The ACE-1 scientific objectives will be addressed in several activities during the intensive measurement campaign and during the transit flights and cruise between the United States and Australia. Six research questions, outlined in section 2.1, have been defined to address these objectives. The strategies for answering these 6 questions are outlined in this chapter. Chapter 5 contains the specific approaches that will be used to implement these strategies, with the necessary instruments and platforms described in Chapter 4.
The ACE-1 scientific objectives and related research questions are directed toward the overall ACE goal: to provide the necessary data to incorporate aerosols into global climate models and to reduce the uncertainty in the calculation of climate forcing by aerosols. A critical part of aerosol radiative forcing calculations involves relating the size-dependent chemical composition of aerosols to their radiative impact. In models, this module makes the critical link between anthropogenic changes in the chemical composition of the atmosphere and the associated impact on the Earth's radiation budget. Questions 1 and 2 address Objective 1 and are meant to provide data to test and validate aerosol/climate models on local and column-integrated scales.
Process studies are also needed to evaluate the factors controlling the sources, formation and fate of aerosols and the way these processes affect the number size distribution, chemical composition, and radiative and cloud nucleating properties of the particles. Questions 3, 4 and 5 address Objective 2 and relate to models of the natural biogeochemical processes that create atmospheric sulfate and its precursors and the processes that control the aerosol population. Finally, question 6, which addresses Objective 3, integrates the closure experiments and process studies into models on a variety of scales.
Objective 1. Document the chemical, physical and radiative characteristics of remote marine aerosols and
investigate the relationships between these aerosol properties.
Figure 3. NCAR C-130 and NOAA Discoverer track lines for the transits between the United States and Australia. The C-130 will stop in Anchorage, Alaska; Oahu, Hawaii; Christmas Island; Pago Pago, American Samoa; and Christchurch, New Zealand. The total transit will require about 2 weeks of time and will yield a free tropospheric latitudinal transect of gas and aerosol measurements from approximately 78°N to 70°S. Measurements will also be made on the north-bound transit with stops in Brisbane, Australia; Nadi, Fiji Islands; Christmas Island; Hilo, Hawaii; and Moffett Field, California. The ship transit from Seattle to Hobart will require 28 days and will yield a boundary layer trans-Pacific transect of gas and aerosol measurements from 48°N to 43°S. Measurements will be made only on the south-bound transit.
2. to test the validity of models to predict:
b) cloud nucleating properties based on Khler theory and measurements of chemical and physical aerosol and cloud properties,
4. to identify areas where improvements in instrumentation or modelling are needed.
3.1.1 Local closure
A local closure experiment is one in which all measurements are made at a single location and time. Several local closure experiments are required to address uncertainties in the estimated climate forcing by sulfate aerosols. The unknowns include the aerosol mass scattering efficiency, aerosol hemispheric backscatter fraction, fractional increase in aerosol scattering efficiency due to hygroscopic growth, and parameters affecting aerosol activation (Penner et al., 1994). All of these experiments relate to one specific question:
Key measurements for each prediction include the aerosol number concentration and chemical composition as a function of particle size. It is important, therefore, to validate these measurements and to estimate their uncertainty. This comparison will address the internal consistency of these measurements, i.e., can the chemically analyzed mass account for the total aerosol mass? Is the mass derived from the aerosol chemical size distribution consistent with that from the aerosol number size distribution? Measurements of total gravimetric aerosol mass, chemically analyzed mass, and mass derived from the number size distribution will be made simultaneously.
2. Measured hygroscopic response of the aerosol size to changes in relative humidity (RH) and the hygroscopic response calculated from the measured aerosol number and chemical mass size distributions, RH and published functional relationships between chemical composition and water uptake.
This intercomparison adds an additional step of complexity to intercomparison 1 by varying the total water content of the aerosol. It will address the ability to parameterize the response of ambient aerosol to changes in RH. The hygroscopic growth characteristics of the aerosol will be measured directly with a tandem differential mobility analyzer (TDMA) (McMurry and Stolzenberg, 1989) and gravimetrically in equilibrium with a range of humidities (Hnel, 1979).
3. Measured aerosol scattering and calculated aerosol scattering derived from Mie theory applied to measured number and chemical mass size distributions.
This intercomparison directly addresses the uncertainty in determining the aerosol/climate parameters of aerosol mass scattering efficiency and aerosol hemispheric backscatter fraction. Scattering and backscattering of visible light by particles will be measured directly with an integrating nephelometer. These direct measurements of light scattering properties will be compared with independent predictions from the aerosol number and chemical mass size distributions, some simplifying assumptions (e.g. spherical particle shape, an externally mixed aerosol, particle refractive index, and density), and Mie theory.
4. Measured increase in aerosol scattering due to hygroscopic growth and the calculated increase based on measured number and chemical mass size distributions, RH, and published functional relationships between particle chemical composition and water uptake.
This intercomparison adds an additional step of complexity to intercomparison 3 by varying the total water content of the aerosol. It will address the aerosol/climate parameter of fractional increase in aerosol scattering efficiency due to hygroscopic growth. Two nephelometers will be operated in parallel, to measure directly the scattering and backscattering of visible light by particles at a low reference RH (40%) and at a higher RH. The increase in aerosol scattering due to the aerosol hygroscopic growth will be independently calculated using the data from intercomparisons 2 and 3.
5. Measured CCN supersaturation spectrum, the CCN supersaturation spectrum derived from Khler theory applied to the measured aerosol number and chemical mass size distributions, the CCN supersaturation spectrum derived from TDMA growth factors, and the CCN super-saturation spectrum derived from cloud droplet number concentration.
This intercomparison will address questions of aerosol activation that affect the indirect radiative influence of aerosols on climate. The droplet-nucleating potential of the aerosol population will be measured directly with a thermal diffusion cloud chamber that subjects the aerosol to a variety of supersaturations and counts the number of droplets that are nucleated. This defines the number concentration of cloud condensation nuclei (CCN) as a function of the supersaturation spectrum, CCN(S%). Growth times will be assessed to determine whether the particles are obtaining a dynamic or equilibrium response. The supersaturation spectrum of the aerosol population will be predicted independently by three methods. The first prediction will use the aerosol number and inorganic and organic chemical mass size distribution, certain simplifying assumptions (e.g. an externally mixed aerosol and insoluble content), and Khler theory. The second prediction will use the aerosol growth characteristics determined from the TDMA (intercomparison 2). Finally, the supersaturation spectrum will be derived from the cloud-base droplet number concentration, cloud microphysics and cloud structure.
6. Measured cloud droplet number concentration just above cloud base and the cloud droplet number concentration calculated from the measured updraft velocity, aerosol number size distribution, aerosol chemical composition, and CCN supersaturation below cloud base.
This intercomparison adds the complexity of real clouds to the measurements of intercomparison 5. It tests the ability of cloud microphysical models to predict cloud droplet number concentrations from properties of the sub-cloud aerosol as well as the suitability of parameterizations of cloud droplet number concentration used in global-scale models. The methods used to determine the CCN supersaturation spectrum do not reproduce the time-history of supersaturation encountered in real clouds. The cloud droplet size distribution just above cloud base (but high enough to be above the level of peak supersaturation) will be calculated using the observed updraft velocity in two ways, from the measured CCN supersaturation spectrum and from the measured aerosol size distribution and chemical composition. The extent of agreement between the measured and calculated droplet number concentrations and size distributions will provide a valuable indicator of the suitability of current models and measurement approaches for representing the initial stages of cloud droplet nucleation and growth.
7. Measured index of refraction and index of refraction derived from the measured aerosol number and chemical mass size distributions, RH and published functional relationships between chemical composition and water uptake.
This intercomparison will address an important link between the physical and chemical properties of the aerosol. The multiangle scattering probe (MASP) determines light scattering at two angles from every particle counted. The scattering data from the MASP can be used to calculate the particle index of refraction. These calculations of refractive index will be compared with independent predictions using the measured aerosol number and chemical mass size distributions, RH and published functional relationships between chemical composition and water uptake.
Aerosol size (number distribution and/or mass) at a range of humidities
Aerosol scattering and backscattering at a range of humidities
Cloud droplet number distribution
CCN supersaturation spectrum
Aerosol index of refraction
A column closure experiment extends the local (zero-dimensional) closure to multiple altitudes (one-dimensional closure) in order to compare and calibrate satellite and surface based column-integrated radiation measurements with in-situ (aircraft) aerosol chemical, physical and radiative measurements. Specifically,
Apart from the direct effect of aerosols on climate, aerosols in the lower atmosphere are known to control climate indirectly through their influence on the cloud droplet number concentration. The depth of the cloud, together with the cloud droplet number concentration define cloud optical depth (essentially cloud column closure) which is a quantity closely related to cloud albedo. By observing aerosol below clouds, cloud droplets at several levels inside the clouds, and up- and down-welling short-wave radiation above clouds, a unique data set is obtained linking subcloud aerosol to a climate parameter (cloud albedo) in the pristine environment of the Southern Ocean. Furthermore, cloud albedo is a parameter which can be retrieved from satellite observations. Thus, the in-situ data set can be used to test optical depth retrieval algorithms. Although the instrumentation aboard the C-130 is not optimized for cloud studies, some flight time will be devoted to this research effort.
Column closure experiments are also needed for reasons totally unrelated to climate. The impact of atmospheric aerosol scattering must be estimated and corrected for in order to derive information such as water-leaving radiances from satellite sensors. The atmospheric correction algorithms that have been developed for this purpose (Gordon and Wang, 1994) require assumptions about the nature and concentration of the aerosol and its resulting integrated radiative impact. The ACE-1 column closure experiments will help to improve the aerosol/radiative corrections. The transit flights between the United States and Australia will provide aerosol chemical and physical data to improve the regional aerosol corrections.
The area near Cape Grim, Tasmania was chosen for ACE-1 in part because it provides an opportunity to characterize the natural aerosol and hence a background from which to quantify anthropogenic perturbations. However, measuring aerosol radiative effects in this background low-optical-depth case (0.04 to 0.05; Forgan, 1990; Durkee et al., 1991) presents a significant challenge requiring extremely careful calibrations and intercalibrations. Current optical instruments (nephel-ometers and sunphotometers) are capable of making accurate and precise measurements in this region, but the signal to noise ratio will be small.
To provide a contrasting natural high-optical-depth case to validate the column closure experimental approach, two additional experiments will be conducted near Hawaii on the transit to Australia. The first experiment will use the Kilauea volcano plume that provides a well-contained, soot-free source of sulfate aerosols with optical depths of 0.1 and higher. The degassing associated with Kilauea's ongoing eruptive phase (since 1983) constitutes a point source for sulfur gases and associated sulfate aerosol into the relatively pristine atmosphere of the central Pacific. The plume provides a strong signal that is readily detected by satellite (Porter et al., 1994) and in-situ measurements (Quinn et al., 1990). Consequently, a radiative closure experiment carried out both during descent through the plume and in the adjacent clean air will provide a range of responses for the in-situ instrumentation aboard the C-130, including the specific effect of markedly increased sulfate concentrations superimposed on a background aerosol. The direct comparison with adjacent clean air will help to quantify the importance of aerosol sulfate in aerosol extinction and will help to refine the algorithms needed to interpret satellite data by providing data at a much larger aerosol optical depth.
The second test of the column closure experimental approach will be performed near Hawaii over the SeaWiFS optical mooring (Hooker et al., 1992). This mooring provides long-term time series comparisons between in-situ and satellite measurements of normalized water-leaving radiance and hence provides an opportunity to intercalibrate the C-130 optical measurements with the future SeaWiFS sensors.
Upward and downward radiances measured at several altitudes
Aerosol number and mass distributions at several altitudes
Aerosol radiative properties at several altitudes
Aerosol optical depth spectra for the column above those altitudes
Lidar observations of aerosol scattering profiles above and below the aircraft altitudes at as many wavelengths as possible
Cloud droplet number distribution
CCN supersaturation spectrum
Objective 2. Determine the key physical and chemical processes controlling the formation and fate of
and how these processes affect the number size distribution, the chemical composition, and the radiative and cloud
nucleating properties of the particles.
Three specific questions will be addressed as part of ACE-1 in order to quantify the key biological, chemical and physical processes controlling the source, trans-formation and fate of aerosol particles in the marine boundary layer. The questions along with relevant background information and measurements needed to study each problem are outlined in the following discussion.
3.2.1 Biological DMS production and air-sea exchange
DMS is one component of an active seawater sulfur cycle (Bates et al., 1994). The precursor of DMS, DMSP (dimethylsulfonium propionate), is produced by many phytoplankton species as an osmolyte. During phytoplankton senescence and/or grazing DMSP is released to the water column and enzymatically cleaved to DMS and other sulfur compounds. DMS can be lost from the water column by air-sea exchange, microbial consumption, or photochemical oxidation. The factors controlling the rate of DMS cycling in surface seawater and hence its seawater concentration strongly affect the amount of DMS that is released to the atmosphere.
The 1991 Pacific Sulfur-Stratus Investigation (PSI) and the 1992 Pacific IGAC-MAGE cruise included studies of the major sulfur reservoirs in surface seawater and the rates of exchange between these reservoirs. The results have shown that in the temperate North Pacific, air-sea exchange is a very small sink for seawater DMS. In contrast, the rates of air-sea exchange, microbial consumption and photochemical oxidation in the Equatorial Pacific are very similar to each other. Similar experiments will be conducted in the South Pacific as part of ACE-1 intensive.
The air-sea flux of DMS is the starting point of the marine atmospheric sulfur cycle and thus limits the production of sulfate aerosol. The need to improve measurements and models of air-sea exchange led IGAC to create the MAGE activity and is one of the major motivators behind ACE-1. During the equatorial Pacific IGAC/MAGE 1992 experiment, this flux was calculated by two independent methods: seawater DMS concentrations coupled with air-sea exchange models and the atmospheric diurnal cycle of DMS concentrations coupled with atmospheric photochemical models (Thompson et al., 1993; Yvon et al., 1994). ACE-1 investigators will repeat this approach during ACE-1 by measuring the concentrations of seawater and atmospheric DMS and the photochemically important atmospheric parameters. DMS surface fluxes will also be determined from the Lagrangian sulfur budget analysis (section 3.2.2).
The ocean surface can also be a source of ammonia and organic material, both of which are potentially important in aerosol formation and growth. Fluxes of these species will be estimated based on air-sea exchange and micro-budget techniques.
Rate measurements of DMSP consumption, DMS production, DMS microbial consumption
Seawater chlorophyll, pigment and nutrient concentrations
Atmospheric DMS concentrations
Atmospheric photochemically active species (CO, CH4, NMHC, H2O2, NO, NO2, NOy, and O3)
Meteorological data (wind speed, boundary layer height, etc.).
Lagrangian experiments offer tremendous potential for studying these atmospheric oxidation processes and chemical budgets. During the Atlantic Stratocumulus Transition Experiment (ASTEX), the MAGE Science Team tested a Lagrangian experimental approach for observing changes in the chemical species in the marine boundary layer near the Azores. Initial results suggest that this approach can yield process information without the confounding effects of air mass changes. The Lagrangian strategy will continue to be developed during ACE-1 and will include gas phase DMS, DMSO, DMSO2, SO2, methanesulfonic acid (MSA), and H2SO4 and particulate phase MSA and SO4. This suite of measurements, in a region far from anthropogenic SO2 sources, will provide data to quantify rates and efficiencies of DMS and SO2oxidation.
A variety of measurements are needed to clarify the oxidation and removal pathways of marine boundary layer sulfur. For the Lagrangian observations the measurements will be repeated in the same airmass over a period of two days.
Gas phase DMS oxidation products (DMSO, DMSO2, SO2, MSA, H2SO4)
Photochemically active species concentrations (CO, CH4, NMHC, H2O2, NO, NO2, NOy, and O3)
Measurements of both sub- and super-micron nss sulfate and MSA.
Mass size distributions of nss sulfate and MSA.
Number size distributions from 3 nm to 10 mm diameter.
Air mass trajectories using smart balloons.
The marine boundary layer aerosol generally offers a large amount of surface area upon which gases such as H2SO4 can condense. It is therefore a rare event when the condensable vapors reach sufficiently high supersaturations to nucleate new particles in large concentrations (Covert et al., 1992). Since precipitation, cloud processing, and coagulation are continually removing particle number, some source of new particles is needed to explain why the total number remains relatively constant over time.
Observations of high ultra-fine particle (3nm < Dp < 20nm) number concentrations in the clean upper tropospheric air and process-model calculations suggest that this region may be a source of new nuclei (Ito, 1985; Clarke, 1993; Raes et al., 1993). Air in cumulus updrafts may be scavenged of nearly all its aerosol mass, resulting in an air mass with low particle surface area. In addition, low temperatures at these altitudes reduce saturation vapor pressures, thus promoting bursts of nucleation, followed by coagulation. Subsidence and entrainment can then bring these fresh nuclei into the boundary layer.
During ACE-1 measurements of the number size distribution between 3 and 600 nm diameter will be used to identify events of new particle production in both the marine boundary layer and free troposphere. By also measuring gas phase aerosol precursors it should be possible to directly compute aerosol nucleation and growth rates. Chemical mass size distributions, the ammonium to nss sulfate molar ratio as a function of size, and single particle analysis will be used to determine the role of specific chemical species in new particle production. These same measurements will be used to define conditions that inhibit particle production.
In addition to homogeneous nucleation and subsequent growth through condensation, the aerosol size distribution can be transformed by cloud processing. Sub-micron aerosols can cycle through non-precipitating clouds many times before being removed from the atmosphere through rain (Hoppel et al., 1986). During a cycle, the larger particles are activated and become cloud droplets. In addition, gas phase species and smaller particles can be absorbed into the droplets. When the cloud droplet evaporates, the residue is larger than the original particle. The result is a size distribution that contains two sub-micron peaks, one due to homogeneous nucleation and one due to cloud cycling. A thin layer of stratocumulus clouds often exists over the proposed study region providing an opportunity to investigate the effect of cloud cycling on the size distribution. These measurements will be used to estimate the fraction of sulfur species oxidized in cloud.
The final stage in the atmospheric aerosol cycle is removal by wet or dry deposition. Although it will not be possible to conduct an extensive deposition experiment, deposition rates will be estimated using precipitation collectors. These deposition rates will be compared with calculated deposition fluxes and aerosol formation rates. These rates can be used to calculate the average lifetime of atmospheric sulfate, a parameter that has a high uncertainty factor in estimates of the climate forcing by sulfate aerosols (Penner et al., 1994).
Mass size distribution of the major cations, anions, and organic species.
The ammonium to nss sulfate molar ratio from 10 to 600 nm diameter.
Single particle analysis and TDMA measurements to determine internal/external mixing
Single particle analysis to identify other chemical components of the aerosol
Number size distribution from 3 to 10000 nm diameter.
CCN number concentration at the supersaturation of stratocumulus clouds.
Satellite measurements of the degree of cloud cover.
Estimates of the mixing time into clouds and the rainfall rate.
Measurements of wet deposition.
Temperature and RH.
Objective 3. Assess the climatic importance of remote marine aerosols.
- How can observations be used to improve the accuracy of aerosol-climate models?The climatic importance of the atmospheric aerosol requires improved paramet-erization of the processes that
control the aerosol sources, properties, evolution, and spatial distribution. While satellites offer observations
on a planetary scale, they lack any capability for chemical analysis of the aerosol particles and for coupling the
spatial distribution of the aerosol to spatially and temporally variable aerosol precursors and source processes.
Modelling of the aerosol system, including gaseous precursors and sources, aerosol production processes and
properties, and the three dimensional spatial and temporal distribution offers the only possibility for coupling
locally measured properties to global forcing and effects.
The observational data collected during ACE-1 will be useful for testing and improving a hierarchy of related models (Penner et al., 1994):
Rather than simply providing the modelling community with data, ACE-1 is being planned in concert with members of the modelling community to ensure that the data will be useful for validating and refining models.