Gregg J. S. Bluth

Global tracking of the SO2 clouds from the June, 1991 Mount Pinatubo eruptions

The explosive June 1991 eruptions of Mount Pinatubo produced the largest sulfur dioxide cloud detected by the Total Ozone Mapping Spectrometer (TOMS) during its 13 years of operation: approximately 20 million tons of SO2, predominantly from the cataclysmic June 15th eruption. The SO2 cloud observed by the TOMS encircled the Earth in about 22 days (∼21 m/s); however, during the first three days the leading edge of the SO2 cloud moved with a speed that averaged ∼35 m/s. Compared to the 1982 El Chichon eruptions, Pinatubo outgassed nearly three times the amount of SO2 during its explosive phases. The main cloud straddled the equator within the first two weeks of eruption, whereas the El Chichon cloud remained primarily in the northern hemisphere. Our measurements indicate that Mount Pinatubo has produced a much larger and perhaps longer-lasting SO2 cloud; thus, climatic responses to the Pinatubo eruption may exceed those of El Chichon.

Lithologic and climatologic controls of river chemistry

Abstract The chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff. In all, 101 different rivers, each draining a primary lithology, were examined across the United States, Puerto Rico, and Iceland. To minimize seasonal effects, only rivers with at least two years of data were used. Basaltic catchments were examined in the greatest detail. In a survey of Hawaiian Island watersheds, the average river chemistries could be related to the distribution of soil associations within each catchment. An analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium (ion exchange) control on cation ratios. Among different lithologies, unique weathering rate relationships were developed with yields at typical present-day runoff rates (1–100 cm/y) increasing in the order sandstones, granites, basalts, shales, and carbonates. The bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation. Our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering; thus, a warm, wet climate, or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal.

Volcanic sulfur dioxide measurements from the total ozone mapping spectrometer instruments

The total ozone mapping spectrometer (TOMS), first flown on the Nimbus 7 satellite, has delivered an unanticipated set of unique information about volcanic plumes because of its contiguous spatial mapping and use of UV wavelengths. The accuracies of TOMS sulfur dioxide retrievals, volcanic plume masses, and eruption totals under low-latitude conditions are evaluated using radiative transfer simulations and error analysis. The retrieval algorithm is a simultaneous solution of the absorption optical depth equations including ozone and sulfur dioxide at the four shortest TOMS wavelengths and an empirical correction based on background condition residuals. The retrieval algorithm reproduces model stratospheric sulfur dioxide plume amounts within ±10% over most central scan angles and moderate solar zenith angles if no aerosols or ash are present. The errors grow to 30% under large solar zenith angle conditions. Volcanic ash and sulfate aerosols in the plume in moderate optical depths (0.3) produce an overestimation of the sulfur dioxide by 15–25% depending on particle size and composition. Retrievals of tropospheric volcanic plumes are affected by the reflectivity of the underlying surface or clouds. The precision of individual TOMS SO2 soundings is limited by data quantization to ±6 Dobson units. The accuracy is independent of most instrument calibration errors but depends linearly on relative SO2 absorption cross-section errors at the TOMS wavelengths. Volcanic plume mass estimates are dependent on correction of background offsets integrated over the plume area. The errors vary with plume mass and area, thus are highly individual. In general, they are least for moderate size, compact plumes. Estimates of the total mass of explosively erupted sulfur dioxide depend on extrapolation of a series of daily plume masses backward to the time of the eruption. Errors of 15–30% are not unusual. Effusive eruption total mass estimates are more uncertain due to difficulties in separating new from old sulfur dioxide in daily observations.

Integrating retrievals of volcanic cloud characteristics from satellite remote sensors: a summary

Volcanic eruptions are events that rapidly and suddenly disperse gases and fine particles into the atmosphere, a process most conveniently studied from the synoptic satellite perspective, where remote sensing offers a practical tool for spatial and temporal measurements. Meteorological satellites offer approximately 20 years of archived data, which can be analysed for measurements of masses of SO2 and fine volcanic ash in spatial two–dimensional arrays and integrated with other meteorological data. The satellite data offer a tool to study volcano–atmosphere interactions in a quantitative way. They provide information of unique value for understanding the fate and transport of fine silicates with significant health hazards and for addressing the problem of volcanic cloud hazards to jet aircraft. Studies of satellite data have demonstrated the following. (1) Volcanic clouds from convergent plate boundary volcanoes contain large and variable excesses of SO2. (2) The second day of atmospheric residence for volcanic clouds has significantly higher SO2 than the first, suggesting that early volcanic H2S may be converting to SO2. (3) Complete conversion of SO2 to sulphate in the stratosphere occurs at an efolding rate of approximately 120 days. SO2 loss from stratospheric volcanic clouds occurs at an e–folding rate of approximately 35 days, and the SO2 loss rate for volcanic clouds which only barely reach the stratosphere is rapid (efolding only a few days). The latter limits the stratospheric aerosol build–up from smaller eruptions. (4) Fine volcanic ash (with diameters of less than ca.25μm) in drifting volcanic clouds retrieved after 10 h or more appear to represent a small fraction (less than 2% of the total mass) of the total mass of magma erupted, and also a small fraction (less than 20%) of the total mass of fine ash erupted. This is probably explained by the fact that the total mass is greatly reduced by aggregation processes within the volcanic cloud. (5) The amounts of fine ash decrease faster in volcanic clouds of larger eruptions, supporting the self–removal processes suggested by Pinto et al. in 1989. (6) Evidence for a strong role of ice in the fallout and aggregation of volcanic cloud ash is considerable. (7) In many cases, volcanic clouds separate into higher SO2–rich portions and lower ash–rich portions. The two portions follow different trajectories and the lower, ash–rich portions are affected by interactions with moist tropospheric air.

Stratospheric loading of sulfur from explosive volcanic eruptions

This paper is an attempt to measure our understanding of volcano/atmosphere interactions by comparing a box model of potential volcanogenic aerosol production and removal in the stratosphere with the stratospheric aerosol optical depth over the period of 1979 to 1994. Model results and observed data are in good agreement both in magnitude and removal rates for the two largest eruptions, El Chichón and Pinatubo. However, the peak of stratospheric optical depth occurs about nine months after the eruptions, four times longer than the model prediction, which is driven by actual SO2 measurements. For smaller eruptions, the observed stratospheric perturbation is typically much less pronounced than modeled, and the observed aerosol removal rates much slower than expected. These results indicate several limitations in our knowledge of the volcano‐atmosphere reactions in the months following an eruption. Further, it is evident that much of the emitted sulfur from smaller eruptions fails to produce any stratospheric impact. This suggests a threshold whereby eruption columns that do not rise much higher than the tropopause (which decreases in height from equatorial to polar latitudes) are subject to highly efficient self‐removal processes. For low latitude volcanoes during our period of study, eruption rates on the order of 50,000 m3/s (dense rock equivalent) were needed to produce a significant global perturbation in stratospheric optical depth, i.e., greater than 0.001. However, at high (>40°) latitudes, this level of stratospheric impact was produced by eruption rates an order of magnitude smaller.

Volcanic eruption detection by the Total Ozone Mapping Spectrometer (TOMS) instruments: a 22-year record of sulphur dioxide and ash emissions

Abstract Since their first depolyment in November 1978, the Total Ozone Mapping Spectrometer (TOMS) instruments have provided a robust and near-continuous record of sulphur dioxide (SO2) and ash emissions from active volcanoes worldwide. Data from the four TOMS satellites that have flown to date have been analysed with the latest SO2/ash algorithms and incorporated into a TOMS volcanic emissions database that presently covers 22 years of SO2 and ash emissions. The 1978–2001 record comprises 102 eruptions from 61 volcanoes, resulting in 784 days of volcanic cloud observations. Regular eruptions of Nyamuragira (DR Congo) since 1978, accompanied by copious SO2 production, have contributed material on approximately 30% of the days on which clouds were observed. The latest SO2 retrieval results from Earth Probe (EP) TOMS document a period (1996–2001) lacking large explosive eruptions, and also dominated by SO2 emission from four eruptions of Nyamuragira. EP TOMS has detected the SO2 and ash produced during 23 eruptions from 15 volcanoes to date, with volcanic clouds observed on 158 days. The EP TOMS instrument began to degrade in 2001, but has now stabilized, although its planned successor (QuikTOMS) recently failed to achieve orbit. New SO2 algorithms are currently being developed for the Ozone Monitoring Instrument, which will continue the TOMS record of UV remote sensing of volcanic emissions from 2004 onwards.

Early evolution of a stratospheric volcanic eruption cloud as observed with TOMS and AVHRR

This paper is a detailed study of remote sensing data from the total ozone mapping spectrometer (TOMS) and the advanced very high resolution radiometer (AVHRR) satellite detectors, of the 1982 eruption of El Chichon, Mexico. The volcanic cloud/atmosphere interactions in the first four days of this eruption were investigated by combining ultraviolet retrievals to estimate the mass of sulfur dioxide in the volcanic cloud [Krueger et al., 1995] with thermal infrared retrievals of the size, optical depth, and mass of fine-grained (1–10 μm radius) volcanic ash [Wen and Rose, 1994]. Our study provides the first direct evidence of gravitational separation of ash from a stratospheric, gas-rich, plinian eruption column and documents the marked differences in residence times of volcanic ash and sulfur dioxide in volcanic clouds. The eruption column reached as high as 32 km [Carey and Sigurdsson, 1986] and was injected into an atmosphere with a strong wind shear, which allowed for an observation of the separation of sulfur dioxide and volcanic ash. The upper, more sulfur dioxide-rich part of the cloud was transported to the west in the stratosphere, while the fine-grained ash traveled to the south in the troposphere. The mass of sulfur dioxide released was estimated at 7.1 × 109 kg with the mass decreasing by approximately 4% 1 day after the peak. The mass of fine-grained volcanic ash detected was estimated at 6.5 × 109 kg, amounting to about 0.7% of the estimated mass of the ash which fell out in the mapped ash blanket close to the volcano. Over the following days, 98% of this remaining fine ash was removed from the volcanic cloud, and the effective radius of ash in the volcanic cloud decreased from about 8 μm to about 4 μm.

Observations of Volcanic Clouds in Their First Few Days of Atmospheric Residence: The 1992 Eruptions of Crater Peak, Mount Spurr Volcano, Alaska

Satellite SO2 and ash measurements of Mount Spurr’s three 1992 volcanic clouds are compared with ground‐based observations to develop an understanding of the physical and chemical evolution of volcanic clouds. Each of the three eruptions with ratings of volcanic explosivity index three reached the lower stratosphere (14 km asl), but the clouds were mainly dispersed at the tropopause by moderate to strong (20–40 m/s) tropospheric winds. Three stages of cloud evolution were identified. First, heavy fallout of large (>500 μm) pyroclasts occurred close to the volcano (<25 km from the vent) during and immediately after the eruptions, and the cloud resembled an advected gravity current. Second, a much larger, highly elongated region marked by a secondary‐mass maximum occurred 150–350 km downwind in at least two of the three events. This was the result of aggregate fallout of a bimodal size distribution including fine (<25 μm) ash that quickly depleted the solid fraction of the volcanic cloud. For the first several hundred kilometers, the cloud spread laterally, first as an intrusive gravity current and then by wind shear and diffusion as downwind cloud transport occurred at the windspeed (during the first 18–24 h). Finally, the clouds continued to move through the upper troposphere but began decreasing in areal extent, eventually disappearing as ash and SO2 were removed by meteorological processes. Total SO2 in each eruption cloud increased by the second day of atmospheric residence, possibly because of oxidation of coerupted H2S or possibly because of the effects of sequestration by ice followed by subsequent SO2 release during fallout and desiccation of ashy hydrometeors. SO2 and volcanic ash travelled together in all the Spurr volcanic clouds. The initial (18–24 h) area expansion of the clouds and the subsequent several days of drifting were successfully mapped by both SO2 (ultraviolet) and ash (infrared) satellite imagery.

Comments on “Failures in detecting volcanic ash from a satellite-based technique”

Abstract The recent paper by Simpson et al. [Remote Sens. Environ. 72 (2000) 191.] on failures to detect volcanic ash using the ‘reverse absorption’ technique provides a timely reminder of the danger that volcanic ash presents to aviation and the urgent need for some form of effective remote detection. The paper unfortunately suffers from a fundamental flaw in its methodology and numerous errors of fact and interpretation. For the moment, the ‘reverse’ absorption technique provides the best means for discriminating volcanic ash clouds from meteorological clouds. The purpose of our comment is not to defend any particular algorithm; rather, we point out some problems with Simpson et al.s analysis and re-state the conditions under which the ‘reverse’ absorption algorithm is likely to succeed.

The February–March 2000 Eruption of Hekla, Iceland from a Satellite Perspective

An 80,000 km 2 stratospheric volcanic cloud formed from the 26 February 2000 eruption of Hekla (63.98° N, 19.70° W). POAM-III profiles showed the cloud was 9-12 km asl. During 3 days this cloud drifted north. Three remote sensing algorithms (TOMS SO 2 , MODIS & TOVS 7.3 μm IR and MODIS 8.6 μm IR) estimated ∼0.2 Tg SO 2 . Sulfate aerosol in the cloud was 0.003-0.008 Tg, from MODIS IR data. MODIS and AVHRR show that cloud particles were ice. The ice mass peaked at ∼1 Tg ∼10 hours after eruption onset. A ∼0.1 Tg mass of ash was detected in the early plume. Repetitive TOVS data showed a decrease of SO 2 in the cloud from 0.2 Tg to below TOVS detection (i.e.<0.01 Tg) in ∼3.5 days. The stratospheric height of the cloud may result from a large release of magmatic water vapor early (1819 UT on 26 February) leading to the ice-rich volcanic cloud. The optical depth of the cloud peaked early on 27 February and faded with time, apparently as ice fell out. A research aircraft encounter with the top of the cloud at 0514 UT on 28 February, 35 hours after eruption onset, provided validation of algorithms. The aircrafts instruments measured ∼0.5-1 ppmv SO 2 and ∼35-70 ppb sulfate aerosol in the cloud, 10-30% lower than concentrations from retrievals a few hours later. Different SO 2 algorithms illuminate environmental variables which affect the quality of results. Overall this is the most robust data set ever analyzed from the first few days of stratospheric residence of a volcanic cloud.