A novel semi-direct method to measure OH reactivity by chemical ionization mass spectrometry (CIMS)

Muller, Jennifer B. A.; Elste, Thomas; Plass-Dülmer, Christian; Stange, Georg; Holla, Robert; Claude, Anja; Englert, Jennifer; Gilge, Stefan; Kubistin, Dagmar

An operational chemical ionization mass spectrometer (CIMS) for hydroxyl radical (inline-formulaOH) and sulfuric acid (inline-formulaH2SO4) concentration measurements was adapted to include observations of inline-formulaOH reactivity, which is the inverse of inline-formulaOH lifetime, for long-term monitoring at the Global Atmosphere Watch (GAW) site Hohenpeissenberg (MOHp), Germany. inline-formulaOH measurement using CIMS is achieved by reacting inline-formulaOH with inline-formulaSO2, leading to the production of inline-formulaH2SO4, which is then detected. The adaptation for inline-formulaOH reactivity consists of the implementation of a second inline-formulaSO2 injection, at a fixed point further down flow in the sample tube to detect the inline-formulaOH decay caused by reactions with inline-formulaOH reactants present in the sample.

The method can measure inline-formulaOH reactivity from less than 1 to 40 sinline-formula−1 with the upper limit due to the fixed positioning of the second inline-formulaSO2 injection. To determine inline-formulaOH reactivity from inline-formulaOH concentration measurements, the reaction time between the two titration zones and inline-formulaOH wall losses in the sample tube need to be determined accurately through inline-formulaOH reactivity calibration. Potential measurement artefacts as a result of inline-formulaHOx recycling in the presence of NO have to be considered. Therefore, NO contamination from gases used in instrument operation must be minimized and ambient NO must be measured concurrently to determine the measurement error.

This CIMS system is shown here to perform very well for inline-formulaOH reactivity below 15 sinline-formula−1 and NO concentrations below 4 ppb, both values that are rarely exceeded at the MOHp site. Thus when deployed in suitable chemical environments, this method can provide valuable continuous long-term measurements of inline-formulaOH reactivity. The characterization utilizes results from chamber, laboratory and modelling studies and includes the discussion and quantification of sources of uncertainties.

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Muller, Jennifer B. A. / Elste, Thomas / Plass-Dülmer, Christian / et al: A novel semi-direct method to measure OH reactivity by chemical ionization mass spectrometry (CIMS). 2018. Copernicus Publications.

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