The crystallochemical characterization of magnetic spinels prepared from aqueous solution has been studied primarily by electron microscopy and 57Fe Mossbauer spectroscopy. Two synthetic routes have been investigated : method 1 ; partial oxidation of Fe" solutions in the presence of NO; at 100 O C , and method 2; reaction of hydrated Ferrr oxide (ferrihydrite) with Fe" ions at room temperature, pH = 7, 8 and 9. Both routes gave small (10-60 nm) irregular magnetite (Fe,O,) particles that were readily oxidised to maghemite (y-Fe,O,). The reaction proceeded via a green-rust intermediate and anions such as inorganic phosphate (Pi) and SO,,-reduced the rate of transformation. Spinel formation at room temperature was severely inhibited by Pi levels of 1 mol%, whereas 20-30 mol % Pi was required for retardation of crystallization at 100 "C. Intermediate levels (5-10 mol YO) resulted in morphological enhancement of the octahedral crystal habit. A similar effect was observed in the presence of SO:-and at neutral pH in method 2.

The interaction of SO(3) and H(2)O at low temperatures upon an inert surface has been studied with infrared spectroscopy and compared to the predictions of recent computational studies. At low temperatures and low water partial pressures, amorphous deposits of molecular H(2)SO(4) complexed with variable amounts of H(2)O in a ratio of between 1:1 and 2:1 are formed. Upon annealing, this material ejects water and converts first to a 1:1 H(2)SO(4).H(2)O complex and subsequently to anhydrous H(2)SO(4). Adding water to the amorphous molecular hydrate results in the formation of a new species, which on the basis of its thermal behavior and by comparison to theoretical predictions can be attributed to a molecular polymer with a repeat unit of (H(2)SO(4).(H(2)O)(2))(n)(). Implications of these observations for the initial stages of the formation of sulfate aerosol in the atmosphere and their surface reactivity are discussed.

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Two un-ionised molecular hydrate complexes, previously observed by Fourier-transform spectroscopy in flat films of vapour deposited sulfuric acid, have been identified directly in aerosols nucleated from SO 3 and H 2 O in a 75 dm 3 coolable spectroscopic cell. Aerosols of a 1 : 1 complex H 2 SO 4 ÁH 2 O have been generated at 195 K and have also been formed in cooled liquid sulfuric acid aerosols. A second polymeric metastable hydrate of the type (H 2 SO 4 .2H 2 O) n has been identified as the initial material formed during the nucleation process of sulfuric acid aerosol at 195 K. These metastable species are likely to play an important role in the formation of sulfuric acid aerosols at the low temperatures found in the upper troposphere and the lower stratosphere.

The interaction of gaseous NH 3 with thin films of sulfuric acid monohydrate (SAM, H 3 O + HSO 4 À ) in the 200-230 K range has been monitored by infrared spectroscopy and secondary ion mass spectrometry. The reaction occurs with the formation of the characteristic IR spectral features of the NH 4 + and '' salt '' bisulfate ions, whilst loss of H 3 O + and an additional surface species is observed. The evolution of the spectral features with time suggest that two processes are occurring at the uppermost surface layers, with a rapid initial reaction which saturates as the amount of available reactants on the surface is reduced and a slower continuous process which depends upon the transport of new reactants to and the removal of reaction products away from the surface. By comparison with the IR spectra of a variety of sulfuric acid hydrates, a surface reaction scheme is proposed and the implications of these observations for the reaction of condensed acids (sulfate aerosol) with bases in the atmosphere is discussed.