In-Situ Field Measurements of Isoprene, Its Oxidation Products and Selected Monoterpenes in a Eucalyptus Forest

Importance of the Topic

Field measurements of biogenic volatile organic compounds (VOCs) such as isoprene and monoterpenes are essential for understanding their role in atmospheric chemistry, air quality and climate processes. Conventional sampling methods rely on cryogenic refocusing, which is impractical for remote locations and may introduce sample degradation. Developing a robust, rapid, on-site analytical procedure allows continuous monitoring of natural emissions, improving data reliability and supporting atmospheric modelling.

Objectives and Study Overview

This study aimed to perform in-situ monitoring of isoprene, its key oxidation products (methacrolein and methyl vinyl ketone) and selected monoterpenes in a Portuguese eucalyptus forest. The goals were to:

• Implement a sorbent tube sampling approach without cryogenic refocusing.
• Use a programmed temperature vaporisation (PTV) injector coupled to a wide-bore GC column for rapid desorption and analysis.
• Evaluate diurnal and multi-day concentration profiles in a remote canopy environment.

Instrumentation Used

• Sampling tower at 16 m canopy height with battery-powered personal air sampler (15 mL/min for 15 min).
• Sorbent tubes packed with Tenax TA and Carbosieve S-III, maintained at –10 °C to prevent breakthrough.
• GC-8000 equipped with Optic 400 PTV injector and CO₂ oven cooling.
• 60 m × 0.53 mm ID dimethyl polysiloxane column (3 μm film).
• Flame ionisation detector (FID) with helium carrier gas at ~28–30 mL/min.

Methodology and Instrumentation

Ambient air was drawn through cooled sorbent tubes to collect 225 mL samples. Breakthrough tests showed no losses at –10 °C with maximum sample volume of 500 mL. Tubes were inserted directly into the PTV injector in place of the liner. A rapid temperature ramp (30 °C to 240 °C at 16 °C/min) desorbed analytes onto the GC column. The GC run started at 5 °C (3 min), then ramped up to 240 °C in three stages. High helium flow ensured rapid sorbent reconditioning, allowing a full analysis cycle every 70 minutes without carry-over.

Key Results and Discussion

• Continuous on-site analysis over five days captured clear diurnal trends: low morning isoprene levels under cloud cover rose sharply after noon alongside MACR and MVK.
• Field calibration for isoprene (0.2–100 ppb) was linear and stable, with detection limits around 0.2 ppb.
• Monoterpene separation was limited under rapid conditions; only well-resolved compounds (e.g. α-pinene) were quantified accurately.
• Results agreed with previous forest measurements, validating the rapid PTV approach.

Benefits and Practical Applications

• Eliminates cryogenic cooling and reduces logistic constraints in remote sites.
• Immediate analysis (<2 min post-sampling) minimizes degradation and artefact formation.
• High sorbent reusability cuts consumable costs and reduces variability between tubes.
• Potential to adapt for automated, unattended field monitoring of biogenic emissions.

Future Trends and Applications

Advancements may include integration with mass spectrometric detectors for compound identification, improved column phases for enhanced monoterpene resolution, miniaturised modular systems for mobile platforms, and full automation through programmable sampling cycles. Coupling real-time VOC data with meteorological and photochemical models will enhance prediction of air quality and climate impacts.

Conclusion

The PTV-GC method with Tenax/Carbosieve sorbent tubes enabled accurate, frequent on-site measurements of isoprene, its oxidation products and selected monoterpenes without cryogenic refocusing. Rapid desorption and high carrier flows allowed a 70-minute sampling–analysis cycle and preserved sorbent integrity. Immediate analysis reduces sample artefacts and supports potential automation for continuous monitoring of biogenic VOC emissions.

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