An increase in the amount of stratospheric aerosol due to the Pinatubo eruption (June 12-15, 1991, 15.14 deg N, 120.35 deg E) was observed from the end of June, 1991 by a lidar in NIES (National Institute for Environmental Studies), Tsukuba (36.0 deg N, 140.1 deg E). After large fluctuations in summer of 1991, the amount of the aerosols increased in mid-September as a result of enhanced transportation from the subtropical region. In autumn and winter of 1991, dense aerosol layers were continuously observed. Aureolemeter (scanning spectral radiometer) measurements were also carried out with lidar measurements and columnar size distribution of stratospheric aerosols was estimated for some cases. Collaborative measurements with the lidar and aureolemeter provided some information on height distribution of the surface area of aerosols in late 1991

Hayasaka, Tadahiro

Hayashida, Sachiko

Matsui, Ichiro

Nakane, Hideaki

Sasano, Yasuhiro

English

NASA Technical Reports Server

N95- 11054
STRATOSPHERIC AEROSOL INCREASE AFTER
ERUPTION OF PINATUBO OBSERVED WITH LIDAR
AND AUREOLEMETER
Sachiko Hayashida, Yasuhiro Sasano, Hideaki Nakane, Ichiro Matsui
National Institute for Environmental Studies (NIES)
16-2 Onogawa, Tsukuba, Ibaraki 305 JAPAN
and
Tadahiro Hayasaka
Center for Atmospheric and Oceanic Studies,
Faculty of Science, Tohoku University
Sendai, Miyagi980 JAPAN
ABSTRACT
An increase in the amount of stratospheric aerosol due to
the Pinatubo eruption (June 12 -15, 1991, 15.14 °N, 120.35
°E) was observed from the end of June, 1991 by a lidar in
NIES (National Institute for Environmental Studies), Tsukuba
(36.0 °N, 140.1 ° E). After large fluctuations in summer of
1991, the amount of the aerosols increased in mid-September
as a result of enhanced transportation from the sub-tropical
region. In autumn and winter of 1991, dense aerosol layers
were continuously observed. Aureolemeter (scanning spectral
radiometer) measurements were also carried out with lidar
measurements and columnar size distribution of stratospheric
aerosols was estimated for some cases. Collaborative
measurements with the lidar and aureolemeter provided some
information on height distribution of the surface area of
aerosols in late 1991.
1. INTRODUCTION
The Mr. Pinatubo eruption in the Philippines on June 15,
1991 was reported as the most violent of this century. The
total amount of SO2 injected into the stratosphere was
estimated to be about 20 megatons from Satellite data (Bluth et
al, 1992; McCormick and Veiga ,1992). These results indicate
the amount of volcanic material transported into the
stratosphere by the Pinatubo eruption was over twice that of
the El Chichon (Mexico) eruption on March 29 and April 4,
1982.
Stratospheric ozone decreased about 10% (Mantis et al.,
1986), after the eruption of El Chichon in 1982. Hofmann and
Solomon (1989) suggested that heterogeneous reactions of
clorine-related substances on the surface of the volcanic
aerosols would cause a significant decrease in the ozone
amount. Therefore, it is speculated that Pinatubo aerosols
would cause ozone depletion of more than 10%.
As the heterogeneous reaction rates are proportional to
the surface area of aerosol particles, it is important to estimate
the surface area of aerosols to examine the effect of
heterogeneous reaction on ozone depletion.
At NIES, Tsukuba Japan, lidar measurements of the
stratospheric aerosols after the Mr. Pinatubo eruption have
been carried out. Aureolemeter measurements have been also
carried out at NIES since November 1991. The aureolemeter
provides information on the cohamnar size distribution of
aerosols although it cannot include information on its height
distribution.
In this paper, we report some results of the lidar
measurements and show the time variation and height
distribution of volcanic aerosols. We also report on some
examples of the estimation of surface area of stratospheric
aerosols derived from collaborative measurements with the
lidar and aureolemeter at NIES.
2. MEASUREMENTS AND DATA PROCESSING
Specifications of the NIES lidar system for aerosol
measurements were described in Shimizu et al. (1985). An
Nd-YAG laser emits laser pulses of about 400 mJ for a 532
nm wavelength with a repetition rate of 25 Hz. The diameter
of the telescope is 1.5 m. Data processing was described in
Hayashida-Amano et al., (1991) in detail. The algorithm of
Fernald (1984) was applied to correct for attenuation of the
laser signal in the atmosphere. The ratio of extinction to
backscatter of aerosols is denoted as S 1 which depends on the
size distribution of the particles. S 1 is essentially important for
determining aerosol profiles after large volcanic eruptions
because the attenuation of signal is strong in the aerosol layer.
The integrated backscattering coefficient (IBC) is defined
as
IBC = _a(Z) dz (1)
1
where 13a denotes the backscattering coefficient for aerosols
and z is altitude. IBC times S1 signifies the optical thickness
of the aerosol layer for the altitude region between Zl and z 2.
Here Zl and z2 were taken as 15 km and 30 km, respectively.
The aureolemeter was set in November 1991 on the top
of the same building of the NIES lidar. Specifications of the
instrument and data processing are described in Nakajima et
al. (1986). The aureolemeter was developed to measure not
only direct solar irradiance but sky radiance in the solar
almucantar. The aureolemeter is equipped with a silicon
photodiode and seven interference filters at wavelengths of
332, 368,500, 675,777, 862 and 937 nm.
An algorithm developed by Nakajima et al., (1986) was
used for retrieving the size distribution of aerosols from the
measurements. The solar aureole including the effect of
multiple scattcring was calculated theoretically and compared
with the observed data to derive scattering phase function P.
635
https://ntrs.nasa.gov/search.jsp?R=19950004641 2020-06-16T11:31:06+00:00Z
ScatteringphasefunctionPatscatteringangleOisgiven
by
f
P(0)=|K(0,1n r) dV dlnr
J din r
(2)
where dV/dlnr is the volume size distribution of aerosols, r is
the radius of an aerosol particle, and K is the kernel function
calculated from Miffs amplitude function. Equation (2) was
solved as an inverse problem to obtain volume spectrum
dV/dlnr.
3. RESULTS OF LIDAR MEASUREMENTS
Stratospheric measurements were routinely carried out
every clear night. The time series of IBC for the altitude region
from 15 to 30 km are shown in Fig. 1. The IBC shown here
includes an extinction correction for S 1 = 50.
First arrival of the volcanic aerosol layer was observed
near 16 km on June 28, 1991: only two weeks after the
eruption. This is due to advection associated with an Asian
anticyclone (Hayashida and Sasano, 1992, McCormick and
Veiga, 1992). In summer of 1991, aerosol layers appeared
sporadically at an altitude of over 20 km. A thick aerosol layer
with a peak at about 22.5 krn was ftrst observed on September
25. Although there were only limited measurements in
September due to bad weather conditions, we could confirm
that there was no distinct layer on September 12. It was
difficult to estimate IBC quantitatively on September 12 and
25, because the measurements were carried out in daytime
with only an insufficient S/N ratio. Therefore, IBCs obtained
only on September 5 (befora the increase of IBC) and on
October 3 (after the increase of IBC) were plotted in Fig. 1.
The increase in the aerosol amount corresponded to the
change in wind system. Transportation mechanism was
discussed in Hayashida and Sasano (1992) with analysis of
meteorological data.
During the autumn and winter of 1991, a thick and dense
aerosol layer was continuously observed from about an
altitude of 17 to 30 km. Although there were still large
fluctuations in the amount of aerosols, IBC increased up to
about 6 x 10 -3 sr -1.
4. SURFACE AREA ESTIMATION
In autumn and winter, tropospheric optical thickness is
usually small and the weather condition is good in Tsukuba.
We were able to make many measurements with the lidar and
aureolemeter in autumn and winter of 1991. As the optical
thickness of tropospheric aerosols was superimposed on to the
stratospheric optical thickness in aureolemeter data,
tropospheric lidar measurements were used to select some
cases for clear troposphere. November 25, November 29 and
December 13 were selected as clear tropospheric cases.
The columnar size distribution was derived from the
aureolemeter measurements. Here, we fitted the derived size
distribution to a composite of Junge and log-normal
distribution functions using the data in the range from 0.2 to
2.0 I_m. Tropospheric aerosols usually have a bi-modal
distribution with a mode radius of about 0.2 ;am and about 3 -
4 lam (Shiobara et al., 1991). Therefore, the size distribution
function adjusted for the range from 0.2 to 2.0 lain represents
mainly contribution from the stratospheric aerosol. After the El
Chichon eruption, stratospheric aerosols had a mode radius of
about 0.5 t_m, and therefore, it was possible to distinguish
stratospheric size distribution from a tropospheric one. The
size distribution observed in late 1991 was similar to that
observed after the E! Chichon eruption.
The distribution function adjusted is expressed as
dV/dlnr = CO +
CI exp [ -1/2 ((In r- In r0 )/In o) 2 ] (3)
where Co represents concentration of the Junge distribution,
and C1, r0 and o represent concentration, mode radius and
dispersion of log-normal distribution, respectively.
Table 1 shows the parameters C0,CI, r0 and o for the
three days. Junge mode is usually observed in the troposphere
(Shiobara et al., 1991). Here we assume the log-normal
distribution mode can be regarded as a representative mode of
stratospheric aerosol.
Table 2 shows some parameters such as IBC, optical
thickness and S 1 for 532 nm which were calculated from Mie
3
J
!0i .....
150
1991 1992 _
J J A S i O N D J F M A -.:
] .... r
2011 250
h. _ i o i .
r
1
300 350 400 450 500
DA'_S SINCE JAN. l, lqql
TABLE 1. LOG-NORMAL MODE DERIVED FROM
AUREOLEMETER MEASUREMENTS
Date Co(cm3/cm 2) Cl(cm3/cm 2) r0(3tm) 0
(x 10 -6) (x 10 "6)
Nov. 25 0.84 1.2 0.47 1.56
Nov. 29 0.48 2.2 0.58 1.67
Dec. 13 0.34 2.7 0.51 1.62
Fig.1. The time series of the integrated backscattering
coefficient (IBC) for the range from an altitude of 15 to 30 km
after the Pinatubo eruption.
636
TABLE2.OtrHCAL PARAMETERS CALCULATED FOR THE LOG-NORMAL
MODE DERIVED FROM AUREOLEMETER*
Date Optical thiqkn¢_** IBC** $1"* _ ***
Nov. 25 0.073 0.0017 43.9 56.2
Nov. 29 0.13 0.0034 36.5 48.6
Dec. 13 0.16 Q,QQ4Q 4Q,9 _,7
* refractive index was assumed for 1.44 - 0.00i
** calculated for 532 nm on the basis of log-normal mode shown in Table 1.
*** the ratio of surface area to backscattering coefficient (sr -1)
35
v
25
2
C-
15
0
91/11/25 19: 18:49
91/12/1 22" 41" 34
91/12/13 17: 10:45
:-
_:::i :;_ii!!!!_i:.........
• = .......... ._..___..
, _. , I , . . . , . , . , I ......... I ......... I ........
10 20 30 40 50
SURFACE AREA (jum2/cm3)
Fig. 2. The surface area of stratospheric aerosols on November 25, December 1 and December 13, 1991
estimated from the lidar and aureolemeter measurements.
637
theory basexlon the log-normaldistributionshown inTable I.
The refractiveindex was assumed to be 1.44 - 0.00ion the
assumption of 75 % sulfuricacid.The ratio_ of surfacearea
(A) tobackscattcringcoefficients(= A/D isalsoshown. Ifsize
distributionof aerosolsdoes not depend on altitudeso much,
the ratio_ times localvalue of _(z)means the surfaceareaat
altitudez.Figure 2 shows thesurfacearea profiles(_x [3(z))
for those three days. These profiles were obtained by
assuming $I = 43.9,36.5,40.9,respectivelyaccordingto the
calculatedvalues forthelog-normal mode ( see,Table 2).As
therewas no stratosphericlidarmeasurement on November
29, the lidardata on December i was compared with the
aureolemetcrdataon November 29.
Even ifthesizedistributionisdependent on altitude,_in
Table 2 should bc takenas a verticallyaveraged valuethrough
the stratosphere.The value of 4= 53 averaged for the three
days should be taken as a representativevaluefor theperiod
during lainNovember to earlyDecember 1991.
The calculatedvaluesof IBC inTable 2 arc not always
consistentwith thoseobtained from the lidardata.One of the
reasons for this discrepancy is because the lidar and
aureolcmetcrmeasurements arcnotcarriedout simultaneously.
Another possiblemason isthe refractiveindex used for the
calculation.The calculationfor refractiveindex =1.44 would
mislead theresultswhen aerosolsincludevolcanicash.
In this paper, the Junge mode was assumed for
tropospheric aerosols and removed from the aurcolemeter
data.Troposphericlidardatashouldbc analyzed inmore detail
to estimate troposphericaerosolcontributionto the optical
thicknessobserved by aurcolcmetcrinmore detailinthefuture
study. Other information on size distribution(multiple
wavelength lidarmeasurements, for example) should be
referredforfurtherstudies.
5. CONCLUDING REMARKS
Lidar measurements over Tsukuba after the Pinatubo
eruption on June 12-15 revealed an increase of stratospheric
aerosol amount since the end of June. After mid-September, a
thick aerosol layer was observed continuously.
Collaborative measurements with the lidar and
aureolemeter were carried out and succeeded in estimating the
profile of the surface area of aerosols. Ratio _ showing the
surface area to backscattering coefficient is about 53 sr-1 in
late 1991. The surface area of Pinatubo stratospheric aerosol
was estimated to be about 40 i.tm2/cm 3 at the peak of the
aerosol layer (about 21 km).
ACKNOWLEDGMENTS.
We wish to acknowledge our indebtedness to H. Satoh
and F. lida from FIT Inc. for their help in the lidar and
aureolemeter measurements.
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Stratospheric aerosol increase after eruption of Pinatubo observed with lidar and aureolemeter

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