The Mount Pinatubo Eruption

Stratospheric aerosols measurements from Camaguey, Cuba, between January 1992 and November 1993 are reported. The observations began six months after the Mount Pinatubo eruption, and show the aerosol characteristics during the period of cloud decay. Mean monthly profiles were employed for the study. The backscattering ratio at the peak of the profile shows a decrease from 14.80 at 25 km height in January 1992 to 3.82 at 22 km in December of the same year and 1.61 at 19 km in November of 1993. Although the decrease in altitude of the peak is 3 km for both years, the decrease in scattering ratio is more pronounced in the first year, around 11 units, than in the second, around 2 units. As expected, the trend for the integral of backscattering ratio between 16 and 33 km shows a similar trend, with values of 4.49 + 10^-3 sr ^-1 for January 1992 to 1.57 + 10^-3 sr ^-1 in December of the same year and 0.40 + 10^-3 sr ^-1 for November 1993. This represents a decrease of 3 + 10^-3 sr ^-1 for the first year and 1 + 10^-3sr^-1 for the second year, with a decrease in the entire period of one order of magnitude. Another fact that is revealed by the profiles is the practically monotorlic decreasing trend in the scattering ratio and of the integral, but the height of the peak presents an oscillatory decreasing tendency. The base of the layer was at 18 km in January, 1992 decreasing to 17 km in April and to 16 km in July, oscillating around that height for the rest of the year and for all of 1993, except for June 1993. Finally, some particular characteristics of the mean monthly profiles are shown.

The Airborne Arctic Stratospheric Expedition II involved measurements of key quantities concerning the chemistry and physics of the stratospheric ozone loss from the NASA operated DC-8 and ER-2 platforms. The series of AASE II flights was conducted between August 22, 1991, until March 26, 1992, from Moffett Field (California), Fairbanks (Alaska), and Bangor (Maine). The timing and location of the AASE II flights permitted to obtain a large data basis pertaining to the effects of the Mt. Pinatubo volcanic plume spreading in the northern hemispheric stratosphere. This contribution presents results obtained from the ER-2 in-situ measurements up to altitudes of ≈ 20 km in the polar stratosphere with respect to: (1) the aerosol microphysics, (2) the stratospheric dynamics, and (3) the heterogeneous chemistry.

Stratospheric aerosol observations have been carried out with three lidars, in the period preceeding and following the eruption of Mt. Pinatubo. The lidars were located at South Pole, Rome and Thule. The detailed analysis of the results is still under way: their general features and highlights are summarised in this paper. The aerosol backscattering data show the global evolution of the volcanic aerosol cloud in relation to the general circulation of the atmosphere and to microphysical processes. Other inferred parameters are the mass, the center of mass of the cloud. and the size distribution of the aerosol. Correlations between the aerosol and the ozone contents. found after all main eruptions since 1962, have been confirmed. Large effects on polar stratospheric cloud activity have been recorded.

Surface area and mass of stratospheric aerosols generated by the June 1991 eruption of Mt. Pinatubo have been estimated by means of a statistical aerosol model applied to midlatitude lidar observations. Surface area results agree well with equivalent ones obtained by balloon-borne particle counters. These observations indicate that ozone destruction induced by heterogeneous chemical reactions proceeding on Mt. Pinatubo aerosols should persist at least until mid-1993. Lidar-derived aerosol loads for Mt. Pinatubo are presented and compared to the ones recorded after the 1982 eruption of El Chichon. Over a two-year period past the eruption the column mass of the Pinatubo cloud was 2–3 times larger than the El Chichon one.

A technique is given to determine the effective radius, aerosol surface area density and aerosol volume density of stratospheric aerosols from the measurements by the Improved Stratospheric and Mesospheric Sounder (ISAMS) of emitted radiance at 12.1 μm. It is shown that the Mt. Pinatubo aerosol cloud had a volume density of about 9 μm³ cm^-3 a surface area density of 42 μm² cm^-3 and an effective particle radius of 0.6 μm. Using an estimate of aerosol acidity from thermodynamic calculations, the liquid H₂SO₄ content within the aerosols can be found. This is shown to be a better tracer of atmospheric motion than the raw extinction value, as variations due to temperature features are reduced. It is also shown that the acid mass contained in the Pinatubo cloud between the 100 and 5 mb levels reached a maximum of ~ 19.5 megatonnes of H₂SO₄ at the start of 1992.

In this paper we consider microphysical processes which affect the formation of sulfate particles and their size distribution in a dispersing cloud. A model for the dispersiorl of the Mt. Pinatubo volcanic cloud is described. We then consider a single point in the dispersing cloud and study the effects of nucleation, condensation and coagulation on the time evolution of the particle size distribution at that point.

Three years of lidar measurements at the northern midlatitude station of Garmisch-Partenkirchen show the evolution, northward spread and decay of the aerosol cloud which had formed in the stratosphere after the explosive eruption of Mt. Pinatubo in mid-June 1991. These lidar data are the basis for calculations of aerosol particle extinction, mass, and surface area, which are important parameters in considerations of climate response and heterogeneous chemistry effects on the stratospheric ozone layer.

With the purpose to study the interaction between stratospheric aerosols and trace gases, a two-dimensional photochemical/transport model has been developed, with the inclusion of a detailed microphysical code for aerosol formation. Here an attempt is made to predict the sulfate aerosol formation and evolution after the Mt. Pinatubo eruption in 1991, starting from the initial cloud of sulfure dioxide.

The relative impact on stratospheric temperature of the volcanic eruptions of Agung, El Chichon, and Pinatubo is estimated using 9-season averages of temperature to minimize the QBO influence. The low-stratospheric warming following the 3 eruptions tends to be greatest in the tropics and decrease poleward, but on average there is a twofold greater warming in the Southern Hemisphere than in the Northern Hemisphere. The warming following the 3 eruptions is essentially the same in the Northern Hemisphere, but in the Southern Hemisphere the warming is half again as great following Pinatubo, mostly due to a much greater warming in south temperate and south polar zones. The tropical stratospheric warming following El Chichon exceeds that following Agung to a height of 30 km, the difference in warming increasing with height. The warming following Pinatubo slightly exceeds that following El Chichon to a height of 24 km, but at greater heights the warming following Pinatubo is indicated to be much less, an unexpected finding in view of satellite evidence that Pinatubo aerosols rose to nearly the 40 km level.

The response of surface air temperatures to four major tropical explosive volcanic eruptions is identified. The common features of the average response (the composite) are then compared with the response to the Pinatubo eruption both for global average temperatures and for the spatial pattern. The historic sample of four eruptions shows significant cooling of about 0.2°C in global mean temperatures during months 13 to 24 following each eruption. The spatial pattern of the composite shows cooling over preferred regions. The Pinatubo eruption has generated a similar magnitude cooling of O.2°C, this being only half that expected based on model results with a General Circulation Model. The cooling after Pinatubo was most marked over continental landmasses, particularly eastern North America, the Middle East and eastern Asia. All three regions were identified in the composite. One major region tends to warm slightly following major volcanic eruptions, northwestern Eurasia, and this also was evident following Pinatubo.

The eruptions of three low-latitude volcanoes (Mt. Agung, El Chichón, and Pinatubo) have in the recent past disturbed conditions in the stratosphere. In particular, the temperature structure in the tropics and the dynamics of the extratropical circulation in winter were affected. We describe some of the effects of the volcanic eruptions as they appeared in the stratosphere on the Northern Hemisphere, and stress those differences between the effects of the three eruptions that are associated with the state of the QuasiBiennial Oscillation during and after each of the eruptions.

After the 1991 Mt. Pinatubo eruption, we started a research project entitled EPIC (Effects of the Pinatubo Eruption on Climate). The vertical profiles and time variations of stratospheric aerosols injected by the eruption have been monitored by a global lidar network. The physicochemical properties of the Pinatubo aerosols were analyzed by balloonborne impactors and a multi-channel sunphotometer. Based on these observational data and satellite data, the effects of increased aerosols on the solar and infrared radiation field, atmospheric temperature, the ozone layer and consequently climate were investigated including numerical simulations.

The decay of the Mt. Pinatubo volcanic cloud was monitored by systematic groundbased aerosol lidar systems implemented at the Observatoire de Haute-Provence (OHP, 44°N, 6°E) and at the Antarctic station of Dumont d’Urville (66.4°S, 140°E) Additional backscatter lidar measurements were also performed during the EASOE campaign in Sodankylä (67°N, 26°E). At northern mid-latitude, comparisons with the El Chichon volcanic cloud indicates similar aerosol loading but a longer residence time of the volcanic aerosols especially in the 15- 20 km altitude range. The analysis of the aerosol measurements obtained at the northern and southern polar latitudes shows that mixing can take place at the edge of the polar vortex in the lower stratosphere whereas the vortex remains mainly isolated above, especially in the southern hemisphere. The measurements performed in the winter and spring of 1992 in Dumont d’Urville allow to evaluate the subsidence of air inside the vortex, at a rate of 1 km/month at the 475 K potential temperature level. Besides, an ozone decrease of about 20 % as compared to the long-term climatology was recorded on the ozone sonde measurements performed at OHP in the 17 km to 22 km altitude range, in 1992 - 1993. The maximum depletion was found at the beginning of 1993, when it reached 30 % around 17 km, below the 2 sigma long term variability.

The eruption of Mount Pinatubo in June 1991 placed a large amount of SO₂ in the stratosphere which was converted to sulfuric acid aerosols in 2-3 months after the eruption. These aerosols remained primarily in the tropical stratospheric reservoir (TSR) for the first 6 months or so after the eruption before being largely dispersed to midlatitudes. The large amount of SO₂ and aerosols in the TSR was sufficient to cause easily observable changes in stratospheric ozone. There was an initial 2% column increase caused by photooxidation of SO₂, followed by a 5% column decrease in the region from the Equator to 10 deg. S in the August-November 1991 period based on total ozone mapping spectrometer (TOMS) data. A comparison of electrochemical concentration cell (ECC) sonde data at Brazzaville (4°s) and Ascension Island (8°s) with the historical pre-Pinatubo stratospheric aerosol and gas experiment II (SAGE II) ozone data showed indicated a 8% column decrease between 16 and 28 km along with a 2% column increase between 28 and 32 km. The decrease from 16 to 28 km is likely due primarily to a combination of aerosol heating and lofting of the associated air mass and aerosol effects on photolysis, with small contributions from heterogeneous chemistry involving the aerosols. The increase above 28 km is primarily due to the removal of NO₂ by the aerosols, permitting the photochemical production of ozone to be more efficient. The phase change of the QBO from easterly to westerly was delayed by the rising motion of the aerosols by about 10 months, resulting in about a 10-D.U. decrease of ozone for that period compared to what would have been the case without the delay. In addition, in the middle of 1992, there was another minimum in stratospheric ozone in the Equator-to-10°S region, beyond that expected from QBO effects, which is not explained.

Following the eruption ofMt. Pinatubo, substantial decreases were observed both in ozonesonde and total column ozone observations. These decreases are illustrated with data from mid-latitudes and polar regions, and are interpreted in terms of heterogeneous chemical processes, both involving hydrolysis of dinitrogen pentoxide and reactions involving chlorine species such as ClONO₂ HOCl, and HCl on sulfate aerosols. In addition, this paper shows in detail for the first time how the ozone budget varies with altitude in response to sulfate aerosol perturbations, revealing the reasons for observed increases in ozone above about 25 km following the eruption.

Pinatubo stratospheric sulfate aerosols have been observed by balloonborne backscattersonde for 3 consecutive winters at Thule. Blocking effects of the polar vortex edge and diabatic descent is observed. A case study of stratospheric chemical ozone depletion, based on ozone soundings of the same air parcel from Reykjavik, Scoresbysund and Thule and analysis of air parcel backward trajectories is presented.

Total ozone measurements from TOMS have shown a continuous negative ozone trend after the Pinatubo eruption. As shown by two-dimensional assessment models, heterogeneous chemistry on the volcanic aerosol surface would produce the maximum depletion during the winter-spring of 1991/92. It is shown here that the volcanic aerosol additional heating may produce anomalies in the stratospheric circulation which may cause the apparent delay in the ozone destruction.

The eruptions of the low latitude volcanoes El Chichón and Pinatubo have disturbed conditions in the stratosphere and the total ozone field. The spatial and temporal distribution of these disturbances is studied using monthly mean total ozone residuals after careful removal of the components of known oscillations (seasonal variation, QBO, El Nino/Southern Oscillation). The total ozone deficiencies attributed to the volcanic effect are found to range from -2% in the tropical region up to about -5% over middle and high latitudes, lasting for 6 or more months after the eruption. These deficiencies are larger than both the noise term and the anticipated error from the aerosol contaminated radiances.

The climatic impact of the Mt. Pinatubo volcanic eruption of June 1991 was studied by observations and model simulation for the first winter and summer after the eruption. A low resolution (T21) atmospheric GCM was driven by additional stratospheric aerosol derived from observations. The main results as observed and simulated are: Warming of the aerosol containing stratospheric layers resulting in an enhanced polar night jet. Reduction of the incoming shortwave radiation by 2 to 4 W/m-2. Modification of the tropospheric planetary wave structure in winter. During winter the induced advection leads to positive temperature anomalies in the lower troposphere over the midlatitude continents, while in summer the local radiation balance determines the volcanic impact and leads to slight cooling.

Global cooling of the Earth’s surface has been observed following the largest volcanic eruptions of the past century, although the average cooling is perhaps less than expected from simple energy balance considerations. The Mount Pinatubo eruption, with both the climate forcing and response observed better than previous volcanoes, allows a more quantitative analysis of the sensitivity of climate to a transient forcing. We describe the strategy and preliminary results of a comprehensive investigation of the Pinatubo case.

The environmental impact of the 1991 Mt. Pinatubo volcano eruption in the stratosphere, as well as relevant consequences for global climate change and ozone layer dynamics have been discussed, with special emphasis on the possibility of identifying the volcanic climatic signal (VCS). The combined results of ground-based, balloon, aircraft, and satellite monitoring of the volcanic cloud propagation, and the scale of ejections have been considered and compared with similar data for other explosive volcanic erutpions to show that the Mt. Pinatubo eruption has doubtless been the most powerful one, especially with regard to the emissions of sulphur-containing gases. The perspectives of future satellite monitoring of volcanic eruptions have been briefly discussed. It has been emphasized that the results of numerical modelling aimed at the assessment of climate change due to eruptions are controversial. It has been shown that it is difficult to identify VCS because of coupling between various climate-forming processes (the El Niño/Southern Oscillation phenomenon, and the Quasi-Biennial Oscillation in the Stratosphere being of special significance).

A regional climate study of the Pinatubo eruption is reported. A parameterization of the interaction between the aerosol layer and the solar and planetary radiation is introduced in the MM4/3-D mesoscale model. Simulations with different optical depth values (0.1 and 0.04), are performed and compared with a reference run without aerosols. Results of the different runs are compared to the ECMWF data analysis to ascertain whether the inclusion of the aerosol radiative forcing may better simulates the observed data. The optical thickness influences the dynamical field, while is marginal for the temperature field although this is better represented by the model simulation when aerosol are included. The model simulation performed using an optical depth value of 0.1 better represents the mid tropospheric wind field, suggesting that the aerosol layer perturbs the troposphere even in the short term.