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Upon exposure to sunlight, or to artificial light at wavelengths longer than 290 nm, sulfur dioxide in air underwent oxidation to give sulfur trioxide in the presence of air pollutants such as biacetyl (2,3-butanedione), benzaldehyde and nitrogen dioxide, but not in their absence. Only nitrogen dioxide completely oxidized sulfur dioxide to sulfur trioxide.
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Nojima et al. (2004) studied this question.
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