Mechanism and Elimination of a Water Vapor Interference in the Measurement of Ozone by UV Absorbance

Abstract

A water vapor interference in ozone measurements by UV absorption was investigated using four different ozone monitors (TEI models 49 and 49C, Dasibi model 1003-AH, and a 2B Technologies model 202 prototype). In the extreme case of step changes between 0 and 90% relative humidity (RH), a large interference in the range of tens to hundreds of ppbv was found for all instruments tested, with the magnitude and sign depending on the manufacturer and model. Considering that water vapor does not absorb at the wavelength of the Hg lamp (253.7 nm) used in these instruments, another explanation is required. Based on experimental evidence and theoretical considerations, we conclude that the water vapor interference is caused by humidity effects on the transmission of uncollimated UV light through the detection cell. The ozone scrubber acts as a water reservoir, either adding or removing water from the air sample, thereby modulating the detector signal and producing a positive or negative offset. It was found for the 2B Technologies ozone monitor that use of a 1-m length of Nafion tubing just prior to the entrance to the detection cell reduces the water vapor interference to negligible levels (+/- 2 ppbv for step changes between 0 and 90% RH) while quantitatively passing ozone.

A Re-Examination of Ambient Air Ozone Monitor Interferences

“…As shown by Wilson and Birks,16 changes in water vapor level can affect the signal of UV O 3 monitors. This interference should be minimized in the UV c monitor by using humidity equilibration.…”

“…Wilson and Birks 16 have described a simple approach to minimize the effect of rapid changes in RH on UV O 3 measurements. They install Nafion tubing immediately upstream of the absorption cell to equilibrate the sample and reference air humidity with the (typically lower) regulated humidity of the shelter housing the monitor.…”

“…However, the fact that a similar instrument equipped with a Nafion inlet tube (UV-Hop-N) did not show the same strong increase in signal suggests that water vapor is the indirect cause of the interference, likely by the cell wall reflectivity changes proposed by Wilson and Birks. 16 The results of the three water vapor interference tests at three different O 3 levels are summarized in Table 4. The interference responses were determined at the highest water vapor concentrations near the end of each test.…”

“…Their difference with the UV-Hop units may be related to the relatively lower waterabsorbing capacity of their scrubber designs. 16 …”

“…Meyer et al 14 reported interferences associated with changing relative humidity (RH) that affected light transmission through scratched optical cell windows. Wilson and Birks 16 reported RH interferences associated with changing transmission of multiply reflected UV light down the optical cell, although a pre-cell humidity equilibrator made of water-permeable material (Nafion) resolved this source of interference.…”