S. Rottenberger

Emissions of volatile organic compounds from Quercus ilex L. measured by Proton Transfer Reaction Mass Spectrometry under different environmental conditions.

Volatile organic compound (VOC) emissions of the Mediterranean holm oak (Quercus ilex L.) were investigated using a fast Proton Transfer Reaction Mass Spectrometry (PTR-MS) instrument for analysis. This technique is able to measure compounds with a proton affinity higher than water with a high time resolution of 1 s per compound. Hence nearly all VOCs can be detected on-line. We could clearly identify the emission of methanol, acetaldehyde, ethanol, acetone, acetic acid, isoprene, monoterpenes, toluene, and C10-benzenes. Some other species could be tentatively denominated. Among these are the masses 67 (cyclo pentadiene), mass 71 (tentatively attributed to methyl vinyl ketone (MVK) and metacrolein (MACR)), 73 (attributed to methyl ethyl ketone (MEK)), 85 (C6H12 or hexanol), and 95 (vinylfuran or phenol). The emissions of all these compounds (identified as well as nonidentified) together represent 99% of all masses detected and account for a carbon loss of 0.7–2.9% of the net photosynthesis. Of special interest was a change in the emission behavior under changing environmental conditions such as flooding or fast light/dark changes. Flooding of the root system caused an increase of several VOCs between 60 and 2000%, dominated by the emission of ethanol and acetaldehyde, which can be explained by the well described production of ethanol under anoxic conditions of the root system and the recently described subsequent transport and partial oxidation to acetaldehyde within the green leaves. However, ethanol emissions were dominant. Additionally, bursts of acetaldehyde with lower ethanol emission were also found under fast light/dark changes. These bursts are not understood.

Exchange of short‐chain monocarboxylic acids by vegetation at a remote tropical forest site in Amazonia

[1] As part of the project LBA-EUSTACH (European Studies on Trace gases and Atmospheric Chemistry as a contribution to the Large-Scale Biosphere-Atmosphere experiment in Amazonia), the exchange of formic acid and acetic acid between vegetation and the atmosphere was investigated in the wet-to-dry season transition and the dry-to-wet season transition periods in 1999 in Rondonia, Brazil. Direct exchange measurements on the branch level mainly exhibited uptake of formic acid and acetic acid for all plant species in both seasons, although diel, seasonal, and interspecies variations were observed. Even though other physiological and physico-chemical parameters may have contributed, the uptake of organic acids was found to be primarily a function of the ambient atmospheric mixing ratios. The linear dependence suggests a bidirectional exchange behavior of the plants and calculated deposition velocities (0.17-0.23 cm s -1 ), compensation point mixing ratios (0.16-0.30 ppb), and potential emission rates under purified air conditions (0.013-0.031 nmol m -2 s -1 ) are discussed. Vertical profile measurements in and above the primary forest canopy further strengthened the assumption that the forest is rather a sink than a source for organic acids. The generally lower mixing ratios observed within the canopy were indicative of an uptake by vegetation and/or the soil surface. Continuous measurements of the ambient atmospheric mixing ratios at the canopy top revealed strong diel variations in both seasons and a marked seasonality with higher mixing ratios during the dry season, both being mirrored in the variation of observed uptake rates of the plants. High atmospheric concentrations during the dry season were attributed to biomass burning. During the wet season, when biomass burning activity was low, indirect emission by the vegetation, i.e., photochemical oxidation of primarily emitted biogenic reactive hydrocarbons, was assumed to dominantly contribute to the atmospheric burden of the organic acids. The high degree of correlation between atmospheric formic acid and acetic acid indicated that similar atmospheric processes were affecting their mixing ratios.

EXCHANGE OF SHORT-CHAIN ALDEHYDES BETWEEN AMAZONIAN VEGETATION AND THE ATMOSPHERE

As a part of the LBA-EUSTACH (EUropean Studies on Trace Gases and Atmospheric CHemistry as a contribution of the Large-scale Biosphere-Atmosphere ex- periment in Amazonia) project, the exchanges of formaldehyde (HCHO) and acetaldehyde (CH3CHO) between Amazonian vegetation and the atmosphere were investigated by branch enclosures and compared with gradient measurements during the wet-to-dry transition and dry-to-wet-transition periods at a remote forest site in Brazil, 1999. Branch enclosure measurements of several tree species showed emission as well as deposition of short-chain aldehydes, but fluxes were clearly dominated by deposition during both seasons. This bidirectional exchange was found to depend mainly on the actual ambient concentrations of these compounds and to exhibit a compensation point below 0.6 ppb of the compound in air with deposition velocities between 0.16 and 0.21 cm/s during the wet-to-dry season. During the dry-to-wet season, the deposition velocities and the compensation point in- creased. Under the clean air conditions of the wet-to-dry season, the major pathway for the aldehyde uptake was via leaf stomata. For HCHO, a mesophyll resistance of the same order of magnitude as the stomatal resistance contributed to the total leaf surface resistance, whereas the mesophyll resistance for CH3CHO was small, allowing a rapid uptake. This finding indicates a major contribution of metabolic consumption processes in addition to physical and chemical processes to the overall resistance. During the dry-to-wet period, when ambient air concentrations substantially increased, we found indications for an ad- ditional deposition to the leaf cuticle. Vertical gradient measurements of ambient air con- centrations in and above the canopy closely agreed with the branch enclosure studies and confirmed that the forest acts rather as a sink than as a direct source for HCHO and CH3CHO. Diel courses of ambient air concentrations and ratios of HCHO and CH3CHO above the canopy suggest photochemical oxidation of biogenically or pyrogenically emitted precursor compounds as the major sources for short-chain aldehydes in the tropical atmosphere.

Root anoxia effects on physiology and emissions of volatile organic compounds (VOC) under short- and long-term inundation of trees from Amazonian floodplains

Volatile organic compound (VOC) emissions are affected by a variety of biotic and abiotic factors such as light intensity, temperature, CO2 and drought. Another stress factor, usually overlooked but very important for the Amazon region, is flooding. We studied the exchange of VOCs in relation to CO2 exchange and transpiration of 8 common tree species from the Amazonian floodplain forest grown up from seeds using a dynamic enclosure system. Analysis of volatile organics was performed by PTR-MS fast online measurements. Our study confirmed emissions of ethanol and acetaldehyde at the beginning of root anoxia after inundation, especially in less anoxia adapted species such as Vatairea guianensis, but not for Hevea spruceana probably due to a better adapted metabolism. In contrast to short-term inundation, long-term flooding of the root system did not result in any emission of ethanol or/and acetaldehyde. Emission of other VOCs, such as isoprenoids, acetone, and methanol exhibited distinct behavior related to the origin (igapó or várzea type of floodplain) of the tree species. Also physiological activities exhibited different response patterns for trees from igapó or várzea. In general, isoprenoid emissions increased within the course of some days of short-term flooding. After a long period of waterlogging, VOC emissions decreased considerably, along with photosynthesis, transpiration and stomatal conductance. However, even under long-term testing conditions, two tree species did not show any significant decrease or increase in photosynthesis. In order to understand ecophysiological advantages of the different responses we need field investigations with adult tree species.

Design and field application of an automated cartridge sampler for VOC concentration and flux measurements

One of the major limitations in advancing the understanding of tropospheric ozone and aerosol generation and developing strategies for their control is the technical ability to accurately measure volatile organic compounds (VOCs). This paper describes the design of a constant flow VOC sampler. The versatile sampler can be used for fully automated concentration and flux measurements of VOCs. The sampler incorporates a microprocessor control unit and provides highly accurate mass flow control and significant ease of operation. Sampling sequences can be programmed directly or by remote control through a PC. All important operational parameters necessary for a complete sampling audit trail are logged. Compact weatherproof housings and low power consumption allow operation at remote sites and locations which are sensitive to disturbances or have restricted access. Inner wetted surfaces of the sampler are constructed from non-contaminating materials that do not sorb or emit VOC, and thus permit the collection of representative samples even in environments with very low VOC concentrations. The cartridge magazine provides a maximum of 20 sequential cartridge samples, which allows for long-term air quality assessments. In the dual channel mode, two samples can be collected simultaneously through two independent sample loops, providing ten sequential sample pairs. This design allows the parallel collection of (a) quality assurance backup samples, (b) samples on two different types of cartridges/sorbents to allow a variety of analyses, or (c) differential samples for flux measurements using enclosure, aerodynamic profile, or relaxed eddy accumulation (REA) methods. Field applications including airplane profile measurements above a tropical rainforest area, as well as gradient and REA measurements over a mid-latitude mixed forest stand are described, and demonstrate the validity and flexibility of the system. In particular, the application of the VOC sampler as an integrated part of a REA system is emphasized.