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

Abstract

Abstract. The measurement of volatile organic compounds (VOCs) can be influenced by ozone (O3), 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 O3 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.

Assessment of aldehyde contributions to PTR-MS m/z 69.07 in indoor air measurements

“…Ozone can play an important role in measurement artifacts indoors ( e.g. Ernle et al 1 ). It is ubiquitously present in outside tropospheric air and is transferred to indoor environments through passive or active ventilation.…”

Identifying and correcting interferences to PTR-ToF-MS measurements of isoprene and other urban volatile organic compounds

“…The key aldehydes observed to produce an interference were nonanal and octanal. The same aldehydes may be produced on inlet surfaces exposed to high ozone concentrations and result in an isoprene artifact (Ernle et al, 2023;Vermeuel et al, 2022).…”

“…Long-chain aldehydes are not routinely reported in urban datasets and the isoprene interference due to aldehyde fragmentation is underappreciated in ambient PTR-ToF-MS datasets. Studies have described how aldehydes produced on the surfaces of inlet tubing interfere with isoprene measured by PTR-ToF-MS in remote forests (Vermeuel et al, 2022) and in the stratosphere (Ernle et al, 2023), and aldehydes emitted from ocean surface waters also interfere with isoprene measurements in laboratory studies and ambient measurements near coastal regions (Kilgour et al, 2021). Longchain aldehydes are likely ubiquitous in cities, and cooking activities are likely a major source of octanal and nonanal resulting in an isoprene interference (Wernis et al, 2022;Peng et al, 2022).…”

How Does Personal Hygiene Influence Indoor Air Quality?

“…In this study, the fast-GCMS collected samples every 3 min using a cryogenic three-step preconcentration followed by GC column separation (DB-624; Agilent Technologies) and detected by a quadrupole mass spectrometer under the selected ion monitoring mode. The operation details and working principles are given elsewhere. − The detection limit (LOD) and the total uncertainty for the measured compounds are listed in the Supporting Information (see Table S4).…”