Aerosol Optical Depth - Atmospheric Turbidity

There is growing awareness of the need to consider atmospheric aerosol particles in climate and global change models. As originally portrayed by Angstrom (1929, 1961, 1964), the central quantity is the aerosol optical depth (AOD), an index of the attenuation of radiation as it passes through the atmosphere due to the presence of suspended particles. AOD and "turbidity" are essentially synonymous quantities, both being logarithmic indices of atmospheric optical attenuation to a vertical beam.

There are several methodologies by which AOD measurements are made. The most common is by measuring the intensity of the direct solar beam and plotting against air mass. The zero intercept of such a Langley plot defines the incoming radiation above the atmosphere, and the slope is the optical depth. If corrections are then applied for Rayleigh scattering and for effects of various trace gases (especially ozone), then the residual is the aerosol optical depth.

International Associations

At the international level, the key organizing body for AOD measurements has been the WMO's Global Atmosphere Watch. GAW is the successor of the earlier WMO Background Air Pollution Monitoring Network (BAPMoN). ARL led an expert review of the operations of the global sunphotometer network run by BAPMON, as a result of which the sunphotometer network has been shut down. In brief, the instruments used employed filters that drifted with time, and the calibration requirements were therefore too much for the network to handle. It has been widely concluded that the instrumentation was deployed before it was ready for prime time. Moreover, the sunphotometer approach is more suited to research-grade observatory stations than to routine operations by technicians rather than experienced and expert scientists. ARL hosted a meeting of experts on the subject in 1994, during which Swiss experts agreed to serve as the developers of new methodologies for advanced sunphotometric methods. This work has progressed well, and a small array of stations is now being instrumented with new sensors, on a trial basis.

Why NOAA, why now, and why ARL?

NOAA is leading contributor to the international climate change debate, and is widely viewed as a source of independent scientific guidance of the highest quality. NOAA is also seen to be a steward of the nation's environment, and a long-term operator of relevant monitoring arrays. The Integrated Surface Irradiance Study (ISIS) is an example of the kind of research- oriented network that is unique to NOAA: NOAA actively integrates scientific research with routine monitoring in many of its continuing observational programs. ISIS operates AOD sensors at this time, on a trial basis.

Because of its air quality focus, ARL is a leading player on the national atmospheric optics and visibility stage, and is intimately involved in activities related to atmospheric particles. ARL operates one of the longest-running sulfate aerosol monitoring networks -- the Atmospheric Integrated Research Monitoring Network (AIRMoN). Studies of AOD are a clear extension of these ongoing network investigations. In essence, the AOD studies are viewed as a linkage between the ISIS and AIRMoN programs.

The ARL Role

The ARL view remains that AOD is a highly variable quantity, and that an emphasis on background conditions at remote and/or high altitude observatories will yield a biased view of the global AOD situation. The ARL focus is on developing simple instrumentation suitable for routine operation with inexpert technical oversight. To this end, ARL is actively exploring the use of rotating shadow band radiometry in place of (or supporting) the more conventional sunphotometric techniques of previous programs.

The approach adopted in ARL explorations has been the use of rotating shadow band systems. There are three different varieties now in trial use. Intercomparisons between pairs of these instruments reveal that they compare well, although each focuses on a different aspect of the overall solar radiation measurement problem. Each device yields a meaningful measure of the aerosol optical depth, without the need for precise orientation of the sensor (as in the case of conventional sunphotometry). It has been concluded that the two configurations of this kind of sensor that are commercially available are adequate for use in two-tiered networks, with the simpler of the devices being used at all stations and with the more complex being deployed at a subset of sites that classify as "observatories."

In recent months, a new sunphotometer instrument is being deployed in a trial test at selected GAW locations. This is currently viewed as a research-grade instrument, for use in sophisticated studies of background conditions. ARL is continuing a search for a simpler device suitable for routine operation.

The different devices now being investigated for potential routine application are designed for different purposes, none specifically for measuring aerosol optical depth. However, each has characteristics that suit it for some specific AOD application. The devices are

• continuously-rotating shadow band instruments (with two configurations now being studied)
• intermittently rotating shadow band devices (typically with multiple wavelength filters), and
• sun-seeking and solar aureole devices.

The ARL interest is in the rotating shadow band devices, of all kinds. It has been concluded that sensors of this kind should be included in field tests of alternative aerosol optical depth sensors, so as to evaluate their utility vis-a-vis sunphotometers. It is anticipated that sensors of the rotating shadow band configuration may well prove adequate for widespread operation in turbidity networks, but that understanding of their answers will require more intensive application of modern sunphotometry methods at a subset of sites, perhaps the GAW global observatories. This is in accord with the conclusions drawn in the earlier examination of BAPMON turbidity procedures -

1. There is little reason to continue turbidity measurements under the original BAPMON protocols, using sunphotometers at regional stations without tight quality control.
2. If turbidity monitoring is to be supported then it may best be served by making different kinds of measurement at GAW global and regional stations. In theory, rotating shadow band approaches appear well suited for application at all sites, whereas sunphotometers require a level of expertise and technical attention that is likely mostly available at global sites.
3. Uniform and policed quality control practices are needed.

It is apparent that the different devices under consideration all have the capacity to yield high-quality aerosol optical depth data. It is also apparent, however, that the complexity of the instrumentation introduces a consideration of some considerable importance. In a two-tiered array (such GAW) it would appear optimal to deploy an instrument of the simpler kind at all locations and to add an additional more complex sensor at the observatories.

REFERENCES

Angstrom, A., 1929; On the atmospheric transmission of sun radiation and on dust in the air. Geograf. Ann. Deut., 11, 156 - 166.

Angstrom, A., 1961; Techniques of determining the turbidity of the atmosphere. Tellus, 13, 214 - 223.

Angstrom, A., 1964; The parameters of atmospheric turbidity. Tellus, 16, 64 - 75.

WMO, 1991; Report of the Meeting of Experts to Assess Available Data and Define the Aerosol Component for GAW-BAPMON Stations (Boulder, 16 - 19 December 1991), WMO Global Atmosphere Watch Report Number 79. WMO Secretariat, Geneva, Switzerland.