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Grants:"Biogenic VOC emissions under present and elevated CO2 conditions: Implications for future air quality and climate", National Science Foundation, , 2004 - 2008
Dramatic increases in atmospheric CO2 concentrations are predictable during this century. Because of this and other climate changes, plant metabolism will be altered. We hypothesize these changes will result in unpredictable changes in biogenic emissions of nonmethane hydrocarbons (NMHCs), oxygenated hydrocarbons (OxHCs) and halocarbons (collectively referred to as volatile organic compounds or VOCs). VOCs play a crucial role in regional and global photochemistry, including organic aerosol formation. These processes greatly impact ozone formation, hydroxyl production and consumption, the abundance of some greenhouse and stratospheric ozone depleting gases, and production of secondary organic aerosols (SOAs). Because only a small number of gases emitted from a few plants have been studied under high CO2 concentrations, it is uncertain what the future biogenic response will be. By measuring fluxes of several gases from numerous plants equilibrated at elevated CO2 levels, we will be able to estimate future biogenic emissions. These data are essential for reliable model simulations of future air quality.
We have two sampling foci for this study: The main focus is measurement of halocarbon, NMHCs, and OxHCs fluxes from plants and soil at existing CO2 enrichment experiments. The FACE (Free-Air-Carbon dioxide-Enrichment) experiments are a unique opportunity to measure emissions from plants in an "equilibrated" high CO2 environment and compare them to background emissions at the same location. We plan to make sampling trips to all six U.S. FACE sites currently operating. We will sample the biogenic emissions from all of the plant species grown at these sites as well as the soils.
At each field site, we will measure the exchange of NMHCs, OxHCs, and halocarbons from the major plant species and the soil surface. The method of measurement of trace gas emissions from leaves is based on that described by Lerdau and Keller . This system design has been used by senior personnel (J. Sparks) for laboratory measurements [Sparks et al. 2003; Teklemariam and Sparks, in review] and during successful field campaigns in Panama [Sparks et al. 2000] and North Carolina [Sparks et al., in review]. Measurements of leaf flux (Fig. 1A) will be made using a leaf chamber that encloses 6 cm2 of leaf area connected to a portable gas-exchange system (model LI-6400, LiCor Inc., Lincoln, Nebraska).
During measurement, the leaf will be monitored for equilibrium by tracking the photosynthetic rate (Amax) and the leaf-internal CO2 concentration (ci) to stability. After stability is reached, the effluent air from the cuvette will be monitored using a can sampling system or a proton transfer reaction mass spectrometer (PTRMS)(Fig. 1A). The source air to the system will be humidity controlled VOC-free zero air. Relative humidity in the source air will be controlled using vaporized permanganate distilled water. The water will be vaporized using a peristaltic pump to flow water through a heated line and then flowing the VOC free air through the water vapor. The relative humidity of the source air will be controlled by the water flow rate through the heated line. As stated previously, the LI-6400 is equipped with an adjustable CO2 mixing system downstream of the inlet of the entry point of humidified VOC-free air. This will allow for extremely low background of NMHCs, OxHCs and halocarbons in the sample pathway.
Soil exchange of NMHCs, OxHCs and halocarbons will be measured using a static soil chamber method. The soil chamber will be constructed of 1/16" silicosteel with a diameter of 25 cm. The chamber will be silonized (Entech Instruments, Inc., CA) to create a non-reactive surface. The system is designed so the LI-6400 sensor head will attach directly to the soil chamber to directly sample the chamber headspace. The humidified zero air (as described for leaf measurements) will flow through the sensor head to the chamber headspace. Sub-samples of the outflow will be taken every five minutes for canister analysis or the effluent airstream will be monitored continuously by the PTRMS. Temperature of the chamber headspace, soil surface and at 10 cm soil depth will be monitored throughout the sample period. Soil moisture manipulation experiments will be performed in the field. After the soil flux measurements are made at the ambient soil moisture, deionized water simulating an average precipitation event for the site will be sprinkled over the same area, allowed to infiltrate and additional flux samples taken. Bulk density, soil moisture and soil pH will be measured by taking soil samples after both the pre-wetted and wetted flux measurements. Soil carbon and nitrogen will be determined using a CHN analyzer. Metal and halide analysis will be performed using extraction and ion chromatography. The Forest Ecosystems Group laboratory and the Water Quality Analysis Laboratory both located at UNH have the facilities to perform the soil analysis and foliar CHN analysis will be conducted at Cornell University using an elemental analyzer.
We propose to use well-established techniques developed at the University of California-Irvine and in the Sive and Varner laboratories at the University of New Hampshire to perform high precision and accurate measurements of C2-C10 NMHCs, C1-C2 halocarbons, C1-C5 alkyl nitrates and selected OxHCs. Samples will be collected in both 1-liter silica lined canisters (Entech Instruments, Simi Valley, CA) and 2-liter electropolished stainless steel canisters (University of California, Irvine, CA). The samples are collected in the field and then returned to the laboratory for analysis by gas chromatography using flame ionization and electron capture detection in conjunction with mass spectrometry.
A plan is presented in this proposal to quantify the biogenic emissions of nonmethane hydrocarbons (NMHCs), oxygenated hydrocarbons (OxHCs) and halocarbons from ecosystems both representative of the present global environment and ecosystems where CO2 is currently being enhanced (~50%) to levels predicted in the middle of this century. Our goal is to create and utilize an extensive database for use in regional and global atmospheric chemistry models. This work is essential for more realistic modeling studies of present and future climate. Anticipated changes in air composition and climate will feedback on the biosphere. More reliable atmospheric models will better predict these changes and aid in the understanding of the future biosphere.