WATER-VAPOR INTERFERENCE IN THE MEASUREMENT OF OZONE IN AMBIENT AIR BY ULTRAVIOLET-ABSORPTION

Atmospheric observations show that ultraviolet absorption ozone monitors used for ambient measurements can, under some conditions, respond to variable water vapor concentrations with quantitative responses varying from instrument to instrument. A combined theoretical and experimental study shows that these monitors should not respond to water vapor. The source of the response appears to be the varying extinction of ultraviolet light by the windows of the optical cells. A change of 6-g H2O kg air-1 gives, at maximum, an extinction equivalent to 800 ppbv ozone. This changing extinction appears to be related to surface contaminants or surface structure on the cell windows. Instruments with such cell windows can give spurious ozone results in some measurement sequences.