Optimization of a gas chromatographic unit for measuring BVOCs in ambient air

Abstract

A new online gas chromatographic method dedicated to Biogenic Volatile Organic Compounds (BVOC) analysis was developed for the measurement of a 20 BVOC gaseous mixture (-terpinene and 2-carene) at a time resolution of 90 minutes. The optimized method includes an online Peltier-cooled thermodesorption system sample trap made of Carbopack B coupled to a gas 15 chromatographic system equipped with a 60 m, 0.25 mm i.d. BPX5 column. Eluent was analysed using a flame ionization detection (FID). Potassium iodide was identified as the best ozone scrubber for the 20 BVOC mixture. In order to obtain an accurate quantification of BVOC concentrations, the development of a reliable standard mixture was also required.Quantification of BVOCs was reported with a detection limit ranging from 4 ppt for α-pinene to 19 ppt for sabinene. The main source of uncertainty was the calibration step, stressing the need of certified gaseous standards for a wider panel of 20BVOCs. This new method was applied for the first time to measure BVOCs in a pine forest during the LANDEX-episode-1 field campaign (summer 2017). All targeted BVOCs were detected at least once along the campaign. The two major monoterpenes observed were β-pinene and α-pinene, representing on average 60% of the measured terpenoïds, while isoprene represented only 17%. Uncertainties determined were always below 13% for the six major terpenes. Uncertainties may be larger for the other compounds especially for those presenting a mixing ratio close to the detection limit. 25

Terpenes and their oxidation products in the French Landes forest: insight from Vocus PTR-TOF measurements

“…In this work, the mixing ratios of 250 sesquiterpenes were found to vary from 8.9 ppt to 408.9 ppt in the Landes forest, with an average of 64.5 ppt during the 251 observations. This sesquiterpene level is comparable to that reported by Mermet et al (2019) Vocus PTR-TOF measurements, the average mixing ratio of diterpenes was around 2 ppt in the Landes forest. Considering the 261 low volatility of diterpenes and their potential wall losses inside the inlet tubing and the instrument, the diterpene concentration 262 might be higher.…”

Answers to RC1

“…As a consequence, the calculated uncertainties are significantly higher for these 2 compounds, for which some isomerization or thermodegradation could occur. Indeed, Tenax TA is another well characterized adsorbent but thermodegradtion of monoteprenes may also occur as reported by Coeur et al (1997).…”

“…For some monoterpenes a low response was observed. It is the case of sabinene, terpinolene, 2-carene for example, but not for the most abundant monoterpenes such as b-pinene, a-pinene, limonene or myrcene (Mermet et al, 2019). While isomerization may be an issue for measuring some monoterpenes with this instrument, the most abundant contributors to the OH reactivity are well measured and this issue does not impact the conclusions of this study.…”