Lisa Horrocks

A miniaturised ultraviolet spectrometer for remote sensing of SO2 fluxes: a new tool for volcano surveillance

Abstract For 30 years, the correlation spectrometer (COSPEC) has been the principal tool for remote monitoring of volcanic SO 2 fluxes. During this time, the instrument has played a prominent role in volcanic hazard assessment. COSPEC data also underpin estimates of the global volcanic SO 2 flux to the atmosphere. Though innovative for its time, COSPEC is now outdated in several respects. Here we report the first measurements with a potential replacement, using a low cost, miniature, ultraviolet fibre-optic differential optical absorption spectrometer (mini-DOAS). Field experiments were conducted at Masaya Volcano (Nicaragua) and Soufriere Hills Volcano (Montserrat). The mini-DOAS was operated from a road vehicle and helicopter, and from a fixed position on the ground, indicating fluxes of ∼4 and 1 kg s −1 at Masaya Volcano and Soufriere Hills Volcano, respectively. Side-by-side observations with a COSPEC on Montserrat indicate a comparable sensitivity but the mini-DOAS offers several advantages, including the collection of broadband ultraviolet spectra. It has immense potential for geochemical surveillance at volcanoes worldwide.

Rethinking adaptation for a 4 ◦ C world

With weakening prospects of prompt mitigation, it is increasingly likely that the world will experience 4°C and more of global warming. In such a world, adaptation decisions that have long lead times or that have implications playing out over many decades become more uncertain and complex. Adapting to global warming of 4°C cannot be seen as a mere extrapolation of adaptation to 2°C; it will be a more substantial, continuous and transformative process. However, a variety of psychological, social and institutional barriers to adaptation are exacerbated by uncertainty and long timeframes, with the danger of immobilizing decision-makers. In this paper, we show how complexity and uncertainty can be reduced by a systematic approach to categorizing the interactions between decision lifetime, the type of uncertainty in the relevant drivers of change and the nature of adaptation response options. We synthesize a number of issues previously raised in the literature to link the categories of interactions to a variety of risk-management strategies and tactics. Such application could help to break down some barriers to adaptation and both simplify and better target adaptation decision-making. The approach needs to be tested and adopted rapidly.

Remote sensing of CO2 and H2O emission rates from Masaya volcano, Nicaragua

Although CO 2 and H 2 O account for more than 90 mol% of volcanic gases, the rates at which these gases are emitted from volcanoes are difficult to determine because of their high atmospheric background levels. We report the first precise field measurements of volcanic CO 2 , and H 2 O, in addition to HCl, HF, and SO 2 , in the plume of Masaya volcano, Nicaragua, a basaltic volcano with a record of Plinian activity. The molar ratios for CO 2 : SO 2 (2.3–2.5) and H 2 O: SO 2 (66–69) observed in February–March 1998 and March 1999 show no significant variation over the 12 month period. The molar composition of the gas is similar to other basaltic arc volcanoes in Central America. Emission rates of SO 2 from the summit crater, determined by correlation spectroscopy, averaged 21 kg s −1 during the study periods, indicating CO 2 , H 2 O, HCl, and HF emission rates of 32–36, 380–420, 7.0–7.8, and 0.86–0.95 kg s −1 , respectively. At these rates it takes only a few years to emit the equivalent volatiles associated with Masaya9s prehistoric Plinian eruptions.

Stable gas plume composition measured by OP‐FTIR spectroscopy at Masaya Volcano, Nicaragua, 1998–1999

We report non-eruptive plume gas composition data collected by open-path Fourier transform infrared (OP-FTIR) spectroscopy at the summit of Masaya Volcano, Nicaragua, in 1998 and 1999. Masayas plume is characterised by SO2/HCl and HCl/HF mass ratios of 2.8–3.0 and 8.1–8.2 respectively. When combined with COSPEC SO2 emission rate data, these ratios indicate average emission rates of HCl and HF of 7.3 kgs−1 and 0.9 kgs−1. The gas composition is remarkably consistent, showing little temporal variation on the scale of days, weeks or even between the two observation periods, one year apart. Such striking uniformity is best explained by steady-state open system degassing.

Diurnal changes in volcanic plume chemistry observed by lunar and solar occultation spectroscopy

We report the first spectroscopic measurements of volcanic gas emissions by lunar occultation. The experiment was carried out at Masaya volcano, Nicaragua in 1998 using a portable Fourier transform infrared spectrometer. Both SO 2 and HCl column concentrations were determined to yield a SO 2 /HCl molar ratio of 2.2 ± 0.28 (± 1σ). This is significantly greater than the equivalent ratio of 1.6 ± 0.02 (± 1σ) derived from solar occultation measurements of the volcanic plume. We propose that the cause of the nocturnal increase in SO 2 /HCl ratio is dissolution of HCI into volcanic water droplets within the plume. This arises because the low saturated vapour pressure of water by night results in strong condensation of plume water vapour whereas by day only negligible plume water vapour condenses.

Compositional variation in tropospheric volcanic gas plumes: evidence from ground-based remote sensing

Abstract Remotely sensed measurements of volcanic plumes have been undertaken for 30 years with instruments such as the correlation spectrometer, and more recently, open-path Fourier transform infrared (OP-FTIR) spectrometers. Observations are typically made several kilometres from the source, by which time chemical reactions may have occurred in the plume, overprinting the source composition and flux. Volcanological interpretations of such data therefore demand an understanding of the atmospheric processes initiated as gases leave the volcanic vent. Ground-based remote sensing techniques offer the temporal resolution, repeatability and quantitative analysis necessary for investigation of these processes. Here we report OP-FTIR spectroscopic measurements of gas emissions from Masaya Volcano, Nicaragua, between 1998 and 2001, and examine the influence of atmospheric processes on its tropospheric plume. Comparisons of observations made at the summit and down-wind, and in different measurement modes confirm that tropospheric processes and local meteorology have only minor impact on gas composition after the plume has left the crater. This study demonstrates that plume monitoring downwind provides a reliable proxy for at-crater sampling, and that volcanological information content is not obscured by the intervening transport. From February 1998 to May 2000, Masaya’s plume composition was strikingly stable and characterized by SO2/HCl and HCl/HF molar ratios of 1.6 and 5.0, respectively. Departures from this stable background composition are likely to signify changes in the volcanic system or degassing regime, as identified in April–May 2001.

Open-path Fourier transform infrared spectroscopy of SO2: An empirical error budget analysis, with implications for volcano monitoring

Fourier transform infrared (FTIR) spectroscopy is increasingly used as a tool for volcano monitoring, allowing measurement of a range of gases including SO2, HCl, and HF. Retrievals are complicated, since the open-path spectra are typically pressure broadened and contaminated hy atmospheric H2O, CO2, etc., and the field instruments employed have low spectral resolution (∼0.5 cm−1). We present a detailed analysis of 0.5 cm−1 resolution infrared spectra of certified SO2 mixtures in order to assess the sensitivity of such instruments and the retrieval procedures used for field spectra. We investigate the effects on retrievals of the SO2 V1+v3 band (centered at 2499.87 cm−1) of varying the retrieval spectral window and background fit, and of errors in the instrument line shape (ILS), temperature, and pressure. Largest deviations in retrieved amounts result from errors in the ILS (1.5–2.1%) and temperature (2.9–3.0% for a 10 K change). The total error estimate associated with these factors is similar to the uncertainty in line parameter data. Overall, the retrieval accuracy was better than 5%, except for the lowest concentration spectra. Errors calculated in the retrieval algorithm were conservative enough to cover these accuracy limits. We estimate that in field spectra, SO2 concentrations above 2.5 × 1017 molecules cm−2 (100 ppm m at room temperature and pressure) should be measurable with high accuracy. These results are encouraging in the context of deployment of open-path FTIR spectrometers for surveillance of active volcanoes, and the findings are equally applicable to other open-path measurements of SO2, for example from anthropogenic sources.

Monitoring gases from andesite volcanoes

Monitoring gases from andesite volcanoes for hazard mitigation or scientific enquiry is complicated by the wide range of eruption styles. Monitoring is aimed at both measuring the rates of gas emission, and changes in their compositions. Direct sampling techniques are restricted to accessible vents, and are unsuitable for syn-eruption monitoring. Correlation spectroscopy is a simple and robust method for measuring emission rates of sulphur dioxide, but is subject to large errors. Open-path Fourier transform spectroscopy provides a remote method for determining plume gas compositions, but requires careful atmospheric radiative transfer modelling. Few andesite volcanoes have been consistently monitored. Published data show that there is no simple general model for volcano degassing: each volcano, and each eruption, presents separate problems, many of them arising from the evolving interaction between magmatic and hydrothermal systems during an episode of activity. Because of its lower solubility in magmas and conservative behaviour in hydrothermal systems, remote measurements of carbon dioxide proportions and emission rates would be extremely valuable for monitoring, but they remain difficult because of its high atmospheric concentration.