The effect of relative humidity on the detection of pyrrole by PTR-MS for OH reactivity measurements

The hydroxyl radical (OH) is the most important atmospheric oxidant. Recently Sinha et al. [V. Sinha, J. Williams, J.N. Crowley, J. Lelieveld, Atmos. Chem. Phys. 8 (2008) 2213] developed a new method to measure the total OH reactivity of ambient air (OH sink) employing a proton transfer reaction mass spectrometer (PTR-MS) as a detector. The new method uses pyrrole (C4H4NH) as a reagent and for an OH reactivity measurement this species must be measured under both dry (∼0% RH) and humid air (>30% RH). Here, we investigate the sensitivity dependence of the PTR-MS for pyrrole, as a function of relative humidity in the sampled air. Various normalizations with respect to the H3O+ ion and its different hydrated cluster ions H3O+(H2O)n=1,2,3 are compared. It is shown that both the primary ion signal (H3O+ ion m/z = 19) and the first water cluster ion H3O+(H2O) (m/z = 37) should be used for pyrrole quantification. However, in spite of using this normalization, the PTR-MS sensitivity for pyrrole changes by as much as 16% between dry (∼0% RH) and humid air (above 30% RH), with higher sensitivity when the sampled air is humid. Thus, for accurate quantification of pyrrole using a PTR-MS, calibration factors appropriate to dry and humid air should be employed. We recommend that humidity dependence of the PTR-MS be taken into account when reactivity measurements are performed using the pyrrole based comparative reactivity method (CRM).

Graphical abstractSensitivity dependence of PTR-MS for pyrrole detection in dry and humid air is reported for the new application of OH reactivity measurements in ambient air.Figure optionsDownload full-size imageDownload as PowerPoint slide