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<p>We present the development of a chemical ionization mass spectrometer ion source specifically designed for in situ measurements of trace gases in the upper troposphere and lower stratosphere. The ion source utilizes a commercially available photoionization krypton lamp, primarily emitting photons in the vacuum ultraviolet (VUV) region at wavelengths of 124 and 117 nm (corresponding to energies of 10 and 10.6 eV, respectively), coupled to a commercially available Vocus proton transfer reaction mass spectrometer. The VUV ion source can produce both negative and positive reagent ions; however, here we primarily focus on generating iodide anions (<span class="inline-formula">I⁻</span>). The instrument's drift tube (also known as ion–molecule reactor) operates at pressures between 2 and 10 mbar, which facilitates ambient sampling at atmospheric pressures as low as 50 mbar. The low operating pressure reduces secondary ion chemistry that can occur in iodide chemical ionization mass spectrometry (CIMS). It also allows the addition of water vapor to the drift tube to exceed typical ambient humidity by more than 1 order of magnitude, significantly reducing ambient humidity dependence of sensitivities. An additional benefit of this ion source and drift tube is a 10- to 100-fold reduction in nitrogen consumed during operation relative to standard <span class="inline-formula">I⁻</span> ion sources, resulting in significantly reduced instrument weight and operational costs. In iodide mode, sensitivities of 76 cps ppt<span class="inline-formula">⁻¹</span> for nitric acid, 35 cps ppt<span class="inline-formula">⁻¹</span> for <span class="inline-formula">Br₂</span> and 8.9 cps ppt<span class="inline-formula">⁻¹</span> for <span class="inline-formula">Cl₂</span> were achieved. Lastly, we demonstrate that this ion source can generate benzene (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">C</mi><mn mathvariant="normal">6</mn></msub><msubsup><mi mathvariant="normal">H</mi><mn mathvariant="normal">6</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="51ca74182adfd3e6b34e5f82c0fce6c9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-1159-2022-ie00001.svg" width="29pt" height="15pt" src="amt-15-1159-2022-ie00001.png"/></svg:svg></span></span>) and ammonium (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="9641cdd414b305565815b5b604dabf23"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-1159-2022-ie00002.svg" width="24pt" height="15pt" src="amt-15-1159-2022-ie00002.png"/></svg:svg></span></span>) reagent ions to expand the number of detected atmospheric trace gases.</p>