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Abstract. Precise and accurate hydroxyl radical (OH) measurements are essential to investigate mechanisms for oxidation and transformation of trace gases and processes leading to the formation of secondary pollutants like ozone (O3) in the troposphere. Laser-induced fluorescence (LIF) isa widely used technique for the measurement of ambient OH radicals and wasused for the majority of field campaigns and chamber experiments. Recently,most LIF instruments in use for atmospheric measurements of OH radicalsintroduced chemical modulation to separate the ambient OH radicalconcentration from possible interferences by chemically removing ambient OHradicals before they enter the detection cell (Mao et al., 2012; Novelliet al., 2014a). In this study, we describe the application andcharacterization of a chemical modulation reactor (CMR) applied to theForschungszentrum Jülich LIF (FZJ-LIF) instrument in use at the atmospheric simulation chamberSAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). Besides dedicated experiments insynthetic air, the new technique was extensively tested during theyear-round Jülich Atmospheric Chemistry Project (JULIAC) campaign, inwhich ambient air was continuously flowed into the SAPHIR chamber. Itallowed for performing OH measurement comparisons with differential opticalabsorption spectroscopy (DOAS) and investigation of interferences in a large variety of chemical and meteorological conditions. Good agreement wasobtained in the LIF–DOAS intercomparison within instrumental accuracies (18 % for LIF and 6.5 % for DOAS) which confirms that the new chemicalmodulation system of the FZJ-LIF instrument is suitable for measurement ofinterference-free OH concentrations under the conditions of the JULIACcampaign (rural environment). Known interferences from O3+H2Oand the nitrate radical (NO3) were quantified with the CMR in synthetic air in the chamber and found to be 3.0×105 and 0.6×105 cm−3, respectively, for typical ambient-airconditions (O3=50 ppbv, H2O = 1 % and NO3=10 pptv). The interferences measured in ambient air during the JULIAC campaign in the summer season showed a median diurnal variation with a median maximum value of 0.9×106 cm−3 during daytime and a median minimum value of 0.4×106 cm−3 at night. The highest interference of 2×106 cm−3 occurred in a heat wave from 22 to 29 August, when the air temperature and ozone increased to 40 ∘C and 100 ppbv, respectively. All observed interferences could be fully explained bythe known O3+H2O interference, which is routinely corrected in FZJ-LIF measurements when no chemical modulation is applied. No evidence for an unexplained interference was found during the JULIAC campaign. A chemical model of the CMR was developed and applied to estimate thepossible perturbation of the OH transmission and scavenging efficiency byreactive atmospheric trace gases. These can remove OH by gas phase reactions in the CMR or produce OH by non-photolytic reactions, most importantly by the reaction of ambient HO2 with NO. The interfering processes become relevant at high atmospheric OH reactivities. For the conditions of the JULIAC campaign with OH reactivities below 20 s−1, the influence on thedetermination of ambient OH concentrations was small (on average: 2 %).However, in environments with high OH reactivities, such as in a rain forest or megacity, the expected perturbation in the currently used chemical modulation reactor could be large (more than a factor of 2). Suchperturbations need to be carefully investigated and corrected for the properevaluation of OH concentrations when applying chemical scavenging. Thisimplies that chemical modulation, which was developed to eliminateinterferences in ambient OH measurements, itself can be subject tointerferences that depend on ambient atmospheric conditions.

atmospheric measurement techniques

Characterization of a chemical modulation reactor (CMR) for the measurement of atmospheric concentrations of hydroxyl radicals with a laser-induced fluorescence instrument