Red Mangroves Emit Hydrocarbons

RED MANGROVES EMIT HYDROCARBONS Author(s): Jordan G. Barr, José D. Fuentes, Daniel Wang, Yolanda Edmonds, Jay C. Zieman, Bruce P. Hayden, and Dan Childers Source: Southeastern Naturalist, 2(4):499-510. Published By: Eagle Hill Institute DOI: http:// dx.doi.org/10.1656/1528-7092(2003)002[0499:RMEH]2.0.CO;2 URL: http://www.bioone.org/doi/ full/10.1656/1528-7092%282003%29002%5B0499%3ARMEH %5D2.0.CO%3B2 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. 2003 SOUTHEASTERN NATURALIST 2(4):499–510 RED MANGROVES EMIT HYDROCARBONS JORDAN G. BARR1,*, JOSÉ D. FUENTES1, DANIEL WANG2, YOLANDA EDMONDS1, JAY C. ZIEMAN1, BRUCE P. HAYDEN1, AND DAN CHILDERS3 ABSTRACT – The objective of this study is to investigate hydrocarbon species and amounts released by red mangrove foliage and determine if these quantities warrant future research on atmospheric chemical processing of these compounds. The field investigation took place during July 2001 at Key Largo, Florida Bay, Florida. Foliage still attached to plants was enclosed in cuvettes while air of known flow rates circulated around leaves to study hydrocarbon emissions. Cuvette air samples underwent gas chromatographic analyses to determine species and amounts of hydrocarbons released by mangrove foliage. Red mangrove foliage emits isoprene and trace amounts of the monoterpenes of α-pinene, β-pinene, camphene, and d-limonene. The mangrove flowers released these latter compounds in amounts ranging from 0.5 to 10 mg (monoterpene) per gram of dry biomass per hour. These fluxes are normalized to the foliage temperature of 30 ûC. When normalized to the foliage temperature of 30 ûC and light levels of 1000 µmol m-2 s-1, isoprene emission rates as high as 0.092 ± 0.109 µg (isoprene) per gram of dry biomass per hour were measured. Compared to terrestrial forest ecosystems, red mangroves are low isoprene emitters. During peak flowering periods in the summertime, however, red mangroves may emit sufficient amounts of monoterpenes to alter ground-level ozone concentrations and contribute to biogenic aerosol formation. INTRODUCTION Plants release a plethora of non-methane hydrocarbons (NMHCs) to the atmosphere (Kesselmeir and Staudt 1999). The most common NMHCs released by vegetation include isoprene (C5H8), monoterpenes (C10H18), sesquiterpenes (C15H28), and oxygen-containing (CnH2n-2O) compounds. Atmospheric scientists and chemists have extensively studied these biogenic NMHCs because once in the atmosphere they are highly reactive with the oxidizing species of hydroxyl radicals, ozone, and nitrate radicals. If introduced in environments rich in nitrogen oxides and under favorable meteorological conditions, these reactions can lead to the production of ground-level ozone (Pierce et al. 1998). In jurisdictions such as the eastern United States, ground-level ozone is a major environmental concern because summer-time ambient ozone levels frequently exceed the National Ambient Air Quality Standard (NAAQS) established by the U.S. Environmental Protection Agency. 1 Department of Environmental Sciences, 291 McCormick Road, University of Virginia, Charlottesville, VA 22904. 2Environment Protection Service, Environment Canada, Ottawa, ON, Canada K1A 0H3. 3Biology Department, Florida International University, Miami, FL 33199. *Corresponding author - [email protected]. 500 Southeastern Naturalist Vol. 2, No. 4 The oxidation of monoterpenes and sesquiterpenes can also produce high yields of fine carbonaceous aerosols (Hoffman et al. 1997, Went 1960). In the atmospheric convective boundary layer, these fine aerosols can modify the quality and quantity of incoming solar irradiance through light scattering and attenuation thereby impacting the energy balance of the lower atmosphere. Because of their size and chemical characteristics, these aerosols can also serve as condensation nuclei and thus influence cloud formation processes. The ecological reasons as to why only certain plant genera produce hydrocarbons are not known (for recent reviews see Fall and Wildermuth 1998, Sharkey and Yeh 2001). Since the discovery of the environmental importance of NMHCs (Went 1960), extensive research has been achieved to learn both what plant species release hydrocarbons and to investigate hydrocarbon production rates (Fuentes et al. 2000, Guenther et al. 2000). To date, field research has demonstrated that deciduous and coniferous tree species release large amounts of NMHCs. In general, deciduous tree species release mostly isoprene whereas coniferous plants emit monoterpenes and sesquiterpenes (Kesselmeir and Staudt 1999). Despite the extensive research reported on phytogenic NMHCs to date, mangrove forests have not been investigated to identify the emitted hydrocarbon species and emission rates. At the global scale, mangrove forests constitute a unique and an ecologically important biome as mangroves occupy environments whose edaphic and physical climate characteristics exhibit mixed continental and maritime attributes. As part of the Long Term Ecological Research program at the Florida Coast Everglades (http://fcelter.fiu.edu/), new research is being pursued to investigate the processes governing the rates of trace gas exchange between mangrove forests and overlying atmosphere. The goal of the present study is to determine the NMHC species and amounts released by red mangrove (Rhizophora mangle L.) leaves and flowers. An additional goal is to investigate the environmental controls on hydrocarbon emission rates. METHODS Air sampling protocols The field experiments took place during the month of July 2001 at a site near Key Largo, Florida (80°29.383' N, 25°10.735’W). To identify emitted hydrocarbon species and establish diel cycles of emission rates, small branches with 5–10 leaves were enclosed in a 4-L chamber. Flowers were also counted if any were present on the branches enclosed in the chamber. Air sampling commenced immediately after sunrise and ended around sunset. Red mangrove branches from different trees were easily reached from a boat. The studied leaves were located mainly in the crown of the mangrove forest. To avoid air leaking into the leaf 2003 J.G. Barr et. al. 501 cuvette, it remained sealed for the duration of each sample. On average, a branch remained in the cuvette for about 0.5 hours after which a new branch was selected to duplicate the experiment. To reduce the leaf boundary resistance to gas diffusion, a fan (whose blades were made of Teflon) kept the air inside the cuvette well stirred. Air entering the cuvette passed through a column containing activated charcoal to remove reactive hydrocarbons and gases such as ozone. With the aid of a pump, air circulated through the cuvette at the flow rate of 4.0 L min-1, which was dictated by a mass flow controller (Fig. 1). The cuvette remained under pressure as only 210 mL min-1 were subsampled and stored in 3.2 L electropolished, stainless steel canisters for subsequent gas chromatographic analyses. A three-way valve was placed between the mass flow controller and the canister to flush the sampling (Teflon) lines before opening the canister valve (Fig. 1). Before their field use, canisters were properly cleaned and fully evacuated. Air gathering pro- Figure 1. Configuration of the leaf cuvette used to determine foliage hydrocarbon emission from red mangrove leaves. The symbol definition is as follows: F = Filter, MFCcu = mass flow controller for air entering the leaf cuvette, and MFCca = mass flow controller for air stream flowing into the canister. 502 Southeastern Naturalist Vol. 2, No. 4 ceeded until the final canister pressure reached 1 atmosphere above ambient pressure. The total air volume per cuvette sample amounted to approximately 6.2 L. Ambient air samples were gathered by disconnecting the cuvette from the flow system (see Fig. 1). Unfiltered air was pumped into the canisters from a 0.5 m height and several meters away from mangrove foliage. Three samples were collected over the course of the study period. While leaf cuvette sampling took place, continuous and concurrent meteorological measurements were made on a 10-m tower located 50 m away from the hydrocarbon-sampling site. Measurements were made every second and one-minute averages were computed and stored in a data logger (model 23X, Campbell Scientific, Logan, UT). Because hydrocarbon emissions also depend on prevailing environmental conditions, the data for photosynthetically active radiation (PAR), net radiation (Rnet), air temperature, wind speed, and specific humidity (q) are reported in this study (see Results section). Hydrocarbon analyses In the laboratory, species and amounts of hydrocarbons were determined using a high-resolution gas chromatograph (GC, model 6890, Agilent Technologies, Palo Alto, CA) equipped with a quadrupole massselective detector (MSD, model 5973, Agilent Technologist). Because ambient hydrocarbons exist in trace amounts, analytes were first cryogenically concentrated (using a preconcentrator, model 7100, Entech Instruments, Inc., Simi Valley, CA) before they were introduced to the chromatographic column. Air from the sample canister was drawn through the preconcentrator’s multi-stage trapping system. A mass flow controller measured sample volume. Normally, 500 ml of air was passed through a glass bead trap. The cryogenic traps were packed with 60/80mesh glass bead and maintained at -165 ûC while concentrating the hydrocarbons. To minimize the shift in hydrocarbon retention times due to the presence of water vapor in air samples, a three-stage concentration technique (called microscale purge and trap) was used to separate water from hydrocarbons. Air samples were first concentrated to about a 0.5 mL in a cryogenic glass bead trap. This trap was then heated to 50 ûC and held there while slowly passing helium through the trap to transfer the hydrocarbons to a secondary trap whose temperature was set at -50 ûC. Sweeping the hydrocarbons from the first to the second trap with only 50 mL of helium resulted in the transfer all the hydrocarbons and less than 1 µL of water. After transfer to the second trap, the trap containing hydrocarbons was back-flushed while heating it to 180oC and further focused on an open-tubular trap at -160 ûC. This cryofocusing trap was rapidly heated to 100 ûC, resulting in rapid injection of hydrocarbons into the analytical column. Hydrocarbon compounds were separated on a 60 m, 0.32 mm ID fused silica capillary column with a 1.0 um film 2003 J.G. Barr et. al. 503 thickness, bonded liquid phase (J&W DB-1). To verify the fidelity of hydrocarbon measurements, gaseous mixtures of internal standards were added directly to the first stage cryogenic trap. Additional analytical details can be found elsewhere (EPA 1997). Hydrocarbon fluxes For the hydrocarbon species identified in the cuvette samples (Table 1), emission rates (E) were estimated based on the mass balance approach. This method is based on the tenet that E is determined by knowing the difference in gas concentration leaving (χOut) and entering (χIn) the cuvette, the flow rate (f) of air stream passing through the cuvette, and the amount of biomass (A) enclosed in the cuvette. This relationship is shown in [1]. E = (χ Out − χ In ) f A [1] In this study, the χIn was assumed to be negligible as the air stream entering the cuvette passed through activated charcoal which removed all biogenic hydrocarbons. The values for the χIn were verified by passing air through the empty cuvette and gathering the air in a canister for subsequent analysis. A mass flow controller provided the flow rate Table 1. Chemical and physical properties of biogenic hydrocarbons released by red mangroves. Compound Molecular structure Chemical formula Molecular weight Life time (g mol-1) (min)† Isoprene C5H10 68.18 26 α-pinene C10H16 136.24 34 β-pinene C10H16 136.24 33 Camphene C10H16 136.24 52 d-Limonene C10H16 136.24 12 † The atmospheric life time (τ) was estimated based on: τ = {kHO [HO] + kO3 [O3]}-1. The kHO and kO3 represent the reaction rate constants for the hydroxyl radical (OH) and ozone (O3) and were obtained from Atkinson (1994). The average daytime concentrations of 6x107 and 12x1011 molecules cm-3 were assumed for HO and O3, respectively. 504 Southeastern Naturalist Vol. 2, No. 4 (f) passing through the cuvette (Fig. 1). The hydrocarbon emissions (E) reported below are expressed per unit biomass enclosed in the cuvette. Amounts of biomass were derived based on the leaves harvested following the cuvette air sampling. Leaves were freeze-dried under vacuum for a period of 24 h (on average, a fully developed red mangrove leaf weighted 0.52±0.15 g). Mangrove flowers were harvested and dried in an oven at 80oC for a period of 48 h, giving an average weight of 109.4±17 mg per flower. RESULTS AND DISCUSSION Atmospheric conditions also influence the rates of hydrocarbon emissions from plants. Therefore, to provide a frame of reference for the hydrocarbon emissions reported below, half-hourly averages for the most relevant atmospheric variables are included in Figure 2. Warm conditions prevailed during the air-sampling period (5 July 2001) in which the air temperature (measured at 1.5 m above the surface) exhibited a strong diurnal variation and ranged from 26 to 32oC. These relatively high temperatures promote large hydrocarbon emission rates from vegetation (Guenther et al. 2000). Throughout the day mostly clear conditions dominated as reflected by the strong and smooth diurnal Figure 2. Half-hourly averaged (a) air temperature (l), (b) PAR(n ) and net radiation (u), (c) specific humidity (l ), and (d) wind speed (n). The PAR values were divided by 3 to fit in the scale for net radiation. Vertical bars denote the standard deviation around the arithmetic mean. 2003 J.G. Barr et. al. 505 signatures of both PAR and Rnet. Large radiation loads were experienced in the early afternoon, with peak PAR levels reaching 2200 µmoles m-2 s-1. Also, the available energy at the research site became large, net radiation attaining maximum values around 600 W m-2. In a separate study (Barr et al. 2002b), it was discovered that these radiation loads impose drastic physiological stresses on red mangrove foliage as manifested in large reductions in both leaf stomatal conductance (close to zero) to water diffusion and photosynthetic rates. The specific humidity diurnal pattern (Fig. 2) exhibited only a small modulation reflecting the maritime influence at the research site, and varied between 17 and 19 g Kg-1. Light wind speeds, ranging from 1 to 4 m s-1, prevailed during the hydrocarbon measurement period. As outlined in the introduction, hydrocarbons are important atmospheric constituents because they serve two key roles in the chemistry of the atmosphere. First, hydrocarbons can be readily oxidized to produce alkyl, alkoxy, and alkylperoxy radicals. These radicals can in turn initiate and drive many chemical reactions, thereby reducing the atmospheric lifetime of gaseous constituents such as greenhouse gases. Second, the hydrocarbon oxidation products break the photostationary state of ground-level ozone formation in the lower atmosphere. Therefore, hydrocarbons can be effective ozone precursors. Despite their atmospheric importance, there is a dearth of speciated hydrocarbon information. Therefore, as part of air sampling done in Key Largo, biogenic and anthropogenic species were identified and quantified (Fig. 3) during 4–6 July 2001 to provide baseline information on the abundance of hydrocarbon species and associated amounts. Anthropogenic species exhibited the highest mixing ratios. These hydrocarbons included the broad families of alkanes, alkenes, and aromatic species. The alkanes were found to be the most abundant, then the aromatic hydrocarbons, followed by the alkenes. Average mixing ratios as high as 20 ppbv were measured for compounds such as dichloromethane. For the alkanes of propane, butane and isobutane the average mixing ratios reached as high as 10 ppbv. These alkanes most likely originated from local households (because of the burning of propane and natural gas) and transportation. Even gases such as toluene exhibited daytime mixing ratio averages greater than 1 ppbv. The strong anthropogenic signature reflects the fact that the site was impacted by local automobile and boat traffic. A boat docking station and a highway were located within 10 m and 200 m from the air-sampling site, respectively. The local biogenic hydrocarbon source was sufficiently strong to yield ambient average mixing ratios in the range from 0.03 to 0.5 ppbv. Even highly reactive biogenic hydrocarbons such as d-limonene and pinenes (Table 1) were present in the ambient samples (Fig. 3). Thus, the integrated emis- 506 Southeastern Naturalist Vol. 2, No. 4 sions of biogenic hydrocarbons indicate that mangroves dictate a measurable fraction of regional atmospheric chemical processes. Figure 3. Average mixing ratios for the most abundant hydrocarbons found in Key Largo, Florida Bay, Florida during 4–5 July 2001. Air samples were obtained during 08:00 to 22:00 h (Eastern Daylight Saving Time). Vertical bars denote the standard deviation around the arithmetic mean. 2003 J.G. Barr et. al. 507 Red mangroves emit biogenic hydrocarbons. As shown in Table 1, the identified biogenic hydrocarbon species include isoprene, αpinene, β-pinene, camphene, and d-limonene. In environments such as the Florida Everglades, these biogenic hydrocarbons have lifetimes less than one hour (assuming local daytime ozone and hydroxyl levels of 80 ppbv and 1 pptv, respectively) due to their rapid reactions with ozone and hydroxyl radicals. Red mangrove leaves only release small amounts of isoprene (Fig. 4). The cuvette measurements (16 in total) made throughout the course of two days were averaged and gave emission rates of 0.1 µg (isoprene) per (gram of dry matter per h). These emission rates (EIsoprene) were normalized to emission rates (E s) at leaf temperature (Tl) and intercepted PAR levels of 30oC and 1000 µmol m -2 s -1 , respectively by applying equation [2] (Guenther et al. 2000). The functions f(PAR) and f(Tl) are adjustments for PAR and leaf temperature, respectively. Es = E Isoprene {f (PAR )• f (Tl )} [2] The adjustment function for PAR is given in [3] where α is expressed in units of m2 s-1 mmol-1 photons and CL is a constant. f (PAR ) = α CL PAR 1+ α 2 PAR 2 [3] The function for leaf temperature is given in [4] where R is 8.314 J mol1 K-1 (the universal gas constant), CT1 and CT2 are constants, Topt is an optimum temperature (in Kelvin) for emission, and TKl is leaf temperature (in Kelvin). Figure 4. Biogenic hydrocarbon emission rates estimated for red mangrove foliage. Vertical bars denote the standard deviation around the arithmetic mean. 508 Southeastern Naturalist  C (T − 303) exp T 1 Kl   R 303TKl  f (Tl ) =  C (T − Topt ) 1+ exp T 1 Kl   R 303TKl  Vol. 2, No. 4 [4] The normalized isoprene emission rates, using the model described above (equations [2] to [4]), are included in Figure 4. Compared to emissions from terrestrial tree species (Geron et al. 2001), whose normalized emission capacity ranges from 44 to 180 µg (isoprene) per (gram of dry matter per h), red mangroves emit only trace amounts of isoprene. The normalized isoprene emission rates amounted to 0.092 ± 0.109 mg (isoprene) per gram of dry biomass per hour. In the case of monoterpenes (Fig. 4), the emission rates from red mangrove flowers reached nearly 10 µg (Camphene) per (gram of dry matter per h). Compared to monoterpene emissions from terrestrial ecosystems (Kesselmeir and Staudt 1999), fluxes from mangrove foliage are in the same order of magnitude. Monoterpene emissions from mangroves, however, are likely to be episodic as flowers represent the primary source of these compounds. If mangrove trees flower at the same time it is then possible that these monoterpene production rates can impact the aerosol formation in the local atmospheric boundary layer. Long-term studies are required to investigate the seasonal monoterpene production patterns from mangroves. SUMMARY The goal of this preliminary investigation was to study whether red mangrove forests emit biogenic hydrocarbons. Red mangrove leaves emit isoprene and the emission rates can be as high as 0.5 µg (isoprene) per (gram of dry matter per h). These emission rates are low compared to those from terrestrial forest ecosystems. The estimated emission rates, however, are sufficiently strong to give rise to ambient isoprene mixing ratios in the 0.2 to 1.0 ppbv range. Unlike mixed deciduous temperate forests, red mangroves remain physiologically active year round and could emit considerable amounts of isoprene to the atmosphere. Because of its double-bond chemical structure, isoprene is a highly reactive chemical species and the measured ambient isoprene levels can influence the local photochemical processes leading to oxidant formation. Red mangrove flowers emit monoterpenes such as α-pinene, β-pinene, camphene, and d-limonene. One conclusion of this study is that monoterpene production from red mangrove forest is sporadic as these plants flower during certain times (July to October) of the growing season. 2003 J.G. Barr et. al. 509 ACKNOWLEDGEMENTS The field research was supported by the National Science Foundation through the Florida Coastal Everglades-Long Term Ecological Research, the Virginia Coast Reserve Long-Term Ecological Research, and the Barley Scholars Program. J.G. Barr received summer support from the Department of Environmental Sciences, the University of Virginia, and obtained funding from N.A.S.A. to conduct data analyses. Y. Edmonds received support from the National Science Foundation. Environment Canada provided support to accomplish the hydrocarbon analyses. T.A. Frankovich is acknowledged for sharing biomass data for mangrove flowers and the picture included in the cover of this issue of the journal. LITERATURE CITED Atkinson, R. 1994. Gas-phase tropospheric chemistry of organic compounds. Journal of Physical Chemistry. Reference Data Monograph No. 2:1–216. Barr, J.G., J.D. Fuentes, J.C. Zieman, and J.W. Bottenheim. 2002a. Radiative forcing by aerosols from plants. Bulletin of the American Meteorological Society 83:667–668. Barr, J.G., J.D. Fuentes, J.C. Zieman, and D. Childers. 2002b. Physiological responses of red mangroves to the physical climate in the Florida Everglades. Tree Physiology. Under review. EPA (Environmental Protection Agency). 1997. Compendium of methods for the determination of toxic organic compounds in ambient air. In 2nd Edition, Compendium Method TO-15, Determination of Volatile Organic Compounds (VOCs) in Air Collected in Specially-Prepared Canisters and Analyzed by Gas Chromatography/Mass Spectrometry, report number EPA/625/ R-96/010b. Fall, R., and M.C. Wildermuth. 1998. Isoprene synthase: from biochemical mechanism to emission algorithm. Journal of Geophysical Research 103:25599–25609. Fuentes, J.D., M. Lerdau, R. Atkinson, D. Baldocchi, J.W. Bottenheim, P. Ciccioli, B. Lamb, C. Geron, L. Gu, A. Guenther, T.D. Sharkey, and W. Stockwell. 2000. Biogenic hydrocarbons in the atmospheric boundary layer: A review. Bulletin of the Amer. Meteor. Soc. 81:1537–1574. Fuentes, J.D., D. Wang, G. den Hartog, H.H. Neumann, and F.T. Dann. 1996. Ambient biogenic hydrocarbon concentrations and isoprene emissions from deciduous forests. Journal of Atmospheric Chemistry 29:1–29. Geron, C., P. Harley, and A. Guenther. 2001. Isoprene emission capacity for US tree species. Atmospheric Environment 35:3341–3352. Gu, L., J.D. Fuentes, H.H. Shugart, R.M. Staebler, and T.A. Black. 1999. Responses of net ecosystem exchanges of carbon dioxide to changes in cloudiness: Results from two North American deciduous forests. Journal of Geophysical Research 104:31421–31434. Guenther, A., C. Geron, T. Pierce, B. Lamb, P. Harley, and R. Fall. 2000. Natural emissions of non-methane volatile organic compounds, carbon dioxide, and oxides of nitrogen from North America. Atmospheric Environment 34:2205–2230. 510 Southeastern Naturalist Vol. 2, No. 4 Hoffmann, T., J.R. Odum, F. Bowman, D. Collins, D. Klockow, R.C. Flagan, and J.H. Seinfeld. 1997. Formation of organic aerosols from the oxidation of biogenic hydrocarbons. Journal of Atmospheric Chemistry 26:189–222. Kesselmeier, J. and M. Staudt. 1999. Biogenic volatile organic compounds (VOC): An overview on emission, physiology and ecology. Journal of Atmospheric Chemistry 33:23–88. Pierce, T., C. Geron, L. Bender, R. Dennis, G. Tonnesen, and A. Guenther. 1998. The influence of increased isoprene emissions on regional ozone modeling. Journal of Geophysical Research 103: 25611–25629. Sharkey, T.D., and S.S. Yeh. 2001. Isoprene emission from plants. Annual Review of Plant Physiology and Plant Molecular Biology 52:407–436. Went, F.W. 1960. Blue hazes in the atmosphere. Nature 187:641–643.