| OH reactivity measurement has become an important measurement to constrain the total OH
loss frequency in field experiments. Different techniques have been developed by
various groups. They can be based on flow-tube or pump and probe techniques,
which include direct OH detection by fluorescence, or on a comparative method,
in which the OH loss of a reference species competes with the OH loss of trace
gases in the sampled air. In order to ensure that these techniques deliver equivalent
results, a comparison exercise was performed under controlled conditions. Nine OH
reactivity instruments measured together in the atmosphere simulation chamber
SAPHIR (volume 270 m3) during ten daylong experiments in October 2015 at ambient
temperature (5 to 10∘ C) and pressure (990-1010 hPa). The chemical complexity of air
mixtures in these experiments varied from CO in pure synthetic air to emissions from
real plants and VOC/NOx mixtures representative of urban atmospheres. Potential
differences between measurements were systematically investigated by changing the
amount of reactants (including isoprene, monoterpenes and sesquiterpenes), water
vapour, and nitrogen oxides. Some of the experiments also included the oxidation of
reactants with ozone or hydroxyl radicals, in order to elaborate, if the presence
of oxidation products leads to systematic differences between measurements of
different instruments. Here we present first results of this comparison exercise. |