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@ARTICLE{Cho:891564,
      author       = {Cho, Changmin and Hofzumahaus, Andreas and Fuchs, Hendrik
                      and Dorn, Hans-Peter and Glowania, Marvin and Holland, Frank
                      and Rohrer, Franz and Vardhan, Vaishali and Kiendler-Scharr,
                      Astrid and Wahner, Andreas and Novelli, Anna},
      title        = {{C}haracterization of a chemical modulation reactor ({CMR})
                      for the measurement of atmospheric concentrations of
                      hydroxyl radicals with a laser-induced fluorescence
                      instrument},
      journal      = {Atmospheric measurement techniques},
      volume       = {14},
      number       = {3},
      issn         = {1867-8548},
      address      = {Katlenburg-Lindau},
      publisher    = {Copernicus},
      reportid     = {FZJ-2021-01589},
      pages        = {1851 - 1877},
      year         = {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.},
      cin          = {IEK-8},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IEK-8-20101013},
      pnm          = {211 - Die Atmosphäre im globalen Wandel (POF4-211)},
      pid          = {G:(DE-HGF)POF4-211},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000626739200002},
      doi          = {10.5194/amt-14-1851-2021},
      url          = {https://juser.fz-juelich.de/record/891564},
}