Abstract
Atmospheric aerosols play an active role in the atmospheric radiative energy transfer by interacting with the solar energy through light scattering and/or absorption. Such an interaction has the potential to counter balance or enhance the warming caused by the greenhouse gases (e.g. carbon dioxide, water vapor, and methane). The interactions would depend on a variety of factors such as the microphysics and chemistry of aerosol particles, and the environmental conditions. To model the energy transfer processes accurately, several aerosol parameters such as aerosol optical thickness (AOT), single scattering albedo, scattering phase function, refractive index, and chemical composi tion of aerosols have to be known. In this report, a robust method for aerosol optical thickness is presented. The method is based on the least median squares (LMS) regression technique. The LMS-based technique is fundamentally different from the traditional least-squares (LS) technique commonly used today to retrieve AOT. The LMS technique was found to resist influential outliers and sustain the impacts of outliers much better than the LS in our application. The outlier-resistance property is an important design consideration for an automatic AOT retrieval algorithm. We demonstrated the strength of the new technique by using the shortwave irradiance measurements taken by one of the Multi-filter Rotating Shadow-band Radiometers installed at the Southern Great Plains in Oklahoma, U.S.A., one of the three sites operated by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program. The data cover a time period from September 1995 to May 1997. The LMS-retrieved mean top-of-atmosphere solar irradiance values (i.e. I0) were stable based on the one-tailed Student's t tests. It was also found that the difference between the 75th and 25th percentiles (a non-parametric robust estimate of variation) of I0 over the 2-year period was 0.2 for the 499-nm channel, and 0.05 for the 860-nm channel. The monthly averaged AOT values over the 2-year period were from 0.05 to 0.30 for the 499-nm channel and from 0.03 to 0.15 for the 860-nm channels. AOT values peaked between May and September and reached minima in between November and January. The synoptic flow from the North might have contributed to the low level of AOT values during the wintertime, while the southerly flow, traversing through more industrial areas before reaching the site, could have contributed to the influx of particles during the summertime.
| Original language | English |
|---|---|
| Pages (from-to) | 805-817 |
| Number of pages | 13 |
| Journal | Journal of Aerosol Science |
| Volume | 30 |
| Issue number | 6 |
| DOIs | |
| State | Published - Jul 1999 |
Funding
Acknowledgements—This research was supported in part by the U.S. Department of EnergyÕs Atmospheric Radiation Measurement Program. The authors are indebted to Drs Joseph Michalsky of the State University of New York at Albany, Rangasayi Halthore of the Brookhaven National Laboratory, and Warren Wiscombe of the NASA/Goddar Space Flight Center for valuable discussion, and Richard Cederwall of the Lawrence Livermore National Laboratory for extensive and patient review of the draft manuscript. Terra Nash was supported in part by an appointment to the Oak Ridge National Laboratory (ORNL) Professional Internship Program administered jointly by the Oak Ridge Institute of Science and Education (ORNL) and the Oak Ridge National Laboratory. Scott Kopetz was supported in part by an appointment to the DOE Summer Student Internship Program administered jointly by the ORISE and ORNL. MDC also acknowledges two anonymous reviewers for their positive suggestions. The authors thank computational supports provided by the members of the ARM Archive located at the Oak Ridge National Laboratory. This is Oak Ridge National Laboratory Environmental Sciences Division publication number 4831. Oak Ridge National Laboratory is operated by the Lockheed Martin Energy Research Corporation, Inc. for the Department of Energy under Contract Number DE-ACO5-96OR22464. * The paper has been authorized by a contractor of the U.S. Government under Contract No. DE-AC05-96OR22464. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. t Author to whom correspondence should be addressed.