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

Abstract. Precise and accurate hydroxyl radical (OH) measurements are essential to investigate how trace gases are oxidized and transformed in the troposphere and how secondary pollutants like ozone (O3) are formed. Laser induced fluorescence (LIF) is a widely used technique for the measurement of ambient OH radicals and was used for the majority of field campaigns and chamber experiments. Recently, most LIF instruments in use for atmospheric measurements of OH radicals introduced chemical modulation to separate the ambient OH radical concentration from possible interferences by chemically removing ambient OH radicals before they enter the detection cell. In this study, we describe the application, characterization, and validation of a chemical modulation reactor (CMR) applied to the Forschungszentrum Julich LIF (FZJ-LIF) instrument in use at the atmospheric simulation chamber SAPHIR. Besides dedicated experiments in synthetic air, the new technique was extensively tested during the year-round Julich Atmospheric Chemistry Project (JULIAC) campaign, in which ambient air was continuously flowed into the SAPHIR chamber. It allowed performing OH measurement comparisons with Differential Optical Absorption Spectroscopy (DOAS) and investigation of interferences in a large variety of chemical and meteorological conditions. A good agreement was obtained in the LIF DOAS intercomparison within instrumental accuracies (18 % for LIF, 6.5 % for DOAS) which confirms that the new chemical modulation system of the FZJ-LIF instrument is suitable for measurement of interference-free OH concentrations. Known interferences from O3 + H2O and the nitrate radical (NO3) were quantified with the CMR in synthetic air in the chamber and found to be 3.0 × 105 cm-3 and 0.6 × 105 cm-3, respectively, for typical ambient air condition (O3 = 50 ppbv, H2O = 1 %, NO3 = 10 pptv). The interferences measured in ambient air during the JULIAC campaign in summer season had the median diurnal variation of the interference with a maximum daytime value of 0.9 × 106 cm-3 and a minimum nighttime value of 0.4 × 106 cm-3. The highest interference of 2 × 106 cm-3 occurred in a heat wave from 22–29 August, when the air temperature and ozone increased to 40 °C and 100 ppbv, respectively. All observed interferences could be fully explained by the known O3 + H2O interference, which is routinely corrected in FZJ-LIF measurements when no chemical modulation is applied. No evidence for unexplained interference was found during the JULIAC campaign. A kinetic chemical model of the chemical modulation reactor was developed and applied to estimate the possible perturbation of the OH transmission and scavenging efficiency by reactive atmospheric trace gases. These can remove OH by gas phase reactions in the reactor, or produce OH by non-photolytical 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 the determination 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) and would need careful analysis and correction. This implies that chemical modulation, which was developed to eliminate interferences in ambient OH measurements, itself can be subject to interferences that depend on ambient atmospheric conditions.