Influence of ozone and humidity on PTR-MS and GC-MS VOC measurements with and without a Na2S2O3 ozone scrubber

The measurement of volatile organic compounds (VOCs) can be influenced by ozone (O-3), resulting in sampling artefacts that corrupt the data obtained. Published literature reports both positive (false enhancements of signal) and negative (loss of signal) interference in VOC data due to ozonolysis occurring in the sample gas. To assure good data quality it is essential to be aware of such interfering processes, to characterize them and to try to minimize the impact with a suitable sampling setup. Here we present results from experiments with a sodium thiosulfate ozone scrubber (Na2S2O3), which is a cost-effective and easily applied option for O-3 scavenging during gas-phase sampling. Simultaneous measurement of selected organic trace gases using gas chromatography-mass spectrometry and proton transfer reaction-mass spectrometry was performed at different ozone levels (0-1 ppm) and different relative humidities (0 %-80 %). In this way both tropospheric and stratospheric conditions were examined. The measured data show that several carbonyl compounds including acetaldehyde, acetone and propanal show artificial signal enhancement when ozone is present at higher concentrations (> 150 ppb) in dry air, while analytes with double bonds like isoprene (measured with GC-MS) and terpenes show lower signals due to reaction with ozone. Both effects can be eliminated or in the case of sesquiterpenes substantially reduced by using Na2S2O3 impregnated quartz filters in the inlet line. With the chosen scrubbing material, relative humidity (RH) substantially improves the scrubbing efficiency. Under surface conditions between 50 %-80 % RH, the filter allows for accurate measurement of all species examined.