Urban hydrogen peroxide

Another nonozone photochemical oxidant observed in urban atmospheres is hydrogen peroxide. Bufalini et found this compound to be present at concentrations up to 0.04 ppm in the air in Hoboken, New Jersey, and up to 0.18 ppm on a smoggy day in Riverside, California. Figure 4-42 shows that the diurnal hydrogen peroxide variation in Riverside on August 6, 1970, nearly paralleled that of total oxidant. Figure 4-43 indicates, however, that on at least one occasion (August 11, 1970) it peaked as early as 10 30 a.m. 

The relative importance of the various heterogeneous oxidation pathways depends on pH. At pH values below —4.5 the hydrogen peroxide pathway typically dominates. In urban areas hydrogen peroxide may not be abundant enough to be the most important oxidant. Here transition metal catalysts can enhance the rates considerably, especially if there are alkaline materials from fly ash or ammonia to neutralize the growing acidity of droplet phases, which otherwise limits SO2 solubility. 

Bufalini JJ, Gay B W. Jr, Brubaker KL. 1972. Hydrogen peroxide formation from formaldehyde photooxidation and its presence in urban atmospheres. Environ Sci Technol 6 816-821. 

Bufalini, J. J., B. M. Gay, Jr., and K. L. Brubaker (1972). Hydrogen peroxide formation from formaldehyde photo-oxidation and its presence in urban atmosphere. Environ. Sci Technol. 6, 816-821. 

Direct measurements have not been made, to my knowledge, regarding the lower limit of partial pressure of H2O in air necessary for formation of hydrogen peroxide. One can reason, however, that the limiting partial pressure ought to be the same as that necessary for a metal to corrode. Based on corrosion information, the critical lower limit for the partial pressure is more properly expressed in terms of relative humidity rather than absolute pressure. The critical relative humidity for corrosion is that which allows moisture to condense on the surface of a metal. This value, in turn, depends on the nature and concentration of hygroscopic impurities present both in the atmosphere and on the metal surface. For commercial steels in ordinary urban air, the critical relative humidity is about 50%, but for high purity metals in filtered air, the critical value is undoubtedly much lower. 

KADLECEK In mountain water there are moderately high concentrations of hydrogen peroxide, and in rain water over clean areas they maintain moderate levels, and over urban areas, the hydrogen peroxide levels drop to only a few parts per billion. 

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