Formal blind intercomparison of HO2 measurements during the HOxComp campaign

Hydroperoxy radical (HO₂) concentrations were measured during the formal blind intercomparison campaign HOxComp carried out in Jülich, Germany. HO₂ was detected by three instruments using chemical conversion of HO₂ to hydroxyl radicals (OH) and subsequent detection of the sum of OH and HO₂ by laser induced fluorescence (LIF). The campaign was divided into two parts: (1) ambient air sampling for three days and (2) six experiments in the atmosphere simulation chamber SAPHIR each lasting one day. The latter provided conditions, during which the air was homogeneously mixed, in order to investigate the performance of instruments and to determine potential interferences of measurements under well-controlled conditions. Measurements are correlated as shown by linear correlation coefficients, R², between 0.82 to 0.98. However, the regression analysis of the entire data set of measurements in SAPHIR yields slopes between 0.69 to 1.26. Furthermore, the agreement between measurements is variable for different experiments. Intercepts are smaller than typical atmospheric daytime concentrations (less than 1 pptv). The quality of fit parameters improves significantly, when data are grouped into data subsets of similar water vapor concentrations. Because measurements of LIF instruments were corrected for a well-characterized water dependence of their sensitivities, this indicates that an unknown factor related to water vapor affected measurements in SAPHIR. Measurements in ambient air are also well correlated, but regression parameters differ from results obtained from SAPHIR experiments. This is most likely caused by sampling different air masses at the slightly distant locations of instruments.