The eruption of Mount Pinatubo (15.1^oN) in the summer of 1991 injected approximately 18 million tonnes of SO₂ in the lower tropical atmosphere Cole-Dai et al., 1997. This was subsequently converted to stratospheric sulphate aerosol. Backscattering of solar radiation from the enhanced stratospheric aerosol cools the troposphere but it also results in increased stratospheric radiative heating rates and heterogeneous chemistry rates. The Canadian Middle Atmosphere Model (CMAM) is a fully interactive 3D model which has been used to study both the short term (a few months after the eruption) and long term (a few years) effects of the eruption of Mount Pinatubo on the atmosphere. The aerosol surface areas were derived from extinction coefficients from SAGE II observations. The model also includes important mid-latitude heterogeneous chemistry reactions.

The CMAM has been used to study how enhanced aerosols, coupled with present-day chlorine and bromine loading, directly and indirectly affect the stratospheric ozone column and distribution. The model results show that global ozone levels decreased soon after the eruption, with the minimum occurring in mid-1993 which is in reasonable agreement with observations. The global ozone deficit persisted until 1995 before levels recovered to pre-Pinatubo values. Time series of some important atmospheric source gases are presented, one in the upper stratosphere and one in the lower- to mid-stratosphere. Results indicate that erroneous trends can be derived from observations due to the lag between the eruption and the time it takes this lower stratospheric air to propagate up to the upper stratosphere. Our model suggests that the discontinuity in linear trends as presented in Randel et al.~(1999) might be an artifact of this time lag following the eruption.

Gaseous SO₂ acts as a precursor for the formation of stratospheric sulphate aerosols, e.g. it Yue, 1981 , which causes a large increase in the amount of available surface area upon which heterogeneous reactions can take place. Also, any changes in the local ozone field can alter the photolysis coefficients which can further perturb chemical partitioning and radiation. This complex interaction between chemistry, radiation and dynamics is well suited to modeling by fully interactive 3-dimensional GCM models which include or are coupled to a comprehensive (gas phase and heterogeneous) photochemical module.

In situ measurements have documented the influence that this and other eruptions (such as El Chichon in April 1982) have had on the stratosphere, e.g. Hofmann, 1990; Gleason et al., 1993; Fahey et al., 1993; Schoeberl et al., 1993; Wilson et al., 1993. The chemical perturbations which occurred subsequent to the Mount Pinatubo eruption has been well documented by a suite of instruments on various satellites.

Some studies have not included the effects of BrONO₂ hydrolysis. It was shown in Slusser et al., 1997 that decreases in summertime NO₂ columns at 65^oS over a five year period from May 1990 to February 1995 could be explained by heterogeneous hydrolysis of BrONO₂ and N₂O₅. Lary, 1996 showed that the mid latitude hydrolysis of BrONO₂ on aerosols is an important source of HO_x in the lower stratosphere. The hydrolysis of BrONO₂ is not subject to the saturation effects that occur with N₂O₅ and ClONO₂ hydrolysis. This is due to the relatively low concentrations of BrO (~5 pptv), which are always smaller than that of NO₂, even under strongly perturbed conditions leading to low NO_x/NO_y ratios Chartrand and McConnell, 1998. Also, it is expected that the hydrolysis of BrONO₂ will become increasingly important under conditions where the rates of hydrolysis of N₂O₅ and ClONO₂ are saturated or when the day length is long (Danilin and McConnell, 1995). This is expected to be true under volcanically-enhanced conditions, at high-latitudes in the summer ( Randeniya et al., 1996).

Derivations of trend information over the past decade been well documented. For example, a slow recovery of global ozone may affect radiation which would affect other chemical species as well as heating and transport.

The CMAM has fixed surface mixing ratios for CFC's, CH₄, H₂O, and N₂O. Since, we have no increases/decreases in these important source gases over time, it is interesting to compare our modeled species trends with that derived from observations over the same time period (1991-1997). In this way, we will elucidate whether the trends attributed to increases and decreases in background levels of these gases are an artifact of the perturbation and recovery of the Mount Pinatubo eruption, or they are real.

The use of the CMAM for the to examination of short- and long-term stratospheric impacts due to the eruption provides a means of studying the interaction of chemical and dynamical changes in the upper part of the stratosphere where these perturbations, for example, may be delayed due to slow Brewer-Dobson circulation.

Global ozone concentrations averaged between 60^oS to 60^oN show an almost immediate decrease which peaks in mid-1992 and begins to slowly recover over the next 3 years. The concentrations recover to within the natural variability of the model control run by the end of 1995. Column ozone levels in general show agreement with the TOMS results presented by Randel et al (1995). These results show a significant ozone decrease near the equator in late 1991 and then again in mid-1992. Also, the observations show a much larger decrease in the northern hemisphere polar region which extended down to about 50^oN in late 1991-early 1992. The most significant decrease occurred in late 1992 and into mid 1993 in the northern hemisphere. This large perturbation is thought to be due to increases in ClO concentrations due to an increased heterogeneous conversion of ClONO₂ and HCl on aerosols and PSCs. Although we did not include heterogeneous chemistry in our model run, the decreases in ozone in the region between 60^oS and 60^oN shows good agreement. Clearly however, we would not be able to capture the mid latitude ozone decrease which would be expected as a result of depletion in the polar region followed by transport towards mid latitudes after the break-up of the vortex.

Randel et al (1999) have indicated that a plausible explanation for the discontinuity in trends on or near 1996 may be due to a radiative-chemical coupling mechanism. In particular, they suggest a long term depression in the amount of column ozone and then concomitant changes that this would have on photochemistry would affect species concentrations long after the eruption of Pinatubo. In this study, we show that global ozone concentrations remain well below "normal" levels and the rate at which the ozone recovers (e-folding time) is on the order of a year and a half. Also, global ozone values do not recover to within natural variability until 1995-1996, which is in good agreement with the timing of the trend discontinuity.

The data also shows that a discontinuity in the linear trend information in the upper part of the stratosphere may be attributable to the perturbation of the lower stratosphere following the eruption and the subsequent upward propagation of this "marked" air. The timing of the discontinuity in the trend information presented by Randel is well correlated to the time where the perturbed air in the lower stratosphere reaches the upper stratosphere and then subsequently begins to recover.

We present results for total hydrogen H,(=CH₄+ H₂O), as well as CH₄, H₂O, HNO₃, and NO₂ at different pressure levels in order to elucidate the influence that the Pinatubo eruption has had upon the trend information derived from data in the 1990s. Interestingly, the downward trend in CH₄ concentrations in the upper stratosphere is captured by our results. The magnitude of the CH₄ changes in the lower part of the stratosphere is smaller than observed, indicating that most changes in the amount of His strictly due to increased water concentrations and not CH₄.

The increases in the water concentrations in the stratosphere are driven by the heating in the tropopause region which occurred as a result of increased aerosol loading. In the lower stratosphere, water vapour levels increase soon after the eruption reaching a maximum in about 1993 (depending on actual altitude) and then subsequently recovering but not quite to pre-Pinatubo levels, at least not on the time frame of our simulation. In the upper part of the stratosphere, the water vapour peak does not occur until several months after the peak at 10 mb and therefore the recovery (trend discontinuity) would not start until much later as well. This temporal behaviour in water increases and so the recovery may skew the trend results determined at various regions in the stratosphere.

The results for the HNO₃, concentrations for the lower stratosphere (45 mb) show good agreement with MLS HNO₃, observations (Figure 22 of Randel). If one ignores the data prior to 1993, the model also shows a decrease in HNO₃, concentrations of about 0.35 ppbv over the 4 year span 1993-1997. The similarities in our results and that presented by Randel suggests the trend is an artifact of HNO₃ recovery after the Pinatubo eruption. Similar findings were shown for NO₂ trends (increases) at the 14 mb pressure level.

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