Volatile organic compounds at the air-sea interface : gas exchange rates, oceanic emissions and the effect of ocean acidification

Oceans are key sources and sinks in the global budgets of significant atmospheric trace gases, termed Volatile Organic Compounds (VOCs). Despite their low concentrations, these species have an important role in the atmosphere, influencing ozone photochemistry and aerosol physics. Surprisingly, little work has been done on assessing their emissions or transport mechanisms and rates between ocean and atmosphere, all of which are important when modelling the atmosphere accurately.rnA new Needle Trap Device (NTD) - GC-MS method was developed for the effective sampling and analysis of VOCs in seawater. Good repeatability (RSDs <16 %), linearity (R2 = 0.96 - 0.99) and limits of detection in the range of pM were obtained for DMS, isoprene, benzene, toluene, p-xylene, (+)-α-pinene and (-)-α-pinene. Laboratory evaluation and subsequent field application indicated that the proposed method can be used successfully in place of the more usually applied extraction techniques (P&T, SPME) to extend the suite of species typically measured in the ocean and improve detection limits. rnDuring a mesocosm CO2 enrichment study, DMS, isoprene and α-pinene were identified and quantified in seawater samples, using the above mentioned method. Based on correlations with available biological datasets, the effects of ocean acidification as well as possible ocean biological sources were investigated for all examined compounds. Future ocean's acidity was shown to decrease oceanic DMS production, possibly impact isoprene emissions but not affect the production of α-pinene. rnIn a separate activity, ocean - atmosphere interactions were simulated in a large scale wind-wave canal facility, in order to investigate the gas exchange process and its controlling mechanisms. Air-water exchange rates of 14 chemical species (of which 11 VOCs) spanning a wide range of solubility (dimensionless solubility, α = 0:4 to 5470) and diffusivity (Schmidt number in water, Scw = 594 to 1194) were obtained under various turbulent (wind speed at ten meters height, u10 = 0:8 to 15ms-1) and surfactant modulated (two different sized Triton X-100 layers) surface conditions. Reliable and reproducible total gas transfer velocities were obtained and the derived values and trends were comparable to previous investigations. Through this study, a much better and more comprehensive understanding of the gas exchange process was accomplished. The role of friction velocity, uw* and mean square slope, σs2 in defining phenomena such as waves and wave breaking, near surface turbulence, bubbles and surface films was recognized as very significant. uw* was determined as the ideal turbulent parameter while σs2 described best the related surface conditions. A combination of both uw* and σs2 variables, was found to reproduce faithfully the air-water gas exchange process. rnA Total Transfer Velocity (TTV) model provided by a compilation of 14 tracers and a combination of both uw* and σs2 parameters, is proposed for the first time. Through the proposed TTV parameterization, a new physical perspective is presented which provides an accurate TTV for any tracer within the examined solubility range. rnThe development of such a comprehensive air-sea gas exchange parameterization represents a highly useful tool for regional and global models, providing accurate total transfer velocity estimations for any tracer and any sea-surface status, simplifying the calculation process and eliminating inevitable calculation uncertainty connected with the selection or combination of different parameterizations.rnrn

Simulating short lived carbonaceous compounds with an atmospheric chemistry general circulation model

Ein neu entwickeltes globales Atmosphärenchemie- und Zirkulationsmodell (ECHAM5/MESSy1) wurde verwendet um die Chemie und den Transport von Ozonvorläufersubstanzen zu untersuchen, mit dem Schwerpunkt auf Nichtmethankohlenwasserstoffen. Zu diesem Zweck wurde das Modell durch den Vergleich der Ergebnisse mit Messungen verschiedenen Ursprungs umfangreich evaluiert. Die Analyse zeigt, daß das Modell die Verteilung von Ozon realistisch vorhersagt, und zwar sowohl die Menge als auch den Jahresgang. An der Tropopause gibt das Modell den Austausch zwischen Stratosphäre und Troposphäre ohne vorgeschriebene Flüsse oder Konzentrationen richtig wieder. Das Modell simuliert die Ozonvorläufersubstanzen mit verschiedener Qualität im Vergleich zu den Messungen. Obwohl die Alkane vom Modell gut wiedergeben werden, ergibt sich einige Abweichungen für die Alkene. Von den oxidierten Substanzen wird Formaldehyd (HCHO) richtig wiedergegeben, während die Korrelationen zwischen Beobachtungen und Modellergebnissen für Methanol (CH3OH) und Aceton (CH3COCH3) weitaus schlechter ausfallen. Um die Qualität des Modells im Bezug auf oxidierte Substanzen zu verbessern, wurden einige Sensitivitätsstudien durchgeführt. Diese Substanzen werden durch Emissionen/Deposition von/in den Ozean beeinflußt, und die Kenntnis über den Gasaustausch mit dem Ozean ist mit großen Unsicherheiten behaftet. Um die Ergebnisse des Modells ECHAM5/MESSy1 zu verbessern wurde das neue Submodell AIRSEA entwickelt und in die MESSy-Struktur integriert. Dieses Submodell berücksichtigt den Gasaustausch zwischen Ozean und Atmosphäre einschließlich der oxidierten Substanzen. AIRSEA, welches Informationen über die Flüssigphasenkonzentration des Gases im Oberflächenwasser des Ozeans benötigt wurde ausgiebig getestet. Die Anwendung des neuen Submodells verbessert geringfügig die Modellergebnisse für Aceton und Methanol, obwohl die Verwendung einer vorgeschriebenen Flüssigphasenkonzentration stark den Erfolg der Methode einschränkt, da Meßergebnisse nicht in ausreichendem Maße zu Verfügung stehen. Diese Arbeit vermittelt neue Einsichten über organische Substanzen. Sie stellt die Wichtigkeit der Kopplung zwischen Ozean und Atmosphäre für die Budgets vieler Gase heraus.

Chemical interaction between ocean and atmosphere

The exchange of chemical constituents between ocean and atmosphere provides potentially important feedback mechanisms in the climate system. The aim of this study is to develop and evaluate a chemically coupled global atmosphere-ocean model. For this, an atmosphere-ocean general circulation model with atmospheric chemistry has been expanded to include oceanic biogeochemistry and the process of air-sea gas exchange. The calculation of seawater concentrations in the oceanic biogeochemistry submodel has been expanded from DMS, CO₂

Measurement of volatile organic compounds and total OH reactivity in the atmosphere

Volatile organic compounds play a critical role in ozone formation and drive the chemistry of the atmosphere, together with OH radicals. The simplest volatile organic compound methane is a climatologically important greenhouse gas, and plays a key role in regulating water vapour in the stratosphere and hydroxyl radicals in the troposphere. The OH radical is the most important atmospheric oxidant and knowledge of the atmospheric OH sink, together with the OH source and ambient OH concentrations is essential for understanding the oxidative capacity of the atmosphere. Oceanic emission and / or uptake of methanol, acetone, acetaldehyde, isoprene and dimethyl sulphide (DMS) was characterized as a function of photosynthetically active radiation (PAR) and a suite of biological parameters, in a mesocosm experiment conducted in the Norwegian fjord. High frequency (ca. 1 minute-1) methane measurements were performed using a gas chromatograph - flame ionization detector (GC-FID) in the boreal forests of Finland and the tropical forests of Suriname. A new on-line method (Comparative Reactivity Method - CRM) was developed to directly measure the total OH reactivity (sink) of ambient air. It was observed that under conditions of high biological activity and a PAR of ~ 450 μmol photons m-2 s-1, the ocean acted as a net source of acetone. However, if either of these criteria was not fulfilled then the ocean acted as a net sink of acetone. This new insight into the biogeochemical cycling of acetone at the ocean-air interface has helped to resolve discrepancies from earlier works such as Jacob et al. (2002) who reported the ocean to be a net acetone source (27 Tg yr-1) and Marandino et al. (2005) who reported the ocean to be a net sink of acetone (- 48 Tg yr-1). The ocean acted as net source of isoprene, DMS and acetaldehyde but net sink of methanol. Based on these findings, it is recommended that compound specific PAR and biological dependency be used for estimating the influence of the global ocean on atmospheric VOC budgets. Methane was observed to accumulate within the nocturnal boundary layer, clearly indicating emissions from the forest ecosystems. There was a remarkable similarity in the time series of the boreal and tropical forest ecosystem. The average of the median mixing ratios during a typical diel cycle were 1.83 μmol mol-1 and 1.74 μmol mol-1 for the boreal forest ecosystem and tropical forest ecosystem respectively. A flux value of (3.62 ± 0.87) x 1011 molecules cm-2 s-1 (or 45.5 ± 11 Tg CH4 yr-1 for global boreal forest area) was derived, which highlights the importance of the boreal forest ecosystem for the global budget of methane (~ 600 Tg yr-1). The newly developed CRM technique has a dynamic range of ~ 4 s-1 to 300 s-1 and accuracy of ± 25 %. The system has been tested and calibrated with several single and mixed hydrocarbon standards showing excellent linearity and accountability with the reactivity of the standards. Field tests at an urban and forest site illustrate the promise of the new method. The results from this study have improved current understanding about VOC emissions and uptake from ocean and forest ecosystems. Moreover, a new technique for directly measuring the total OH reactivity of ambient air has been developed and validated, which will be a valuable addition to the existing suite of atmospheric measurement techniques.