Assessments of volcanic aerosol mass loading of the stratosphere

In order to estimate time series of the spatial patterns of volcanic aerosol optical depth ($AOD$) variations of the stratosphere we assume that $AOD$ is linearly proportional to the volcanic aerosol mass concentration $a$,

$$AOD = p_{1}\cdot a,$$ (2)

where $p_{1}$ is a scale factor. To calculate horizontal space-time patterns of the volcanic aerosol mass concentration following an eruption, we use a stratospheric transport parameterization to describe the spreading of the aerosol within the lower to mid-stratosphere, presented in section 2.2. To describe the production and sedimentation of the aerosol we use a time function which depends on latitude as well as the season and time after eruption, introduced in section 2.3. As an assessment of the strength $a^{*}$ of a volcano eruption, the Volcanic Explosivity Index ($VEI$), provided by Simkin et al. (1981) and actualized by Siebert (1993), is used where $a^{*}=10^{VEI}$ for large eruptions with a column height of at least 10 to 25 km ($VEI \ge 4$) and zero otherwise. We neglect eruptions with $VEI \le3$ because they are not supposed to inject precursors of aerosols into the stratosphere (Simkin et al., 1981). The actualized version also takes into account the results of Robock and Free (1995) who argue that the Mount St. Helens eruption (1982) was only of $VEI=4$ instead of 5 and Agung eruption (1963) was of $VEI=6$ instead of 4. These two examples show that it is very difficult to get reliable information about the strength of eruptions before the period of detailed instrumental observations. Nevertheless, for some strong volcanic eruptions the stratospheric mass loading is known from observations or is estimated indirectly. Since the middle of the nineteenth century six volcanic eruptions occured with a $VEI$ of 6. For all these eruptions estimates of stratospheric aerosol mass loading are provided by several researchers using different methods. Stothers (1996) presents an overview of the results and a comparison with his own investigations. We list these estimated stratospheric aerosol loading in Table 1. The averaged stratospheric aerosol loadings of the six most recent eruptions with $VEI=6$ amounts to $\mu = 24.8 Tg$. Assuming that a volcanic eruption with $VEI=6$ leads exactly to an aerosol loading of 25 $Tg$ we obtain the following equation to correct the $VEI$ data in respect to the estimated stratospheric aerosol loading $M$:

$$VEI_{correct} = 4 + \log_{10} (4 Tg^{-1} \cdot M).$$ (3)

The corrected $VEI$ values are listed in the last column of Table 1. For volcanoes with $VEI < 6$ estimates of stratospheric mass loading exist, too (Stothers, 1996). We also correct the $VEI$ of these volcanoes with respect to the observations using equation (3). The results for the strongest observed eruptions since 1880 are listed in Table 2. For our further investigations we use the series with corrected $VEI$ values.