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@ARTICLE{Marno:890286,
      author       = {Marno, Daniel and Ernest, Cheryl and Hens, Korbinian and
                      Javed, Umar and Klimach, Thomas and Martinez, Monica and
                      Rudolf, Markus and Lelieveld, Jos and Harder, Hartwig},
      title        = {{C}alibration of an airborne
                      ${HO}\<sub\>\<i\>x\</i\>\</sub\>$ instrument using the {A}ll
                      {P}ressure {A}ltitude-based {C}alibrator for
                      ${HO}\<sub\>\<i\>x\</i\>\</sub\>$ {E}xperimentation
                      ({APACHE})},
      journal      = {Atmospheric measurement techniques},
      volume       = {13},
      number       = {5},
      issn         = {1867-8548},
      address      = {Katlenburg-Lindau},
      publisher    = {Copernicus},
      reportid     = {FZJ-2021-00867},
      pages        = {2711 - 2731},
      year         = {2020},
      abstract     = {Laser-induced fluorescence (LIF) is a widely used technique
                      for both laboratory-based and ambient atmospheric chemistry
                      measurements. However, LIF instruments require calibrations
                      in order to translate instrument response into
                      concentrations of chemical species. Calibration of LIF
                      instruments measuring OH and HO2 (HOx) typically involves
                      the photolysis of water vapor by 184.9 nm light, thereby
                      producing quantitative amounts of OH and HO2. For
                      ground-based HOx instruments, this method of calibration is
                      done at one pressure (typically ambient pressure) at the
                      instrument inlet. However, airborne HOx instruments can
                      experience varying cell pressures, internal residence times,
                      temperatures, and humidity during flight. Therefore,
                      replication of such variances when calibrating in the lab is
                      essential to acquire the appropriate sensitivities. This
                      requirement resulted in the development of the APACHE (All
                      Pressure Altitude-based Calibrator for HOx Experimentation)
                      chamber to characterize the sensitivity of the airborne
                      LIF-FAGE (fluorescence assay by gas expansion) HOx
                      instrument, HORUS, which took part in an intensive airborne
                      campaign, OMO-Asia 2015. It utilizes photolysis of water
                      vapor but has the additional ability to alter the pressure
                      at the nozzle of the HORUS instrument. With APACHE, the
                      HORUS instrument sensitivity towards OH
                      (26.1–7.8 cts s−1 pptv−1 mW−1, $±22.6 \%$
                      1σ; cts stands for counts by the detector) and HO2
                      (21.2–8.1 cts s−1 pptv−1 mW−1, $±22.1 \%$
                      1σ) was characterized to the external pressure range at the
                      instrument nozzle of 227–900 mbar. Measurements
                      supported by a computational fluid dynamics model, COMSOL
                      Multiphysics, revealed that, for all pressures explored in
                      this study, APACHE is capable of initializing a homogenous
                      flow and maintaining near-uniform flow speeds across the
                      internal cross section of the chamber. This reduces the
                      uncertainty regarding average exposure times across the
                      mercury (Hg) UV ring lamp. Two different actinometrical
                      approaches characterized the APACHE UV ring lamp flux as
                      6.37×1014(±1.3×1014) photons cm−2 s−1. One
                      approach used the HORUS instrument as a transfer standard in
                      conjunction with a calibrated on-ground calibration system
                      traceable to NIST standards, which characterized the UV ring
                      lamp flux to be 6.9(±1.1)×1014 photons cm−2 s−1.
                      The second approach involved measuring ozone production by
                      the UV ring lamp using an ANSYCO O3 41 M ozone monitor,
                      which characterized the UV ring lamp flux to be
                      6.11(±0.8)×1014 photons cm−2 s−1. Data presented
                      in this study are the first direct calibrations of an
                      airborne HOx instrument, performed in a controlled
                      environment in the lab using APACHE.},
      cin          = {IEK-8},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IEK-8-20101013},
      pnm          = {243 - Tropospheric trace substances and their
                      transformation processes (POF3-243)},
      pid          = {G:(DE-HGF)POF3-243},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000537699800002},
      doi          = {10.5194/amt-13-2711-2020},
      url          = {https://juser.fz-juelich.de/record/890286},
}