Characterization of a chemical modulation reactor (CMR) for the measurement of atmospheric concentrations of hydroxyl radicals with a laser-induced fluorescence instrument
Authors: Cho Changmin , Hofzumahaus Andreas , Fuchs Hendrik , Dorn Hans-Peter , Glowania Marvin , Holland Frank , Rohrer Franz , Vardhan Vaishali , Kiendler-Scharr Astrid , Wahner Andreas , Novelli Anna 
Published: March 2021

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) 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 (Mao et al., 2012; Novelli
et al., 2014a). In this study, we describe the application and
characterization of a chemical modulation reactor (CMR) applied to the
Forschungszentrum Jülich LIF (FZJ-LIF) instrument in use at the atmospheric simulation chamber
SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). Besides dedicated experiments in
synthetic air, the new technique was extensively tested during the
year-round Jülich Atmospheric Chemistry Project (JULIAC) campaign, in
which ambient air was continuously flowed into the SAPHIR chamber. It
allowed for performing OH measurement comparisons with differential optical
absorption spectroscopy (DOAS) and investigation of interferences in a large variety of chemical and meteorological conditions. Good agreement was
obtained in the LIF–DOAS intercomparison within instrumental accuracies (18 % for LIF and 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 under the conditions of the JULIAC
campaign (rural environment). 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 and 0.6×105 cm−3, respectively, for typical ambient-air
conditions (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 by
the 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 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 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 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). Such
perturbations need to be carefully investigated and corrected for the proper
evaluation of OH concentrations when applying chemical scavenging. 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.