Kippzonen BSRN Scientific Solar Monitoring System Uživatelský manuál stáhnout pdf (Strana 65)

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and archive for aerosol characterization. Rem. Sens. Environ., 66, 1 - 16.

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Radiation Network Technical Plan for BSRN Data Management (Version 2.1). W CRP W MO/TD-no. 443.

W orld Climate Research Programme, 2001: Report of the Sixth Science and Review W orkshop,

Melbourne, Australia, 1 - 5 May 2000 (Annex IV). W CRP Informal Report No. 17/2001.

7.0 Measurement of Aerosol Optical Depth

7.1 Introduction

The monitoring of aerosol optical depth (AOD) has been considered an important, but difficult, observation

that is necessary if there is to be an increase in understanding of the surface radiation budget. While

the optical depth provides information on spectral atmospheric extinction, a number of inversion algorithms

have been developed to use this information to produce data on the columnar aerosol number-size

distribution, volume distribution and concentration. Along with other measurements of the solar aureole

and almucantar the data has also been inverted to provide information on aerosol absorptivity. By

measuring, or more normally assuming, a change in aerosol concentration with height above the surface,

these optical properties can be used to better understand the radiative regime of the atmosphere. Over

time, more and more user communities have expressed the need for this type of data. Several global

and regional aerosol measurement networks have been established in response to this need. The

two most significant of these, with respect to the ongoing work of the BSRN are the Global Atmosphere

Watch (GAW) sunphotometer network, which is being established at global background stations, and

the NASA Aerosol Robotic Network (AERONET) .

At the inception of the BSRN the importance of aerosol optical properties on the radiation budget was

recognized and the BSRN Archive was designed to include values of spectral AOD . The sunphotometers

available in the early 1990s were found to be too unstable and the measurement protocols’ unacceptable

to determine changes in global and regional climates, as was the mandate of the BSRN. Instrument

instability was due in large part to the rapid aging of the interference filters used in the instruments,

especially at shorter wavelengths, while many measurement protocols consisted of only a few

measurements per day because they were designed around the use of handheld instruments. Changes

in instrument design, including the reduction of the exposure time of filters to UV radiation and the

stabilization of instrum ent temperature have improved the perform ance and stability of new instruments.

Accurate tracking system s have improved the repeatability of observations by decreasing pointing

uncertainty, while the increase in sampling frequency provides a means of tracking changes in instrument

and atmospheric conditions that could not be accomplished with only several or even tens of observations

per day.

At the 6 BSRN Science and Review Workshop the Working Group on Aerosol Optical Depth proposed,

th 15

and the BSRN accepted, specifications related to AOD measurements. The objective of the BSRN

measurement program is to provide AOD measurements at selected wavelengths between 360 and

1100 nm with an uncertainty no greater than 0.01 under near ideal conditions, decreasing to no greater

than 0.02 for all conditions when water clouds do not interfere with the direct line-of-site to the solar

disk.

The calculation of AOD, although straightforward in principle, varies because of the number of

approximations that are used in its calculation. Small differences in the methods used to calculate

sun-earth distance, Rayleigh optical depth, or the ozone absorption coefficients and scale height all

increase the overall uncertainty in the calculation of AOD. Individual differences between methods

of calculation alter the overall AOD. In combination, one set of algorithms, compared to a different

set will act as a bias. This will be of greater significance to the determination of small AODs, as measured

at background stations, than those in more turbid areas. The Working Group on Aerosol Optical Depth

Measurements determined that because of the possibility of such discrepancies, the submission of

AODs to the archive could not provide users with an accurate long-term, spatially diverse database.

Therefore, it was decided that spectral transmission data, with ancillary information, be transmitted

to the BSRN Archive. The archive would then calculate the aerosol optical depth using a common

set of algorithms. Once determined the algorithms will be made available to BSRN scientists, but all

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