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

Abstract

Abstract. 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 ProtonTransfer-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.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 −1 ) 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.

What effect does VOC sampling time have on derived OH reactivity?

“…Using PTR-MS and the CRM method in a boreal forest in Finland during August 2008, Sinha et al (2010 reported missing OH reactivity of approximately 50 %. This site was revisited in 2010, when missing OH reactivity of 58 to 89 % was recorded (Nölscher et al, 2012a). Similar results in a mixed deciduous forest were obtained by Hansen et al (2014), who reported missing OH reactivity of 46 to 65 %.…”

“…1; see also Nölscher et al, 2012a). Indices denote the origin of the data (PTR (PTR-ToF-MS) or GC (DC-GC-FID) and CL (ClearfLo) or PAR (PARADE)).…”

“…Period 1 was mainly influenced by continental air masses and only towards the end by air that travelled over the UK and the English Channel (UK-marine). Period 2 was dominated by UK-marine air, but was also influenced by air masses that travelled over the Atlantic (see Phillips et al, 2012).…”

“…canister samples once every hour (Kovacs et al, 2003;Sadanaga et al, 2005;Yoshino et al, 2006), when compared to measured total OH reactivity. However, the sampling time during one GC cycle is often shorter than the analysis time, and thus any high temporal variability in measured OH reactivity is not easily captured when it is derived from GC data (Nölscher et al, 2012a). When measured and calculated OH reactivity are compared, high-time-resolution data are often averaged over intervals that correspond to the GC cycle.…”

“…The reaction takes place in a glass vessel and is most commonly probed by PTR-MS. Recently, Nölscher et al (2012b) presented a GC-PID for the detection of pyrrole for CRM. These techniques enable comparison of directly measured OH reactivity to calculated OH reactivity using Eq.…”