EMISSIONS OF BIOGENIC OXIDANT AND PM PRECURSORS: VERY HIGH REACTIVITY VOCS AND SURFACE LAYER CHEMISTRY ABOVE FORESTS
Contact
phone: 919-541-2433
email: coutros.katherine@epa.gov
Description:
Biogenic emissions of volatile organic compounds (VOCs) -- chemicals emitted naturally by the green foliage of a forest, for example -- have been repeatedly shown to be important contributors to ozone pollution levels in many parts of the country. Recently, both the National Research Council and a NARSTO (North American Research Strategy for Tropospheric Ozone) review of the state of the science identified issues with the non-isoprene portion of the biogenic emission inventory to be both important and highly uncertain. In addition, reaction of ambient ozone with biogenic compounds at and near the foliage may be causing very high reactivity emissions to be incorrectly disregarded and their oxidation products not represented in emission models. This task focuses on making measurements of biogenic VOC emissions over a range of environmental conditions, so that emission models (algorithms) can be developed to accurately describe these emissions in EPA air quality models. To perform this task, we will be applying the new technique of relaxed eddy accumulation (REA) which we have developed to measure multi-component fluxes (emissions) from area sources. Recent changes to the EPA air quality model ( MODELS-3) with regard to generation of secondary organic aerosol have made the model highly dependent on the accuracy of the aerosol precursors present in the biogenic emission inventory in predicting fine particle formation. Improvements to the model thus will give EPA a more accurate tool for predicting the effect of emission changes on ambient fine (PM2.5) particulate matter.
Purpose/Objective:
The goals of this research are to reduce uncertainties in the non-isoprene portion (ozone and aerosol precursors) of the biogenic emission inventory used in EPA air quality models through measurements of emissions from forest canopies and determination of their dependence on growth cycles and environmental variables.
Record Details:
Record Type: PROJECTStart Date: 04/01/1997
Projected Completion Date: 09/01/2004
Record Last Revised: 06/12/2006
Record Created: 05/05/2003
Record Released: 05/05/2003
OMB Category: Other
Record ID: 56251
Organization:
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
NATIONAL EXPOSURE RESEARCH LAB
HUMAN EXPOSURE AND ATMOSPHERIC SCIENCES DIVISION
ENVIRONMENTAL CHARACTERIZATION & APPORTIONMENT BRANCHKeywords:
VOLATILE BIOGENIC EMISSIONS, OXIDANT PRECURSORS, RELAXED EDDY ACCUMULATOR, PM PRECURSORS, Project Information:
Progress: A second generation data acquisition and control system has been built and tested for the relaxed eddy accumulator (REA) flux measurement system. The new system provides enhanced tuning and control capabilities of the REA system and allows the concurrent collection of radiation, atmospheric stability and mass flux data. Over the winter of 2002/2003 the system was disassembled and reconstructed in a new field enclosure to improve accessibility for calibration and servicing. In addition improvements were made to the gas flow control system (reduced drift due to ambient temperature fluctuations and removal of flow uncertainty due to water vapor), provision for heating the inlet air stream for increased transmission of semi-volatiles organic compounds and isolation of the REA and eddy correlation (water vapor and carbon dioxide) sample streams to avoid injection of REA valve switching transients (pressure fluctuations) in the infrared gas analyzer signal. All these changes have been implemented to prepare us for making long term (seasonal) emission measurements.
In July 2003 we participated in a joint NSF/EPA sponsored study at our research site. Entitled CELTIC (Chemical Emission, Loss, Transformation and Interactions within Canopies). See: http://acd.ucar.edu/~cstroud/CELTIC/. Research groups from the National Center for Atmospheric Research, University of Colorado, University of South Dakota, Georgia Tech, Cornell University, the University of Virginia and others collaborated on a month long study examining the emission and transformation of biogenic VOCs using an array of gas and particle measurement techniques. Resulting collaborative papers will discuss comparability of flux measurement techniques and comparison of flux measurements with leaf level cuvvette emission measurements.
An initial experiment has been performed to assess whether ozone contacting leaf surfaces (dry deposition) may oxidize reactive species (volatile and nonvolatile) resulting in the release of volatiles. To test this hypothesis a loblolly pine seedling was placed in an illuminated Bell jar and clean, ozone-free air was passed through it. The emissions without the presence of ozone consisted of 37% B-caryophyllene, 23% a-pinene, 7% b-pinene and methyl salicylate, 6% a-humulene and several montoterpenes at the 0.5-3% level. Upon fumigation with 115 ppb of ozone, the emissions of B-caryophyllene dropped to 5% and the a-pinene to 17% while the methyl salicylate increased to 18%. Several new compounds which were not present in the ozone-free treatment included: nonanal at 17%, decanal and hexanal at 3%, and heptanal & octanal at 2% along with 6-methyl-5-hepten-2-one at 3%. If this process is common to other foliage surfaces, then the current air quality models are not accounting for a significant source of emissions.
Relevance: EPA and the States use Air Quality models to understand the sources of air pollutants and to develop strategies to protect the air that people breathe. These models are no better than the input data that are used to develop the implementation plans. Understanding biogenic emissions is important in developing effective implementation strategies, especially for ozone and particulate matter. Currently, most natural emissions from forests are computed by using data that are based on experiments on small seedlings, leaves or branches. Many of these early measurements suffered from deficiencies that were only recently appreciated such as non-recognition of emissions unintentionally excluded by analytical design, poorly quantified illuminating radiation, limited measurements over the growth cycle of the vegetation and failure to account for release of volatiles due to stressors such as ozone and drought. In order to address these issues, measurements of the actual movement of these chemicals from forest canopies to the atmosphere are needed. Transport of these chemicals measured in the free atmosphere from above the canopy reduce biases associated with limited spatial sampling since mixing of air integrates the emissions from within and among tree crowns. Sampling a whole forest canopy also obviates problems associated with simulating real conditions and disturbing the vegetation. To make such measurements, a new technique called relaxed eddy accumulation (REA) has been developed. This 2nd generation system is now making measurements, utilizing numerous additions to improve emission data quality and characterize environmental conditions during measurement periods. Field measurements began in the summer 2003 to characterize diurnal and seasonal emissions from a loblolly pine forest. These measurements will be compared with leaf level (field and laboratory) emissions to develop process level understanding of the mechanisms controlling atmospheric emissions. The resultant models will be used to improve biogenic emission algorithms used in EPA's air quality models. This is a particularly timely effort since the accuracy of the non-isoprene portion of the biogenic emission inventory has been assessed to be highly uncertain by the NRC and a recent NARSTO review. Furthermore, recent changes to the EPA air quality model ( MODELS-3) with regard to generation of secondary organic aerosol have made the model highly dependent on the accuracy of the aerosol precursors present in the biogenic emission inventory in predicting final particle formation. Improvements to the model thus will give EPA a more accurate tool for predicting the effect of emission changes on ambient fine (PM2.5) particulate matter. Lastly, the inclusion of sesquiterpenes in the measurements is especially timely since a recently published paper identifies sesquiterpene ozononation as being responsible for new particle formation in forested areas and not monoterpene oxidation.
Project IDs:
ID Code: 3914
Project type: OMIS