NOAA OGP Aerosol Climate Program
An integrated set of field and modelling studies that focuses on reducing uncertainties in estimates of the climate forcings from anthropogenically-influenced aerosols
Timothy S. Bates (PMEL), John A. Ogren (CMDL), and V. Ramaswamy
(GFDL)
September 15, 1995
Atmospheric aerosols affect the Earth's radiation budget directly through the scattering and absorption of solar and thermal radiation and indirectly through their effects on the microphysical and radiative properties of clouds and their effects on atmospheric trace gases. Present estimates of the direct radiative forcing for the sulfate component of the aerosol, yield a forcing over industrialized regions (eastern U.S.A., Europe, eastern Asia) that is greater in magnitude, but opposite in sign, than the radiative forcing from anthropogenic greenhouse gases. This sulfate forcing, along with the forcing from other components of the atmospheric aerosol, may have masked detection of the effects of greenhouse warming in the long-term temperature records. Clearly, a forcing component of this magnitude must be properly represented in global climate models in order to provide reliable guidance for policy-makers responding to concerns about anthropogenic effects on climate. NOAA's strategic plan addresses the need to assess and predict changes in climate on time scales of 10 to 100 years. NOAA's OGP has strong Atmospheric Chemistry and Ocean CO2 Programs studying the climatic impact of anthropogenically-produced greenhouse gases. However, the climatic effect of these gases must be coupled with those from anthropogenically-produced aerosol particles in general circulation models (GCMs) in order to accurately explain the Earth's surface temperature record over the past 100 years and to predict future chages in climate (IPCC, 1994 & 1995; Mitchell et al., 1995; Karl et al., 1995). NOAA scientists, with funding from the OGP, have demonstrated their expertise in aerosol/climate research and have taken a clear lead nationally and internationally in developing research strategies, organizing research programs, and contributing to IPCC assessment efforts. NOAA's OGP clearly is a lead agency in aerosol/climate research. This brief report summaries the NOAA OGP Aerosol Climate Program. More detailed progress reports are included as separate documents. What began as a NOAA OGP Marine Sulfur and Aerosol Program in 1990 has expanded to an integrated set of short-term intensive field experiments, long-term continuous observations, and radiative transfer and global climate modeling studies. The focus of this program is to understand aerosol properties and processes and to reduce uncertainties in estimates of the climate forcing and response resulting from anthropogenic aerosols. The ultimate goal of this integrated set of studies is to incorporate aerosols in three-dimensional chemical transport models that can be used to calculate the direct and indirect radiative forcing by the particles and determine the meteorological and climatic response to these aerosol forcings. Rationale and Objectives Atmospheric aerosol particles affect the Earth's radiative balance both directly through the upward scatter of solar radiation and indirectly as cloud condensation nuclei (CCN). The natural aerosol has been substantially perturbed by anthropogenic aerosols, particularly sulfates from SO2 emissions and organic condensates and soot from biomass and fossil fuel combustion. The global mean radiative forcing due to the direct effect of anthropogenic sulfate aerosol particles is calculated to be of comparable magnitude (approximately -0.3 to -1.1 watt/m-2) but opposite in sign to the forcing due to anthropogenic CO2 and other greenhouse gases (Charlson et al., 1991, 1992; Kiehl and Briegleb, 1993; Penner et al., 1994). More uncertain is the radiative forcing due to the indirect cloud-mediated effects of aerosol particles (Boucher and Lohmann, 1995). Although aerosol particles have a potential climatic importance over and down wind of industrial regions that is equal to that of anthropogenic greenhouse gases, they are still poorly characterized in global climate models. This is a result of a lack of both globally distributed data and a clear understanding of the processes linking gaseous precursor emissions, atmospheric aerosol properties, and aerosol optical depth and cloud reflectivity. At this time, tropospheric aerosols pose one of the largest uncertainties in model calculations of the climate forcing due to anthropogenic changes in the composition of the atmosphere. To reduce the uncertainties in estimates of aerosol forcing of climate the NOAA OGP Aerosol/Climate Program is addressing three specific objectives:
Objective 1: Characterize the Aerosol A simultaneous chemical, physical and radiative characterization of the atmospheric aerosol is needed in a variety of different air masses to develop and test model calculations of aerosol distributions and radiative effects. NOAA's research strategy for this objective is twofold:
The current ground-based network involves research groups at NOAA/CMDL, NOAA/PMEL, the University of Washington (UW), and the University of Illinois at Urbana-Champaign (UIUC) and includes four stations at Cheeka Peak, WA; Niwot Ridge, CO; Bondville, IL; and Sable Island, Nova Scotia. Since climate forcing is defined as the anthropogenic perturbation to the background atmosphere, field studies are needed in regions of both natural and perturbed air masses in order to quantify the non-linear changes to the background atmosphere resulting from anthropogenic emissions. The regional network sites therefore were chosen to characterize clean marine, clean continental, polluted continental and polluted marine air masses.
Short-term intensive campaigns provide a means of obtaining distributions of aerosol properties on larger scales. PMEL cruises during the past two years have transited the mid-Pacific from 55N to 70S, providing a long latitudinal "snap-shot" of aerosol chemical, physical and radiative properties. Future cruises will include a transit from Seattle to Tasmania as part of ACE-1 and a characterization of the North Atlantic aerosol as part of ACE-2. The short-term intensive field studies make use of closure experiments. Since aerosol chemical, physical and radiative properties are not independent of one another, it is possible to over-determine aerosol properties or related processes using a variety of measurement and modeling techniques in order to examine the internal consistency of these different measurement and modeling strategies. These over-determined data sets are referred to as closure experiments and are useful in estimating measurement uncertainties, testing the validity of models to predict aerosol radiative properties from measurements of chemical and physical aerosol properties, and in identifying areas where improvements in instrumentation or modeling are needed.
Objective 2 - Quantify the Processes
Prognostic aerosol climate models must be able to parameterize realistically the processes that control the formation, transformation and fate of atmospheric aerosols in order to calculate direct and indirect radiative forcing accurately. Model parameterizations need to start with an emissions inventory and calculate what fraction of the gas phase precursors is transformed to aerosol particles, how the size distribution of these particles evolves, and how long these particles remain in the atmosphere. NOAA's research strategy for this objective is to conduct short-term intensive field studies to quantify the fundamental processes controlling the formation, growth, cloud nucleating and radiative properties, and spatial distributions of important chemical components of the atmospheric aerosol, particularly anthropogenic sulfur and organic compounds.
Field process studies generally require a large number of investigators in order to measure simultaneously all the necessary gas and aerosol parameters. NOAA has made and can continue to make major contributions to these studies by participating in multi-agency aerosol field programs such as the International Global Atmospheric Chemistry Program's (IGAC) Aerosol Characterization Experiments (ACE). These experiments seek to understand and quantify the pathways, rates and yields of sulfur gas oxidation and the formation, growth, evolution and deposition of aerosol particles.
The remote marine atmosphere was chosen for the first ACE (October-December, 1995). Non-sea-salt (nss) sulfate aerosol particles in the remote marine atmosphere are thought to have only one primary gaseous precursor, dimethylsulfide (DMS), thereby simplifying studies involving the formation and growth of the aerosol. This environment also provides an opportunity to establish the chemical, physical and radiative properties of the natural aerosol system and thus provides a background from which to compare and quantify any anthropogenic perturbation. ACE-2 (June-July, 1997) will extend these characterization and process studies to the North Atlantic Ocean with an emphasis on the anthropogenic perturbation of the background aerosol.
The NSF atmospheric chemistry program and NOAA Aerosol Climate Program are the two major US contributors to ACE-1. NSF is providing approximately $4M over a two-year period in PI grants as well as 295 hours of C-130 time and two integrated sounding systems. NOAA is contributing 59 days of class one ship-time and PI support via PMEL. ACE-2 already has the strong support of the European community. The combined support from the European Commission and individual European countries for ACE-2 is approximately $10M (USD). NOAA's shipboard aerosol measurement program is a central part of the planned experiment.
Objective 3. Quantify the Climatic Forcing and Response
An accurate determination of the radiative forcing and the climate response due to aerosols necessarily requires the use of high-precision radiative transfer models and three-dimensional general circulation models (GCMs). The modelling component of the NOAA Program is directed towards improving the radiative transfer computations of the aerosol forcing and towards evaluating the regional-scale to planetary-scale responses of the climate system to aerosol radiative forcing. There are two issues related to modelling aspects that are of significance for the aerosol climate problem. The first concerns the accuracy of the radiative fluxes and heating rates and their quantitative dependence on the aerosol single-scattering properties. This involves conducting detailed sensitivity studies using the GFDL line-by-line, doubling-adding model which takes into account the spectrally-dependent properties of all the atmospheric constituents. This high-precision 'benchmark' computational algorithm is unique and offers an unparalleled capability to calibrate and establish the robustness of the aerosol radiative parameterizations routinely used in GCMs. An additional feature of this algorithm is the means it provides to compare the computed radiative quantities with those measured, thus building a firm basis for the model's radiative forcing determination. The second issue concerns the need to investigate the nature of the response of GCM to aerosol forcings (Ramaswamy et al., 1995) - e.g., the regional changes in the climate caused by aerosols, equilibrium versus transient responses, comparison with the response to greenhouse gas forcings, and comparison of model simulations with the observed climate record (including surface temperature changes, changes in the vertical profile of the atmospheric temperature, precipitation, soil moisture).
Advancements in modelling, together with concurrent progress in the observational and monitoring activities will enable the development of robust predictive capabilities for assessing aerosol effects on a spectrum of timescales (diurnal to few years to multidecadal).
NOAA - a lead agency in aerosol climate research
With the merger of the International Global Aerosol Program (IGAP) into the International Global Atmospheric Chemistry Program (IGAC) there is now one international voice for atmospheric aerosol research. While IGAC's primary focus is on atmospheric chemistry, the program leaders recognize that aerosols must be studied as a system with chemical, physical, radiative and cloud nucleating properties. In recognition of the importance of aerosol particles to climate, IGAC created an aerosol Focus comprised of 4 new aerosol activities. Two NOAA PIs serve as co-convenors on two of these activities. In addition, an aerosol task has been added to the Global Modelling activity which is being led by a NOAA PI. NOAA is also a lead agency in organizing and participating in the Aerosol Characterization Experiments (ACE) which will be an integrating task shared by the several aerosol activities. Accompanying this document are copies of the ACE-1 and ACE-2 Science and Implementation Plans. NOAA PIs have also taken lead roles nationally as part of the National Academy of Sciences Panel to recommend an inter-agency aerosol climate research program. The Academy report is currently under review. Internationally, NOAA scientists have participated in the IPCC assessments (1994 & 1995) and the Dahlem conference on Aerosol Forcing of Climate. NOAA has several unique capabilities which are clearly necessary components in national and international aerosol climate research programs. PMEL has developed a shipboard aerosol measurement capability which can provide oceanic-scale distributions of aerosol chemical, physical, and radiative properties to test and validate GCMs. PMEL's shipboard measurement program is an essential component to both ACE-1 and ACE-2. No other group within the US currently has this capability. CMDL and PMEL have developed a ground-based aerosol monitoring network using standardized sampling protocols in collaboration with the World Meteorological Organization/Global Atmospheric Watch (WMO/GAW) program to obtain aerosol properties in locations dominated by key aerosol types. These standardized sampling protocols are now being adopted by aerosol programs sponsored by other agencies (DOE-ARM, NSF-AEROCE). GFDL's high-precision 'benchmark' computational algorithm for the solar spectrum is unique in the world and offers an unparalleled capability to calibrate and establish the robustness of the aerosol radiative parameterizations routinely used in GCMs. The GFDL GCMs, widely used in climate assessments of greenhouse gas increases, are being adapted for aerosol research and are extremely well-suited to address the vast range of questions concerning aerosol-climate interactions.
The primary goal of the NOAA-OGP aerosol/climate research project has been to provide the necessary data to incorporate aerosols into global climate models and to reduce the overall uncertainty in the calculation of climate forcing by aerosols. Our highest priority has been to build a strong aerosol measurement program and to enhance the effectiveness of this program through international collaboration and leadership within the IGAC and WMO/GAW programs.
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