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The primary and most important oxidant in the atmosphere is  the hydroxyl radical (OH). Currently OH sinks, particularly  gas phase reactions, are poorly constrained. One way to  characterize the overall sink of OH is to measure directly the  ambient loss rate of OH, the total OH reactivity. To date,  direct measurements of total OH reactivity have been either  performed using a Laser-Induced Fluorescence (LIF) system  ("pump-and-probe" or "flow reactor") or the Comparative  Reactivity Method (CRM) with a Proton-Transfer-Reaction Mass  Spectrometer (PTR-MS). Both techniques require large, complex  and expensive detection systems. This study presents  a feasibility assessment for CRM total OH reactivity  measurements using a new detector, a Gas Chromatographic  Photoionization Detector (GC-PID). Such a system is smaller,  more portable, less power consuming and less expensive than  other total OH reactivity measurement techniques. <br><br>  Total OH reactivity is measured by the CRM using a competitive  reaction between a reagent (here pyrrole) with OH alone and in  the presence of atmospheric reactive molecules. The new CRM  method for total OH reactivity has been tested with parallel  measurements of the GC-PID and the previously validated PTR-MS  as detector for the reagent pyrrole during laboratory  experiments, plant chamber and boreal field studies. Excellent  agreement of both detectors was found when the GC-PID was  operated under optimum conditions. Time resolution  (60–70 s), sensitivity (LOD 3–6 s<sup>−1</sup>) and  overall uncertainty (25% in optimum conditions) for total  OH reactivity were similar to PTR-MS based total OH  reactivity measurements. One drawback of the GC-PID system was  the steady loss of sensitivity and accuracy during intensive  measurements lasting several weeks, and a possible toluene  interference. Generally, the GC-PID system has been shown to  produce closely comparable results to the PTR-MS and thus in  suitable environments (e.g. forests) it presents a viably  economical alternative for groups interested in total OH  reactivity observations

Total OH reactivity measurements using a new fast Gas Chromatographic Photo-Ionization Detector (GC-PID)