GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 21, PAGES 4063-4066, NOVEMBER 1, 2001

Anti-correlation between stratospheric aerosol extinction
and the Ångström parameter from multiple wavelength
measurements with SAGE II − a characteristic of the
decay period following major volcanic eruptions
Sachiko Hayashida and Mariko Horikawa
Faculty of Science, Nara Women’s University

Abstract. Stratospheric Aerosol and Gas Experiment(SAGE) II data at multiple wavelengths were analyzed toshow how aerosol extinction decays with time following major volcanic eruptions. Comparisons were made between thelowest background level in 1999 and the past data record of
the stratospheric aerosol layer. The time variation of theextinction coeﬃcients was compared with the Ångström parameter, which is a good indicator of particle size. A clear
anti-correlation was found between the extinction and theÅngström parameter after the eruption of Mt. Pinatubo.
Comparison of the extinction coeﬃcients and Ångström parameters in 1989 and 1999 made it clear that the aerosol
layer was aﬀected by volcanic eruptions in 1989. The distinguishing negative correlation is a characteristic feature of
decay periods following volcanic eruptions.

1. Introduction
Hofmann [1990] noted that the amount of stratosphericsulfate aerosol measured by balloon-born OPCs at 41◦ N increased by 30 - 50% between 1979 and 1989. Hofmann [1991]
suggested that this increase in background aerosols could be
attributed to increased jet fuel emission in the lower stratosphere.
Thomason et al. [1997] compared global stratosphericaerosol levels from the Stratospheric Aerosol and Gas Experiment (SAGE) in 1979 with those from SAGE II in 19891991, and concluded that 1989-1991 was characterized byan ongoing global recovery from the eruptions of El Chichon(1982), Nevado del Ruiz (1985), and Kelut (1990). The major eruption by Pinatubo in June 1991 made it impossible
to follow these measurements through the 1990s to conﬁrm
the changes in background levels. With the decay of the
Pinatubo perturbation, the investigation of the backgroundcan now be continued.
As part of the 1998 WMO/UNEP ozone assessment,
long-term records of the integrated stratospheric aerosol column observed with some lidars were compared with an OPC
data record at Laramie, showing their consistency and thelevels in late 1997 being well below pre-Pinatubo values.
Therefore, the estimate of the anthropogenic eﬀect basedon the 1979-1989 comparison is now considered an overestimate [WMO, 1998]. At the time of the WMO report in late

Copyright 2001 by the American Geophysical Union.
Paper number 2000GL012826.
0094-8276/01/2000GL012826$05.00

1997, the stratospheric background aerosol level was stilldecreasing, and we continued to track this trend from 1997
to the end of 1999.
In this study, we analyzed SAGE II aerosol extinction
data at multiple wavelengths from January 1985 throughDecember 1999. This includes a later period than that analyzed by Thomason et al. [1997] and WMO [1998]. We
present the time variation of the extinction coeﬃcient, andthe changes in particle size indicated by changes in theÅngström parameter. This study demonstrates that the decay terms after volcanic disturbances are characterized not
only by a decrease in the mass of aerosols, but also by achange in particle size.

2. Analysis method
SAGE II measures aerosol extinction at four wavelengths,(1.019, 0.525, 0.452, and 0.386 µm), at altitudes from 10 to
40 km, using solar occultation technique. We used version
6.0 of the aerosol data in this study. The extinction coeﬃcients were averaged for each ﬁve-degree latitude band from80◦ N to 80◦ S, at each altitude, and for each month, where
the number of measurements exceeded ten per grid, after
screening out the data with more than 80% relative error.
The wavelength dependence of the extinction was approximated asσ = σo λ−α(1)−1where σ is an extinction coeﬃcient (km ), λ is wavelength(microns), and α is the Ångström parameter. The Ångströmparameters were derived from all events, and are averaged
for the ﬁve-degree latitude bands. Because of concerns as
to quality, the extinction data at the shortest wavelength,0.386 µm, were not used.

3. Results and Discussion
3.1. Long-term variation of the 1.0-micron
extinction coeﬃcient and Ångström parameter
Figure 1 shows the time series of the 1.0-µm extinction
coeﬃcient (σ1.0 ) at 20 km, from January 1985 through December 1999, as observed by SAGE II. An enormous increasein σ1.0 occurred after the eruption of Mt. Pinatubo (June1991, Philippines), although there was a short-lived data
void immediately following the eruption, because of data
truncation [McCormick and Veiga, 1992]. The subsequent
decay phase lasted for more than seven years, as indicatedin the ﬁgure.

4063

4064

(c)

(d)

(e)

(f)

85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

5
4
3
2
1
0
-1

Ångström Parameter

(b)

5
4
3
2
1
0
-1

Ångström Parameter

10-1
10-2
10-3
10-4
10-5
10-6
10-7
0.2
0.4
0.6

10-1
10-2
10-3
10-4
10-5
10-6
10-7
0.2
0.4
0.6

(a)

5
4
3
2
1
0
-1

Ångström Parameter

Effective Extinction(1019nm) Effective Extinction(1019nm)
radiusradius

10-1
10-2
10-3
10-4
10-5
10-6
10-7
0.2
0.4
0.6

Effective Extinction(1019nm)
radius

HAYASHIDA & HORIKAWA: ÅNGSTRÖM PARAMETER OF STRATOSPHERIC AEROSOL

85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

Figure 1. (upper panels) Time series of the 1.02-µm extinction coeﬃcient at 20-km altitude, observed with SAGE II (black). The
horizontal bars indicate the range of the average over one month. Ångström parameters are also plotted in the same panel (red). The
panels are for (a) 0-5◦ N, (b) 0-5◦ S, (c) 30-35◦ N, (d) 30-35◦ S, (e) 55-60◦ N, and (f) 55-60◦ S. (lower panels) Time series of the eﬀective
radius. Black shows the unimodal aerosol type and red corresponds to the bimodal aerosol type. All data are plotted with one-sigma
error bars. The eﬀective radius is inversely plotted to compare with the Ångström parameters.

The time series of the Ångström parameter (α) in Figure
1 (red crosses) shows a signiﬁcant negative correlation with
σ1.0 , suggesting that particle growth was enhanced by thesulfur compounds emitted from the volcanic eruptions. In
particular, α dropped remarkably (down to zero) after the
eruption of Mt. Pinatubo. Although the ﬁgures at otheraltitudes are not shown here, there was a similar tendency
at altitudes from 17 to 26 km.
Figure 1 shows that the extinction coeﬃcient continued
to decrease after 1997, when the WMO [1998] comparison was made. The unusually prolonged period of disturbance following the Pinatubo eruption was caused by thelarge amount of sulfate injection [McCormick et al., 1995].
By 1999, the stratospheric aerosol extinction was as low as∼10−4 /km at 20 km, the lowest value recorded in the entire
period of SAGE II observations.

3.2. Relationship between the Ångströmparameter and particle size
What is the relationship between particle size and α? Yue[1999] proposed a practical method that derives particle size
distribution from multiple wavelength extinction coeﬃcientsfrom SAGE II; he showed that the number density can be expressed as a linear combination of the extinction coeﬃcientsobserved with SAGE II. For this study, we used coeﬃcients
C(k,i,t,p) that were oﬀered by Glenn K. Yue [personal communication, 2000], deriving the size distribution. The de-

rived size distribution yields other parameters, such as theeﬀective radius or sonde ratio. The correlation between α
and eﬀective radius is very compact; however, the relationship depends on aerosol type: non-volcanic (unimodal) or
volcanic (bimodal).
The time series of the eﬀective radius (ref f ) derived
by Yue’s method is plotted in the lower panels of Figure
1. Results for both unimodal (black crosses) and bimodal(red crosses) models are shown. In some period after the
Pinatubo eruption, the data for only the bimodal type (red)are shown, because unimodal type does not give reasonablesize distribution. The behaviors of ref f and α are very similar.
We also simulated the extinction coeﬃcient in SAGE II
channels based on Mie theory for various values of the parameters rm and s for the lognormal size distribution function: where rm is mode radius and s is standard deviation.
The refractive index of 1.44 was taken from Russell et al.
[1996] (at 223K, 5 ppm of water vapor). We made a diagram showing the correlation between α and σ1.0 for various size distribution parameters rm and s (not shown here).
The correlation between σ1.0 and the total mass of aerosols
was the better than the other wavelengths, as expected, so
σ1.0 can be used as a parameter representing aerosol mass.
On the diagram of α and σ1.0 obtained from Mie calculations, we plotted the observed σ1.0 and α data with data
uncertainties. All candidates for rm and s that are expected

4065

mode radius mode radius mode radius

HAYASHIDA & HORIKAWA: ÅNGSTRÖM PARAMETER OF STRATOSPHERIC AEROSOL
(a)

(b)

(c)

(d)

(e)

(f)

85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

0.0
0.2
0.4
0.6
0.0
0.2
0.4
0.6
0.0
0.2
0.4
0.6

Figure 2. (The time variation of the estimated mode radius, r̄m in Equation (2) (black crosses) and the estimated eﬀective radius(red) at an altitude of 20 km corresponding to the variation of the extinction and the Ångström parameter shown in Figure 1. The
time variation of the eﬀective raius is comparable to that determined by Yue’s method, as shown in Figure 1.

3.3. Anti-correlation of the extinction and theÅngström parameter during the decay period
Figure 3 is a scatterplot of α vs. σ1.0 on an isentropic
surface during the decay period after Pinatubo. The diﬀerent colored dots represent data for diﬀerent years. Linear
regression lines for each year are shown in the same color
as the dots for that year, and their lengths show the rangeof values. For example, the dark blue line in each panel depicts the data observed in 1993 for all latitudes, with a larger
σ1.0 and smaller α at lower latitudes, and a smaller σ1.0 andlarger α at higher latitudes. The spatial anti-correlation
between σ1.0 and α is now apparent.
As already discussed, σ was large and α was small during
the period of disturbance after the Pinatubo eruption, theopposite of the conditions during the background period. In
1999, all the extinction data were at the background level,and scattered over the range reﬂecting seasonal variation.
The range of the seasonal variation of α is relatively large
depending on latitudes, resulting in an unclear negative correlation.
The negative correlation is clearer at higher altitudes(compare Fig. 3(a) and (c)). Hamill et al. [1999] discussedchanges in the size distribution due to diﬀerences in the rateof descent, which depends on particle size, during the decay
period. The more rapid descent of larger particles causes
extinction to decay faster at longer wavelengths, increasing
α. In other words, the changes in α with time after volcanic
disturbances provide an indicator of the decay phase. The

10-1(a)550K

10-2
10-3
10-4
10-5

Extinction(/km)

theoretically in the range of data uncertainties were averaged to determine the best-estimated size parameters, r̄m
and s̄. Figure 2 depicts the time variation of r̄m and thecorresponding eﬀective radius at an altitude of 20 km, determined in this way. The trend in the time variation of
ref f is comparable to that for a unimodal size distribution
derived by Yue’s method, as shown in Figure 1.
When the size distribution function is expressed by aJünge distribution function, the relationship between α andthe power law exponent p is expressed as α = p - 3. In the
background period, the size distribution function is similar
to the Jünge distribution, as rm is small enough, comparedto the wavelength region of the SAGE II measurements. After large volcanic events, a bimodal size distribution with alarge mode radius is often observed, so that the wavelength
dependence of extinction is sometimes not linearly related to
the logarithm of the wavelength, resulting in non-negligible
residuals in Eq. (1). Therefore, during periods of disturbance α is not a straightforward indicator of particle size.
However, the clear negative correlation between σ and α,
shown in Figure 1, suggests that there is a strong connection between aerosol mass loading and particle growth.

10-1(b)475K

10-2
10-3
10-4
10-5
10-1

(c)415K

10-2
10-3
10-4
10-5
0

2

Ångström Parameter
92 93 94 95 96 97 98 99
Figure 3. ((a)- (c) Scatter plot of the extinction coeﬃcientand Ångström parameter on the same isentropic surfaces at (a)550 K (∼23 km), (b) 475 K (∼20 km), and (c) 415 K (∼17 km).
Diﬀerent colors are used to represent data (dots) and linear regression lines for diﬀerent years. The lengths of the lines indicate
the extent of the values.

4066

HAYASHIDA & HORIKAWA: ÅNGSTRÖM PARAMETER OF STRATOSPHERIC AEROSOL

negative correlation is less clear at lower altitudes, implying
settling of larger particles from higher altitudes.
Jäger et al. [1995] traced the temporal evolution of
the height of the maximum backscattering coeﬃcient overGarmish-Partenkirchen (47.5◦ N), Tsukuba (36.1◦ N), andNaha (26.2◦ N) using lidar (see Fig. 2 of Jäger et al., [1995]).
At all three latitudes, the height of the maximum backscattering coeﬃcient descended from around 23 km in mid-1991
to about 16 km in mid-1993. Our analysis of SAGE II data
showed similar results. The heights of the maximum of both
σ1.0 and α descended for about 2 years after the eruption,
after which time they rose. This suggests that the larger
particles generated from sulfate matter from volcanic injection gradually fall to the tropopause; i.e., large particles
are removed by mixing into the troposphere. Similarly, thenegative correlation at upper altitudes in the early stageof decay after the Pinatubo eruption can be interpreted as
the removal of large particles due to their gravitational settling. At lower altitudes, the negative correlation is lessclear, as the inﬂux of particles from upper altitudes and the
downward outﬂow cancel each other out, complicating thesituation.
The scatterplot for 1988-1989 (not shown here), before
the Pinatubo eruption, shows a negative correlation thatis similar to that observed during the decay period after
Pinatubo. This is further evidence that the period before
Pinatubo was a decay phase and not background.
When examined more closely, in 1999 α shows a latitudinal gradient even in the background period. Although
the classic theory of aerosol microphysics depicts growth
to ’mature particles’ in a poleward meridional circulation,
this study suggests that particles diminish in size toward
poles. Future studies should consider other microphysicalprocesses, such as evaporation, as well as meridional transport.

4. Concluding Remarks
SAGE II version 6.0 extinction data from 1985 through1999 were analyzed, focusing on the Ångström parameter.
The Ångström parameter decreases after major volcanic
eruptions, apparently corresponding to enhancement of extinction. A clear anti-correlation between extinction and
the Ångström parameter was observed during the decay after the Pinatubo eruption. The level of extinction in 1999
was the lowest in the history of systematic measurement of
the stratospheric aerosol layer, while the Ångström parameter was the highest. Comparison of the extinction coeﬃcients and Ångström parameters in 1989 and 1999 made it

clear that even in 1989 the aerosol layer was still aﬀectedby previous eruptions, and that a negative correlation is adistinguishing feature of a decay period after volcanic eruptions.
Comparisons of the Ångström parameter with the eﬀective radii obtained by the method of Yue [1999] and derived
from simple Mie-calculations proved that the Ångström parameter is an eﬀective indicator of particle size. More extensive analysis, including microphysical considerations, would
shed light on the particle formation processes and the mechanism of their loss from the stratosphere.
We thank L. W. Thomason at
Acknowledgments.
NASA Langley for oﬀering us the SAGE II dataset and useful discussions. We also thank Glenn K. Yue at NASA Langley for providing the parameters to retrieve size distribution from SAGE II
extinctions. This study was partly supported by EORC/NASDA.

References
Hamill, P. et al., Are we now at a background stratosphericaerosol level?, Eos Trans. AGU, 80(17), Fall Meet. Suppl.,
A22E, 1999.
Hofmann, D. J., Increase in the stratospheric background sulfuric
acid aerosol mass in the past 10 years, Science, 248, 996-1000,
1990.
Hofmann, D. J., Aircraft sulphur emissions, Nature, 349, 659,
1991.
Jäger, H. et al., Ground-based remote sensing of the decay of the
Pinatubo eruption cloud at three northern hemisphere sites,
Geophys. Res. Lett., 22, 607-610, 1995.
McCormick, M. P., and R. E. Veiga, SAGE II measurementsof early Pinatubo aerosols, Geophys. Res. Lett., 19, 155-158,
1992.
McCormick, M. P. et al., Atmospheric eﬀects of the Mt Pinatubo
eruption, Nature, 373, 399-404, 1995.
Russell, P.B. et al., Global to microscale evolution of the Pinatubo
volcanic aerosol, derived from diverse measurements and analyses, J. Geophys. Res, 101, 18745-18763, 1996.
Thomason, L. W. et al., A comparison of the stratospheric aerosol
background periods of 1979 and 1989-1991, J. Geophys. Res.,
102, 3611-3616, 1997.
World Meteorological Organization, Scientiﬁc Assessment of
Ozone Depletion,1998.
Yue, G. K., A new approach to retrieval of size distributions andintegral properties from SAGE II aerosol spectra, J. Geophys.
Res, 104, 27491-27506, 1999.
S. Hayashida, Nara Women’s University, Kita-uoya Nishimachi, Nara 630-8506, Japan
M. Horikawa, Currently at Fijutsu FIP corporation, 135-8686
Tokyo, Japan(Received January 4, 2001; revised June 13, 2001;
accepted August 13, 2001.)