| Abstract
| - A water vapor interference in ozone measurements by UVabsorption was investigated using four different ozonemonitors (TEI models 49 and 49C, Dasibi model 1003-AH,and a 2B Technologies model 202 prototype). In the extremecase of step changes between 0 and 90% relativehumidity (RH), a large interference in the range of tens tohundreds of ppbv was found for all instruments tested,with the magnitude and sign depending on the manufacturerand model. Considering that water vapor does notabsorb at the wavelength of the Hg lamp (253.7 nm) usedin these instruments, another explanation is required.Based on experimental evidence and theoretical considerations, we conclude that the water vapor interference iscaused by humidity effects on the transmission of uncollimatedUV light through the detection cell. The ozone scrubberacts as a water reservoir, either adding or removing waterfrom the air sample, thereby modulating the detectorsignal and producing a positive or negative offset. It wasfound for the 2B Technologies ozone monitor that useof a 1-m length of Nafion tubing just prior to the entranceto the detection cell reduces the water vapor interferenceto negligible levels (±2 ppbv for step changes between 0 and90% RH) while quantitatively passing ozone.
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