A52B-01 INVITED 10:20h
Emissions of highly reactive BVOCs are an order of magnitude larger than above canopy terpene flux
Biogenic emissions of VOCs play important roles in the chemistry of the atmosphere, with terpenes and isoprene affecting regional air quality, atmospheric radiation, and global climate through secondary organic aerosol formation. In 2002 we began field and laboratory measurements with a fast response Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) of biogenic terpenes, and their oxidation products. Measurements were made in a California pine forest at the Blodgett Forest Research Station. Field measurements of vertical concentration profiles through the forest canopy of terpene oxidation products revealed the presence of large amounts of previously unreported compounds consistent with those recently observed in smog chamber studies. In addition, we have recently shown that about half the ecosystem scale ozone flux in summer is actually due to chemical reactions occurring between terpenoid compounds and ozone within the forest canopy. Taken together, this new information suggests that the flux of terpenes leaving the forest canopy represents at most 10% of the terpenoid compounds actually emitted, and the rest is chemically processed within the forest canopy. Branch enclosure measurements confirm more than 100 BVOCs are emitted but not typically observed above the forest. The implication is that the source of secondary aerosols from biogenic terpene oxidation is likely much larger than previously estimated. We have also consistently observed fine aerosol growth events at the Blodgett Forest site, which we believe are related to the terpene oxidation occurring in the forest canopy. Similar observations of aerosol growth events, non-stomatal ozone deposition, and missing OH reactivity at forested sites around the world suggest unmeasured reactive BVOC emission is common. The unmeasured BVOCs represent a previously unquantified carbon loss from ecosystems and a potentially major source of secondary organic aerosols, oxygenated VOCs (OVOCs), and OH radicals.
A52B-02 10:50h
Exploring the Missing Link(s): Insights Into Unmeasured Reactive Chemical Species in Forested Environments
Indirect evidence for missing (unmeasured) reactive hydrocarbons in forests has recently been presented in the literature; researchers employing independent and unrelated reactivity measurements arrived at similar conclusions. If their interpretations are correct, then conventional measurements are missing a significant portion of the total reactive hydrocarbons present in these environments. This presentation will explore the full set of speciated monoterpene and grouped monoterpene measurements that were obtained at the PROPHET site in northern Michigan for clues to missing reactivity and the relative importance of various nighttime oxidants. Recent progress in quantifying hard to measure low volatility reactive biogenic compounds will be discussed as well as progress in estimating emission factors for selected species. Implications of these recent findings for our current understanding of chemistry in forested environments will be discussed.
A52B-03 11:05h
Seasonal variation of biogenic VOCs and oxidation products over a pine forest in California
Vertical gradients of volatile organic compounds, using PTR-MS,, and water and CO2, using a LICOR 6262, were measured in a Ponderosa pine forest in the Sierra Nevada, California (38.90° N, 120.63° W, 1315m) continuously for 11 months in 2003 and 2004. We have selected two periods representing typical summer (Jun 26 - Sept 23, 2003) and winter (Nov 21, 2003 - Jan 10, 2004) conditions at the site. Median daytime temperatures were 6°C and 26°C in winter and summer, respectively. Vertical fluxes and gradients of water and CO2 revealed significant photosynthetic activity in winter. Concentrations of biogenically emitted 2-methyl-3-buten-2-ol (MBO) and isoprene were a factor of 10-20 lower in winter than in summer. The isoprene oxidation products methyl-vinyl-ketone and methacrolein were 30 times lower showing that their production was slower, consistent with oxidation capacity of the atmosphere being lower in winter. Monoterpene concentrations were only 2-3 times lower in winter. This may reflect either a longer lifetime or a relatively higher emission (compared to MBO or isoprene) of these reactive compounds in winter. The seasonal variation of local emissions, chemistry, and transport will be discussed.
A52B-04 11:20h
A Vertical Integration of the Forest Canopy to Assess the Atmospheric Impacts of Terpenes
Models predicting secondary organic aerosol production require input from measurements of the total terpenes emitted by an ecosystem, not just measurements of the few species typically obtained by above-canopy flux measurements. While these above-canopy fluxes are a useful measure of the terpenes that escape the forest canopy, they provide no information on the terpenes that react within the canopy. The different lifetimes of "reacted" and "escaped" terpenes result in different scales of impact (local versus regional) on the chemistry of the atmosphere. To improve our understanding of the atmospheric impacts of terpenes, we conducted measurements that vertically integrate over the forest canopy, using branch enclosure techniques, vertical gradient measurements, and above-canopy flux measurements using both PTR-MS and GC-FID. Measurements were conducted in a Ponderosa pine plantation on the western slope of the Sierra Nevada, California, in the summer of 2003. This presentation will focus on changes in the monoterpene species composition as we ascend the canopy, comparing the branch-level fluxes with the vertical concentration gradients and above-canopy fluxes.
A52B-05 11:35h
Disjunct Eddy Covariance Measurements of Monoterpene Fluxes From a Norway Spruce Forest Using Proton-Transfer-Reaction Mass Spectrometry
Interest in reliable quantification of organic trace compounds released from terrestrial ecosystems stems from their impact on oxidant levels such as ozone and hydroxyl radicals and on secondary organic aerosol formation. In an attempt to quantify these emissions, a disjunct eddy covariance (DEC) sampling system was coupled to a proton-transfer-reaction mass spectrometer (PTR-MS). In the DEC method, an instantaneous grab sample is taken at intervals of tens of seconds and vertical wind speed at the instant of sample collection is recorded. Intermittent periods are used for sample analysis by a moderately fast chemical sensor, in this case a PTR-MS, which allows for fast and highly sensitive detection of biogenic volatile organic compounds. The vertical turbulent transport of a trace compound is then calculated from the covariance of the fluctuations in vertical wind speed and compound mixing ratio. Fluxes of monoterpenes from a Norway spruce forest were measured during the 2002 summer intensive field campaign of BEWA2000 and results compared well with data obtained using the relaxed eddy accumulation and enclosure approach. In addition to this field experiment, a laboratory test was carried out to validate the disjunct sampling procedure.
A52B-06 11:50h
Biogenic sources of formaldehyde and acetaldehyde during summer conditions.
Photochemical modeling estimated contributions to ambient concentrations of formaldehyde and acetaldehyde from biogenic emissions over the continental United States during January 2001 (Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract A52B-0117). Results showed that maximum contributions occurred over states along the Pacific and southeastern Atlantic coasts. Biogenic contributions were between 10-50 percent and 20-90 percent over urban and rural areas, respectively. For formaldehyde, biogenic contributions were equally divided between direct emissions and photochemical production. Photochemistry accounted from 50 to 80 percent of acetaldehyde concentrations. Both compounds had highest production yields from Nonmethane Volatile Organic Compounds (NVOCs) that contain double bonds between carbon atoms. Ethene emissions were specifically identified to significantly contribute to formaldehyde concentrations. Acetaldehyde production also had large yields from NVOCs containing single bonds between carbon atoms. Isoprene emissions played a minor role compared to other biogenic NVOCs because winter temperatures and solar irradiances reduced its biogenic sources. We have conducted further modeling to assess biogenic contributions during July 2001. Results show that biogenic contributions dominated at most locations in the continental US. In the southern states, maximum contributions moved westward to Louisiana and Arkansas but the states along the Pacific coast remained as locations of maximum contributions. Photochemical production controled most concentrations but up to one third of acetaldehyde can be traced to direct emissions. Isoprene emissions generally produced over 50 percent of formaldehyde concentrations. Acetaldehyde retained the same patterns in production yields from biogenic NVOCs as January 2001. Terrain and its vegetation type affected how these contributions distributed among specific compounds in biogenic emissions. Disclaimer - This paper has been reviewed in accordance with the United States Environmental Protection Agency's peer and administrative review polices and approved for presentation and publication. Although it has been reviewed by EPA, the paper does not necessarily reflect EPA policies or views.
A52B-07 12:05h
Impact of Aromatics, Terpenes, and Additional Organic Nitrates on Global Tropospheric Chemistry
Aromatics, monoterpenes, and ethene are sometimes regarded as relatively unimportant to tropospheric chemistry. These species and their reaction products are sometimes omitted from global chemistry/transport models [e.g., {\it Bey et al.}, 2001] or else are represented in approximate form. Here, we show the changes that result in a global model when aromatics, terpenes, and ethene are included. The model is IMPACT, a global 3-dimensional chemistry/transport model developed at Lawrence Livermore National Laboratories [{\it Rotman et al.}, 2004], with a modified numerical solution for photochemistry. The model has been exercised under the the NASA Global Modeling Initiative (GMI, http://gmi.gsfc.nasa.gov) using the photochemical representation from {\it Fiore and Jacob} [2003]. This has been compared to a modified calculation using extended photochemistry. The extensions include three primary aromatic species and two terpenes, along with various secondary reaction products. Isoprene nitrates are assumed to react with OH to produce NO$_{2}$ and secondary organics rather than decompose into HNO$_{3}$ directly. Results show that O$_{3}$ increases by up to 20% in source regions and 10% over much of the northern hemisphere when the additional organic species are included. The change in O$_{3}$ varies seasonally. PAN also increases by 20% in much of the northern hemisphere. NO$_{x}$ decreases by 20% in source regions and increases in remote locations, reflecting increased transport of NO$_{x}$ away from source regions by organic nitrates. Bey, I., D. Jacob, R. Yantosca, J. Logan, B. Field, A. Fiore, Q. Li, H. Liu, L. Mickley, and M. Schultz (2001), Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation, {\it J. Geophys. Res.}, {\it 106}, 23,073-23,096. Fiore, A., and D. J. Jacob (2003), {\it The GEOS-CHEM chemical mechanism version 5-07-8}, Harvard University, Cambridge, MA, USA. Rotman, D. A., et al. (2004), IMPACT, the LLNL 3-D global atmospheric chemical transport model for the combined troposphere and stratosphere: Model description and analysis of ozone and other trace gases, {\it J. Geophys. Res.}, {\it 109}, D04303, doi:10.1029/2002JD003155.