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Issue No. 18, May 1999
Laboratory studies on new particle formation processes induced by hydrocarbon oxidation (NUCVOC) |
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Current aerosol climate models focus mainly on sulfur chemistry, especially when considering the formation of new particles in the troposphere. Nucleation events are usually discussed in terms of sulfuric acid nucleation, either as a binary (H2SO4/H2O) or ternary (H2SO4/NH3/H2O) system. However, recent aerosol formation studies at rural sites indicate a link between new particle formation and oxidation of volatile organic compounds (VOCs). Measurements of nanometer size particles at the SMEAR station in southern Finland provide an example of particle formation events over boreal pine forests [Mäkelä et al., 1997]. Similar observations have been made in other forested areas in Portugal [Kavouras et al., 1998], Greece [Kavouras et al., 1999], Canada [Leaitch et al., 1999] and the US [Marti et al., 1997]. In each case the observation of nucleation events took place in remote forested areas, where the release of highly reactive VOCs from trees followed by a rapid oxidation to less volatile products has to be considered a potential source of nucleating vapors. Unfortunately, the amount of condensable material needed to form nanometer particles is extremely smalland the chemical identification of the nucleating species is rather difficult. Therefore, the present project, Nucleation Processes from Oxidation of Biogenic Volatile Organic Compounds (NUCVOC), concentrates on a series of laboratory studies to model the chemical transformation of natural VOCs into condensable species, and to predict their atmospheric fate and relevance in the formation of particulate matter. The effort to understand particle formation from biogenic VOC oxidation is driven not only by the field observations mentioned above, but also by certain characteristics of this group of hydrocarbons. First, it is well established that terrestrial vegetation releases into the atmosphere a tremendous amount of organic compounds (e.g., isoprene, monoterpenes, sesquiterpenes, oxygen-containing compounds). The large quantities of biogenic VOCs emitted globally in comparison with the release of anthropogenic VOCs stimulated the research into the atmospheric chemistry of these compounds. Secondly, the aerosol formation potential of biogenic VOCs with more than six carbon atoms, as measured in various smog chamber experiments, is generally high and, for specific biogenics, the major fraction of products can convert to the particle phase [Griffin et al., 1999; Hoffmann et al., 1997]. Based on information regarding fractional aerosol yields and on available emission inventories, the global production of secondary organic aerosols (SOA) from oxidation of natural VOC is estimated to be between 30 270 Tg per year [Andreae and Crutzen, 1997]. Since aerosol formation from biogenic VOC oxidation represents a natural source contributing to the continental background aerosol, knowledge of its contribution to the total particle burden of the troposphere is crucial in order to determine the relative importance of anthropogenic versus natural particle production. The question of whether the oxidation products of VOCs simply add to the tropospheric aerosol mass by condensation on pre-existing particles, or whether they also contribute to the aerosol number concentration by homogeneous nucleation of non-volatile products, has to be answered to evaluate climatic effects of tropospheric aerosols. The NUCVOC-Project Since dozens of precursors are involved in particle formation from VOC oxidation, an experimental approach based on a set of well-defined laboratory studies was chosen to examine the role of low-volatility organics in tropospheric nucleation processes. Likewise, the fact that the production of condensable species from unsaturated VOCs is associated with three major oxidation pathways (ozonolysis, OH, and NO3-initiated VOC degradation) favors experiments in reaction chambers. The primary objectives of the NUCVOC project can be summarized as follows:
Preliminary ResultsResults to date represent essential steps towards a better understanding of aerosol formation from hydrocarbon oxidation. For example, the application of newly developed analytical methodologies, designed to sample, separate and detect very low volatility compounds, resulted in the identification of numerous individual oxidation products. Gas-phase ozonolysis has been investigated in detail and new products identified, such as multifunctional carboxylic acids, including various dicarboxylic acids [e.g., C8- and C9-diacids (Fig. 1)] [Christofferson et al., 1998; Glasius et al., 1998; Calogirou et al., 1999]. These acids were produced from all investigated precursor molecules, a finding recently confirmed by other groups [Yu et al., 1999; Jang and Kamens, 1999]. Degradation mechanisms capable of explaining the formation of carboxylic diacids from the gas phase reaction of biogenic VOCs with ozone have been developed and attempts to estimate their vapor pressure are in progress [Winterhalter et al., 1999]. Due to their low volatility, the diacids are particularly interesting candidates in terms of their ability to form new particles by homogeneous nucleation.
Clearly, formation of condensable vapors such as dicarboxylic
acids (described above) is a necessary prerequisite for new particle
formation by homogeneous nucleation. However, condensation of
existing aerosol products is always in competition with the nucleation
of new aerosols. Therefore, the production rate for new particles
depends upon the formation rate of condensable vapors, temperature,
and characteristics of pre-existing aerosol particles (i.e.,
their number concentration and size distribution). Furthermore,
classical homogeneous nucleation theory assumes a free energy
barrier during the initial phase of particle formation, inhibiting
the build up of small clusters in the gas phase. Considering
the products observed from the gas phase oxidation of biogenic
VOCs, it seems worthwhile to ask if this model accurately describes
new particle formation from biogenic systems. As mentioned above, the occurrence of nucleation in the atmosphere is also dependent upon a variety of conditions such as the number concentration and size distribution of the pre-existing atmospheric aerosol. Nevertheless, fast reacting biogenic VOCs are again suitable potential candidates, since the time scale of the formation of condensable species from these systems might be shorter than the characteristic time scale to reach gas/particle equilibrium, resulting in supersaturated gas phase concentrations [Bowman et al., 1997]. Therefore, heteromolecular homogeneous nucleation of natural VOC oxidation products might provide the basic mechanism in the formation of new terrestrial biogenic particles. However, although progress has been made to understanding the fundamental reactions and products involved in aerosol formation from VOC oxidation, the whole process of new particle production is still poorly understood, primarily due to the large number of gas phase species which can participate and the difficulties of modeling the underlying processes. Considering the above discussion, there is little doubt that research into new particle formation processes from natural precursors has to be intensified. A key aspect is the connection of the available laboratory data with field observations, such as from PARFORCE and BIOFOR campaigns. This seems especially important because present aerosol climate models ignore organic aerosols and because pressing scientific questions-e.g., to what extent anthropogenic aerosols counteract radiative forcing by greenhouse gases-can only be answered if the organic aerosol phase, although difficult to measure, is taken into account. ReferencesNUCVOC homepage: http://www.isas-dortmund.de/ wgroups/ag_321/projects/nucvoc/nucvoc_home.html.
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