DIRECT AND INDIRECT AEROSOL EFFECT:
Scientific Issues Addressed by the Various Instruments
(A. Clarke)

Direct Effect and Instrumentation Summary

The so-called "direct effect" of radiative forcing on climate relates to the changes in net radiative fluxes in the atmosphere caused by the modulation of atmospheric scattering and absorption properties due to changes in the concentration and optical properties of the aerosol. An issue of current concern is whether such a forcing might be caused by particle emissions from anthropogenic activities. The assessment of this issue will depend upon how mean clear-sky regional radiative transfer is perturbed by anthropogenic aerosol relative to the "natural" aerosol characteristic of a region. Hence, a region that exhibits both gradients in anthropogenic aerosol and variations in aerosol types is well suited to this objective. The suite of instruments and investigators assembled for INDOEX are focused on this issue and the conditions in the Indian Ocean (IO) study region will provide the set of conditions ideal for addressing the problem.

In addition to the instruments listed below, the direct effect requires measurement of the radiative fluxes, i.e. forcing, which are described under the RAMS section.

The combination of platforms and coordinated observations will link this data set to concurrent data on in-situ aerosol radiative properties but also their optical effects as detected by satellites. The intent is not only to determine whether their is evidence for the "direct effect" but to quantitatively link any observed effect to the physics and chemistry of the aerosol in this polluted region and to other "natural" aerosol such as sea-salt or dust. The instrumentation and proposed approach is designed to obtain and relate data that will address the following questions related to the direct forcing issue.

Aerosol Instrumentation and Applicability for Related Questions

What is the size distribution and size-resolved chemical composition of the aerosol [point
measurements - LASX, RDMA; integrated chemical properties - Ionic Composition, Carbonaceous Composition; continuous "fast" characterization of aerosol fields - CN counters, Nephelometer]?
These are needed to describe fundamental properties of aerosol size distributions and chemistry that must be used as inputs for modeling and linking aerosol radiative effects to specific aerosol characteristics (eg. sulfate, soot, salt, dust etc.) and their refractive index.

LASX - Laser Aerosol Spectrometer (or Laser Optical Particle Counter - LOPC) with thermal conditioning of aerosol to obtain size distributions of dry larger aerosol [0.15RDMA - Radial Differential Mobility Analyzer. This instrument provides size distribution of smaller aerosol )[.007Ionic composition - (filter/impactor) Aerosol ionic chemistry for all sizes above 1 Ám and all sizes below 1Ám. Time averaged data over about 1/2 hr legs (200 km). Needed for chemical assessment of bulk composition of major aerosol types and confirmation / validation of rapid inferred methods (LAS-X). Also need in conjunction with nephelometers to get optical characteristics such as the mass scattering coefficients.

Carbonaceous aerosol - elemental and organic carbon analysis will be done on filter integrated over about 1hr legs. Continuous aethaelometer (light absorption) will be used to continuously infer black carbon (soot) concentrations on about 5 min time periods for comparison to filter measurements. Needed for light absorption coefficient, particularly for pollution plumes and for assessment of organic aerosol constituents.

What are the optical properties of the dry aerosol? A fundamental optical property of the dry aerosol often used in modeling is the Single Scatter Albedo (SSA), a ratio of the scattering to the sum of scattering and absorption coefficients. Pollution, dusts and salts have SSA values that range for about 0.7 to 0.95 and 0.99 respectively. A combination of multiwavelengh nephelometer (scattering coefficient) and aethaelometer (absorption) will be used to continuously characterize this parameter. This nephelometer also measures the hemispheric backscatter coefficient that can be used to help constrain the "backscatter fraction" that is returned to space under ambient conditions. These measurements also provide a constraint (closure) upon modeled optical properties of the dry aerosol that must be achieved before modeling ambient or column properties.

What are the optical properties of the "wet" ambient aerosol? Since both natural and anthropogenic aerosol generally contain deliquescent components that pick up water with increasing humidity, the ambient optical properties are dramatically affected the associated water that is a function of the dry aerosol chemistry and its size variation. In order to measure and account for this effect we propose to fly two nephelometers with one at low (dry) and one at high (wet) conditions so that the direct effect of water uptake on the scattering coefficient will be measured. These nephelometers will be operated with and without an impactor (5-% cut at 1 Ám) so that coarse and fine aerosol behavior will be separately determined. This provides an added constraint on the aerosol optical model since it must show consistency on predicted optical behavior for both total and backscatter coefficients (closure)at three wavelengths for coarse and fine aerosol under both wet and dry condition. A model that matches these measured optical properties (and size distributions) provides confidence that it can be applied successfully to the ambient atmosphere.

What is the state of mixing of aerosol types {ie. is soot and sulfate mixed in each particle (internal mix) or are the particles separate populations (external mix)} Tandem -RDMA]? The Tandem RDMA is really two RDMA'S in the same instrument package. When operated in this mode the first RDMA selects an aerosol size for analysis. This selected aerosol is then heated to drive of sulfate ant then the residual sizes are scanned to see the change upon heating. If the loss of mass results in sizes that are smaller then the particles were internally mixed (such a s soot coated with sulfate). If on the other hand the sizes stay the same but only the number is decreased then the particles were separate chemical species (externally mixed). This is important for determining their cloud nucleating potential since this is governed by both size and the soluble fraction of the particle.

How do these integrated measurements change for very different regions down the continental pollution gradient and across the ITCZ in clean southern Indian ocean air? We will use LIDAR measurements to identify homogeneous conditions suitable for column radiative closure measurements and to establish a means of scaling any variability in closure region to the column measurements. The integration of all of the above will be employed in various region of the IO in order to assess differences that can be attributed to the aerosol and other factors. Detailed characterization provided by the "closure" profiles will by extended to representative regions though a combination of in-situ and Lidar profiling along gradient legs.

How will rapid changes in aerosol character be detected in-situ along horizontal or vertical legs? We anticipate that aerosol will change along the gradients and that changes will be rapid in the vertical. It is not unlikely that cleaner marine air may be near the surface overlain by a pollution layer and possibly dust layer above that. We will use the rapid in- situ instrumentation to establish the boundaries and changes in their character and not just their concentrations. For example the wavelength dependence of the nephelometer responds quickly to changes in the size distribution say on the transition from pollution to dust. The ratio of Condensation Nuclei, CN, measured hot (300 C)and cold (40 C) also shows rapid changes from pollution to clean air that is not always seen in the concentration alone. These rapid signals (up to 1 Hz) can be used to scale more intermittent measurements over appropriate spatial scales.

Wing Particle probes (MASP, FSSP, PCASP): This instrument is needed for in-situ characterization of aerosol size distribution of "wet" aerosol and for aerosol sizes too large to get into aircraft inlet (e.g., coarse dust and sea-salt).

Indirect Effect and Instrumentation Summary
The so-called "indirect effect" of radiative forcing on climate relates to the changes in net radiative transfer in the atmosphere caused by the modulation of cloud properties due to changes in the concentration of cloud condensation nuclei, CCN. An issue of current concern is whether such a forcing might be caused by particle emissions from anthropogenic activities. These fall into two broad categories: a) changes in cloud albedo caused by the increases in the number of cloud droplets, and b) changes in cloud lifetime caused by modification of cloud properties and precipitation processes. Complex and non- linear process are involved in these effects that are at present poorly understood and which have large uncertainties associated with them. Progress on this challenging issue will involve the coordination of instrumentation and observations that are interdisciplinary and well integrated. The suite of instruments and investigators assembled for INDOEX are well suited to making progress on this issue and the conditions in the INDOEX study region will provide the set of conditions suitable for addressing the problem.

The National Research Council has recognized the importance of cloud radiative forcing on climate and has outlined the issues and suggested approaches in the document "Aerosol Radiative Forcing and Climate Change" (NRC, 1996). The full complement of information required to make global assessments of indirect forcing on global scales are beyond the scope of any one program. NRC recommendations suggest the coordination of the major federal agencies to establish programs over a period of years in order to carry this out. However, the first requirement for modeling radiative forcing recommended by the NRC states that: "major advances in the representation of indirect climatic effects of aerosols in global climate models by treating in a fundamental manner the relationship between aerosol mass and aerosol number, aerosol number and CCN number, CCN number and cloud droplet number concentration (CDNC) and CNDC and cloud optical properties" At present the data base upon which a suitable representation can be built is inadequate or nonexisting. The measurements proposed under INDOEX will be designed to help fill this gap with an integrated data set that will be collected in a region presently exposed to significant anthropogenic impact which is expected to increase markedly in the future. The combination of platforms and coordinated observations will link this data set to concurrent data on in-situ cloud radiative effects but also their optical properties as detected by satellites. We will do this over the Indian Ocean (IO), a region that provides an opportunity to characterize and compare this system for large gradients in anthropogenic aerosol and anticipated effects. The intent is not only to determine whether there is evidence for the "indirect effect" but to quantitatively link any observed effect to the physics and chemistry of the aerosol in this polluted region. The instrumentation and proposed approach is designed to obtain and relate data that will address the following questions related to the indirect forcing issue.

Instrument Justification and Applicability for Related Questions
What is the size distribution and size-resolved chemical composition of anthropogenic aerosol introduced into IO cloud systems [point measurements - LASX, RDMA; integrated chemical properties - Ionic Composition, Carbonaceous Composition; continuous "fast" characterization of aerosol fields - CN counters, Nephelometer]?


These are needed to describe fundamental properties of aerosol size distributions and chemistry that must be related to cloud drop spectra if links to sulfate, soot etc. are to be achieved and for required model input.

LASX - Laser Aerosol Spectrometer (or Laser Optical Particle Counter - LOPC) with thermal conditioning of aerosol to obtain size distributions of dry larger aerosol [0.15RDMA - Radial Differential Mobility Analyzer. This instrument provides size distribution of smaller aerosol ) [.007Ionic composition - (filter/impactor) Aerosol ionic chemistry for all sizes above 1um and all sizes below 1um. Time averaged data over about 1/2 hr legs (200 km). Needed for chemical assessment of bulk composition.

Carbonaceous aerosol - elemental and organic carbon analysis will be done on filter integrated over about 1 hr legs. Continuous aethaelometer (light absorption) will be used to continuously infer black carbon (soot) concentrations on about 5 min time periods for comparison to filter measurements. Needed for light absorption coefficient and for modeling refractory nuclei in CCN.

What is the measured CCN saturation spectra for this aerosol below and around cloud [CCN Spectrometer]?

CCN spectrometer - This instrument determines the number of CCN activated at a range of supersaturation under ideal stable laminar flow conditions. Provides and index of aerosol likely to be activated in clouds at various supersaturation.

What is the actual CDNC spectra resulting from this aerosol introduced into cloud [FSSP, 1D PROBE, 2D PROBE, PVM 100, PCASP]?

These are the actual drop size distributions present in the cloud and represent not only the effect of supersaturation but also entrainment and turbulent mixing processes etc.

What is the size and composition of the aerosol responsible for the CDNC spectra [CVI with CNN, RDMA, OPC, etc.]?

The Counterflow Virtual Impactor (CVI) allows only the large activated cloud droplets to enter the counterflow tip after which they are evaporated to a dry state for subsequent analysis by other instrumentation. This allows the physical-chemical properties of the aerosol that actually formed the cloud droplets to be determined.

What is the state of mixing of aerosol types in the CCN size range {ie. is soot and sulfate mixed in each particle (internal mix) or are the particles separate populations (external mix)} [Tandem - RDMA]? The Tandem RDMA is really two RDMA's in the same instrument package. When operated in this mode the first RDMA selects an aerosol size for analysis. This selected aerosol is then heated to drive of sulfate ant then the residual sizes are scanned to see the change upon heating. If the loss of mass results in sizes that are smaller then the particles were internally mixed (such a s soot coated with sulfate). If on the other hand the sizes stay the same but only the number is decreased then the particles were separate chemical species (externally mixed). This is important for determining their cloud nucleating potential since this is governed by both size and the soluble fraction of the particle. What are the optical properties of clouds (measured simultaneously above and below them) for the CDNC characterized above? Optical propeties will be measured using the Radiometric Measuring System (RAMS). The flux radiometers will determine parameters like albedo, trasmition and absorption. The spectral Total-Direct-Diffuse radiometers will measure radiative fluxes in narrow spectral bandpasses, direct and diffuse components of the radiation filed and in some cases (thin clouds) spectral optical depths. The RAMS have a solid history of performance during FIRE, ASTEX, TOGA-COARE, CEPEX, ARESE and other projects.

How do these integrated measurements change for clouds forming in very different regions down the continental pollution gradient and across the ITCZ in clean Southern Indian Ocean air? The integration of all of the above will be employed in various region of the IO in order to assess differences that can be attributed to the aerosol and other factors.