-phase water-dimerization equilibrium constant within non-rigid BJH- and MCY-type potentials // Int. J. Thermophys. 1992. V. 13 pp. 465–476.

6.       Goldman N., Fellers R.S., Leforestier C., Saykally R.J. Water dimers in the atmosphere:  equilibrium constant for water dimerization from the VRT(ASP-W) potential surface // J. Phys. Chem. A, 2001, V. 105. pp 515–519.

7.       Kulmala M., Pirjola L., Makela J.M. Stable sulphate clusters as a source of new atmospheric particles // Nature 2000. V. 404. pp. 66–69.

8. Kulmala M., Lehtinen K.E.J., Laakso L., Mordas G., Hameri K. On the existence of neutral atmospheric clusters // Boreal Env. Res. 2005. V. 10. pp. 79–87.
9. Hanson D.R., Eisele F.L. Measurement of prenucleation clusters in the NH₃, H₂SO₄, H₂O system // J. Geophys. Res. 2002. V. 107. doi:10.1029/2001/JD001100.
10. Akhmatskaya E.V., Apps C.J., Hillier I.H., Masters A.J., Watt N.E., Whitehead J.C. Formation of H₂SO₄ from SO₃ and H₂O, catalysed in water clusters // Chem. Commun. 1997. V. 107. P. 707.