Description:
Recent analysis of ambient fine particulate matter (PM2.5) has found that significant portions of the organic matter contained therein are of biogenic origin. Radiocarbon (C-14) measurements of the bulk organic matter in fine particles collected near Nashville, TN, found that 40-70% of the total was of biogenic origin. Other studies have found oxidation products of natural emissions (monoterpenes) present in the organic fraction of ambient fine particulate matter. In addition positive condensation nuclei fluxes (small particle emissions) have been recently measured over forest canopies. These measurements collectively indicate that at least a portion of ambient fine particulate matter may be of natural origin and therefore not easily controlled. In order to simulate the formation of aerosols resulting from atmospheric oxidation of volatile organic compounds (VOCs) , air quality modelers need to know the composition of chemicals released from natural sources, variables controlling their rate of release and the physics and chemistry of particle formation from these compounds. Without this knowledge, the models used by EPA for making regulatory decisions will be less accurate, resulting in uncertain gains in human health for control measures implemented for ozone and fine particles. Recent reviews by the National Research Council (NRC) and NARSTO have concluded that the non-isoprene portion of the biogenic emission inventory (which includes monoterpenes) is highly uncertain and is in need of improvement.
This task addresses the detection and quantification of biogenic volatile organic emissions currently not included in (e.g., sesquiterpenes), or poorly defined by (e.g., monoterpenes) these inventories and the development of empirical relationships between these emissions and observed small particle fluxes. Through measurement of above forest canopy fluxes (VOCs, small particle, ozone, solar radiation, energy and photosynthetic) and possible rate controlling environmental variables (ozone stress, temperature, water availability) across all seasons, algorithms will be developed to describe how much, what type, and what transformations occur when volatile organic emissions are released to the atmosphere from forest canopies. The resulting algorithms can then be incorporated into EPA's air quality models (e.g., CMAQ) to improve its accuracy in predicting ambient ozone and secondary organic aerosol.
Keywords:
OZONE, BIOGENIC VOC, OZONE DRY DEPOSITION,
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Project Information:
Progress
:FY05
Winter: completed debugging of laboratory automation software used for analyzing biogenic VOC samples.
Spring: completed development of optimum conditions for automated thermal desorption and analysis of biogenic VOC sample tubes (optimum temperatures, gas transfer volumes etc.)
Spring: Evaluated use of nitric oxide titration of ozone vs manganese dioxide screen ozone scrubber methods for effect on sampled biogenic VOCs. The manganese dioxide scrubber was far superior for monoterpenes. Further experiments defined the optimum number of manganese dioxide screens to maximize VOC collection.
Summer: Began biogenic VOC flux measurements and leaf level emission measurements.
Fall: Tested continuous addition of trans-2-butene to sample inlet as a technique to remove ozone. Technique shows promise for use with sesquiterpenes which are semi-volatile and can partially be removed by manganese dioxide screen ozone scrubber.
FY06
Continue flux (VOC, particle, heat, radiation and photosynthetic) measurements to capture emissions over range of environmental conditions and phenological events.
Continue leaf level emission measurements (cuvvette) to establish source strength without transport chemistry.
Prepare journal article on adsorbent testing for biogenic VOCs.
Initiate contract to develop canopy chemistry and dispersion model and add measurements necessary for running model, i.e. ozone flux, NOx gradient.
FY07
Add fast ozone sensor and NOx gradient instrumentation to tower (contractor).
Develop canopy chemistry/dispersion model to include observed monoterpenes, 2-methyl-3-buten-2-ol and sesquiterpenes (contractor). Add state of the science secondary organic aerosol generation process to allow empirical fit to measured small particle fluxes.
Continue flux (VOC, particle, heat, radiation and photosynthetic) measurements to capture emissions over range of environmental conditions and phenological events. Adjust measurement campaign in response to model guidance.
Prepare journal article on second generation relaxed eddy accumulator system design and testing.
FY08
Prepare journal article describing synthesis of leaf level emissions, flux measurements and modeling of VOC emissions, transport losses and particle generation. Evaluate impact of gas phase canopy chemistry on uncorrected ozone dry deposition rates.
Relevance
:Relevance
For the air quality models such as a EPA's Community Multi-scale Air Quality (CMAQ) model to accurately capture the influence of background, natural vegetative emissions on ambient ozone and particulate matter, they need: 1) emission models that describe the composition and rate of release of biogenic VOCs to the atmosphere and 2) a model that simulates the chemical transformation of these emissions to gaseous and nonvolatile, particle products. The biogenic VOC emission models comprising the non-isoprene portion of the inventory are presently recognized to be poorly defined with respect to seasonal and environmental drivers (other than temperature). Furthermore, it has been recently recognized that sesquiterpene (C15H24) emissions were missed in most of emission studies due to deficiencies in sampling and analytical procedures. Hence, sesquiterpene emissions are not represented at all in the current models. In addition the CMAQ model, to simulate conversion of monoterpenes to secondary organic aerosol, uses smog chamber experimental data which had starting concentrations of biogenic VOCs much higher than ambient levels. It is recognized that processes observed at these concentrations (gas to particle conversion especially) may not scale accurately to ambient conditions. These uncertainties derive in part from whether the products formed are at sufficiently high enough concentrations at ambient concentrations to condense to form aerosols or whether they may coalesce on existing particulate matter.
Significance
This effort is directed to reduce some of the uncertainties described above through observation of emissions of biogenic VOCs from a forest canopy accompanied by observation of small particle emissions. Through statistical analysis of these fluxes we will identify and weigh environmental variables which contribute to the distribution and quantity of the biogenic VOC emissions. A canopy air chemistry and dispersion model will be developed to incorporate the emissions we observe from leaf level cuvvette measurements. This model will then use meteorological and environmental conditions from actual flux measurement periods to predict biogenic VOC fluxes. These comparisons will be used to verify model accuracy and to identify and correct deficiencies. The validated model will then be used to compare with measured particle fluxes in order to develop empirical relationships between the biogenic VOC fluxes and small particle fluxes of secondary organic aerosol.
Impact
The impact of this work will be that new algorithms improving currently poorly defined emissions (monoterpenes) and currently non-represented emissions (sesquiterpenes) will be incorporated in biogenic emission inventories for use in air quality models. Because these compounds can be ozone and particulate matter precursors, the accuracy with which the air quality model can simulate ozone and secondary organic aerosol formation will be improved. In view of the current global estimates of biogenic VOC far exceeding anthropogenic atmospheric loading, these improvements to the models will make them more accurate thus improving their efficacy as a tool for regulatory decision for achieving cleaner air.
Clients
:Thomas Pierce (541-1375) and Kenneth Schere (541-3795) NOAA/AMD, Christopher D Geron (541-4639) NRMRL, NARSTO, Alex Guenther () IGAC/GEIA(NCAR)
Project IDs:
ID Code
:20556
Project type
:OMIS