Determination of Sulphur Dioxide by Pulsed UV-Fluorescence

Abstract
We have investigated the cross-sensitivity of a commercially available SO₂ monitor (TEI 43C-FG) with respect to the fluorescence of NO, CO and CO₂. Quenching of the NO and the SO₂ fluorescence by O₂ and CO₂ was also determined. The results were implemented in a set of equations and subsequently used to correct the monitor’s output for possible interferences. For highly variable O₂ concentrations, quenching by molecular oxygen was
controlled by dilution of the sample gas with synthetic air.

The response of an UV-fluorescence monitor to various gases is largely determined by the light spectrum used for excitation and detection. Therefore, absorption spectra of the main components were measured. Furthermore, fluorescence spectra of SO₂ and NO were obtained as a function of the excitation wavelength. Based on these data, the advantages and limitations inherent to the selection of filters are discussed and further improvements suggested.

1. Introduction
Sulphur dioxide (SO₂) is the most abundant anthropogenic sulphur compound in the troposphere, and is mainly emitted through coal and petroleum combustion, petroleum refining, and metal smelting operations. In Switzerland, SO₂ emissions were reduced by more than 75 % over the past twenty years. However, further improvements are necessary to meet international obligations and to avoid acidification of ecosystems. Therefore, continuing efforts on measurement and control of sulphur oxide emissions are needed [1].

Increasingly stringent emission limits for vehicle engines and industrial applications often require appropriate (catalytic) after-treatment systems. These systems are frequently sensitive to sulphur compounds. The precise determination of concentrations and reactions of sulphur oxides in (flue) gas treatments are therefore a novel application for sensitive SO₂ measurement techniques. This is e.g. illustrated by a study indicating oxidation and storage of sulphur oxides in a continuously regenerating trap (CRT)-system which is employed in diesel engines for the reduction of particulate matter emissions [2].

Commercial pulsed UV-fluorescence detectors offer an inexpensive and efficient way to measure SO₂, and are widely used both in ambient air and flue gases. In these instruments SO₂ is electronically excited by the radiation of an UV lamp, yielding an excited species which fluoresces in the UV.

This technique is very sensitive but other gaseous components may interfere. Quenching (e.g. by O₂ or H₂O) of the excited species can lead to an underestimation of the SO₂ concentration. Positive interference occurs by molecules which absorb radiation and fluoresce in the same region than SO₂ (e.g. NO and HC) [3]. In exhaust gases, nitric oxide (NO) is usually the main positive interferences, while oxygen is the most important
quenching species if water vapour is removed by a Nafion dryer.