Technical Note: Formal blind intercomparison of OH measurements: results from the international campaign HOxComp

Eric Schlosser ,T Elste,H.-P Dorn,M Rudolf,F Holland,U Schurath,T Brauers,G Stange,H Berresheim,K Miyamoto,R Häseler,A Hofzumahaus,Hartwig Harder ,Satoshi Nishida ,Keisuke Watanabe ,Andreas Wahner ,Yoshizumi Kajii ,Akira Yoshino ,Hendrik Fuchs ,Dagmar Kubistin ,Mònica Martínez ,C Plaß-Dülmer ,Yugo Kanaya

doi:10.5194/acp-9-7923-2009

Abstract

Abstract. Hydroxyl radicals (OH) are the major oxidizing species in the troposphere. Because of their central importance, absolute measurements of their concentrations are needed to validate chemical mechanisms of atmospheric models. The extremely low and highly variable concentrations in the troposphere, however, make measurements of OH difficult. Three techniques are currently used worldwide for tropospheric observations of OH after about 30~years of technical developments: Differential Optical Laser Absorption Spectroscopy (DOAS), Laser-Induced Fluorescence Spectroscopy (LIF), and Chemical Ionisation Mass Spectrometry (CIMS). Even though many measurement campaigns with OH data were published, the question of accuracy and precision is still under discussion. Here, we report results of the first formal, blind intercomparison of these techniques. Six OH instruments (4~LIF, 1~CIMS, 1~DOAS) participated successfully in the ground-based, international HOxComp campaign carried out in Jülich, Germany, in summer 2005. Comparisons were performed for three days in ambient air (3~LIF, 1 CIMS) and for six days in the atmosphere simulation chamber SAPHIR (3~LIF, 1~DOAS). All instruments were found to measure tropospheric OH concentrations with high sensitivity and good time resolution. The pairwise correlations between different data sets were linear and yielded high correlation coefficients (r2=0.75−0.96). Excellent absolute agreement was observed for the instruments at the SAPHIR chamber, yielding slopes between 1.01 and 1.13 in the linear regressions. In ambient air, the slopes deviated from unity by factors of 1.06 to 1.69, which can partly be explained by the stated instrumental accuracies. In addition, sampling inhomogeneities and calibration problems have apparently contributed to the discrepancies. The absolute intercepts of the linear regressions did not exceed 0.6×106 cm−3, mostly being insignificant and of minor importance for daytime observations of OH. No relevant interferences with respect to ozone, water vapour, NOx and peroxy radicals could be detected. The HOxComp campaign has demonstrated that OH can be measured reasonably well by current instruments, but also that there is still room for improvement of calibrations.

Highlights

The hydroxyl radical (OH) is the key reactant for the degradation of most compounds emitted from biogenic and anthropogenic sources into the troposphere, e.g. sulfur dioxide, nitrogen dioxide, carbon monoxide, methane, and volatile hydrocarbons (Ehhalt, 1999; Lelieveld et al, 2004)
Related to the different t is the mean of precisions (σ ), which ranged between 3.3×105 cm−3 (FZJ-Laser-Induced Fluorescence Spectroscopy (LIF)-ambient) to 17×105 cm−3 (MPI-LIF) for this campaign’s data
The ambient conditions were moderately polluted with substantial levels of biogenic VOCs

Summary

Introduction

The hydroxyl radical (OH) is the key reactant for the degradation of most compounds emitted from biogenic and anthropogenic sources into the troposphere, e.g. sulfur dioxide, nitrogen dioxide, carbon monoxide, methane, and volatile hydrocarbons (Ehhalt, 1999; Lelieveld et al, 2004). Most of these compounds and their degradation products have adverse impact on the environment because of their toxicity, global warming potential, or their stratospheric ozone depletion capability. OH radicals are primarily produced by photolysis of ozone and the subsequent reaction of the formed. Schlosser et al.: Formal blind intercomparison of OH measurements excited oxygen atoms with water vapour.

Results

Discussion

Conclusion