Fourth Study Conference on BALTEX Scala Cinema Gudhjem

In the case of an ideal fit, the correlation coefficient R
between ln δAER (λ) and ln λ should be equal to minus one.
Lower absolute values, ⏐R⏐ < 1, indicate deviations from
linearity or errors in the determination of aerosol optical
thicknesses δAER (λ) or both. Figure 1 gives a visual
impression of all 1602 correlation coefficients R for summer
2002.
In 98.5 % of the considered cases (1579 series), the
correlation ⏐R⏐was stronger than 0.97. Only in the rest of
23 of cases the correlation was 0.93–97%. High correlations
indicate, on one hand, the applicability of the Ångström
formula at TOravere in summer 2002, and on the other, the
high accuracy of measurements performed by the
AERONET system.
The Ångström wavelength exponent α, varying in the range
0.54–1.96 (Fig. 2), had monthly mean values α = 1.45, 1.48
and 1.42, respectively, in June, July and August. The mean
for all three months was α = 1.45, which exceeds
Ångström’s classic value of α = 1.3.
alpha
2.50
2.00
1.50
1.00
0.50
Ångström alpha, 340-1020 nm, summer 2002, Toravere,
Estonia
0.00
June July August
151 182 213 244
Julian day
Figure 2. Ångström wavelength exponent α in summer
2002 at Toravere, 1602 points shown.
The Ångström turbidity coefficient β, found according to Eq
(4), varied during summer 2002 between 0.018 and 0.602.
The mean β values from June to August were 0.053, 0.087
and 0.132. The total summer mean was β = 0.0907. As seen
in Fig. 3, higher peaks occurred in July and August,
especially in the second half of August. Apparently, these
peaks were related to extended forest and bog fires in
Estonia and nearby Russian territories.
beta
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
Ångström beta, summer 2002, Toravere, Estonia
151
June
182
July
Julian day
213
August
244
Figure 3. Temporal variation of the Ångström turbidity
coefficient β in summer 2002 at Toravere.
Deviation from the Ångström formula occurs on very clear
days, i.e. in cases of very low AOT. As seen in Fig. 4, all
values of lower correlation, ⏐R⏐< 0.97 take place when the
AOT(500 nm) < 0.2. On these days spectral behaviour of
AOT(λ) often is anomalous in region 670–1020 nm and
does not fit the Ångström formula. This may be the result of
several factors: 1) errors in the AOD observations
themselves; 2) uncertainties in estimating light absorption
by water vapor and ozone: and 3) unique aerosol size
distributions that are not approximated by a power law
- 83 -
(Junge) distribution. In the cases of greater turbidity, when
the AOT(500) > 0.2, the Ångström formula fits well,
correlation ⏐R⏐ > 0.97.
R
Correlation coefficients for the Ångström formula, 340-1020
nm, summer 2002, Toravere, Estonia
-1.01
-1.00
-0.99
-0.98
-0.97
-0.96
-0.95
-0.94
-0.93
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
AOT(500 nm)
Figure 4. Correlation coefficients R versus AOT (500
nm). The Ångström formula fits better at greater turbidity.
4. Conclusion
Analysis of 1602 observations made by the AERONET
sunphotometer at Toravere in summer 2002 proved that
spectral behaviour of aerosol optical thicknesses can be
expressed by the Ångström formula. Monthly mean values
of the Ångström wavelength exponent were: α = 1.42–
1.48, and of the turbidity coefficient: β = 0.053–0.132.
The Ångström formula fits better at greater turbidity.
Acknowledgements
This investigation was supported by national grants No.
4140 and No 5857 of the Estonian Science Foundation.
The AERONET team and the Estonian Principal
Investigator Dr. O. Kärner, together with Dr. M. Sulev, are
highly appreciated for installation and maintainance of the
device, and making accessible the unique observation
data.
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