Beschreibung:
<jats:p>Abstract. An operational chemical ionization mass spectrometer (CIMS) for hydroxyl
radical (OH) and sulfuric acid (H2SO4) concentration
measurements was adapted to include observations of OH reactivity,
which is the inverse of OH lifetime, for long-term monitoring at the
Global Atmosphere Watch (GAW) site Hohenpeissenberg (MOHp), Germany.
OH measurement using CIMS is achieved by reacting OH with
SO2, leading to the production of H2SO4, which is then
detected. The adaptation for OH reactivity consists of the
implementation of a second SO2 injection, at a fixed point further
down flow in the sample tube to detect the OH decay caused by
reactions with OH reactants present in the sample. The method can measure OH reactivity from less than 1 to 40 s−1
with the upper limit due to the fixed positioning of the second SO2
injection. To determine OH reactivity from OH concentration
measurements, the reaction time between the two titration zones and OH
wall losses in the sample tube need to be determined accurately through
OH reactivity calibration. Potential measurement artefacts as a result
of HOx recycling in the presence of NO have to be considered.
Therefore, NO contamination from gases used in instrument operation must be
minimized and ambient NO must be measured concurrently to determine the
measurement error. This CIMS system is shown here to perform very well for OH reactivity
below 15 s−1 and NO concentrations below 4 ppb, both values that are
rarely exceeded at the MOHp site. Thus when deployed in suitable chemical
environments, this method can provide valuable continuous long-term
measurements of OH reactivity. The characterization utilizes results
from chamber, laboratory and modelling studies and includes the discussion
and quantification of sources of uncertainties.
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