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The hydroxyl (OH) radical is an important oxidant in the troposphere, whichcontrols the lifetime of most air quality- and climate-related trace gases.However, there are still uncertainties concerning its atmospheric budget, andintegrated measurements of OH sinks have been valuable to improve thisaspect. Among the analytical tools used for measuring total OH reactivity inambient air, the comparative reactivity method (CRM) is spreading rapidly inthe atmospheric community. However, measurement artifacts have beenhighlighted for this technique, and additional work is needed to fullycharacterize them. In this study, we present the new Mines Douai CRM instrument, with anemphasis on the corrections that need to be applied to ambient measurementsof total OH reactivity. Measurement artifacts identified in the literaturehave been investigated, including (1) a correction for a change in relativehumidity between the measurement steps leading to different OH levels, (2)the formation of spurious OH in the sampling reactor when hydroperoxyradicals (HO2) react with nitrogen monoxide (NO), (3) not operating theCRM under pseudo-first-order kinetics, and (4) the dilution of ambient airinside the reactor. The dependences of these artifacts on various measurableparameters, such as the pyrrole-to-OH ratio and the bimolecular reactionrate constants of ambient trace gases with OH, have also been studied. Basedon these observations, parameterizations are proposed to correct ambient OHreactivity measurements. On average, corrections of 5.2 ± 3.2,9.2 ± 15.7, and 8.5 ± 5.8 s−1 were respectivelyobserved for (1), (2) and (3) during a field campaign performed in Dunkirk,France (summer 2014). Numerical simulations have been performed using a box model to check whetherexperimental observations mentioned above are consistent with ourunderstanding of the chemistry occurring in the CRM reactor. Two differentchemical mechanisms have been shown to reproduce the magnitude ofcorrections (2) and (3). In addition, these simulations reproduce theirdependences on the pyrrole-to-OH ratio and on bimolecular reaction rateconstants of gases reacting with OH. The good agreement found betweenlaboratory experiments and model simulations gives us confidence in theproposed parameterizations. However, it is worth noting that the numericalvalues given in this study are suitable for the Mines Douai instrument andmay not be appropriate for other CRM instruments. It is recommended thateach group characterize its own instrument following the recommendationsgiven in this study. An assessment of performances for the Mines Douai instrument, including apropagation of errors from the different corrections, indicates a limit ofdetection of 3.0 s−1 and total uncertainties of 17–25 % for OHreactivity values higher than 15 s−1 and NOx mixing ratios lowerthan 30 ppbv

Detailed characterizations of the new Mines Douai comparative reactivity method instrument via laboratory experiments and modeling