Indian Journal of Radio & Space Physics Vol. 33, June 2004, pp. 170- 175

Effect of volcanic aerosols on stratospheric ozone at Kodaikanal

Meena Jain & Namita Kundu

Radio & A tmospheric Science Di vision, National Physical Laboratory, New Delhi 110 012

Received 20 Dece111ber 2002: revised 20 January 2004; accepted 4 February 2004

Nimbus 7 S!3UV data (average of zonal means for 5- 10°N and IO-l 5°N) and Dobson data over Koda ikanal ( I 0° N latitude) for the period 1979- 1994 have been analysed to study the effect o f El-Chichon and Pinalllbo volcanic eruptions on ozone quasi-bienn ia l osc illation (QI30) . The ozone Q!30 has been found to be disturbed up to the height of 34 km following both the eruptions and the effect persisted even after one year of the event. The impact of EI-Chichon volcanic eruption on ozone shows a depletion up to 34 km with maximum effecl at 29 km, whereas the impact fo llowing Pinatubo eruption shows ozone depletion up to 29 km above which an increase in ozone is observed. The cause of depleti on below about 27 km is due to well known heterogeneous chemistry. The cause of ozone depletion above 27 km fo llowing the two major eruptions has been investigated. The pcrcemage change in ozone has been estimated after taking the warming due to aerosol into considerati on. The theoretically ca lculated and the observed percentage dev iations of ozone show good agreement in the case of EI-Chichon erupti on, wh ile for Pinatubo erupti on the calculated values show more negati ve values.

Keywords: Ozone, QBO, EI-Chichon erupti on, Pinatubo cwpti on, Ozone depletion l'ACS No.: 92.60.Mt ; 94. 10.Fa; 92.60.Sz Il'C Code: G 01 W 1/00; G 0 1 W 1/17

I Introduction Volcanic erupti ons play a major role in changing

earth 's atmosphere and atmospheric constituent budget especially ozone. The sulphuric acid aerosol s injected into the stratosphere by major volcanic erupti ons des troy ozone through different mechani sms such as: (i) increase in available aerosol surface aren and chlorine injected by volcano-enhance · heterogeneous react ions, (ii ) increased opti ca l. ex tinction , thereby increas ing change in local heating which can alter the mean circul ation and temperature distributi on; strengthening of the mean circulation can uplift the ozone layer caus ing a localized depletion and (iii ) in upper stratosphere the large sulphate aeroso ls induce radiati on changes such as heating due to scattering. The sli ght ri se in temperature affects both the production and loss rates of ozone g iving a net ozone depletion.

A significant decrease of to tal ozone following El­Chichon eruption in Mexico (17 .3°N, 93.2°W) on March 1982 was observed, even after about a year, using ground-based Dobson and sate llite measurements 1

-5

• Three months after the eruption, the cloud hacl c ircl ed the g lobe between 0° and 30°N. The altitude of the peak aeroso l concentrati on had dropped to 27 km after 8 weeks. The Mt. Pinatubo eruption ejec ted about 12-20 MT of sulphur dioxide into the stratosphere. The clouds were initi a lly confined in

tropics between 20° and 30° N and subsequent ly spread as far as the outer edge of the Antarcti c polar vortex. The aerosol layer was located between I 0 and 30 km with the maximum loading at around 22 km.

Various measurements show decrease in ozone fol lowing Pinatubo eruption in June 199 1 at Philippines6

-10 (15.1 °N, 120°E). An increase in ozone

above 28 km was also observed by vanous researchers6

·12

"13

.

T he existence o f an appreciable quasi-biennial oscillation (QBO) in total ozone is well known. In the present study, the effect of EI-Chichon and Pinatubo eruptions on QBO of ozone is in vestigated and the impact of volcanic aerosols on total ozone and stratospheric ozone has been estimated.

2 Disturbed ozone QBO A SBUV instrument on Nimbus-7 satellite

measures the solar ultra-viole t radiation backscattered by the earth 's atmosphere and refl ected by clouds and the teiTestrial surface . The measure ments are made within 12 wavelength channels from 256 to 340 nm. From these measure ments the total column ozone and vertical pro file are estimated . These SBUV data arc available from National Space Science Data Centr (NSSDC), USA. The monthly mean zonal average data for 5-10° N and 10-15° N from SBUV/2 instrument onboard Nimbus-7 satellite is downl oaded

JAIN & KUNDU: EFFECT OF VOLCANIC AEROSOLS ON STRATOSPHERIC OZONE 171

from NSSDC (http://nssdc.gsfc.nasa.gov) website. Dobson spectrometer is a double quartz prism spectrometer, which isolates two narrow bands in the ultra-violet (Huggins bands) so chosen that the longer wavelength is very little affected by ozone absorption, while the shorter one is greatly absorbed. Two wavelength pairs [generally, A (305.5/325.4) and D (317.6/339.8)] are used to eliminate the effect of aerosol scattering. Measurement of the ratio of the intensities of these two bands allows one to calculate the amount of ozone through which light has passed. India Meteorological Department (IM D) contributes ozone data over various Indian stations to World Ozone & Ultraviolet Radiation Data Centre (WOUDC) for the data archival on routine basis.

The data are analysed for the period from November 1978 to September 1994 to get the QBO in ozone. The time series data are detrended by subtracting long term linear trend and smoothed by 13 month running average. A time series is residual by subtracting a long term mean for each month of the year. The missing value between two existing residual s in a time series is supplied by linear interpolation. The resulting anomaly time series has been smoothed by a 7-month running mean operator to reduce high frequency variability and noi se. Percentage deviation of residual for Dobson values over Kodaikanal are shown in Fig. I. Zonal mean ozone QBO using SBUV data for 10° N is depicted in Fig. 2. Both Figs I and 2 show disturbed QBO-a prominent decreasing effect up to the winter of 1983-84 and less prominent effect for 1992-93. The normal trend of QBO over Kodaikanal (excluding the effect of eruption) is to produce minima during January of 1982 and 1992 and maxima during January of 1983 and 1993. It can be seen from Figs I and 2 that a minimum is obtained in January 1982 and February 1992. The ozone deviation starts decreasing from July 1982 and October 1992 producing a minimum in place of expected maximum in QBO, implying thereby the effect of the eruptions to cause depletion in QBO. The minimum, just after the Pinatubo eruption , is less pronounced as compared to that observed during 1982-1983.

The satellite observations of ozone mixing ratio at different heights are further analysed to determine the altitude up to which the eruptions had affected the QBO. It is seen that the QBO peak which should have occurred after EI-Chichon eruption is missing up to 34 km and reappears above it, showing that the aerosols thrown up by the volcano have affected

4.------------------------------ ---~

3

~ 2 ::> 0

z 0

~ > ~ o+m~-h4-.-,-.-~~r-rl-r-v~-.-,-,+-rt-.~~ w .,.. m

§ ::J r -3 ~----------------------------------~

Fig. I -Ozone QI30 (DU) using Dobson data at Kodaikanal ( I0°N)

3,-----------------------------------~

2

::> 0

z 0 Q m ... .... <( c.

"' > w -1 => 0 w z 2-2 0

-3

"' m c. ~

~ ~----------------------------------~

Fig. 2-Zonal mean QBO (DU) for I 0° N representing average global variations at thi s latitude using SBUV data

atmospheric ozone up to 34 km he ight. The QBO peak after Pinatubo eruption has been found to be affected up to 31 km. The variations at some representative heights such as 27, 29, 31 , 34, 36 and 38 km are depicted in Fig. 3. It may be noted that the depletion at 31 and 29 km is much larger than that at 27 km following El-Chichon eruption, whereas the depletion decreases steadily following Pinatubo eruption. The broadness of QBO following Pinatubo is much larger than EI-Chichon indicating that the disturbance persi sted longer following Pinatubo eruption.

3 Estimated ozone change The impact of volcanic eruptions is estimated after

taking QBO into account. The duration of depletion is known to be of the same time span as that of QBO.

172 INDIAN J RADIO & SPACE PHYS, JUNE 2004

Feb-90 Feb-92 Feb-94

36 km

Feb-90 Feb-92 Feb-94 29 km

Feb-90 Feb-92 Feb-94 27 km

Feb-86 Feb-88 Feb-90 Feb-92 Feb-94

MONTH

Fig. 3- Height-wi se variation of QBO of ozone using Nimbus-7 SBUV data for I 0°N

The data are divided into nine season intervals, each season consisting of three months. The volcanic interval is taken as the one in which the eruption took place and the other eight seasons are considered following the eruption. The mean ozone of the nine season intervals proceeding the volcanic interval in which eruption does not occur is calculated. The difference between the mean ozone of the volcanic interval and that of average of other intervals represent the ozone depletion. The est imated volcanic impact on ozone is depicted in Table I.

A large depletion of 1.89 per cent of total ozone is est imated following El-Chichon as compared to that of Pinatubo eruption. Height-wise analysis shows that ozone depletion exists up to 34 km with a maximum impact at 29 km after El·-Chichon eruption, whereas a steady ozone depletion up to 29 km and an increase above this height are observed following Pinatubo eruption . The results are comparable with the es timates made by Angell 10

'14 and Stolarski et at. The

magma of the El-Chichon and Pinatubo eruption reaches up to 27 km and 22 km, respectively, and S02

is converted to su lphate aerosols providing a large surface area for heterogeneous reaction to take place. The aeroso ls particles in the Junge layer are of much smaller (about 0.05-0.2 ~tm) radius than the hydrated sulphate ae rosols (about 2-10 ~m) and thus provide almost no surface area for heterogeneous reaction to

take place above the height of volcanic magma. Thus, the disturbance in QBO at an altitude above 27 km is due to the change in ozone production and loss rates due to the radiative warming by large aerosols. The sulphate aerosols (of nearly a micron sized) scatter solar radiation back to space, cooling the earth's surface and absorb ing infrared upwelling radiation, thereby warming the stratospheric layers where they reside. The time required to reach the maximum temperature is about two months after depletion. The stratospheric warming due to El-Chichon and Pinatubo eruptions has been reported by various researchers6

·9

·15

'16

'18

. Very little warming due to Pinatubo aerosol was reported above 24 km, since the maximum altitude at which the debris were thrown up were less than that of El-Chichon, while El-Chichon warming is observed up to 35 km height9

·17

. At the height range of 30-35 km where the temperature increase is only a fraction of a degree, the ozone photochemical reactions are affected, produci ng enough depletion to influence QBO. A 2-D model is used to calculate the change in ozone.

In a 2-D model, the zonally averaged distribution of all chemical species (i) are governed by continuity equation of the form

an v·an .an s -' +---' +w --' = +D. a{ a arp a Z I

... ( 1)

JAIN & KUNDU: EFFECT OF VOLCANIC AEROSOLS ON STRATOSPHERIC OZONE 173

Table !-Estimated percentage ozone change over Kodaikanal

Volcano Total Ozone change(%) at 27 km 29 km 3lkm 34 km 36 km

El Chichon - 1.89 -2.85±0.26 -4.53±0.37 -4.24±0.43 -2.02±0.31 -0.65±0.21

Pinatubo - 1.16 -2.57+0.25 -1.57+0.31 +0.32+0.37 +1.03+0.34 +0.56+0.28

where, II; is the mtxmg ratio of the species. The parameters S, the zonally averaged production/loss rate and D;, the diffusion term which represents the flux divergence due to a small scale disturbances, can be written as

.. . (2)

where P; is the production rate and L; the loss rate for the species i

and

D. = I

I ark an;J I a( kan;) 1 ',.vCOSqJ- --- p, .. --a·COS(/Ja(/J ·· a({J p, az · -- az . .. (3)

where kc~ and kvy are the vertical and meridional diffusion coefficients, a the radius of earth and p, the air density. Ignoring the first term which is not much significant, the diffusion term can be written as

D. =--- pk -' Ia( an) ' P,. a z ·' ~ a z

.. . (4)

The values of kc~ generally do not vary with the annual cycle, but are different within the troposphere and stratosphere. The values of kcc in lower stratosphere are taken as 0.1 E+4 cm2/s . The velocities v* and w* are derived directly from the knowledge of diabatic heating rate 19 and are computed using the equations

... (5)

and

1 a ( . ) 1 a ( *) ----- v COS(/) +-- Ps w =0 acosqJ acp Ps a z

... (6)

Upward transport occurs when the net diabatic heating is positive, while downward transport is found when Q is negative. The model assumes chemical reactions in oxygen-hydrogen-nitrogen atmosphere.

The ozone-mixing ratio is taken as a function of time t, latitude qJ and altitude z. A 2-D grid plane (latitude (/J, altitude z) is formed by taking 19 equal points in the latitude ranging from 0° N to 90° N and 31 equally spaced points in the altitude range 20-50 km. Initial values of ozone mixing ratio 11 for 19x3 1=15431 points of the grid plane are assumed and the model is solved by grid point method. The time, altitude and latitude steps are taken as 1 day, lkm and 5° latitude, respectively. The values for dry adiabatic lapse rate is taken as 10°/km and the actual lapse rate as a function of height is taken from U S Standard Atmosphere. The heating rate is computed using the parametrization given by Schoeberl and Strobel20

. The cooling produced by radiative emission due to carbon dioxide at 15 Jlm wavelength band has been taken into account. Contribution from 80 Jlm band of water vapour in middle stratosphere is relatively very small2

1.22 and is not taken into account.

The ozone values over Kodaikanal are calculated for the altitudes of 27 , 29, 31 , 34 and 36 km, respectively. Under normal condition thi s gives an ambient ozone abundance. The volcanic impact is calculated by taking the aerosol heating into account. Angell9

·17 computed the warming in seven climatic

zones (north and south poles, north and south temperate, north and south subtropics and equatorial zones) following EI-Chichon and Pinatubo eruptions after taking QBO into account in the same way as is done for this study to find estimated ozone change (Table I). Since the warming for equatorial regi on as reported by AngeiJ9

·17 has been included in the model,

the seasonal and QBO signals arc automatically taken care of. Since the rocketsonde data observations are available up to 30 km as reported by Angell 9

, the warming above 30 km is taken from radiosonde data 17

• The observed and the calculated values are given in Table 2. While converting the percentage change from concentration to mass mixing ratio, the change in air density due to enhanced temperature is taken into account. In this present study, the ozone change beyond 31 km has not been calculated as no heating was observed above thi s height foll owing the

174 INDIAN J RADIO & SPACE PHYS, JUNE 2004

Table 2 - EI-Chichon and Pinatubo volcani c impact on stratospheric ozone [~Ti s the change in temperature as observed by Angeii9·17

;

0 .1 is the ca lculated ambient ozone number density , after taking radiat ion imo account and ~03/03 the percentage change in ozone number density. The quantity in the bracket indicates power.]

Ambient EI-Chi chon Pinatubo Height T 0 , /cnr' ~T 0 .1 /em· ~0~03 ~T 0 3 /em· ~Oi0.1

km OK OK % OK %

27 223.8 4.67( I 2) 1.1 4.55( I 2) -2.56 1.6 4.51 ( 12) -3.42

29 228.0 4.27( 12) 1.2 4.13(12) -3.26 1.7 4.14( 12) -3.04

3 1 232.2 3.92 ( 12) 1.3 3.78( 12) -3.57 1.8 3.77(12) -3.82

34 235.5 3.66( 12) 0.4 3.63 ( 12) -0.819

36 239.8 3.49( 12) 0.1 3.49( 12) 0

Pinatubo eruption . The model results for El-Chichon agree well with the estimated values (Table 1), whereas for Pinatubo the model results g ive more negative values. The order of magnitude of the estimated values agree very well with the observed values as seen at 27 km, 3 1 km and 34 km heights after El-Chichon eruption . The estimated values for Pinatubo show an increase in ozone after 29 km, while the model results do not show such increase. The increase in ozone above 29 km may be due to lofting and cannot be explained by photochemistry. The Pinatubo magma of about 20 MT was much larger than that of El-Chichon (13.4 MT) and was injected up to a maximum height of 22 km. The high impact at 22 km produces heating of aeroso l layer at 25-27 km, lifting the airmass and depleting ozone at layers below it. So lofting is the upward motion of airmass transporting ozone at the higher heights. We have not considered lofting in this paper.

4 Discussion and conclusion Several processes occur follow ing a major volcanic

eruption:

(i) Particulate aerosol loading at tropospheric heights effectively reduce solar radiation reaching the earth's surface. This aerosol cloud gets diffused globally after some time.

(ii) The S02 emitted by volcano is converted to large hydrated sulphate aerosols in stratosphere and provides an increase in aerosol surface area for heterogeneous chemistry to take place.

(iii ) Number of large aerosols thus created increase optical thickness of the atmosphere, thereby enhancing the scattering and absorption of solar flux which results in increase in local heating and alter the mean circulation and temperature

distribution. Strengthening the mean circulation can uplift the ozone layer, causing a localized depletion. The height up to which thi s effect can be een depends on the magnitude of a particular eruption.

(iv) The sulphate aerosols induce radiation changes such as heating due to scattering. The slight ri se in temperature affects the temperature dependent reaction rate constants of ozone chemistry. Both the production and loss reactions are affected giving a net ozone depletion at thi s height.

In case of El-Chichon eruption, the present model results tally with the estimated values. The total ozone depletion has been calculated as 1.89% after taking QBO into account and is comparable with the findings of other researchers5

.t0

.23

. The magma of the eruption reached up to 30 km with a maximum impact at 18-20 km. The aerosol surface area was observed to

increase to about 50 11m2/cm3 (background value 0.75) at 18-20 km in midlatitude. The increased aerosol surface area provides site for heterogeneous chemistry to take place for ozone depletion at this altitude24

.

U mkehr data and ozonesonde data show a more pronounced ozone depletion between 16 and 19 km in midlatitudes after one year of eruption2

. Since El­Chichon aerosols had reached 30 km height they produce enough warming to deplete ozone, though the peak warming reported was at altitudes between 20 and 25 km (height of maximum aerosol loading). A fractional increase in temperature can affect temperature dependen t reaction rates and photodissociation rates to deplete ozone. 1t has been shown in Table 2 that ozone depletion between 27 and 34 km is due to fractional increase in temperature. Since no change in ozone QBO was observed above 34 km (Fig. 3) and no warming was reported above

JAIN & KUNDU: EFFECT OF VOLCANIC AEROSOLS ON STRATOSPHERIC OZONE 175

this altitude, it can be inferred that the ozone depletion at these altitudes was caused by radiative change only.

A decrease in ozone below 29 km and an increase above this height are observed at Kodaikanal after Pinatubo eruption. This feature is observed by various researchers6

·12

. The increase may be due to enhanced circulation owing to aerosol heating in tropics shortly after the eruption, which resulted in upward motion of

. . h. h h . h 13 25 Th atr transporting ozone at tg er etg ts · · . . e Pinatubo magma of about 20 MT was much larger than that of El -Chichon magma (13.4 MT) and was injected with a maximum loading at 22 km, while El­Chichon magma reached up to 30 km with a maximum loading at 27 km. The high impact at 22 km produces heating of 2-3°C in the region of aerosol layer (20-25 km) causing an uplift of airmass and thereby depleting ozone at layers below it. Heterogeneous chemistry on the surface of Pinatubo aerosol droplets and change in photochemical production/loss rate may be other reasons for ozone depletion 1

1.25

. The present model results do not tally with the estimated values as we have not calculated the effect due to heterogeneous chemistry and lofting in this paper. Chakrabarty and Peshin26 reported an insignificant change in Dobson ozone following Pinatubo eruption. The reason for discrepancy may be attributed to the method of analysis. They have considered averaging of Dobson data without taking QBO into account26

.

Thus, it can be concluded that various volcanic eruptions affect ozone by various mechanisms depending upon the amount of the magma, the time of the eruption, and the latitude at which the eruption occur.

5 A-cknowledgements The authors are thankful to the World Ozone

Ultraviolet Radiation Data Centre (WOUDC), Canada, for providing Dobson data and to the National Space Science Data Centre (NSSDC), USA, for providing SBUV data.

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