AERONET—A Federated Instrument Network and Data Archive for Aerosol Characterization
Introduction
Accurate knowledge of the spatial and temporal extent of aerosol concentrations and properties has been a limitation for assessing their influence on satellite remotely sensed data (Holben et al., 1992) and climate forcing (Hansen and Lacis, 1990). With the exception of the AVHRR weekly ocean aerosol retrieval product (Rao et al., 1989), the voluminous 20-year record of satellite data has produced only regional snapshots of aerosol loading, and none have yielded a database of the optical properties of those aerosols that are fundamental to our understanding of their influence on climate change. With the advent of the EOS era of laboratory quality orbiting spectral radiometers, new algorithms for global scale aerosol retrievals and their application for correction of remotely sensed data will be implemented (Kaufman and Tanré, 1996). However, the prospect of fully understanding aerosols influence on climate forcing is small without validation and augmentation by ancillary ground-based observations as can be provided by radiometers historically known as sun photometers. Following is a description of a new Sun–sky scanning radiometer system that standardizes ground-based aerosol measurements and processing, can provide much of the ground-based validation data required for future remote sensing programs and may provide basic information necessary for improved assessment of aerosols impact on climate forcing.
Section snippets
Background
The technology of ground-based atmospheric aerosol measurements using sun photometry has changed substantially since Volz (1959) introduced the first handheld analog instrument almost 4 decades ago. Modern digital units of laboratory quality and field hardiness can collect data more accurately and quickly and are often interfaced with onboard processing Schmid et al. 1997, Ehsani et al. 1998, Forgan 1994, Morys et al. 1998. The method used remains the same, that is a filtered detector measures
Automatic sun and sky scanning spectral radiometer
Most if not all sun photometer networks have had limited success when people are required to make routine observations. Therefore, an automatic instrument is a fundamental component for routine network observations. The measurement protocol must be reasonably robust such that unwanted data may be successfully screened from useful data, data quality, and instrument functionality may be evaluated and the instrument should be self-calibrating or at the least collects data for its calibration.
Data transmission
Data are transmitted from the memory of the sun photometer via the Data Collection Systems (DCS) to the geostationary satellites GOES-E, GOES-W, or METEOSAT (GMS is anticipated in 1998) and then retransmitted to the appropriate ground receiving station. The data can be retrieved for processing either by modem or Internet linkage, resulting in near real-time acquisition from almost any site on the globe excluding poleward of 80° latitude. The DCS is a governmental system operated for the purpose
Processing system
A fundamental component of the AERONET system is a package of user-friendly UNIX software that provides near real-time information on the status and calibration of the instruments, data processing with referenced and generally accepted processing algorithms, an orderly archive of the data, and convenient electronic access for all users to the raw and processed database. We shall discuss these aspects of the current operational state of the software and future enhancements.
Global perspective
Through 1997 approximately 100 instruments have been included in the network and 60 instruments were deployed world-wide on various islands, North America, South America, Europe, Africa, and the Middle East, fostered by collaboration between international, national, and local agencies, private foundations, and individuals (Fig. 5). As the database continues to expand, the processing system becomes more sophisticated, and more users have access to the database, the need to provide better access
Conclusion
We believe that a successful system for long-term monitoring and characterization of aerosols requires automatic low maintenance radiometers, real time data reception, and processing as well as an easily accessible database for the scientific community. We have combined commercially available hardware, international agency collaborations, a public domain software, and a collaborative philosophy among investigators to form a network that has yielded regionally based aerosol amounts and
Acknowledgements
The authors wish to thank Diane Wickland and Tony Janetos of NASA Headquarters for providing the initial support for this project, Michael King of NASA’s EOS Project Science Office for continued support, Chris Justice for contributing to the vision of the network, and John Vande Castle and Gunar Fedosejevs for actively participating in development of the network. Many thanks to Bruce Forgan for his detailed constructive recommendations to this manuscript and the other anonymous reviewers for
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