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Research Recommendations

The following 19 research recommendations are divided into near- (1 to 2 years), middle- (3 to 5 years), and long- (5 to 10 years) time periods.  Most recommendations cut across the different topics addressed at the workshop.  The recommendations emerged out of the plenary brainstorming session at the end of the workshop in which each participant present shared their thoughts on what needs to be done.

Ideally, each recommendation specifies an expected product, an approach to obtaining that product, and a summary of how the product might be used to support other research recommendations and practical applications.

Near-Term Recommendations

Identify primary and secondary organic compounds and their properties.

    This project would produce a data base of specific organic compounds and compound groups along with important properties.  The data base would include Chemical Abstract Service and common names for identified compounds, references to reports of their detection, reported concentration ranges, water activities, melting point, boiling point, vapor pressures, codes indicating primary or secondary or both, codes indicating potential sources of precursors, potential quantification methods, and detection limits.  The data base would be updatable as new information became available and downloadable from a central location.  Queries would allow users to extract data and to place it into usable formats.  This data base would be assembled from existing tables created by atmospheric organic chemistry researchers via a survey of these researchers.  It would be used to identify which compounds are lacking data that need to be quantified in subsequent experiments.  It could also be used by decision-makers to determine organic compounds that might result from different source emissions (John Watson).

Specify thermal evolution carbon temperature fractions that separate organic compounds into more logical groupings than currently applied carbon fractions.

Review, evaluate, and compare light scattering and absorption models.

Document and evaluate procedures for detection of secondary organic compound quantification.

Define reporting conventions, data base, and priorities for aerosol smog chamber experiments and results.

    This project would provide a consistent set of reporting conventions for smog chamber secondary organic aerosol experiments.  Currently smog chamber experiments tend to fall into two groups, those characterizing the dynamics of aerosol formation and those emphasizing aerosol chemical speciation.  Data acquired during these experiments are not always presented in a consistent format.  Possible common reported data might include the following information: temperature, type of lightsource, NO₂ photolysis rate, humidity, seed particle concentration and type, chamber volume, material and surface/volume ratio, initial and final concentrations of VOC, NO, NO₂, O₃ and aerosol. If a
public database becomes available, it could also include detailed particle distribution and speciation data and intermediate data in addition to the initial and final values.  The establishment of smog chamber research priorities would provide direction for experiments leading to a better understanding of the origins of secondary organic aerosol formation and the atmospheric conditions that affect aerosol growth.  The goals of this project could be accomplished by surveying current investigators in the field, however a meeting of these individuals would also prove valuable (Michael Hurley).

Evaluate  methods to measure black carbon as a normalization for primary and secondary organic carbon.

Define and organize follow-on topical workshops on organic aerosol issues.

Evaluate national networks for optimal resource allocation.

Middle-Term Recommendations

Measure and tabulate vapor pressures and water activities.

    Most of the SOA compounds have intermediate volatilities and therefore exist in both the gas and particulate phases in the atmosphere. Their fraction in the particulate phase depends strongly on temperature and on the concentrations of other organic PM components, and also somewhat on relative humidity. While the framework for understanding these partitioning processes exists, there is little information about the physical properties of the SOA compounds (volatility, behavior in organic and aqueous solutions, etc.). We recommend the measurement of these parameters and their dependence on temperature and composition.  A variety of approaches can be used including the investigation of individual compounds, or the analysis of appropriate smog chamber measurements.

Determine shapes, sizes, and surface reaction properties of particles.

Create and disseminate calibration and performance testing standards.

Develop and apply extraction and derivatization procedures that optimize organic aerosol recovery and quantification.

    Organic compound speciation provides the most valuable information about organic aerosol composition, sources, and atmospheric transformation processes. The molecular level methods usually require extraction of a sample with organic solvent(s), followed by analysis by gas chromatography/mass spectrometry (GC/MS), GC/FTIR/MS, GC with various detectors, HPLC/MS and other methods. Sequential extractions with solvents of increasing polarity and liquid chromatographic separations are frequently used prior to GC/MS analysis to simplify complex organic mixtures. There is a need to optimize the selection of solvents and extraction procedures to assure the integrity of less stable organic compounds, as well as a need for development of more selective separation methods (particularly solid phase extraction methods).
   Highly polar compounds (especially multifunctional) do not elute through a GC column. They require derivatization prior to analysis, to be converted into less polar and more volatile derivatives that will elute through a GC column.  The derivatization techniques are compound-class specific and thus several different methods may be required for a comprehensive analysis of one ambient sample.  The derivatization reagent by-products, the complexity of derivatization products, lack of standards, and limited mass spectral libraries makes these analyses difficult and time consuming. Since the derivatization methods are currently the main tool for polar compound analysis, research is needed to simplify and standardize the derivatization procedures. There is a need for better and more universal derivatization reagents and less laborious procedures.

Field measurements of secondary precursors and end-products in locations with contrasting source emissions and meteorology.

(Paul V. Doskey, Environmental Research Division, Argonne National Laboratory)

   Monoterpenes, which are emitted by vegetation, and aromatic compounds, originating from the production and consumption of petroleum fuels, are two classes of gas-phase organic compounds that have been found to produce high aerosol yields in chamber experiments. Aerosol formation events should be investigated in locations where emission rates of these precursors and levels of atmospheric oxidants are expected to be large. Monoterpene emission rates in the United States are greatest in the southeast, northeast Texas, central and northern California, the Pacific Northwest, and high elevations of the Southwest.  Forested ecosystems in the southeast, northeast Texas, central California, and the southwest are likely receptors of a complex mixture of atmospheric oxidants from major urban areas.  Aromatic compound emissions in Houston and Mexico City are large and produce ambient levels in air that frequently exceed 5 ppbv. These urban areas would be good locations for studies of aerosol formation from anthropogenic precursors.  Field experiments should focus on measuring precursors, oxidants, and the likely products of the chemical oxidations to generate data for aerosol model evaluation.  Meteorological measurements to support the chemical measurements are essential.  It would be desirable to have surface sites established at the candidate locations for long-term monitoring and for conducting intensive field campaigns when, e.g., measurements from aircraft could supplement the surface observations. 

Field and laboratory measurements of particulate hygroscopic properties.

Characterize primary emissions of secondary organic precursors and primary oxygenated compounds.   

Source types of particulate carbon in the majority of field studies have not been characterized very well. For example, in one recent source apportionment study from Los Angeles, only two medium-duty (rather low mileage) diesel vehicles were used to collect source samples to construct a source signature to represent diesel exhaust.
   It is critical to adequately sample the most important source types of primary (and emissions of secondary organic) precursors from the sources thought to be most important contributors to ambient PM2.5.  To do so, examine the data from the NFRAQS (http://www.nfraqs.colostate.edu) for mobile sources, and the new Gasoline/Diesel PM Split Study in Los Angeles, to get an idea of what sample sizes and characteristics are needed to represent the on-road mobile fleet.  In addition, samples from important off-road sources are needed; for example, from locomotives and ship emissions, as well as maybe some data from other source types. We need to understand the relative importance of on- and off-road mobile source contributions to ambient data. This previous discussion covered only mobile sources; what's also important are the other contributors, which may already be sufficiently characterized. One could examine the source profiles from the most recent PM blame apportionment studies to see whether there are chemical differences between similar sources from different studies.
   Regarding oxygenated compounds, this whole issue became very much more complicated once the regulators started mandating oxygen contents of fuels. We know already, for example, that diesels are important sources of primary formaldehyde, and once oxygenates were added to gasoline, formaldehyde and acetaldehyde became important emissions from spark ignition vehicles.
   From Eric Fujita's work in the NFRAQS, we learned that along with PM, we need to pay special attention to SVOC measurement and characterization, not only because these are important emission species, but we don't know how to apportion them between particle and vapor phase, and therefore, we don't know well how to apportion them in ambient studies.  So the best approach is to collect the total exhaust as the sum of PM and SVOCs.
   IMPROVE EC/OC data hinted that SOA might not be very important once one understands the importance of smoke and other primary emission sources at the regional sites.
   Because the source selection and exhaust collection is so costly, I recommend sacrificing some of the ambient measurements in favor of adequate/sufficient source collection for development of source profiles.  We will not be able to do apportionment properly, and thereby properly characterize the relative importance of primary and secondary organic aerosols, until this is done correctly.

Develop improved information extraction methods for current analytical methods.

Develop and test operational mechanisms for secondary aerosol formation for forecasting models.
    Aerosol extinction and other optical properties affect the photolysis rate parameters. For example, the presence of soot particles may decrease photolysis rates by absorbing solar radiation. On the other hand, the presence of other types of particles, particularly fine particles, could increase photolysis rates through increases in radiation scattering. The effect of aerosol particles on photolysis rates is especially important with respect to the formation of secondary organic aerosols because these aerosols are formed through photochemically driven processes. Measurements of spectrally resolved actinic flux should be made in conjunction with other aerosol optical properties. Photolysis rate parameters could then be directly calculated under different aerosol containing atmospheric conditions. Comparison of these kinds of measurements made in urban areas such as Los Angeles in the United States this those made in much more polluted regions such as Mexico City or Shanghi would be helpful in determing the effects of aerosols on photolysis rates.

Long-Term Recommendations

Develop measurement methods that minimize changes in organic composition from that in the atmosphere.

Develop and apply analytical methods to identify and quantify a larger fraction of organic compounds and groups of compounds in suspended particles. 

Develop detailed mechanisms and models for secondary organic aerosol formation.

While the description of the formation of the SOA compounds with a constant yield (e.g., 2% of the oxidized precursor) is a first step it is an oversimplification of the chemical processes leading to these products. Several reactions steps are, in general, needed for the formation of the SOA species. Development and testing of the chemical mechanisms leading to these compounds based on smog chamber work will be necessary for the efficient control of these compounds (for example for understanding the effect of NOx on the formation rates of SOA). The corresponding detailed models should be evaluated against field measurements of the concentrations of these compounds.