Aerosol Issues
Atmospheric aerosols play an important role in radiative forcing and climate change. In addition, they are responsible for visibility impairment and have significant implications for human health, particularly those particles containing poly-aromatic hydrocarbons (PAHs). Recent epidemiological studies show that daily mortality is associated with relatively low particle pollution concentration levels typical of air quality in U.S. cities at the present time. (These results have apparently motivated the EPA to re-examine the particulate PM-10 air quality standard). Aerosols also are associated with acid deposition and, therefore, effects on terrestrial and aquatic ecosystems.
Aerosols are substantially different from gas-phase air pollutants and contaminants, which require usually only the specification of species type and air concentration to compute a radiative effect or dose to humans or other biota. In contrast, for aerosols, one must specify not only the concentration, but also the chemical composition as a function of size, size distribution, solubility, and shape of the particles to estimate relevant parameters, such as particle growth as a function of relative humidity, aerosol optical and radiative properties, deposition rates, formation of cloud droplets and cloud droplet size distributions, and cloud radiative properties.
Much progress has been made over the past decade in understanding the physics, chemistry and thermodynamics of aerosol systems, especially those containing the inorganic components of sulfates, nitrates, ammonium, hydrogen ion (acidity), and other salts such as NaCl, in the presence of water. Considerable effort has gone into laboratory studies of aerosol chemistry and thermodynamic properties as a function of relative humidity for pure salts and for mixtures and a number of thermodynamic equilibrium models have been developed to account for the competition between various chemical species in dry aerosol particles, liquid droplets and gas phase concentrations. Although much work has been done in field studies, both through intensive measurement campaigns and dedicated monitoring activities, complete closure between the laboratory and field measurements and the theoretical and modeling studies has not been achieved.
Areas where further work is needed include:
€ non-sulfate aerosols, particularly organic and dust aerosols,
€ aerosol nucleation,
€ cloud condensation nuclei characteristics and cloud droplet nucleation,
€ improved measurement techniques, including accurate, sensitive and specific single particle analysis,
€ the development and application of new aerosol tracers,
€ closure of aerosol chemical, physical, and optical properties with laboratory-field measurements and microphysical-thermodynamical models, and
€ development and evaluation of global aerosol models capable of treating the full suite of aerosol types and estimates of their evolving size distributions and chemical effects.
These studies must, as much as possible, include all major inorganic and organic aerosol species. Each of these areas are discussed in more detail in the sections below.
Air chemistry models and global chemistry-climate models have been evolving in complexity to capture in a more process-oriented way improved understanding of atmospheric processes, including aerosol mass loadings and some representation of aerosol size. There continue to be issues related to the integration of the body of aerosol knowledge in global models. Physically-based parameterizations of aerosol processes and properties are needed for these models, and issues of scale need to be addressed.
These models rely on air concentration measurements of many species to evaluate their performance. Mostly these observations come from surface measurements of aerosol composition and concentrations. Although there have been a substantial number of marine measurements reported in the literature, there still seems to be too few systematic measurements in the vertical and in marine locations downwind of continental sources for thorough model evaluation. In addition, model evaluations are further limited by a systematic lack of aerosol microphysical properties, such as size distribution, CN (condensation nuclei) count, CCN (cloud condensation nucleus) count at a specified supersaturation, chemical composition (including the organic component) as a function of size, and aerosol light scattering at one or more wavelengths as a function of RH and size distribution and chemistry. Although such continuous systematic measurements in three dimensions even for a one-month period, for example over the eastern US and the North Atlantic, would be of enormous value for model development and evaluation, process studies are also needed to determine the accuracy with which the models represent these processes.
Laboratory studies of aerosol nucleation, especially the [H2O, H2SO4, NH3] system, and aerosol growth in the presence of H2O are needed to test aerosol modules against these experiments and to incorporate new understanding of these processes into them. Targeted process field studies along the lines conducted by Eisele and McMurry would be highly complementary to the laboratory work. Other activity areas, where further work would greatly benefit the development and testing of process modules, including smog chamber nucleation studies relating new particle formation to specific model organic compounds and oxidation rates, new particle formation in the vicinity of clouds and in the free troposphere, and aerosol removal processes through incorporation into cloudwater.
There is also a need to measure and identify reactive organic gases which may serve as precursors for aerosol organics. Organic gaseous precursors are known in urban areas and to a degree in forested areas. Such information, however, is essentially nonexistent in marine environments. Aspects of these areas will be highlighted in the sections below.
Non-Sulfate Aerosols
Much work to date in estimating the effect of aerosols on direct radiative forcing has dealt with sulfate aerosols. Similarly, for visibility impairment and assessment studies, sulfate aerosol has played a major, if not dominant role. In the last few years, however, there has been a growing awareness of the importance of other aerosol components, such as carbonaceous (organic and black carbon) materials and mineral dust. In recent visibility studies, for example, organic carbon aerosol has been found to contribute up to 30-40% of the total particulate light scattering in certain regions.
There is also increasing evidence that organic material may be a significant component of remote marine aerosol, which, up to now, has been assumed to be primarily sea salt and sulfate aerosol. Based on available measurements, the light scattering properties of organic aerosol appear to be similar to those of sulfates, and that the aerosol absorption is governed largely by black carbon. There are uncertainties associated with the size distributions and relative humidity growth dependence of the organic carbon particles that need to be addressed. There is also accumulating evidence that organic aerosol material associated with biomass smoke and marine aerosols, including biogenically-derived sulfur compounds, may be a significant contributor to CCN number concentrations. In addition, sea-salt produced halides (chlorine and bromine) appear to have a significant role in CCN production.
Novakov reported on the results of aerosol measurements made at Cape San Juan on the northeastern corner of Puerto Rico under typical Atlantic trade winds conditions. He finds organic submicrometer aerosol mass concentrations frequently exceed sulfate concentrations, but very little black carbon. Indications are that these particles are water soluble (over 70% of the total filterable organic particulate is water soluble) and consistent with his earlier findings on large contribution of organic aerosol fraction to marine CCN concentrations. Based on the low black carbon level, it is likely that the material is not from combustion sources. By comparing 24-hr samples with 12-hr samples, the data suggest that photochemical processes of gas-to-particle conversion are responsible for particle production. Novakov concludes that organic aerosol cannot be neglected and that more effort by ACP is needed in this area.
Similar results are being found at continental sites. Studies of visibility-impairing hazes at the Grand Canyon, Shenandoah and Great Smoky National Parks (not supported by ACP) have identified organic carbon aerosol as a significant component of the total light extinction budget. It appears that some fraction of the organic aerosol is hygroscopic, while the remainder is less-hygroscopic or hydrophobic. There is some evidence suggesting that the hygroscopic fraction consist of oxidation products of terpenes and terpenoids, whereas the less soluble fraction consists of plant waxes and other similar compounds.
Laulainen called attention to a study (Southeastern Aerosol and Visibility Study -- SEAVS) conducted during the summer of 1995 in the southeastern US at the Great Smoky National Park to determine the chemical composition and physical properties of the aerosol haze in relationship to haze optical properties and the light extinction budget. One part of the study, supported by DOE Fossil Energy's Visibility Assessment studies and conducted by Hildemann at Stanford University, was to collect samples and to analyze them with respect to water soluble and insoluble fractions and to further distinguish for each fraction their biogenic or fossil carbon origin by means of isotopic carbon (14C/12C) signatures. The results of this study are not yet available. Another part of the SEAVS effort, also supported by DOE Fossil Energy and performed by Covert at the University of Washington, was to measure aerosol light scattering with a RH-controlled integrating nephelometer. Preliminary results of this study and other SEAVS activities are expected in the spring of 1996.
In urban areas, Gaffney noted that appreciable soot (black carbon) has been found in precipitation, even though soot is essentially insoluble, and in ambient air filter samples, presumably arising from diesel exhaust. He also noted that some studies done in Alaska using 14C as a tracer showed 30-40% of the soot is of recent origin, implying wood smoke as a source. He suggested a possible application of the tracer technique to the Mexico City study. He further noted possible health effects of organic aerosols, including PAHs and mutagens.
In studies at the Grand Canyon, Laulainen (reporting for McMurry) noted that particles (presumably a combination of sulfates and organics) have been found with an internal carbon seed. Marlow pointed out that even hydrophobic material can serve as a CCN, because of shape effects and, thus, physical effects may have chemical consequences. For example, chain aggregates (e.g., freshly emitted soot) may take on water molecules within their irregular interstitial gaps and, once enough water is taken on, the entire structure collapses to a more compact form. Laulainen cited some experimental work confirming this behavior. Saxena noted that even hydrophobic particles can become coated with other substances, thereby changing their water activation properties.
DeLuisi called attention to desert aerosols. He noted that wind-blown dust represents a variable and, as yet, difficult to quantify component of the planetary atmospheric aerosol burden and may have a significant impact on direct radiative forcing. He mentioned a recent UNEP report that stated the desert regions of the world were increasing and efforts to contain the spread were not working and further noted that the drier regions of the middle US may become even drier as the globe warms up. Work done by the Japanese on Gobi dust aerosols shows that the dust burdens correlate with temperature, usually showing a marked increase in the spring, after temperatures are above freezing. These Gobi dust episodes have been seen as far away as Mauna Loa. Similar results have been observed in the US, where springtime maxima in aerosol loadings have been observed in the western and midwestern States. It is also known that more crustal dust appears in ice core samples during ice ages, suggesting that atmospheric dust levels might be used as a climate change indicator.
Carmichael showed results of model calculations indicating that mineral (soil) aerosol can be a significant sink for certain species, implying that heterogeneous surface reactions on such particles need to be taken into account. His calculations show that nitric acid vapor condenses on the surfaces of the coarse particles, where it then reacts to form aerosol phase nitrate. The calculations appear to be corroborated by measurements by the Japanese, who have shown that a substantial fraction of aerosol nitrate, as determined through size-resolved chemical measurements, resides on the large particles. Also, it appears that even large particles can get transported over substantial distances. Finlayson-Pitts noted that the photochemistry of nitrate on surfaces can give rise to NO2 and O-, which can react with surface water to produce OH + OH-. The bottom line is that dust aerosol appears to play a significant role in radiative forcing and in atmospheric chemistry, and, therefore, needs to have further attention by the ACP community.
Outstanding questions that should be addressed by ACP include:
€ How important are oceanic organic aerosols in radiative transfer and indirect climate forcing?
€ What effect does the organic component of ambient aerosols have on particle solubility and growth as a function of relative humidity?
€ What are the gaseous precursors of organic aerosols in marine atmospheres?
€ What is the role of isoprene, terpenes, and monoterpenoids in production of organic and inorganic peroxides that lead to secondary sulfate aerosol formation?
€ What new measurement methods that minimize sampling and analytical artifacts are needed to characterize organic and other carbonaceous aerosols?
€ What is the global distribution of wind-generated surface erosion aerosols?
€ Can the origin, transport and demise of wind-generated aerosols, derived primarily from the great arid regions of the various continents, be modeled?
€ How can the climatological variance of these aerosols and the relationships to short-term climate variations such as drought and wet seasons be understood? Would a drier climate in some regions mean more wind-generated dust?
€ If there is a climate change, then will the global climatology of these aerosols likely change? Would this in itself be an indicator of climate change, if it can be discerned from a change in the "normal climatology?"
€ Do some desert aerosols contain more ccn than others? Can something be done about desert aridity to reduce its impact?
€ What role do various types and sizes of particles play in heterogenous chemical reactions?
Nucleation, CCN, and Cloud Formation
Recent field work by McMurry and Eisele (supported in part by ACP) has shown that the formation of new particles by nucleation from the gas phase occurs the atmosphere. Other recent laboratory work, e.g. by Mikheev (supported in part by DOE Fossil Energy), on binary and ternary nucleation in laminar flow tube reactors and in smog chambers suggests that certain trace contaminants, such as organic compounds, play an important role in the formation of new particles and that nucleation rates can be considerably faster than predicted by classical binary nucleation theory. Mikheev has also found experimental evidence that uv radiation can enhance particle nucleation rates. There is a need to study these processes in greater detail, both in laboratory settings and in targeted field experiments, to understand various particle formation pathways in chemical systems of atmospheric interest (H2O, H2SO4, NH3, organics, and others).
McMurry indicated that measurements on Mauna Loa and in the Rocky Mountains have shown evidence for nucleation in clear air, while aircraft measurements in the marine atmosphere have shown that nucleation occurs in the vicinity of cumulus and stratus clouds. These data suggest that nucleation occurs at rates that are considerably faster than can be explained by a process that includes only H2SO4 and H2O, as is typically assumed in models. It is generally accepted that nucleation in the marine atmosphere likely affects climate through its influence on cloud condensation nuclei (CCN) concentrations and, therefore, it is important to improve our understanding of the nucleation process.
Marlow cited theoretical work on particle formation at the molecular level, i.e., with small clusters of 13 atoms or less per cluster, and noted the applicability of Lennard-Jones potentials in calculating monomer-cluster collision rates. It was shown for the first time that many-body potentials may be required for calculating cluster-cluster rates, but that the simple 2-body Lennard-Jones potential is entirely adequate for monomer-cluster rates. This result suggests considerable simplification of calculations required for cluster formation in most atmospheric processes. Moreover this work has shown that the simple unit sticking efficiency universally employed for aerosols may also be employed for collision rates of clusters as small as 8-10 monomers each. Together, these results have already proved helpful in data interpretation.
Marlow called attention to a related project for NASA (Atmospheric Condensation Properties of Ultrafine Chain and Fractal Aerosol Particles), where calculations of equilibrium vapor pressures of water over single and adhering particles with fixed quantities of sulfuric acid on their surfaces are performed. Here, in a conceptual extension involving entirely different calculational methodology than used in the ACP work, equilibrium vapor pressures of water over adhering triplets and quadruplets of spheres are calculated to provide the basis for estimating the equilibrium vapor pressures over fractal aggregate aerosol particles such as produced by combustion processes. Also a significant fundamental advance has been made in the calculation of the van der Waals interaction between a molecule and condensed substrate. These new results are being utilized in the current ACP work in calculations of physical adsorption of water on nanometer substrate particles, as well as in interparticle force calculations.
Although the role of CCN in cloud formation and their eventual removal by precipitation seems well established, the chemical and microphysical details are not yet fully implemented into models, partly because the detailed processes for some cloud types seem to be inconsistent with observations or still unknown and partly because global aerosol models that include parameterizations of the known processes have not yet been fully developed. For example, many models assume that all (or perhaps 90%) of the particle mass is incorporated into cloudwater and its fate is the same as that of the cloud water. What is still needed are targeted process studies that elucidate a clearer microscopic understanding of the cloud nucleation process with respect to dependence on size, chemical composition, mass and number loading, and environmental variables, such as updraft velocity and cloud type.
Several studies have shown that biomass smoke particles, composed mostly of organic material, are efficient CCN. However, because smoke particles consist of mixtures of organic and inorganic species, it has not been established whether the organic component is intrinsically CCN active or is intrinsically inactive and rendered CCN active only through association with water-soluble inorganic species. An approach used by Novakov was to perform laboratory experiments with smoke particles practically free of inorganic impurities, generated by combustion of pure cellulose, a surrogate biomass fuel. The results of these experiments demonstrate that the organic component of biomass smoke particles is water soluble and intrinsically CCN active, even when virtually free of inorganic water-soluble impurities.
Saxena presented some results showing CCN formation in the vicinity of dissipating clouds at the Palmer station in Antarctica, a remote marine site. These results showed CCN enhancement by factors of 4 to 7 at a supersaturation of 1%. There is evidence of photochemical processes occurring in the clouds, suggesting that the clouds themselves act as producers-enhancers of CCN. Similar results have been reported for the Arctic.
The Boston College-Aerodyne group (Davidovits, Worsnop, et al.) has been investigating the effects of biogenic sulfur and sea-salt produced chlorine and bromine on CCN production and composition. Biogenically produced reduced sulfur compounds from the marine envirnoment, including dimethylsufide (DMS), hydrogen sulfide(H2S), carbon disulfide (CS2), methyl mercaptan (CH3SH) and carbonyl sulfide (OCS), deliver a sulfur burden to the atmosphere which is about half as large as that due to sulfur oxides produced by fossil fuel combustion. These species and their partial oxidation products dimethyl sulfoxide (DMSO), dimethyl sulfone (DMSO2) and methane sulfonic acid (MSA) contribute to aerosol and CCN production in clean marine air. Furthermore, oxidation of reduced sulfur species will be strongly influenced by NOx-O3 chemistry in marine atmospheres. The multiphase chemical processes for these species must be understood in order to model and predict the relative roles of biogenic and combustion produced sulfur oxides over the oceans.
Inorganic halogens are injected into the troposphere primarily via sea salt aerosols generated by breaking waves on ocean surfaces. Although most of these species remain in the condensed phase and are returned to the ocean by dry and wet deposition, a significant fraction (3 to 20% for Cl) is converted to inorganic halogen vapor via heterogeneous mechanisms, such as acid displacement reactions and reactions involving gaseous NO2, ClNO3, and N2O5. However, the measured inorganic halogen level in the marine boundary layer is not consistent with these known heterogeneous mechanisms. The lack of reliable data for halogen sources and sinks has made it difficult to model the global halogen cycle. More detailed understanding of the heterogeneous chemistry of halogenated species in the marine boundary layer is required.
In this context, Davidovits et al. have carried out a series of experiments to study the uptake of Cl2 by aqueous surfaces as a function of aqueous Br-, and I- concentration, and the uptake of Br2 as a function of I- concentration. Since the solubility of the halogen molecules X2 (X = Cl or Br) is low, the measured uptake is primarily a result of the aqueous reaction of the species with the halide ion Y- (Y = Br or I) via X2 + Y- ---> XY + Y-. The magnitude of the measured halogen uptake and its functional dependence on ion concentration are not in accord with a simple bulk phase reaction mechanism. The data indicate that reactions at the gas-liquid interface have a significant role in the gas uptake process. Our experiments show that the Cl2-Br- and Cl2-I- surface reaction becomes significant for bulk ion concentrations greater than about 0.05M. It is unlikely that the Br- (or I-) concentration in marine aerosols ever reaches such levels. However, the concentrations of other ions are certainly found at such levels and higher. Therefore other possible surface effects relevant to atmospheric processes need to be examined.
Schwartz suggested that the counterflow virtual impactor (CVI), developed by Stockholm University and the University of Washington, ought to be used in conjunction with single particle analysis as a means of addressing the chemical and microphysical characteristics of CCN.
The effect of CCN concentrations on precipitation efficiency is still largely uncertain. Harrison noted some concerns regarding cloud number and particle number and suggested that radiation thermodynamics needs to be sorted out, especially in layer clouds. CCN effects may have the strongest effect on precipitation efficiency of tropical clouds, suggesting that CCN may have an indirect effect on climate other than optics via the Twomey effect. He also pointed out that soot appears to have a longer lifetime in the atmosphere than sulfate, based on ratios of sulfate to soot in the Arctic, suggesting the selective scavenging of sulfate. This question is related to the lack of knowledge about the state of mixing of hydrophobic and hydrophilic aerosol chemical components.
The relationship between cloud droplet number and CCN number concentrations is also important, particularly for estimating the indirect radiative forcing by aerosols by altering cloud albedo. Recent evidence suggests that cloud dynamical factors such as turbulence and entrainment mixing may influence the CCN droplet relationships. Saxena presented results of work at Mt. Mitchell (NC) that appears to confirm the relationship between cloud droplet concentrations and albedo. He further mentioned the work of Fouquart and Isaka, who suggest that a 1.7% increase in albedo is sufficient to cancel a doubling of CO2. Marlow called attention to some preliminary calculations of nonstochastic and stochastic condensation under cloud-base conditions. The calculations are based upon the model for polydisperse aerosols developed by his group. The results show that initial cloud condensation nucleation broadens rather than narrows the aerosol size distribution, as is generally assumed based upon earlier, less-detailed modeling calculations.
Outstanding questions that should be addressed by ACP include:
€ What are the species that are involved with the formation of new particles?
€ What role do "contaminant" species play in aerosol nucleation? What is the effect of uv radiation on enhancing or inducing aerosol nucleation?
€ What is the mechanism of particle production?
€ How is CCN activity related to particle size and the mixing state of hydrophilic and hydrophobic aerosol chemical components?
€ What is the effect of CCN on precipitation efficiency?
Aerosol Thermodynamic and Optical Properties
Aerosol thermodynamic and optical properties are important parameters needed for estimating water uptake by particles, cloud nucleation and radiative transfer through atmospheric haze. For example, recent laboratory measurements on water activities, densities, and refractive indices of solution droplets containing either single or mixed salts of chlorides, sulfates, and nitrates have provided the extensive thermodynamic and optical data required in model computations for predicting the dynamic behavior, visibility reduction, and radiative effects of atmospheric aerosols. Tang showed results of extinction measurements as function of RH for common sulfate and nitrate aerosols as external and internal mixtures. From these results, it appears that the chemical effect on light scattering is outweighed by the size effect of the aerosols, with both external and internal aerosol mixtures exhibiting similar light scattering response for the same size distributions. Such experimental optical and thermodynamic data for organic aerosols, however, is practically non-existent.
Gaffney reported on other laboratory studies involving the examination of the absorptivities of aqueous aerosol species in the infrared, that included inorganic as well as organic species. In these studies, cylindrical internal reflectance spectroscopy is used to determine infrared absorption cross sections in aqueous solutions. Some efforts are also directed in studying the ultraviolet photophysics and photochemistry of dissolved organics and inorganics, especially those having important consequences in aerosol chemistry and dynamics as well as radiative balance questions. Novakov presented some measurements of the absorption coefficient of soot as a function of wavelength. The absorption efficiency varied from about 10-12 m2 g-1 at 550 nm to about 22 m2 g-1 at 250 nm. Noteworthy was that the uv absorbing component was soluble in acetone. DeLuisi also noted results of absorption measurements of dust aerosol and cloud droplet residues from Whiteface Mountain (NY), showing single scattering albedos as low as 0.65.
Field measurements, such as those being made in visibility (e.g., IMPROVE) and air quality and precipitation chemistry (e.g., EMEP, CAPMoN, CASTNet, etc.) networks and other intensive field campaigns, are providing data for evaluating aerosol optical and radiative properties. Recent experiments are beginning to capture enough physical, chemical and optical data on aerosols, including RH dependence, to perform closure experiments, but more targeted experiments and data analysis are needed. Such data are needed, for example, for use in ACP's Global Aerosol Model Evaluation project. Unequivocal determination of the RH dependence of organic aerosol light scattering coefficient and/or efficiency is not yet possible with current techniques and needs further attention.
There is also a growing body of radiometric data, from which aerosol optical depths may be extracted. These include the ARM instruments at SGP, the former Quantitative Links Network, and newer networks operated by the Department of Agriculture (UV-B Trends Network), NOAA (SurfRad), and others (University of Miami's AEROCE network in the North Atlantic). Radiance observations from orbiting satellites from which aerosol optical depths can be extracted are also available. Systematic analysis of these data sets in combination with other field observations are needed for visibility assessment activities and estimating the direct radiative forcing of aerosols. Laulainen noted that the Quantitative Links Network was just getting to the point of having a useful time series for the analysis of aerosol optical depth, cloud optical depth, and diffuse-to-direct irradiance ratios, when the Quantitative Links Program came to an end in September 1995.
The optical and radiative properties also impinge on the activities of the Ozone and UV-B working group. Of interest to this group with respect to ozone trends is the wavelength dependence of the scattering efficiency of stratospheric aerosols. The problem is quite complex as a result of the ever changing aerosol content. The difficulty revolves around the need to remove the aerosol signal from measurements like SAGE and those from the ground that use UV absorption to determine ozone densities. The changing aerosol, especially after volcanic injection, is a source of time-dependent error in the ozone retrievals, and thus seriously affects calculated ozone trends, especially in the lower stratosphere.
With respect to other topics related to UV-B radiation and aerosols, DeLuisi pointed out that TOMS can be used to infer UV radiation at the surface, as well as the presence of absorbing aerosols, such as dust and soot. Saxena reported on the aerosol optical depth difference as a function of wavelength in the visible between two stations (one on top of Mt. Mitchell) separated in the vertical by 1 km and related this difference to UV-B transmission.
Outstanding questions that should be addressed by ACP include:
€ What additional measurements are needed to determine the thermodynamic and optical properties of organic aerosols?
€ How can we predict the light scattering properties of mixed aerosol particles composed of both inorganic and organic compounds present either as soluble or as surface-active components? What field data are needed to test these predictions?
€ How can the relative humidity response factor for organic carbon aerosols (particle growth, light scattering efficiency) be determined unambiguously?
€ How can data from existing radiometric and surface chemistry measurement programs be used effectively for inferring critical aerosol radiative properties and parameters? What additional measurements can be added to these programs for effective column closure studies?
€ What is the role of aerosols in UV-B extinction? Can the scattering and absorption coefficients and single scattering albedos measured in the visible be extrapolated to the UV-B region?
€ How are inferred ozone trends corrected for aerosol effects?
Measurement Techniques and Strategies
Central to all of the issues and measurement requirements discussed in the preceding sections is what additional measurement techniques and/or instruments are needed and what strategies should be used to maximize the scientific return on investment, understanding that a number of other programs exist with similar or related measurement strategies. With respect to measurement techniques, aerosol tracers that can simulate various chemical and physical properties would be useful. Some examples of aerosols include fluorescent dyes and soluble salt perfluorocarbon compounds.
The issue of rapid determination of chemical composition of the aerosol, especially organic forms versus sulfates, or the absorption index/efficiency of the aerosol remains an important area of study and development. In other cases, more widespread or creative use of existing instruments, such as the CVI, RH-scanning (and/or temperature-scanning) integrating nephelometer (single or multi-wavelength versions), or multi-filter rotating shadowband radiometer (MFRSR), could be used in conjunction with other more standard aerosol size and composition measurements.
With respect to measurement strategies, a clear need is evident to coordinate ACP work with other DOE programs, such as ARM, NIGEC, and the possible Mexico City study, as well as other national and international programs, such as NARE, NARSTO, SOS, and the new initiative IGAC/DARF (International Global Atmospheric Chemistry/Direct Aerosol Radiative Forcing) activities. For example, at the ARM site in Oklahoma, a suite of optical and radiative properties of the atmosphere (including aerosol properties such as aerosol optical depth) are being made. ACP could support detailed aerosol chemistry and size distribution measurements, at least on a campaign basis, and thus provide a coupling or closure between the optical and radiative properties with the chemical and physical properties of the aerosols to test radiative transfer models. The data would also be available to test process models.
Another crucial element of an ACP aerosol program is "staying power". A case in point is the continuity of the data gathering and analysis activities of the Quantitative Links Network (QLN). As mentioned previously, just as the network was beginning to produce a meaningful time series of information, this portion of the Quantitative Links program was phased out. Although the data base is available for analysis over the period 1992-1995, there are no plans within any DOE programs to continue the network or to continue with the analysis and interpretation of the data, much of which would have direct relevance to the closure work needed for the direct aerosol radiative forcing. It is clear that the QLN effort demonstrated the viability of a new generation of radiometric instruments for use in a network setting, while providing valuable meaurements for obtaining key aerosol optical parameters. What is lacking now, however, is the continuity of the measurements at the different sites. At this time, it appears that other agencies may continue measurements at some QLN sites and, resources permitting, expand them to other sites within the United States.
In addition to staying power of key measurement programs, there needs to be more attention given to the analysis (or even reanalyis) and interpretation of existing data sets. All too often, great effort is expended in obtaining data for various purposes, either from intensive field campaigns or from extensive network measurements, but then only a fraction of the data is used for testing some specific hypotheses, before going on to design and conduct yet other field programs, leaving the remainder of the data virtually untouched.
An area worth noting is DOE's historical role as a key sponsor of international conferences, such as the five on carbonaceous aerosols and the five on precipitation scavenging, dry deposition, and surface exchange processes. These conferences have provided a unique forum for scientific exchange and a basis for international collaboration.