It is proposed that DOE/ESD (Department of Energy/Environmental Sciences Division) initiate a research program directed to the processes controlling the mass loading, composition, and microphysical properties of tropospheric aerosols, the Tropospheric Aerosol Program (TAP).

This document is meant to serve as a framework for specifying the elements of TAP.

One can turn to the similar air-quality problem of tropospheric ozone to gain some insight into what is required to develop the knowledge base required to effectively address the aerosol problem. Ozone air quality standards have been in place since the 1970's, yet aside from very few places very little progress has been made in reducing ozone. It is only through years of research that we have recently come to appreciate that many of the measures that were implemented were ineffective, or at least not optimally effective. This research is by no means finished, and there are many remaining crucial questions. But we are at least at the point where the last remaining questions are well defined. The most recent research results are beginning to firm up the notion that previous approaches to ozone reduction have been inadequate. New approaches are being considered. The DOE Atmospheric Chemistry Program (ACP) is one of the leading research programs in this field. The field measurements, laboratory investigations, and modeling activities in this program have shaped the conclusions and the remaining questions. How did we get to where we are today in ozone research? It has taken a major national effort that included the key federal agencies NSF, EPA, NOAA and state and local agencies. The task is too big for and too complex for a single agency to master. For such complex research to succeed there must be cross checks and balances. For example one of the reasons that the ACP field program has been so successful was the challenge, competition, and collaboration with the NOAA field program, and the NOAA program has likewise benefited from the interaction with the DOE program.

Understanding the processes that control the loading, distribution, and properties of submicrometer aerosols, the other major unresolved component of air pollution, is going to be at least as hard as ozone, if not harder. Aerosols are chemically and physically much more complex than ozone. They are very heterogeneous in composition and sources, ranging from seasalt, dust and tire particles, to sulfates, nitrate, organics and soot, as well as mixtures of these materials. Some of the aerosols are emitted directly as particles whereas others form in the atmosphere from gaseous precursors, both anthropogenic and natural. While in the atmosphere aerosol particles grow in size and evolve in composition through gas adsorption and coagulation and can change phase by deliquescence and efflorescence. They are removed through wet and dry deposition processes which are dependent on the composition and size of the particles.

There are many aspects to understanding and quantitatively describing the loading and properties of sub 2.5 µm particles from the generation of emission inventories to regional monitoring to detect noncompliance, to developing the knowledge of the fundamental process that control loading of tropospheric aerosols and concentrations of specific classes of compounds, the processes that constitute what might be denoted the "atmospheric life cycle" of these aerosols. A recent NRC report (March 1998) called for development of advanced mathematical, modeling, and monitoring tools to represent the relationships between specific sources of particulate matter and human exposures and for linking sources of toxicologically important constituents and characteristics of particulate matter to exposed individuals and populations. But this cannot be done without understanding of processes that control these relationships. This is where DOE comes in. We will draw on our recent experience with the ozone program, in which we have demonstrated the ability to address this aspect of the problem very effectively. We fully expect other agencies, principally NOAA, NASA, NSF, and EPA, to develop similar programs, as these agencies have declared PM-2.5 as high priority research project for the near future. As with the ozone problem coordinated work by multiple agencies is required to make real progress.

It is clear that to resolve the PM-2.5 problem there will be a call for changes in the energy industry which will place constraints on the energy economy. DOE shares the responsibility to ensure good air quality but has the further responsibility to assure that this is achieved without imposing unnecessary burdens on the energy economy that result from lack of ability to quantitatively describe the loading of PM-2.5 and the ability to develop optimum strategies to meet the new standards.

Climate influence of anthropogenic aerosols. Anthropogenic aerosols scatter solar radiation and modify cloud reflectivity, thereby exerting a cooling influence on climate. This cooling influence is thought to be offsetting a substantial fraction of anthropogenic greenhouse warming, but its magnitude is highly uncertain. This uncertainty precludes confident empirical detection of climate change due to increased greenhouse gases and quantitative inference of climate sensitivity. It also precludes evaluation of performance of global climate model simulations so that it is impossible at present to answer the question of how much of the 0.6 degree temperature rise over the past century can be ascribed to anthropogenic activities and how much to natural variation. Much of this uncertainty arises from present limited ability to quantitatively describe the aerosol loading and microphysical properties that govern their light scattering and cloud nucleating ability.

Sub-micrometer aerosols are highly efficient scatterers of solar radiation; this is the phenomenon that is responsible for impairment of visibility associated with urban photochemical smog. A further consequence of this light scattering is that some of the scattered radiation is returned to space instead of being absorbed at the surface of the earth, as would otherwise be the case. This direct light scattering by aerosols thus exerts a cooling effect on climate change (climate forcing); the upper limit of present estimates of this forcing is thought to be comparable in magnitude over the industrial period to the perturbation in the infrared budget associated with increased concentrations of greenhouse gases, while the lower limit is near zero. To make matters more complex, aerosol particles serve as the nuclei on which cloud droplets form in the atmosphere (cloud condensation nuclei, CCN). This is the process whereby these particles are removed from the atmosphere in precipitation, but it also has the effect of increasing multiple scattering of solar radiation by clouds, thereby making them more reflective, leading to an additional cooling influence on climate. The shortwave radiative forcing of climate by aerosols by these two mechanisms has been identified by the Intergovernmental Panel on Climate Change (IPCC) as the greatest uncertainty in forcing of climate change over the industrial period. A major consequence of this uncertainty is that it limits the ability to empirically detect the climate change due to increased greenhouse gases or to quantitatively infer the sensitivity of climate to greenhouse gas forcing. It also precludes evaluation of performance of global climate model simulations of climate change over the industrial period. In 1996 a NRC panel, convened at the behest of DOE, NASA, NOAA, and NSF, issued an urgent call for a coordinated program of research to quantify this aerosol forcing. Since that time virtually nothing has been initiated. As we get ready to face the next millennium and draft a policy to mitigate or accommodate to the anticipated climate change, it is imperative to resolve this largest scientific uncertainty in climate forcing.

Much of the uncertainty in this forcing arises from present limited ability to quantitatively describe the aerosol loading and microphysical properties that govern their light scattering and cloud nucleating ability. While other programs that are dealing with climate change research like the DOE Atmospheric Radiation Measurement Program (ARM) are focusing on the radiative effects of aerosols, there is a major gap with respect to research to quantitatively describe the atmospheric life cycle of tropospheric aerosols as it pertains to this radiative forcing. This research is again very much in the domain of atmospheric chemistry, building upon the techniques and capabilities of this discipline. Ultimately what is required are models that accurately represent the controlling properties. However representation of aerosol processes in climate modeling is in its infancy, and to the extent that this is done, it is done in a quite simplistic way that does not take into account the heterogeneity of size and composition that is characteristic of aerosols. For example, because of lack of knowledge thus far there has been a tendency, to the extent that aerosol properties are addressed in models at all, to assume treat all aerosols as if they are composed of ammonium sulfate at 25C. However it is known from recent results that aerosol optical and properties depend markedly on composition and temperature. It is therefore necessary to develop the knowledge base that can handle the complexity of actual aerosols.

It seems likely that meeting the climate change issue will also require a significant change in the nation's energy economy at a cost of billions of dollars. Moreover, by virtue of the fact that aerosols couple air quality with climate change our actions to meet one of these challenges will affect the other. In this context it is urgently imperative that the aerosol forcing be placed on a much more quantitative foundation than is available at present.

TAP will develop the fundamental scientific knowledge base required to construct tools for simulating the loading and chemical and microphysical properties of tropospheric aerosols. Only through process level understanding is it possible to construct models that can be used with confidence to devise energy-efficient and cost effective strategies to meet PM-2.5 standards or to evaluate aerosol influences on climate at the present time and for past and prospective future emissions scenarios.

TAP would thus be directed to improved understanding and quantitative description of the life cycle of these aerosols--the processes controlling their mass loading, composition, and microphysical properties, all as a function of time, location, and altitude--as distinguished from, for example, their optical properties, their radiative effects, their health influencing properties, their health effects, etc. TAP would work closely with other programs in DOE and in other agencies, directed to related issues of atmospheric chemistry, to aerosol loading and life cycle, and to the influence of tropospheric aerosols on health, climate, visibility, and the like.

The objective of TAP should be viewed in the context of the national portfolio of measurements and research on atmospheric aerosols. There are many elements that must be implemented in order to be able to meet this country's requirements, and the several Federal agencies have differing responsibilities in this respect, although to be sure the boundaries that separate these responsibilities are somewhat ill defined. EPA's responsibilities include development and evaluation of emission inventories of aerosols and aerosol precursors, monitoring to determine the extent of compliance/non-compliance with air quality standards, and epidemiological studies to quantify aerosol health effects. NOAA has responsibility for long term monitoring of aerosol loading and properties at a small number of sites. Some of these elements are in place now and some of them are being enhanced. A national monitoring network will undoubtedly produce invaluable data, but without the supporting mechanistic knowledge, its value is very limited in providing guidance for developing an efficient control strategy or in evaluating present, past, or future aerosol influences on climate.

If the goal of a national aerosol program includes, as it should, developing the capability to devise effective strategies for control of aerosol loadings and the ability to quantitatively estimate aerosol influences on climate, it is essential to acquire fundamental understanding of the processes governing the loading and properties of tropospheric aerosols. This process-level understanding is fundamental to constructing models that are generally applicable, not just to a limited range of conditions, so that they may be applied to a wide variety of situations, for example modeling for previous emission scenarios in order to develop a historical record of forcing, or modeling for various emission scenarios to answer "what if" type questions. DOE ACP has demonstrated outstanding capability to perform process-level atmospheric research. To date this program has focused largely on the fundamental processes that control photochemical oxidants. It would be a natural extension to adopt the strategies and construct a program aimed at understanding the other major unsolved component of energy-related air pollution, aerosols.

TAP must be viewed as one component of an integrated national program of research on atmospheric aerosols. It is essential that the programs of the several agencies be closely coordinated. The DOE component of the national program is process research to determine, quantitatively describe, and represent in models the mechanisms governing the mass loading, composition, microphysical properties of tropospheric aerosols, and their geographical and vertical distribution.

TAP would contribute to satisfaction by DOE/ESD of its statutory obligation to conduct research directed to understanding the basic chemical and physical processes of the Earth's atmosphere and how these processes may be affected by energy production and use, and ultimately to helping to understand and mitigate the long-term health and environmental consequences of energy production and the use of differing technologies.

TAP would consist of linked laboratory, field and modeling projects focused on the life cycle of tropospheric aerosols--formation, growth, transport, and deposition as these processes govern the loading of these aerosols, their properties, and geographical distribution. The geographical scale to be considered ranges from urban (tens of km) to regional (hundreds of km). The following are classes of questions that must be addressed in a program directed to understanding the processes that control loading and properties of tropospheric aerosols.

Projects to be conducted within the umbrella of TAP would include the following elements:

Laboratory studies: This element is designed to generate the fundamental physical, thermodynamic, and kinetic information that is relevant to unraveling the life cycle of aerosols. These may include particle growth rates, phase transformations, heterogeneous rate constants, accommodation coefficients etc. These projects can be reasonably free standing but need to be coupled to the measurement and modeling needs of TAP.

Boundary-layer meteorology: A major components of any atmospheric program must be meteorology. Although emissions and chemical and physical processes clearly play an important role in determining concentrations of air pollutants generally and of aerosols in particular, these fundamental processes do not vary greatly from day to day. What does change is the meteorology. This meteorological variability makes it crucial to accurately describe the meteorology in order to establish the accuracy of model representations of aerosol chemical and physical processes. As indicated above it is intended that TAP closely coordinate its activities with the DOE-ESD program in vertical mixing and transport.

Field measurements: This component must clearly be the center of TAP. It is only through field measurements that confidence can be developed in the physical understanding and model representation of processes responsible for aerosol loading and properties. This evaluation starts with emission inventories, for example by comparisons of measured and modeled loadings of specific aerosol components, or of ratios of loadings of aerosol components to those of co-emitted tracers of opportunity or of introduced tracers. Theories of various stages of the aerosol life cycle these can be tested only through field measurements. Another important part of field measurements is instrument intercomparisons. The development of the field program would draw heavily on experience with the ozone research in ACP.

Field campaigns would take place in coordination with other major participants in the program like NOAA, EPA, local air monitoring authorities etc. This clearly calls for a program that has the flexibility to conduct campaigns in locations in which other mutually contributing activities are taking place.

TAP field campaigns will require surface and airborne measurements. Surface units must therefore be relocatable so that they can be positioned in multiple strategic locations. By providing continuous high-frequency data 24 hrs a day for long periods (e.g., one month) surface measurements provide essential context for the airborne measurements that are restricted to limited time periods of a few hours.

Intensives would consist of coordinated efforts of various agencies focusing on a geographical area and a scientific objective, for example aerosol evolution in power-plant plumes or plumes from isolated urban areas. Surface measurement stations would be positioned in the vicinity of the emitting source to provide continuous information on upwind and downwind aerosol loading and properties. The aircraft would fly missions at certain times of the day to probe specific questions such as particle growth rates downwind from the source, nighttime processes, boundary layer formation and the influence of these processes on aerosol loading, composition, and microphysical properties. Other components of field studies might focus on the role of clouds in transforming and scavenging aerosols and aerosol precursors; mechanistic description of dry deposition of aerosols and precursor gases. The DOE G-1 aircraft would be the prime airborne platform for TAP measurements, but it anticipated that additional platforms would be available from other Federal, state and local agencies, as well as universities.

It is important that the field campaigns be designed around specific scientific objectives that are relevant to TAP. Depending on funding level it might be possible to field up to three major efforts a year.

Modeling: TAP will develop modeling capabilities on relevant scales. The modeling activities must be carried out in tight collaboration with the field program. Modeling can be used to develop a program plan before the field campaign and afterwards it will be a major part of data analysis. It is therefore imperative that the modeling activities focus on a regional and not global scale. A very important component of the model development must be characterization of the meteorological variables that are responsible for aerosol transport and representation of this transport in the model.

Interaction between modelers and laboratory and field investigators is envisioned as a two-way street in which the modelers provide questions that need to be addressed by the laboratory and field measurements and the measurements provide input and insight for the modelers.

Large Scale Computation: It is reasonable to assume that to develop a complete regional model with a resolution fine enough to include the important processes will require enormous computational resources. This should not be a limiting factor not when the resources are becoming available. It is important that TAP establish links with computational efforts being developed in DOE and pushing to be part of this effort.

Meeting the requirements outlined here is viewed as requiring a mix of principal-investigator led projects, highly focused on the objectives of the overall program, together with supporting infrastructure activities.

Proposal-based initiation of projects to be conducted within the umbrella of TAP. Might be lab, theory, modeling, field. If field, might be component of major campaigns, or might be free-standing. All projects need to be directed to answering key questions and to folding results into the requisite enhanced process-level understanding and ultimately into model-based representation of these processes. Care must be taken in construction of requests for proposals and in award of grants that necessary research components are being addressed, without overemphasis of some components with underemphasis of others.

A chief scientist would exercise responsibility for overall scientific direction of program.

A supporting infrastructure is seen as crucial to achieving the objectives of TAP. This infrastructure would have responsibility for:

Facilities. Provide and maintain aircraft, relocatable aerosol measurement facilities, instrumentation for field measurements.

Support for campaigns. Campaign planning and logistics; calibrations, measurement audits.

Data system and archive. Incorporate measurements into data base/archive to facilitate model development and testing.

Support for modelers. Preparation of meteorological drivers and gridded data sets for comparisons; establishing and maintaining community models supporting swap in/out of modules.