eruptive and passive degassing of sulphur dioxide at nyiragongo ...

S. A. CarnTable 2. Degassing of SO 2from Nyiragongo magmas, 7 October 2002 - 17 November 2003.Sulphur degassed (wt %) SO 2emission per km 3 Volume of degassed magmadegassed magma (Mt)* (km 3 )* Assumes a density of 2.65 for Nyiragongo nephelinite.0.01 0.53 9.1 – 16.80.05 2.65 1.81 – 3.370.21 11.13 0.43 – 0.800.25 13.25 0.36 – 0.67SO 2degassing associated with renewed lavalake activity (October 2002 - present)Following the 17 January 2002 eruption, fresh lava wasnot observed within the summit crater until mid-May2002 (Table 1), with no reports of substantive degassingin the intervening months. Reports of an emission plumeand the odour of SO 2were then communicated in mid-July 2002, evolving to high pressure degassing in lateSeptember (Table 1). modis ir thermal alerts also indicatean absence of thermal anomalies at the summit ofNyiragongo from 24 February - 12 June 2002 (gvn 2003a).More frequent alerts began to be observed in earlySeptember 2002 (gvn 2003a), indicating that persistentmagmatic activity in the summit crater began aroundthis time, although cloud cover may have obscuredsome activity. Since September 2002 near-continuouslava lake activity has continued in the crater ofNyiragongo, producing a persistent, strong plume abovethe volcano (e.g., gvn 2003a). This activity represents areturn to the vigorous lava lake behaviour observed atNyiragongo in the 1970s.Renewed SO 2emissions from Nyiragongo were notdetected by ep toms until 7 October 2002, although theonset of SO 2discharge likely coincided with the appearanceof new lava in the summit crater. Other sensordata for this period of unrest (e.g., modis) has not beenextensively studied to date, but the generally poor performanceof ir instruments under the atmosphericconditions prevailing in central Africa may prevent furtherrefinement of the existing chronology. Since 7October 2002 persistent SO 2plumes from Nyiragongohave been detected by ep toms (Figure 3), typicallyextending west from the volcano across D. R. Congoand often tracked several hundred kilometers downwind,and this activity is ongoing at the time of writing.ep toms cannot distinguish between SO 2emitted byNyiragongo and any gas originating from Nyamuragira,but in the absence of eruptive activity at the latter allthe observed SO 2is attributed to Nyiragongo. Althoughthe ep toms record is not continuous owing to data gapsand meteorological factors (e.g., rainy season cloudcover), intermittent plume and crater observations bygvo, low-level SO 2signals in ep toms data and SO 2detection by the gome instrument strongly support continuousdegassing (Figure 3). These emissions have elevatedNyiragongo to its current status as one of thestrongest sources of volcanic SO 2on the planet in 2002-2003.The ep toms data provide the only estimates of SO 2emissions from Nyiragongo available at present, sinceground-based instrumentation is yet to be deployed.Although toms measures the total SO 2mass within ascene rather than the SO 2flux from a volcanic source,SO 2fluxes can be estimated if some assumptions aremade concerning SO 2lifetime and hence plume dispersion.A common assumption for volcanoes in tropicalenvironments is that rapid SO 2removal (e.g.,Oppenheimer et al. 1998) results in no SO 2persisting formore than 24 hours, and in this case the daily tomstotals would represent a minimum daily SO 2flux. Thisis a significant assumption, and is dependent on plumeheight and environmental factors, which are often poorlyconstrained. Applying this reasoning to the recent tomsdata for Nyiragongo produces SO 2fluxes in excess of50 kt day -1 (~580 kg s -1 ) on several occasions (Figure 3).The altitude of the persistent plume from Nyiragongohas been estimated by gvo to reach 4-6 km, though itis often lower (gvo, personal communication, 2002).Altitudes of ~5 km or more are above the freezing levelin a standard tropical atmosphere (e.g., Figure 2) andsuggest that ice formation probably occurs within higherplumes as emitted and entrained water vapour freezes.Scavenging of SO 2in explosive eruption clouds hasbeen linked to formation of ice (e.g., Rose et al. 1995),with subsequent SO 2release on ablation of the ice, andat Nyiragongo this process may act to reduce the SO 2mass detected by toms close to the vent. At times thedetected SO 2is a distinct mass situated some distancefrom the volcano (as opposed to a plume ‘attached’ tothe volcano), suggesting that downwind release of SO 2due to ablation of ice may be occurring. More detailedinvestigations of this effect are planned for the future.The cumulative SO 2emissions from Nyiragongo observedby ep toms to date (7 October 2002 – 17 November2003) amount to ~2 Mt. However, due to data gaps,meteorological factors and variations in plume heightthis does not represent the total mass of SO 2emittedby the volcano; quantifiable SO 2plumes fromNyiragongo were observed on only 117 days during this406-day period. Extrapolation of this figure to an estimateof the actual SO 2production is difficult, particularlyfor the longer measurement gaps such as January-April 2003 (Figure 3). In the absence of a more robusttechnique the data gaps have been filled using an averageSO 2emission computed from the sequence ofmeasurements closest to each gap. Using this method,the estimated total SO 2mass released in the ~13 monthsamounts to 6.9 ± 2.1 Mt (30% error on toms SO 2retrievals assumed), with an average SO 2emission of~16 kt day -1 (~185 kg s -1 ). This daily average is compa-8