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<strong class="journal-contentHeaderColor">Abstract.</strong> A novel technique has been developed to measure sulfur dioxide (SO<span class="inline-formula">₂</span>) using a modification of the existing electrochemical concentration cell (ECC) ozonesonde technology. The current sonde-based method to measure SO<span class="inline-formula">₂</span> (i.e., the dual-sonde approach) involves launching two ozonesondes together, with one of the sondes having a filter to remove SO<span class="inline-formula">₂</span> at the inlet. The SO<span class="inline-formula">₂</span> profile is determined by taking the difference between the measurements from the two instruments. The dual-sonde method works well in typical tropospheric conditions when <span class="inline-formula">[O₃]&gt;[SO₂]</span> but saturates when <span class="inline-formula">[SO₂]&gt;[O₃]</span> and has large uncertainties in the upper troposphere and lower stratosphere that would limit its effectiveness in measuring SO<span class="inline-formula">₂</span> from an explosive volcanic eruption. Due to these limitations, several modifications were made to create a single-sonde system that would directly measure SO<span class="inline-formula">₂</span> (i.e., the SO<span class="inline-formula">₂</span> sonde). These modifications included (1) a positively biased ECC current, (2) the addition of an O<span class="inline-formula">₃</span> removal filter, and (3) the addition of a sample dryer. The SO<span class="inline-formula">₂</span> sonde measures SO<span class="inline-formula">₂</span> as a reduction in the cell current. There was a strong correlation (<span class="inline-formula"><i>r</i>²&gt;0.94</span>) between the SO<span class="inline-formula">₂</span> sonde and a Thermo 43<span class="inline-formula"><i>c</i></span> analyzer during controlled laboratory tests and pre-flight tests. Varying humidity levels affected the SO<span class="inline-formula">₂</span> sonde's sensitivity (avg <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mn mathvariant="normal">84.6</mn><mo>±</mo><mn mathvariant="normal">31.7</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="5cba66665eac0fd201aad0189738d8ad"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-4373-2022-ie00001.svg" width="66pt" height="10pt" src="amt-15-4373-2022-ie00001.png"/></svg:svg></span></span> <span class="inline-formula">ppbv µA^−1</span>, 1<span class="inline-formula"><i>σ</i></span> RSD <span class="inline-formula">=37</span> %) during initial field tests, which was resolved by adding a sample dryer upstream of the O<span class="inline-formula">₃</span> removal filter and pump inlet. This modification significantly reduced the variability and increased the sensitivity of the SO<span class="inline-formula">₂</span> measurements (avg <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mn mathvariant="normal">47</mn><mo>±</mo><mn mathvariant="normal">5.8</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="51pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="b34d07d0be39310766f1f9a17d57fe7b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-4373-2022-ie00002.svg" width="51pt" height="10pt" src="amt-15-4373-2022-ie00002.png"/></svg:svg></span></span> <span class="inline-formula">ppbv µA^−1</span>, 1<span class="inline-formula"><i>σ</i></span> RSD <span class="inline-formula">=12</span> %). Field tests included measurements near KÄ«lauea volcano (before and during the 2018 eruption of the Lower East Rift Zone), Costa Rica's Turrialba volcano, and anthropogenic plumes from the Athabasca oil sands region of Alberta, Canada. This single-SO<span class="inline-formula">₂</span>-sonde system is an effective, inexpensive instrument for measuring both ground-based and vertical profiles of SO<span class="inline-formula">₂</span> from anthropogenic and natural sources (i.e., volcanic eruptions) over a wide range of concentrations.

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