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Abstract. We present the development of a chemical ionization mass spectrometer ion source specifically designed for in situ measurements of trace gases inthe upper troposphere and lower stratosphere. The ion source utilizes acommercially available photoionization krypton lamp, primarily emittingphotons 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 acommercially available Vocus proton transfer reaction mass spectrometer. TheVUV ion source can produce both negative and positive reagent ions; however,here we primarily focus on generating iodide anions (I−). Theinstrument's drift tube (also known as ion–molecule reactor) operates atpressures between 2 and 10 mbar, which facilitates ambient sampling atatmospheric pressures as low as 50 mbar. The low operating pressure reducessecondary ion chemistry that can occur in iodide chemical ionization mass spectrometry (CIMS). It also allows theaddition of water vapor to the drift tube to exceed typical ambient humidityby more than 1 order of magnitude, significantly reducing ambient humiditydependence of sensitivities. An additional benefit of this ion source anddrift tube is a 10- to 100-fold reduction in nitrogen consumed duringoperation relative to standard I− ion sources, resulting insignificantly reduced instrument weight and operational costs. In iodidemode, sensitivities of 76 cps ppt−1 for nitric acid, 35 cps ppt−1 for Br2and 8.9 cps ppt−1 for Cl2 were achieved. Lastly, we demonstrate that thision source can generate benzene (C6H6+) and ammonium(NH4+) reagent ions to expand the number of detected atmospherictrace gases.

A versatile vacuum ultraviolet ion source for reduced pressure bipolar chemical ionization mass spectrometry