Technical Note: Formal blind intercomparison of HO<sub>2</sub> measurements in the atmosphere simulation chamber SAPHIR during the HOxComp campaign

Abstract

Abstract. Hydroperoxy radical (HO 2 ) concentrations were measured during the formal blind intercomparison campaign HOxComp carried out in Jülich, Germany, in 2005. Three instruments detected HO 2 via chemical conversion to hydroxyl radicals (OH) and subsequent detection of the sum of OH and HO 2 by laser induced fluorescence (LIF). All instruments were based on the same detection and calibration scheme. Because measurements by a MIESR instrument failed during the campaign, no absolute reference measurement was available, so that the accuracy of individual instruments could not be addressed. Instruments sampled ambient air for three days and were attached to the atmosphere simulation chamber SAPHIR during the second part of the campaign. Six experiments of one day each were conducted in SAPHIR, where air masses are homogeneously mixed, in order to investigate the performance of instruments and to determine potential interferences of measurements under well-controlled conditions. Linear correlation coefficients (R 2 ) between measurements of the LIF instruments are generally high and range from 0.82 to 0.98. However, the agreement between measurements is variable. The regression analysis of the entire data set of measurements in SAPHIR yields slopes between 0.69Correspondence to: T. Brauers (th.brauers@fz-juelich.de) to 1.26 and 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 could have been caused by differences in HO 2 concentrations in the sampled air at the slightly different locations of instruments.

Investigation of potential interferences in the detection of atmospheric RO<sub><i>x</i></sub> radicals by laser-induced fluorescence under dark conditions

“…An interference by NO 3 could be a possible reason for systematic differences between HO 2 measurements reported for dark conditions during the comparison campaign HOxComp in 2005 (Fuchs et al, 2010). Good agreement was observed in the sunlit SAPHIR chamber for three LIF HO xinstruments, when the chamber contained synthetic air, NO x (up to 300 pptv), and ozone (up to 150 ppbv).…”

Detection of HO<sub>2</sub> by laser-induced fluorescence: calibration and interferences from RO<sub>2</sub> radicals

“…The measurement intercomparison for HO 2 by different LIF instruments during HOxCOMP revealed discrepancies that were apparently correlated with the atmospheric water vapor (Fuchs et al, 2010). This result was surprising, because all instruments had corrected their measurements for the known influence of OH fluorescence quenching by water vapor.…”

“…HO 2 concentration measurements of three LIF instruments were compared during the HOxCOMP campaign in 2005 (Fuchs et al, 2010). Measurements were partly conducted in ambient air and partly at the atmosphere simulation chamber SAPHIR.…”

“…Good agreement was also found for measurements from ROxLIF and MIESR in chamber measurements, during which HO 2 , CH 3 O 2 and C 2 H 5 O 2 were specifically measured ). However, unexplained differences were observed between three LIF instruments in the international comparison campaign HOxComp (Fuchs et al, 2010), when measurements were taken in ambient air and in chamber experiments. Although the data from different instruments were well-correlated, the linear regressions showed sometimes differences that exceeded the combined estimated measurement errors and exhibited an unexplained water vapor dependence.…”

“…Experimental investigations in the laboratory for C 1 -C 4 alkyl peroxy radicals and results from field campaigns (Stevens et al, 1994;Mather et al, 1997;Kanaya et al, 2001;Creasey et al, 2002;Fuchs et al, 2010) did not hint towards a significant interference. For example, for the instrument characterized here, it was shown that the upper limit for an interference from methyl peroxy radicals (CH 3 O 2 ) is 5 % (Weber, 1998;Holland et al, 2003).…”