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Application of the Leaf Wax-aerosol Method to Assess Spatial and Temporal Patterns of Carbon Isotopic Fractionation of Atmospheric CO2 by Terrestrial Photosynthesis

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Leaf Wax-aerosol Figure 1

Figure 1. Atmospheric sampling towers at Bermuda and Barbados, and portable aerosol sampler installation in Amazon rainforest canopy tree in French Guiana. Typical HiVol aerosol samplers are used for the tower sampling. Left photo shows mounting the sampler on the canopy platform and the cabled walkway in French Guiana. Vacuum hose leads to the pump house at the end of the walkway. Bottom right photo shows flagpole designed to raise and lower filter holders (located under rain shield in photo) to change the sample filters.

Leaf Wax-aerosol Figure 2

Enlarge Image

Figure 2. Upper panel: Seasonal patterns in wax aerosol-derived estimates of the terrestrial photosynthetic discrimination (∆) of source vegetation in continental footprints of Bermuda and Barbados aerosols, and in wax aerosols collected at other North American continental locations. Legend: AK (Alaska tundra), ALB (Alberta, northern short grass C3 prairie), ME (Maine, sub-boreal forest/northern hardwoods), BER (Bermuda-predominately North America witha easterlies from Africa during summer/fall), FL (Florida- North America with southeasterlies in summer/fall), BARB (Barbados-easterlies from N. Africa year-round). The seesaw variations observed in the Florida record (day 180-170) are caused by shifts in air mass trajectories from westerlies to south easterly flow (higher δ13C ). At Barbados, the extremely low ∆ estimates (day 275-350) correspond with the peak in the (C4) crop burning season in the subSahara.

July 1, 2006 through June 30, 2007

Dr. Maureen H. Conte
Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543 (also at Bermuda Institute of Ocean Sciences, Bermuda)

Program Manager:  Dr. Kathy Tedesco,
NOAA Office of Global Programs (Global Carbon Cycle Program)

Related NOAA Strategic Plan Goal:
Goal 2. Understand climate variability and change to enhance society’s ability to plan and respond.

Project Overview
Temporal and spatial variations in the concentration and isotopic composition of atmospheric carbon dioxide can be used to estimate the relative magnitudes of the terrestrial and oceanic carbon sinks. Although a powerful approach, sizeable uncertainties in model-derived estimates exist because of our relative lack of understanding of the extent and causes of variability in isotopic fractionation by terrestrial photosynthesis and respiration.  An important model parameter is the terrestrial photosynthetic discrimination of CO2 (∆), yet no satisfactory method exists to measure this on large spatial scales. Terrestrial biospheric discrimination of CO2 (∆) reflects the integrated consequences of environmental and physiological factors on photosynthetic processes, and the extent to which ∆ varies on global scales can significantly alter model conclusions.

The major goal of this study is to validate and refine a novel “biomarker”-based method I am developing that uses the carbon isotopic composition of higher plant-derived leaf waxes in aerosols to directly estimate ∆ and its temporal variation on large spatial scales. The revised operational plan entails continuous bulk aerosol sampling at three strategically located sites (Bermuda, Barbados, French Guiana) that receive spatially integrated, well-mixed air masses from key regions representative of major ecosystems. This study will produce the first, direct quasi-hemisphere scale data on spatial and temporal variations in terrestrial photosynthetic discrimination of CO2. The data will be used in conjunction with air mass trajectory analyses to improve current estimates of the magnitude and geographical pattern of carbon sinks. Results will additionally provide information on nonvolatile organic aerosols and spatial and temporal variability associated with changing air transport and temporal trends, and thus complement other atmospheric studies of long-range transport of continental emissions and their impact on regional atmospheric chemistry. These products are directly relevant to the NOAA CGCP goal of improving our ability to observe and understand changes in the global environment and the GCC focus on carbon fluxes.

I have established ongoing aerosol sampling programs on Bermuda (July 04-Aug 06), Barbados Apr 05-present) and French Guiana (Oct 06-present). Collections on Bermuda were made at the Tudor Hill tower (SW end of Bermuda). The open ocean air sector facilitates collection of North American air masses and in summer Northern African air masses. Collections on Barbados are made at the Univ. Miami Ragged Point tower on the eastern end of Barbados. Trade winds in Barbados bring North African air year-round. Sampling in French Guiana is being conducted at the Nouragues field station, a a remote camp located in the pristine, northeastern Amazon rainforest. The prevailing winds sampled at Nouragues blow across an essentially uninhabited region of the Amazon in French Guiana and northeastern Brazil. Sampling will continue in French Guiana until Sept 2007.

The methodology used for aerosol sampling at the Nouragues station is unique. The aerosol filter head has been mounted in a 30 meter tall rainforest canopy tree, with an extension to raise the sampler several meters above the crown (Fig. 1). The canopy platform itself is accessed using a rudimentary cabled walkway extending from the side of a steep hill, where the pump house is located. Our portable sampler is powered exclusively by Nouragues’s solar panels and small hydroelectric generator, so there is no local signal contamination. This is the first time a continuous biogenic aerosol sampling campaign has been conducted in the remote Amazon and, as far as I am aware, it is the first time an aerosol sampler has been mounted in a tree and run using exclusively “clean” power generation.

1. First Accomplishment
Generated continuous time-series of bulk carbon and plant wax aerosol molecular and isotopic composition at Bermuda and Barbados. We have received about six months of samples from French Guiana and analysis of these samples is underway. Initial results are excellent, with good signal intensity.

Results to date from Bermuda and Barbados (and data from other continental sites) confirm the large regional scale of the wax aerosol “footprint”. Seasonal variations observed in wax aerosol molecular composition and wax d13C in the Bermuda and Barbados samples reflect seasonality in the plant ecosystems sampled (e.g summer/fall increase in plant species having the C4 photosynthetic pathways, decreased C3 plant photosynthetic discrimination during the dry season) as well as seasonal changes in air mass trajectories. As previously observed at Bermuda (1995-1998 sampling campaign), there is minimal correlation between wax and dust loading indicating that waxes are not carried primarily as a component of the dust. This confirms that the long-range transport of plant waxes across the North Atlantic basin is largely independent of Saharan dust transport (although of course, both are carried by African air masses).

2. Second Accomplishment
Photosynthetic discrimination (∆), derived using the wax aerosol n-alcohol isotopic composition, ranges from 10 to 19.5 ‰ (Fig. 2a). These wax aerosol estimates of ∆ are very consistent with modeled estimates of annual ecosystem discrimination, but importantly the wax aerosols uniquely provide information on temporal variability (shown for comparison in Fig 2b is the annual ecosystem discrimination from the model of Scholze et al., 2003).

At Bermuda, the new wax aerosol data agree well with wax aerosol data previously collected in 1995-1998, but there are also differences providing the first indications of inter-annual variability. The Barbados results are the first detailed seasonal data on the organic composition of biogenic aerosols originating from North Africa. Barbados wax aerosols are isotopically enriched relative to those at Bermuda (and other North American sampling sites), as predicted given the predominance of C4 vegetation in North Africa. At Barbados, there is a 6 per mil seasonal range in the photosynthetic discrimination. The wax aerosol molecular composition suggests that the wintertime minimum in discrimination (nearly a pure C4 plant signature) is caused by enhanced wax emissions during the seasonal burning of C4 cereal crop fields after harvesting, another indication of a major influence of biomass burning on emissions and long-range transport of continental organic carbon.

Conte, M. H. and J. C. Weber. 2007. Large spatial-scale and temporal variability in carbon isotopic fractionation of atmospheric CO2 by the terrestrial biosphere: A plant wax-aerosol proxy approach. NOAA Global Carbon Cycle PI Meeting, Silver Spring, Md., Sep 07.

Conte, M. H. 2007. Assessing large-scale regional and temporal patterns of terrestrial ecosystem discrimination using molecular tracers in continental aerosols. MBL Ecosystems Center Seminar Series, May 07.

Weber, J. C. and M. H. Conte. 2006. Spatial and Temporal Patterns in the Carbon Isotopic Signal of Leaf Wax Aerosols in Continental Air Masses: Linkages with Ecosystem Discrimination. Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract B21E-06.

Conte, M. H. and J. C. Weber. 2005. Quantification of Isotopic Fractionation of Atmospheric Carbon Dioxide by Terrestrial Photosynthesis Using the Carbon Isotopic Composition of Plant Wax Aerosols. Eos Trans. AGU, 86(52), Fall Meet. Suppl., Abstract B13B-02.

Summary of Interaction with NOAA
The project uses the NOAA air trajectory model HYSPLIT to compute back air mass trajectories and establish the potential aerosol “footprint” of each sample.

Dr. Joe Prospero, the director of the NOAA/Univ. Miami collaborative institute, runs the Barbados tower facility and has assisted with logistical support for field activities there.

Summary of Education and Outreach Activity
The project has sponsored undergraduate and high school students in my lab (Becky Clarkson, Brown University; Don Inglis, Mashpee High School; Rachel Franzblau, Ann Arbor Pioneer High School, Ann Arbor, Michigan). Their main activities have been general laboratory support, computer processing of the organic geochemical data generated by GC and GC-irMS, and use of the NOAA HYSPLIT model to compute back air mass trajectories. The HYSPLIT analyses in particular have given the students a much better appreciation of atmospheric circulation patterns and atmospheric modeling.

Last updated: August 19, 2008

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