Biogenic volatile organic compounds from green and blue oceans
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Abstract
The biosphere emits a diverse suite of biogenic volatile organic compounds (BVOCs) into the
atmosphere. Although their atmospheric mixing ratios are typically below a few tens of ppb
(parts per billion, 10-9), they play pivotal roles in the Earth system: they modify greenhouse
gas concentrations, contribute to aerosol and cloud formation, and drive stratospheric ozone
depletion. Consequently, a quantitative understanding of BVOC emissions, transport and loss
pathways is a prerequisite for assessing the present-day impact of human activities on the Earth
system.
This dissertation presents a comprehensive investigation of three key BVOCs—chloromethane
(CH3Cl), dimethyl sulfide (DMS) and isoprenoids (isoprene, its oxidation products, and total
monoterpenes)—using Fast GC-MS and PTR-TOF-MS instruments deployed on the HALO research
aircraft, together with the EMAC (ECHAM5/MESSy2) atmospheric chemistry model. Data from
27 measurement flights spanning 0.2–14 km altitude above two contrasting tropical regions,
the "green ocean" of the Amazon rainforest (CAFE-Brazil) and the "blue ocean" of the Pacific
(CAFE-Pacific), are used to study emission patterns, atmospheric chemistry and transport of those
compounds under near-pristine conditions.
A previously unaccounted humidity interference of the Fast GC-MS setup has been identified
and corrected employing stable tropospheric background mixing ratios of Chlorofluorocarbons
(CFCs) as internal standards.
CH3Cl mixing ratios rise to 700 pptv near the Amazon surface and exceed 1000 pptv over the Papua
New Guinea rainforest, well above the tropical background of roughly 550 ± 50 pptv, confirming
both ecosystems as net sources. The markedly higher values over PNG reveal substantial variability
among rainforests. When combined with ten earlier airborne campaigns, the data show that the
tropics dominate global CH3Cl emissions but exhibit strong longitudinal heterogeneity. Uppertropospheric mixing ratios above the western Pacific are roughly 10% higher than ground-based
estimates, implying a larger stratospheric chlorine input than presently assumed.
DMS measurements reveal frequent upper-tropospheric enhancements (up to 56 pptv) in marine
convective outflows, while they rarely exceed the detection limit in Amazonian convective
outflows. EMAC model-observation comparisons show that the Hulswar et al. (2022) marine
inventory best reproduces the CAFE-Pacific data, while the sole global terrestrial inventory (Spiro
et al. 1992) overestimates Amazonian emissions by a factor of three. Among convection schemes,
the Tiedtke-Nordeng parameterisation yields the closest match to the observed vertical DMSiv
profile. These results point to the need for refined terrestrial DMS emissions and more realistic
convection representations in global models.
Observations of isoprene, its oxidation products (isoprene-OP) and total monoterpenes provide
the first continuous vertical and diel profiles throughout the Amazonian troposphere. Night-time
deep convection transports substantial amounts of those compounds (mean±𝜎 of 860 ± 34 pptv
isoprene, 560 ± 200 pptv isoprene-OP, 50 ± 20 pptv monoterpenes) into the upper troposphere,
priming it for rapid photo-chemistry at sunrise. A sensitivity study with EMAC shows that
a 75% reduction in isoprenoid emissions (simulating deforestation) raises lower-tropospheric
hydroxyl radicals and ozone, thereby shortening the methane lifetime by 0.3–0.4 years, but reduces
secondary organic aerosol, leading to a modest net warming effect.