JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. D5, PAGES 5277-5286, MAY 20, 1988

Atmospheric Effects of the E1 Chich6n Volcanic Eruption Observed by a Multiwavelength Sun-Photometer, 1982-1985

V. LEVIZZANI AND F. PRODI 1

Reparto Nubi e Precipitazioni, Istituto FISBA T, Consi•tlio Nazionale della Richerche, Bolo•Ina, Italy

A long-term analysis of turbidity data, obtained by operating a multiwavelength sun-photometer at a mountain station in northern Italy, has been carried out to investigate the influence of the El Chich6n volcanic eruption on the optical characteristics of the atmosphere. An estimate of the vertical load of aerosol particles indicates that it exceeds 2-12 times the normal background value for the station. Aerosol size distributions, obtained by inversion of spectral optical thickness measurements, show a clearly bimodal nature during the period of greater volcanic influence, with the appearance of a distinct large-particle mode centered between 0.4 and 0.5/•m, revealing the presence of sulphate aerosol particles of volcanic origin in the stratosphere above the station. Sun-photometric measurements prove to be an effective technique for the analysis of the stratospheric aerosol characteristics and their influence on the Earth's radiation budget. The eight wavelengths of the instrument provide a whole series of spectral information which is not ordinarily obtainable from lidar measurements and leads to specific consider- ations on the suspended aerosol and its variations in time.

1. INTRODUCTION

The series of violent eruptions of the volcano E1 Chich6n (March 28, April 3 and 4, 1982) injected into the atmosphere an enormous amount of volcanic dusts and gases, estimated to be as high as 20 Tg of stratospheric mass about 45 days after the event [Hofmann and Rosen, 1983-], introducing consider- able modifications of the atmospheric composition, especially at stratospheric levels.

Lidar measurements performed by many groups were able to trace the latitudinal dispersion and height evolution of the volcanic cloud, showing the formation of a worldwide aerosol veil mainly located between 20 and 30 km altitude [Hayashida and lwasaka, 1985; Hirono and Shibata, 1983; Swissler et al., 1983]. In particular, different researchers agree that the cloud had already reached Europe by May 1982 [-D'Altorio and Vis- conti, 1983; Reiter et al., 1983; Adriani et al., 1983].

Particles samplings in the stratosphere, using both balloon- and aircraft-carried particle counters, were conducted to in- vestigate the nature and size distribution of the particulate. The most relevant results are perhaps (1) the detection of the bimodal nature of the aerosol size distribution, even several

months after the eruption; and (2) the evidence of predomi- nance of sulphate compounds after the fall of the silicates, during the first 3 months following the eruption [Hofmann and Rosen, 1983; Oberbeck et al., 1983; Knollenber•7 and Huffman, 1983].

Solar spectral extinction measurements indicated a con- siderable increase in the aerosol optical thickness of the atmo- sphere during the volcanic period, when comparing the data with background values [Spinhirne, 1983; Hay and Darby, 1984 ; Asano et al., 1985-1.

The present work deals with the atmospheric effects of the aerosol cloud, as observed by a multiwavelength sun- photometer, from October 1982 to June 1985 at a mountain

• Also at Department of Physics, University of Ferrara, Ferrara, Italy.

Copyright 1988 by the American Geophysical Union.

Paper number 8D0024. 0148-0227/88/008 D-0024505.00

station located on the top of Monte Cimone (at the Air Force Meteorological Station, 2170 m above mean sean level (msl) 44•12fN, 10ø4TE) in the northern Apennine chain. A prelimi- nary study of the data [Levizzani and Prodi, 1984] has been extended as to include a wider time period and to introduce new results regarding aerosol size distributions, as obtained by inverting aerosol optical thickness measurements.

2. EXPERIMENTAL TECHNIQUE

2.1. The Instrument

The instrument (sun-photometer FISBAT, named after the Istituto FISBAT (Istituto per lo Studio dei Fenomeni Fisici e Chimici della Bassa e Alta Atmosfera)) has been described in detail by Tomasi et al. [1983]' the specific instrument used for the present study is the one referred to as sun-photometer E. It has the following main features' (1) a field of view of 1ø08 ' full angular diameter' (2) seven narrow-band interference fil- ters in the visible-near-infrared spectral region (see Table 1); (3) an additional band-pass filter with transmission curve peaked at about 0.948 •m to measure total atmospheric pre- cipitable water' (4) a highly sensitive, linear response sensor (UDT 500 Photop) combined with a precision amplifier' (5) a precise thermostating system, maintaining the internal temper- ature at 50øC, with a precision of ___ IøC, to prevent temper- ature variations from affecting the sensor response during measurements' (6) a precise Sun-sighting system with a vernier to obtain an accuracy of __+3 arc min when reading solar elevation angles.

The accuracy of aerosol optical thickness determination is based upon the instrument calibration coefficients, the trans- mission function of the filters, and the correction for gaseous absorption [Tomasi et al., 1983' 1986]' typical error mag- nitude for the present measurements is 0.01. The calibration of the instrument was performed using the Langley plot method at a mountain station near Monte Cimone during highly stable anticyclonic conditions. The window transmission func- tions of the interference filters were determined by means of a microdensitometer and periodically checked with high preci- sion. Corrections for ozone absorption were applied, using ozone columnar measurements performed by means of a

5277

5278 LEVIZZANI AND PRODI: EFFECTS OF EL CI-IICH6N

TABLE 1. Spectral Channels and Half-Bandwidths of the FISBAT Sun-Photometer

Wavelength, Half-bandwidth, Channel txm txm

1 0.3988 0.0094

2 0.4414 0.0088

3 0.5137 0.0118 4 0.5512 0.0102

5 0.6694 0.0108

6 0.8811 0.0132

7 1.0418 0.0094

Dobson spectrophotometer at the nearby station of Sestola (1000 m msl on the foothills of Monte Cimone).

2.2. Turbidity Evaluation

The total spectral optical thickness •:(2) of the atmosphere above the station at the relative air mass m is derived from the

Lambert-Beer law:

•:(2) = -- In [J(2)/RJo(2)]/m (1)

where ,/(2) is the direct solar irradiance, R the correction term for the Earth-Sun distance at observation time, and ,/o(I) the zero air mass intensity at mean Earth-Sun distance, as ob- tained by calibrating the instrument using the Langley plot method. The correspondent aerosol optical thickness %(2) for each measurement is then calculated by subtracting the single contributions produced by the absorption and scattering of gases:

ß ,(2) = •(2)- •o(2) -- rw(2) -- [r•(2)p/po] (2)

where %(2) is the optical thickness induced by ozone absorp- tion, rw(2) is the water vapor optical thickness, r•(2) is the Rayleigh scattering optical thickness, as computed according

to Fr6hlich and Shaw [1980], applying Younq's [1981] depo- larization factor, and p and Po the atmospheric pressure at station level and sea level, respectively [Prodi et al., 1984].

The aerosol optical thickness %(2) is then examined in terms of the power law suggested by ,,fnqstr6m [1929]:

r•(;0 = fi2 -• (3)

where c• and fi are the so-called ,•ngstr/Sm parameters, com- monly used to describe the columnar distribution of aerosol particles as derived from sun-photometric measurements. ,•ngstrfm's law represents a mean behavior of atmospheric aerosol extinction and therefore does not necessarily describe every situation, such as those presenting a maximum in the middle of the spectrum. The two parameters c• and fi, however, provide an immediate picture of the changes occurring in the atmospheric aerosol and are especially used to describe the time trend of solar extinction caused by aerosols. In the pres- ent study, correlation coefficients of the best fit to the spectral curves ranged between 0.85 and 0.96.

The values of fi and • at 1100 LT for each measurement day from October 16, 1982 to June 16, 1985, are plotted in Figure 1 and Figure 2, respectively; the fixed time was chosen in preference to the daily average because the turbidity variables normally show a diurnal evolution. In Figure I the monthly average of parameter fi, as derived from measurements taken at Monte Cimone with the Air Force Volz-type sun- photometer during the period 1980-1981 [Colombo et al., 1985], is also reported; the curve, obtained by averaging the two monthly average values of fi in 1980 and 1981, provides the background mean values for the nonvolcanic period im- mediately before the eruption and also shows the normal sea- sonal oscillations of atmospheric turbidity for the station.

Parameter fi is directly proportional to the amount of aero- sol particles along the Sun path, and its minimum values at

.20

.15

.05

• FISBAI sun-photometer

m---- Monte Cimone 1980/81 background mo

<

•_ i\l

I•l • • I• l & I• It

............. i i I i i i i i, I I I I I

0 N 1982

J FMAMI J ASOND

1983

J FMAMI J ASOND

1984

J FMAMJ

1985

Fig. 1. Daily values of J•ngstr6m's parameter fi at 1100 LT, measured at Monte Cimone with the FISBAT sun- photometer between October 1982 and June 1985 (solid line). Mean monthly values of fi for the period 1980/1981 as derived using a two-wavelength Volz sun-photometer [Colombo et al., 1985] are reported, providing the background prevolcanic seasonal trend (dashed line).

LEVIZZANI AND PRODI: EFFECTS OF EL CHICH6N 5279

3.0

2.5

2.0

1.5

1.0

0.5

OND J FMAMI J AS OND

1982 1983

J F MAM J J A S O N D

1984

J FMAMI

1985

Fig. 2. Daily values of Angstr6m's parameter y at 1100 LT for the same measurements as Figure 1.

Monte Cimone for the posteruption period range between 0.03 and 0.04, as indicated by the two minima of the curve in Figure 1 on October 16, 1982, and October 13, 1983. An increasing trend after October 1982 is clearly evident, though with locally decreasing daily features, up to July 1983, when an absolute maximum of 0.18 is reached, never recorded before at this mountain station (T. Colombo, private com- munication, 1985). A steep decrease then took place until Oc- tober 1983, followed by an annual oscillation during 1984 and 1985, which ranged between 0.05 and 0.1. The behavior of parameter fl during 1982 and 1983 suggests that a noticeable change had occurred in the atmospheric aerosol population above the station, not attributable to any tropospheric source. Two different features are clearly evident: (1) the values of fl exceed by 2-12 times the correspondent monthly mean of the prevolcanic period; (2) instead of presenting a minimum during the first 3 months of 1983, the curve still follows an upward trend, with values 12 times higher than the seasonal minimum in February 1983.

From the data it would seem that an absolute maximum of

the volcanic turbidity had occurred around July 1983. The comparison with the background turbidity of Monte Cimone, however, indicates that this maximum is not entirely due to volcanic aerosol, but also includes the tropospheric seasonal peak. It thus becomes clear that the maximum of turbidity occurred in the first months of 1983 and the subsequent in- crease was only caused by the superimposition of tropospheric effects.

The time evolution of/•ngstr6m's parameter y at 1100 LT, given in Figure 2, also exhibits a very peculiar behavior. In fact, during the period between October 1982 and January 1983, y reached values smaller than 1, with an absolute mini- mum of 0.5 on January 6, 1983. During the ALPEX SOP experiment in March-April 1982 [Prodi et al., 1984] regular measurements of solar extinction were performed, giving much

higher values of y (1.5-2). The very low values of y during this period reveal an enhancement of the role of large particles with respect to standard background conditions. The mini- mum of January 6, 1983, confirms the consideration that the maximum load of volcanic aerosol above the station occurred

at the beginning of 1983. The monthly mean values of the aerosol optical thickness at

the seven window wavelengths of the FISBAT sun-photometer are reported in Table 2 for the whole measurement period. Table 3 shows the background monthly mean at the wave- lengths 0.38 and 0.5 pm of the Air Force Volz-type sun- photometer measured at Monte Cimone during the pre-E1 Chich6n period, 1980/1981.

As a side remark, it is interesting to notice the two peaks of fi (Figure 1) on November 9, 1984, and April 21, 1985, repre- senting isolated episodes against a much lower background. The reason for the sudden increase of the aerosol mass on

these occasions is most certainly to be found in the transport of Saharan dust from Africa toward Europe. The first one, in particular, has already been identified (T. Colombo, private communication, 1985; L. Stefanutti, private communication, 1985) on the basis of aerosol filter samplings and lidar measurements.

3. COMPARISON WITH MEASUREMENTS FROM

OTHER STATIONS

Figure 3 shows a comparison of the time pattern of parame- ter • at Monte Cimone with the corresponding patterns of vertically integrated lidar backscatter a at the two stations of Frascati (42øN) in Italy [Adriani et al., 1983] and Garmisch- Partenkirchen (47.5øN) in West Germany [Reiter et al., 1983; Reiter and d&der, 1986]. It must be considered that parameters • and a refer to the whole column of the atmosphere above the station, and to the stratospheric fraction, respectively; moreover, the three sets of data are taken at different latitudes.

5280 LEVIZZANI AND P•,om' EFFECTS OF EL CmcI-I6N

TABLE 2. Monthly Mean Value of Aerosol Optical Thickness at the Seven Window Wavelengths of the FISBAT Sun-Photometer for the Measurement Period

Wavelengths, tzm

Month 0.3988 0.4414 0.5137 0.5512 0.6694 0.8811 1.0418

Oct. 1982 Nov. 1982

Dec. 1982 Jan. 1983

Feb. 1983 March 1983

April 1983 May 1983 June 1983

July 1983 Aug. 1983 Sept. 1983 Oct. 1983 Nov. 1983 Dec. 1983

Jan. 1984

Feb. 1984

April 1984 June 1984

Sept. 1984 Oct. 1984

Nov. 1984

Dec. 1984

Jan. 1985

Feb. 1985 March 1985

April 1985 May 1985 June 1985

0.198

0.262 0.133

0.173

0.247

0.227 0.337

0.299

0.320

0.545 0.408

0.256 0.207

0.203

0.183

0.181

0.252

0.353

0.390

0.611

0.481 0.438

0 331

0 333

0 466

0 622

0 716

0 420

0.519

0.232 0.141 0.129 0.100 0.286 0.190 0.174 0.137 0.164 0.111 0.098 0.086 0.205 0.164 0.135 0.126

0.293 0.215 0.193 0.179 0.286 0.202 0.163 0.149 0.403 0.299 0.233 0.195

0.371 0.242 0.185 0.164 0.401 0.275 0.212 0.183 0.602 0.449 0.383 0.332 0.449 0.318 0.244 0.198 0.293 0.197 0.126 0.108 0.223 0.149 0.087 0.072 0.202 0.148 0.105 0.094

0.173 0.128 0.095 0.086 0.173 0.132 0.100 0.090 0.242 0.178 0.129 0.109 0.328 0.229 0.147 0.123 0.361 0.241 0.147 0.105 0.503 0.336 0.227 0.175

0.381 0.256 0.174 0.132

0.348 0.240 0.171 0.138 0.247 0.157 0.099 0.072 0.262 0.176 0.122 0.096 0.327 0.240 0.146 0.125 0.463 0.316 0.189 0.167 0.558 0.387 0.250 0.223 0.358 0.284 0.247 0.185 0.425 0.316 0.266 0.185

0 066

0 096

0 070

0 113

0 157 0 135

0 167 0.148

0.167

0.262

0.175 0.114

0.080

0.093

0.086

0.089 0.107

0.130 0.130

0.214

0.159

0.163 0.097

0.116

0.163

0.219

0.268

0.121 0.108

0.069 0.070

0.052 0.104

0.136

0.108 0.127

0.105

0.109

0.188 0.115

0.060

0.042

0.068 0.065

0.076 0.068 0.098 0.055

0.061

0.049 0.067 0.017

0.050

0.045

0.073

0.098

0.093

0.078

Lidar data clearly indicate that the enormous increase of parameter fi observed at Monte Cimone during 1982 and 1983 is mainly due to stratospheric aerosols of the E1 Chich6n cloud. In fact, lidar-integrated backscatter a increased by more than an order of magnitude between January 1982 and January 1983, when it reached its maximum.

During the period January-July 1983 parameter fi contin- ued to increase at Monte Cimone; the data from Frascati and Garmisch show, on the contrary, a decreasing trend of the stratospheric aerosol content, starting from January 1983. This again confirms our statement in section 2.2: the maxi- mum of E1 Chich6n particle load above the station occurred in January 1983, and the increasing trend observed afterward at Monte Cimone results from the superimposition of the yearly tropospheric cycle.

Useful information can be derived from the comparison of present data with those of Michalsky et al. [1984b], and Hay and Darby [1984], referring to the latitudes of Richland, Washington (46.4øN), and Vancouver, British Columbia (49.25øN), respectively. The following time patterns are plotted in Figure 4:(1) the aerosol optical thickness at wavelength 0.7 #m at Monte Cimone (daily values); (2) the aerosol optical thickness at 0.7/•m at Richland (mean monthly or bimonthly values); (3) the aerosol optical thickness obtained at Vancou- ver with an Eppley pyrheliometer in the wide-wavelength range of this instrument (mean monthly values). Therefore the strictly homogeneous quantities to be compared are the opti- cal thicknesses of Monte Cimone and Richland; nevertheless, a comparison with Vancouver data is interesting as regards to the general trend of aerosol load during the volcanic period. The FISBAT sun-photometer is not equipped with a 0.7-btm

filter, and so, in order to compare our data with those of Richland, the optical thickness at this wavelength was com- puted through (3) for each measurement day, using the values of the/•ngstr6m parameters cz and fl.

The three data sets plotted together enforce the previously mentioned considerations on the causes of the time trend of

atmospheric turbidity during the volcanic period. The vol- canic cloud was the primary cause of the very high values of aerosol optical thickness observed at Monte Cimone at the end of 1982 and the first few months of 1983. Then the annual

cycle, which was a superimposed mechanism during the period

TABLE 3. Monthly Mean Value of Aerosol Optical Thickness at the Two Window Wavelengths of the Air Force Volz-type

Sun-Photometer Measured at Monte Cimone During 1980-1981

Wavelengths,/•m

Month 0.3800 0.5000

January 0.104 0.055 February 0.085 0.051 March 0.087 0.053

April 0.191 0.120 May 0.196 0.136 June 0.226 0.153

July 0.195 0.128 August 0.269 0.185 September 0.129 0.090 October 0.097 0.055 November 0.073 0.042 December 0.101 0.064

The data are representative of the background values of the station.

LEVIZZANI AND PRODI: EFFECTS OF EL CHICH6N 5281

o Mt. Cimone •

] .

ß

ß

.03

4'10-3 : : : : : : : : : ' ' I•• [] Garmisch-Partenkirchen 10 -3 .-

10 -4 9.10-5 .....................................

I FMAM] ] ASON D I FMAM] I ASOND ] FMAM] I AS OND I FMAMI

1982 1983 1984 1985

Fig. 3. Comparison of the time pattern of parameter fi at 1100 LT at Monte Cimone (upper panel) with that of vertically integrated lidar backscatter a measured at Garmisch-Partenkirchen (Reiter and diiger [1986]; lower panel, solid line) and Frascati (Adriani et al. [-1983]; lower panel, dashed line).

of major volcanic influence, took over again as the most im- portant effect: this comes out very clearly from the compari- son between Monte Cimone and Richland data. The observed

discrepancies in the three data sets have to be ascribed to the fact that different instruments, based on quite different con- struction features, are intercompared; moreover, localized dif- ferences may occur between daily and mean monthly trends and also between data taken at different latitudes.

4. AEROSOL SIZE DISTRIBUTIONS

The size distributions of the aerosol particles provide first- hand information on the physico-chemical mechanisms oc- curring in the aging volcanic cloud.

For the present study, aerosol size distributions were ob- tained by inverting spectral optical thickness measurements, following the method proposed by King et al. [1978]. The relationship between aerosol optical thickness and particle size distribution is represented by the following Fredholm integral equation of the first kind, after having performed height inte- gration through the atmosphere:

ra()C) = :rcr2Qext(r, 2, m)nc(r ) dr (4)

where nc(r ) is the unknown columnar aerosol size distribution (the number of particles per unit area per unit radius interval It, r + dr] in a vertical column through the atmosphere), and Qext(r, 2, m) is the extinction efficiency factor from Mie theory. The numerical inversion technique is based upon the general solution of this type of integral equation as given by Phillips [1962] and Twomey [1963]. The result of the computation, which must be applied with considerable caution to each par-

ticular case to avoid unphysical solutions to the problem, is a valid distribution for the chosen radius interval in terms of

number of particles per square centimeter. Error analysis and criteria adopted for the selection of integration parameters are described in detail by King et al. [1978] and King [1982]. The refractive index of the aerosol is assumed to be 1.506-0.016i,

as a result of averaging over the spectral range of the sun- photometer [Tomasi and Vitale, 1983].

The results of the above-mentioned inversion procedure are shown in Figures 5-10, together with the corresponding spec- tral measurements of aerosol optical thickness; six repre- sentative days are examined in order to describe the' overall range of situations encountered during the analysis of the op- tical characteristics of the volcanically perturbed atmosphere above Monte Cimone. Error bars associated to aerosol optical thickness measurements and standard deviations relative to

the inversion procedure are reported. Since these inversions were performed using the optical thickness measurements of the whole atmospheric column above the station, they are subject to a high degree of uncertainty when trying to draw conclusions regarding stratospheric aerosols. Local injection of particles due to special meteorological conditions in the region can strongly affect the transparency of the troposphere, thus not allowing any inference on the particulate at high altitude. Therefore only a few very clean days were selected among the whole series shown in Figure 1, to ensure the smallest possible interference of the local tropospheric phe- nomena on the radiative information relative to the strato-

sphere. Moreover, the height of Monte Cimone contributes to keep the air above the station relatively free from most of the anthropogenic aerosol produced in the nearby Po Valley.

5282 LEVIZZANI AND PRODI' EFFECTS OF EL CHICH6N

z

,> Mt. Cimone

m-- - Richland

m .- Vancouver

10

/

05 ..................

1982 1983 1984

Fig. 4. Comparison of the time pattern of aerosol optical thickness at wavelength 0.7 ttm at Monte Cimone (solid line) with the monthly mean of the same quantity measured in Richland (Michalsky et al. [1984b]; dashed line), and the monthly mean of the aerosol optical thickness measured over the entire spectrum of an Eppley pyrheliometer in Vancou- ver (Hay and Darby [1984]; dot-dashed line), between October 1982 and June 1984.

In Figure 5, aerosol optical thickness and inverted size dis- tribution for October 16, 1982, are plotted. The form of the distribution is essentially Junge-type, typical of unperturbed continental conditions, indicating that the effects of the vol- canic cloud had not yet reached a maximum at Monte Cimone in this period. Figure 1 seems to confirm this con-

sideration, since the value of • on October 16, 1982, is the lowest for the period under study. Measurements made later on the same day indicate, however, that the distribution began to show a slightly bimodal aspect, with a large particle mode centered around 0.2-0.3 #m superimposed on the normal background mode of small particles.

.•_

.Ol

Wavelength [/•,m ]

108

107

.1 1 5

Radius [/•,m ]

Fig. 5. Spectral aerosol optical thickness at the seven window wavelengths of the FISBAT sun-photometer, measured at Monte Cimone on October 16, 1982, and relative inverted aerosol size distribution. Standard deviations of the optical thickness measurements are plotted on the left and estimated errors associated with the inverted size distribution shown on the right.

LEVIZZANI AND PRODI' EFFECTS OF EL CHICH6N 5283

.Ol

10 s •

10 7

10 6

10 4

10 3

Jan 11 1983

Wavelength [•m] Radius Fig. 6. Same as in Figure 5, except for January 11, 1983.

A completely different situation is depicted in Figure 6 for January 11, 1983: the size distribution is completely unimodal within the integration range, with a mode radius around 0.6 pm. The monthly mean size distributions for the stratosphere of Michalsky et al. [1984a], and Asano et al. [1985] have exactly the same shape, though differing in the total amount of particles. The first impression that one gains from this com- parison is that the troposphere above Monte Cimone on this particular day was practically transparent, allowing the sun- photometric technique to acquire information on the strato-

spheric aerosol. The distribution shows a good resemblance to the large particle mode of the typical stratospheric sulphate aerosol, as described by Friend [1966]. It thus follows that at this time, there seems to be a continuous condensation of

sulphuric acid vapor onto existing nuclei [Hidy et al., 1978], as has been noted by Holmann and Rosen [1983]. In fact, new particle formation by heteromolecular nucleation from the gas phase appears to have ceased about 3 months after the erup- tion [Hofmann and Rosen, 1983], but the previously nucleated small sulphate particles act as condensation nuclei up to this

.Ol

Wavelength [•m] Fig. 7.

8

10 7

10 6

10 4

10 3

10 •

.1

Apr 16 1983

Radius

Same as in Figure 5, except for April 16, 1983.

1

5284 LEVIZZANI AND PRODI' EFFECTS OF EL CHICH6N

.1 .

.01

8

lO

lO' .

lO 6

lO s

lO 4

lO 3

lO 2

.3 1 2 .1 1 5

Wavelength [ •m] Radius [p,m] Fig. 8. Same as in Figure 5, except for May 17, 1983.

time in the stratosphere, thus giving rise to a large-particle mode sustained by the very high concentration of sulphuric acid in the stratosphere.

The typical bimodal size distribution of the volcanic aero- sol, as observed by means of ground-based techniques, repre- sents the aerosol population above the station common to April 16 and May 17, 1983 (Figures 7 and 8). The bimodality of the distribution suggests that the optical effects of a strato- spheric sulphate aerosol population are superimposed along the light path on those of background tropospheric aerosol.

The mode radius of the large particles is located between 0.4 and 0.5 /•m. Similar results were obtained with sun- photometric techniques by Michalsky et al. [1984a] for the March 1983 monthly mean, and by Spinhirne and King [1985] in the stratosphere at 45øN in May 1983. The contribution of the large-particle mode to the total number of particles is almost the same as the one given by the distribution of Janu- ary 11, 1983 (Figure 6), which consisted of this mode only. This consideration leads us to suppose that the sulphate aero- sol formation in the stratosphere was still in progress up to

.__.

.Ol

Wavelength [•m]

Fig. 9.

z

108

10'

106

10 s

104

103

102

2 .1 1 5

Radius [p,m]

Same as in Figure 5, except for October 13, 1983.

LEVIZZANI AND PRODI' EFFECTS OF EL CI-UCI-I6N 5285

.Ol

Wavelen9th [p,m]

o

10"

Apr 20 1984 10 ?

10" .

10=! ........ , . , , .1 1 5

Radius [p,m]

Fig. 10. Same as in Figure 5, except for April 20, 1984.

the middle of 1983. Hofmann and Rosen [1984] have found that the aerosol load above 20 km altitude steadily increased after the eruption until January 1983, and afterward was sub- ject to a very slow decrease, maintaining quite high values even in September. The same authors report that the mean radius of the stratospheric aerosol (above 17 km) after the months immediately following the eruption, when many com- bined nucleation-sedimentation mechanisms were still active,

maintained a constant value of around 0.2 #m. The accord- ance with these authors is therefore rather good, also con- sidering that they observed a very rapid decay of the total aerosol mass during summer 1983, which is in agreement with the steep decrease of parameter fl at Monte Cimone, starting in July 1983 and not entirely attributable to the annual cycle of the station.

Finally, the aerosol size distributions of 2 days subsequent to the period of major volcanic influence are examined in Figures 9 and 10. October 13, 1983, corresponds to the mini- mum value of mass loading attained at the end of the rapid decrease starting in July (see Figure 1); the size distribution reveals a continental Junge-type form. The same is to be noted in Figure 10 for April 20, 1984, more than 2 years after the eruption, which represents the first local peak of aerosol mass loading after the minimum of October 13, 1983. It follows that, even if a residual of volcanic aerosol was still present during 1984 and 1985, its relative weight on the global popu- lation of atmospheric aerosol was not so important as to be detected by the sun-photometer. Therefore from the point of view of a ground-based measuring instrument, the aerosol size distributions were back to the normal continental shape, sug- gesting that the particle formation mechanisms, which main- tained the sulphate aerosol load unaltered in the stratosphere, had lost most of their efficiency.

5. SUMMARY AND CONCLUSIONS

The series of turbidity measurements, obtained by means of a multiwavelength sun-photometer at the mountain station of

Monte Cimone (2170 m msl, 44ø12'N) in northern Italy over the period from October 1982 to June 1985, proved the ef- fectiveness of this ground-based technique for analyzing changes in the optical characteristics of the atmosphere, caused by large-scale events such as the eruption of E1 Chich6n.

The analysis of turbidity data in terms of the aerosol optical thickness and the Angstr6m parameters cr and/• indicates that a considerable increase in the aerosol load above the station

occurred following the E1 Chich6n eruption. The influence of the volcanic aerosol on the atmospheric turbidity is evident from the very high values of parameter/• and the correspon- dent very low values of parameter cr up to the first few months of 1983. The maximum of volcanic turbidity was found around January 1983; the subsequent persistent increase of turbidity is mainly due to the annual cycle of the station superimposed on the volcanic effects.

Lidar data from the stations of Frascati (42øN, in central Italy; Adriani et al. [1983]) and Garmisch-Partenkirchen (47.5øN, in West Germany; Reiter and diiqer [1986]) are in good agreement with the time pattern of the aerosol load at Monte Cimone during the examined period. A comparison with radiometric data from Richland (46.4øN, Washington; Michalsky et al. [1984b]) and photometric data from Vancou- ver (49.25øN, British Columbia; Hay and Darby [1984]) shows a considerable agreement between results of ground-based tur- bidity stations operating during the E1 Chich6n event.

The spectral behavior of the measurements and the aerosol size distributions, computed by inverting the spectral optical thickness measurements, provide a description of the time variation of the size and composition of the aerosol suspended above the station. It has been possible to document the con- version of the particle population from a continental Junge- type distribution of small particles, prior to the passage of the volcanic cloud, to a bimodal distribution including large stratospheric sulphate particles of volcanic origin with mode radius around 0.4-0.5 #m. This is in good agreement with particle samplings [Hofmann and Rosen, 1984] and sun-

5286 LEVIZZANI AND PRODI.' EFFECTS OF EL CHICH6N

photometric measurements from different sites around the world [Michalsky et al., 1984a; Asano et al., 1985].

Acknowledgments. The research was partly funded by the Ministry of Education and partly under a contract between the European Eco- nomic Community (EEC) and the Italian National Research Council (CNR). It was also part of a joint program, between FISBAT/CNR and the Italian Air Force Meteorological Service. The authors would like to thank Marco Pioppi for his help in data processing, and Captains Tiziano Colombo and Vincenzo Cundari of Monte Cimone Air Force Base for their cooperation regarding the background measurements of the station. Finally, the authors are indebted to the reviewers for their particularly appropriate suggestions.

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(Received June 30, 1986; revised December 31, 1987; accepted January 5, 1988.)