Kipp&Zonen BSRN Scientific Solar Monitoring System User Manual

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H olben, B.N ., T .F.Eck, I. Slutsker, D . T anré, J.P. Buis, A. Setzer, E. Verm ote, J.A. R eagan, Y.J. Kaufman,

T . N akajim a, F. Lavenu, I Jankowiak, and A. Sm irnov, 1998: AER O N ET - A federated ins trum ent network
and archive for aerosol characterization. R em. Sens. Environ., 66, 1 - 16.

G ilgen, H ., C .H . W hitlock, F. Koch, G . Müller, A. O hm ura, D . Steiger, R . W heeler, 1995: Baseline Surface

R adiation N etw ork Technical Plan for BSR N D ata M anagem ent (Version 2.1). W C R P W M O /T D -no. 443.

W orld C lim ate R esearch Program m e, 2001: R eport of the Sixth Science and R eview W orkshop,

M elbourne, Australia, 1 - 5 May 2000 (Annex IV). W C R P Inform al R eport N o. 17/2001.

7.0

Measurement of Aerosol Optical Depth

7.1

Introduction

The m onitoring of aerosol optical depth (AOD) has been considered an im portant, but difficult, observation
that is necessary if there is to be an increase in understanding of the surface radiation budget. W hile
the optical depth provides inform ation on spectral atm ospheric extinction, a num ber of inversion algorithm s
have been developed to use this inform ation to produce data on the colum nar aerosol num ber-size
distribution, volum e distribution and concentration. Along with other m easurem ents of the solar aureole
and alm ucantar the data has also been inverted to provide inform ation on aerosol absorptivity. By
m easuring, or m ore norm ally assum ing, a change in aerosol concentration with height above the surface,
these optical properties can be used to better understand the radiative regim e of the atm osphere. Over
tim e, m ore and m ore user com m unities have expressed the need for this type of data. Several global
and regional aerosol m easurem ent networks have been established in response to this need. The
two m ost significant of these, with respect to the ongoing work of the BSRN are the Global Atm osphere
W atch (GAW ) sunphotom eter network, which is being established at global background stations, and
the NASA Aerosol Robotic Network (AERONET) .

At the inception of the BSRN the im portance of aerosol optical properties on the radiation budget was
recognized and the BSRN Archive was designed to include values of spectral AOD . The sunphotom eters

available in the early 1990s were found to be too unstable and the m easurem ent protocols’ unacceptable
to determ ine changes in global and regional clim ates, as was the m andate of the BSRN. Instrum ent
instability was due in large part to the rapid aging of the interference filters used in the instrum ents,
especially at shorter wavelengths, while m any m easurem ent protocols consisted of only a few
m easurem ents per day because they were designed around the use of handheld instrum ents. Changes
in instrum ent design, including the reduction of the exposure tim e of filters to UV radiation and the
stabilization of instrum ent tem perature have im proved the perform ance and stability of new instrum ents.
Accurate tracking system s have im proved the repeatability of observations by decreasing pointing
uncertainty, while the increase in sam pling frequency provides a m eans of tracking changes in instrum ent
and atm ospheric conditions that could not be accom plished with only several or even tens of observations
per day.

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

and the BSRN accepted, specifications related to AOD m easurem ents. The objective of the BSRN
m easurem ent program is to provide AOD m easurem ents 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 num ber of
approxim ations that are used in its calculation. Sm all differences in the m ethods 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 m ethods
of calculation alter the overall AOD. In com bination, one set of algorithm s, com pared to a different
set will act as a bias. This will be of greater significance to the determ ination of sm all AODs, as m easured
at background stations, than those in m ore turbid areas. The W orking Group on Aerosol Optical Depth
Measurem ents determ ined that because of the possibility of such discrepancies, the subm ission of
AODs to the archive could not provide users with an accurate long-term , spatially diverse database.
Therefore, it was decided that spectral transm ission data, with ancillary inform ation, be transm itted
to the BSRN Archive. The archive would then calculate the aerosol optical depth using a com m on
set of algorithm s. Once determ ined the algorithm s will be m ade available to BSRN scientists, but all