Trace Gas Dynamics: VOCs & BVOCs
A dual-track interactive platform — foundational concepts for newcomers, advanced kinetics and measurement science for researchers.
What Are Volatile Organic Compounds?
VOCs are carbon-based molecules with high vapour pressure at ambient temperatures. They partition readily into the gas phase and participate in atmospheric chemistry across scales from local air quality to global climate.
Phase
& Emission
VOC
Reactions
Volatility-driven phase transfer links emission sources to atmospheric impacts across all scales.
BVOCs: The Plant–Atmosphere Interface
Biogenic VOCs are synthesised primarily through the methylerythritol phosphate (MEP) pathway in chloroplasts. Emission rates are controlled by temperature and photosynthetically active radiation (PAR).
CH₂=C(CH₃)–CH=CH₂ · MW = 68 g mol⁻¹
α-Pinene (C₁₀H₁₆):
Bicyclic monoterpene — major SOA precursor
Limonene (C₁₀H₁₆):
Monocyclic monoterpene — citrus / forest scent
Why They Matter
- Tropospheric O₃ formation: VOCs + NOₓ + hν → O₃ (ground-level smog)
- Secondary Organic Aerosol (SOA): Oxidation products condense to the particle phase, affecting air quality and climate
- OH radical budget: VOCs are the dominant sink for atmospheric OH — the atmosphere's "detergent"
- Climate feedback: SOA from BVOCs alters cloud condensation nuclei (CCN) concentrations and regional precipitation
- Methane lifetime: Amazon BVOC reductions shorten CH₄ lifetime by ~0.3–0.4 yr (Tripathi et al., 2025)
How We Measure Them
- GC-MS / FID: Gas chromatography with mass spectrometry — gold standard for speciation; requires pre-concentration on adsorbent traps.
- PTR-TOF-MS (Ionicon 8000): Proton-transfer-reaction time-of-flight — real-time detection using H₃O⁺ reagent ions; 1 s time resolution; used aboard HALO aircraft in CAFE-Brazil (Tripathi et al., 2025).
- FTIR / DOAS: Open-path optical methods for column-integrated measurements.
- Satellite (TROPOMI HCHO): Column formaldehyde as proxy for isoprene emission; complements aircraft in-situ data.
Atmospheric VOC Removal Pathways
Relative contribution of each sink to global VOC removal, based on modelling studies.
Impacts of Convection, Chemistry & Forest Clearing on BVOCs over the Amazon
Nidhi Tripathi, B.E. Krumm, A. Edtbauer, A. Ringsdorf, N. Wang, M. Kohl, R. Vella, L.A.T. Machado, A. Pozzer, J. Lelieveld & J. Williams · Max Planck Institute for Chemistry, Mainz
DOI: 10.1038/s41467-025-59953-2 · CAFE-Brazil campaign (Dec 2022–Jan 2023) · HALO research aircraft · 15 flights · 0.3–14 km altitude
Night-time deep convection exports unreacted BVOCs to 9–14 km; daytime convection is rendered ineffective by photochemical loss (OH lifetime ~1–2 h).
Isoprene and monoterpenes ~4× higher over pristine forest vs. deforested pasture at 900 m (Flight 06, Rondônia).
Pre-dawn BVOC accumulation (peak 03:00–06:00 at 9–14 km) primes the upper troposphere for rapid photochemistry and new particle formation at sunrise.
75% BVOC reduction → ~6 W m⁻² regional surface warming; −0.3–0.4 yr methane lifetime; net effect: enhanced climate warming through O₃ and aerosols.
Vertical profiles of isoprene & isoprene-OP — all CAFE-Brazil flights (Fig. 2; Tripathi et al., 2025)
Oxidation Kinetics & Lifetimes
The atmospheric lifetime τ of a BVOC against the three major oxidants:
Upper-tropospheric [OH] is 10–50× lower than at the surface, extending BVOC lifetimes by 2–5× (Tripathi et al., 2025).
| Compound | kOH (cm³ s⁻¹) | τ surface | τ UT (9–14 km) |
|---|---|---|---|
| Isoprene | 1.0 × 10⁻¹⁰ | ~1.4 h | ~4–6 h |
| α-Pinene | 5.3 × 10⁻¹¹ | ~2.6 h | ~8–12 h |
| β-Caryophyllene | 2.0 × 10⁻¹⁰ | ~42 min | ~2–3 h |
| Benzene (ref.) | 1.2 × 10⁻¹² | ~9.4 d | ~25–30 d |
*UT lifetimes based on reduced [OH] at altitude; Tripathi et al. (2025)
Isoprene Oxidation: NOₓ-Dependent Fate
Fate of ISOPO₂ determines O₃ vs. SOA formation. In the Amazon UT (high-NOₓ from lightning), convected ISOPO₂ drives photochemical O₃ production (Tripathi et al., 2025).
Isoprene
6 isomers
products
ISOPO₂ + NO → ISOPO + NO₂
NO₂ + hν → O(³P) + NO → O₃
→ Net O₃ formation
ISOPO₂ + HO₂ → ISOPOOH
ISOPOOH + OH → IEPOX → SOA
→ Aerosol dominates
Diel Cycle of BVOCs at Three Altitude Regimes (Fig. 3 — Tripathi et al., 2025)
Surface peak (12:00–16:00) is solar-driven; the upper-troposphere peak shifts to 03:00–06:00 due to nocturnal convective export and near-zero OH at night. Three altitude regimes: 320 m (ATTO), 3–9 km, 9–14 km.
Isoprene — Fig. 3a
Isoprene-OP — Fig. 3b
Monoterpenes — Fig. 3c
BVOC Emission Sensitivity: O₃ & OH Profiles (Fig. 5 — Tripathi et al., 2025)
EMAC model (200×200 km, 90 levels): relative change in O₃ and OH over the Amazon for −50%, −75%, +50% BVOC perturbation.
Vertical profiles of relative change (%) from Figs. 5a & 5b; EMAC nudged to ERA5 reanalysis (Tripathi et al., 2025)
Convective vs. Non-convective Isoprene Profile
~20% of boundary-layer isoprene reaches the UT during convective outflow. Night-time UT peak (0.57 ppbv at 12–13 km) exceeds daytime (0.38 ppbv) because photochemical loss is absent overnight.
Based on Fig. 2 main profiles and day/night UT insets (Tripathi et al., 2025)
Interactive: NOₓ-Dependent SOA Yield
Move the slider to explore isoprene SOA yield as a function of NOₓ regime. Amazon pristine forest sits at the left; deforested/biomass-burning regions shift right.
Based on chamber studies (Kroll et al. 2006, Xu et al. 2015); Amazon constraints from Tripathi et al. (2025)
Global BVOC Emission Budget
Total ~760 Tg C yr⁻¹; isoprene 70% and monoterpenes 11% of terrestrial BVOC (Guenther et al. 2012). Amazon = 40% of global BVOC (Tripathi et al., 2025).
Isoprene Emission: Guenther Temperature Response
Tripathi et al. (2025) applied MEGAN/G93 with MODIS land surface temperature. Emission peaks ~38–40 °C then falls steeply above TM = 314 K.
VOC Fingerprint: Pristine vs. Deforested Amazon (Flight 06)
At 900 m over Rondônia: isoprene and monoterpenes ~4× higher over pristine forest; isoprene-OP only ~1.7× higher due to advection from adjacent forest.
Relative composition from Flight 06 airborne measurements (Fig. 4; Tripathi et al., 2025)
Total OH Reactivity Partitioning (Amazon)
OH reactivity (s⁻¹) measures the total OH sink rate. BVOCs dominate; ~18% remains unidentified. Hover bars for values.
ATom campaign data; adapted from Pfannerstill et al. (2021) and Tripathi et al. (2025)
Evolution of BVOC Measurement Techniques
Canister + GC-FID — Offline analysis; cryogenic preconcentration; ppb detection limits.
PTR-MS (Lindinger et al.) — Real-time H₃O⁺ CIMS; no sample prep; limited mass resolution.
PTR-TOF-MS (Ionicon 8000) — High resolution (m/Δm >5000); 1 s data; used aboard HALO in CAFE-Brazil (Tripathi et al., 2025).
CAFE-Brazil campaign — 0.3–14 km diel profiles; PTR-TOF-MS + GC-MS cross-calibrated with ATTO ground site; 15 research flights from Manaus.