Comparison of OH reactivity measurements in the atmospheric simulation chamber SAPHIR

Abstract

Abstract. Hydroxyl (OH) radical reactivity (k OH ) has been measured for 18 years with different measurement techniques. In order to compare the performances of instruments deployed in the field, two campaigns were conducted performing experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich in October 2015 and April 2016. Chemical conditions were chosen either to be representative of the atmosphere or to test potential limitations of instruments. All types of instruments that are currently used for atmospheric measurements were used in one of the two campaigns. The results of these campaigns demonstrate that OH reactivity can be accurately measured for a wide range of atmospherically relevant chemical conditions (e.g. water vapour, nitrogen oxides, various organic compounds) by all instruments. The precision of the measurements (limit of detection < 1 s −1 at a time resolution of 30 s to a few minutes) is higher for instruments directly detecting hydroxyl radicals, whereas the indirect comparative reactivPublished by Copernicus Publications on behalf of the European Geosciences Union. H. Fuchs et al.: OH reactivity comparison in SAPHIRity method (CRM) has a higher limit of detection of 2 s −1 at a time resolution of 10 to 15 min. The performances of the instruments were systematically tested by stepwise increasing, for example, the concentrations of carbon monoxide (CO), water vapour or nitric oxide (NO). In further experiments, mixtures of organic reactants were injected into the chamber to simulate urban and forested environments. Overall, the results show that the instruments are capable of measuring OH reactivity in the presence of CO, alkanes, alkenes and aromatic compounds. The transmission efficiency in Teflon inlet lines could have introduced systematic errors in measurements for low-volatile organic compounds in some instruments. CRM instruments exhibited a larger scatter in the data compared to the other instruments. The largest differences to reference measurements or to calculated reactivity were observed by CRM instruments in the presence of terpenes and oxygenated organic compounds (mixing ratio of OH reactants were up to 10 ppbv). In some of these experiments, only a small fraction of the reactivity is detected. The accuracy of CRM measurements is most likely limited by the corrections that need to be applied to account for known effects of, for example, deviations from pseudo first-order conditions, nitrogen oxides or water vapour on the measurement. Methods used to derive these corrections vary among the different CRM instruments. Measurements taken with a flowtube instrument combined with the direct detection of OH by chemical ionisation mass spectrometry (CIMS) show limitations in cases of high reactivity and high NO concentrations but were accurate for low reactivity (< 15 s −1 ) and low NO (< 5 ppbv) conditions.

Summertime OH reactivity from a receptor coastal site in the Mediterranean Basin

“…An intercomparison exercise with another CRM instrument carried out before the campaign demonstrated that the measured reactivities were in good agreement (linear least squares fit with a slope of 1 and R 2 value of 0.75). An intercomparison study carried out with other instruments based on the CRM technique and on the laser-induced fluorescence-based technique showed that the measured OH reactivities agree with each other (Fuchs et al, 2017). The same study showed the following limitations of CRM instruments compared to laser-based techniques: (i) higher limit of detection (2 s −1 vs. < 1 s −1 ), lower time resolution (10-15 min vs. 30 s to a few minutes), lower accuracy due to the required corrections to determine the final OH reactivity value (pseudo-first-order deviation and OH recycling for environments exposed to high NO x concentrations).…”

Water Vapor Does Not Catalyze the Reaction between Methanol and OH Radicals

“…[12,16] We have also repeated experiments using the same relative method as Jara-Toro et al,a nd in addition to varying RH, we have also largely varied the gas volume in the Te flon bag to vary the surface to volume ratio.F inally, theoretical calculations were carried out for arefined analysis of possible H 2 Oc atalysis pathways. TheF AGEt echnique,i nitially developed for the quantification of OH radicals in the atmosphere,allows monitoring of the OH concentrations in as elective and sensitive way, even under atmospheric conditions,that is,high O 2 and H 2 O concentrations,q uantification which proves difficult with in situ LIF because of strong quenching of OH fluorescence by O 2 and H 2 O. Here,F AGEh as been coupled to al aser photolysis reactor ( Figure 1) [16,17] for apulsed initiation of the reaction between OH and CH 3 OH. Briefly,OHradicals were generated by pulsed photolysis of am ixture of ozone and…”

Global modelling of the total OH reactivity: investigations on the “missing” OH sink and its atmospheric implications

“…Over the last two decades, techniques capable of measuring the total OH reactivity directly, without the need to quantify individual sinks, have become available: these rely either on direct measurements of the OH decay rate (Di Carlo et al, 2004;Ingham et al, 2009;Kovacs and Brune, 2001) or on the comparative reactivity method (Sinha et al, 2008), in which k OH is determined from the reactivity of a reference species (typically pyrrole). A review (Yang et al, 2016) recently described these techniques in detail, whilst the various instruments developed for direct measurements of k OH have been the subject of an extensive intercomparison (Fuchs et al, 2017). These techniques, when deployed in the field along with instruments for the detection of trace species, have enabled the comparison of direct measurements of the total k OH with the sum of reactivities of the individual OH sinks.…”