the sun, absorption or diffusion infrared radiation emitted by the
terrestrial surface, as well as by increase in the albedo of the
Earth/atmosphere system( 19 ).
In an effort to understand atmospheric chemical and photo-
chemical behaviors, chemists have traditionally examined homo-
geneous gas-phase radical (mainly OH) reactions of natural and
anthropogenic pollutants. In subsequent studies the role of mul-
tiphase chemistry within the atmosphere was taken into
account. Thus, although in general the homogeneous gas-phase
model has been successful, some observations suggest that other
pathways exist that may contribute significantly to the overall
mechanism. These observations include processes occurring
within cloud droplets or raindrops or layers at the surface of ice
crystals, inside which the photochemical reactions are much
stronger than in interstitial air. Among others, in atmospheric
waters beside the OH radicals, also superoxide radical ions,
O 2 , are active in redox reactions: they can be the dominant
source of transition metal compounds at low oxidation states
(e.g., Cu(I)), whereas hydroperoxyl radicals, HOO, or organic
peroxyl radicals, ROO, can be involved in oxidation of reduced
forms of metals such as Cu(I) or Fe(II)( 20 ).
The role of multiphase photochemistry in the atmosphere needs
to be evaluated, especially as surface of metal oxide particles and
other solid aerosols exceeds by far the total surface of the globe.
Atmospheric photochemistry, which alters dust properties, may
influence atmospheric metal cycling. Mixing of mineral dust with
acid aerosol in result of coalescence processes within clouds may
be an important mechanism of increasing the solubility of atmo-
spheric iron in cloud water and photochemical reduction of iron
(III) to iron(II) during the long-range transport of particles. Thus,
the mobilization of metals might be strongly dependent on the his-
tory of the aerosol particles in the atmosphere. Consequently, we
can assume that iron probably undergoes repeated cycles of very
rapid Fe(III) photoreduction and slower Fe(II) oxidation on a time
scale of a few minutes( 19 ). Photolysis plays a similar role in atmo-
spheric pathways of copper compounds.
A. VOLATILEORGANICCOMPONDS
The described processes result in changes in the atmospheric
composition, which can be observed in the case of trace gases,
such as volatile organic compounds (VOCs), sulfur oxides, nitric
oxides, etc.
300 ZOFIA STASICKA