Construction of an airborne chemiluminescence ozone monitor for volcanic plumes

<p>Volcanic plumes contain traces of bromine monoxide, BrO, which catalyze destruction of ozone, O<sub>3</sub>, mixed into the plume. Therefore, local depletion of O<sub>3 </sub>in the plume could be possible. However, calculations comparing mixing with the rate of O<sub>3 </sub>destruction suggest that no significant decline in the O<sub>3</sub> concentration should be expected. On the other hand several studies at different volcanoes have found varying degrees of O<sub>3</sub> depletion inside the plume. So far, ozone and its concentration distribution in volcanic plumes have only been insufficiently determined. Reliable ozone measurements would make a decisive contribution to the understanding of volcanic plume chemistry.</p> <p>The standard technique for ambient O<sub>3</sub> monitoring is the short-path ultraviolet (UV) absorption instrument. But in volcanic plumes this technique suffers from strong interference of the overlapping SO<sub>2</sub> absorption features in the UV. SO<sub>2</sub> is one of the major compounds in volcanic plumes.</p> <p>We want to overcome this problem by relying on the chemiluminescence (CL) reaction between ozone and ethene, a standard technique for O<sub>3</sub> measurement in the 1970s and 1980s, which we found to have no interference from trace gases abundant in volcanic plumes. The key component of a CL O<sub>3</sub>-instrument is a reaction chamber, where ethene is mixed into the ambient air and a photomultiplier tube detects the resulting photons.</p> <p>Field measurements with existing CL O<sub>3</sub>-monitors are complicated, because they are usually heavy and bulky. Therefore we designed a more compact and lightweight version (10 kg backpack size CL instrument), which was used in a field study at Mount Etna. However, the campaign was restricted to plumes that are pushed down to ground in areas accessible by foot.</p> <p>Here we report on a further improved version of the instrument weighing around 1 kg, which we can mount onto a drone to carry it into the plume. In particular, we describe the design advances making the reduction in weight and size possible.</p>

Dynamics of Large, Wet Volcanic Clouds : The 25.4 ka Oruanui Eruption of Taupo Volcano, New Zealand

<p>This work investigates the dynamics of large-scale, ‘wet’ volcanic eruption clouds generated by the interaction of silicic magma with external water. The primary case study draws from a detailed record of non-welded pyroclastic deposits from the ~25.4 ka Oruanui eruption of Taupo volcano, New Zealand, one of the largest phreatomagmatic eruptions documented worldwide. This research uses a three-pronged approach, integrating results from (i) field observations and textural data, (ii) mesoscale numerical modeling of volcanic plumes, and (iii) analogue laboratory experiments of volcanic ash aggregation. This interdisciplinary approach provides a new understanding of dynamic and microphysical interactions between collapsing and buoyant columns, and how this behavior controls the large-and small-scale nature of phreatoplinian eruption clouds. Stratigraphic field studies examine the styles of dispersal and emplacement of deposits from several phases of the Oruanui eruption (primarily phases 2, 3, 5, 6, 7 and 8). Detailed stratigraphic observations and laser diffraction particle size analysis of ash aggregates in these deposits clarify the evolution of aggregation mechanisms with time through the relevant eruption phase, and with distance from vent. Deposits of the wettest phase (3) show the key role of turbulent lofting induced by pyroclastic density currents in forming aggregates, particularly those with ultrafine ash rims (30-40 vol.% finer than 10 μm) which are uniquely formed in the ultrafine ash-dominated clouds above the currents. Drier deposits of phases 2 and 5, which also saw lower proportions of material emplaced by pyroclastic density currents, contain fewer aggregates that are related to low water contents in the medial to distal plume. Discovery and documentation of high concentrations of diatom flora in the Oruanui deposits indicates efficient fragmentation and incorporation of paleo-lake Taupo sediments during the eruption. This highlights the potential for incidental contamination of volcanic deposits with broader implications for correlation of distal tephras and possible contamination of paleoenvironmental records due to incorporation of diachronous populations of volcanically-dispersed diatoms. The impact of extensive surface water interaction on large-scale volcanic eruptions (>108 kg s-1 magma) is examined by employing the first 2-D large-eddy simulations of ‘wet’ volcanic plumes that incorporate the effects of microphysics. The cloud-resolving numerical model ATHAM was initialized with field-derived characteristics of the Oruanui case study. Surface water contents were varied from 0-40 wt.% for eruptions with equivalent magma eruption rates of c. 1.3 x108 and 1.1 x109 kg s-1. Results confirm that increased surface water has a pronounced impact on column stability, leading to unstable column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents (PDCs). Contrary to the suggestion of previous studies, however, abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds, owing to vigorous microphysics-assisted lofting of PDCs. Key processes influencing the aggregation of volcanic ash and hydrometeors (airborne water phases) are examined with a simple and reproducible experimental method employing vibratory pan agglomeration. Aggregation processes in the presence of hail and graupel, liquid water (<30 wt.%), and mixed water phases are investigated at temperatures from 18 to -20 °C. Observations from impregnated thin sections, SEM images and x-ray computed microtomography of these experimental aggregates closely match natural examples from phreatomagmatic phases of the ~25.4 ka Oruanui and Eyjafjallajökull (May 2010) eruptions. These experiments demonstrate that the formation of concentric, ultrafine rims comprising the outer layers of rim-type accretionary lapilli requires recycled exposure of moist, preexisting pellets to regions of volcanic clouds that are relatively dry and dominated by ultrafine (<31 μm) ash. This work presents the first experimentally-derived aggregation coefficients that account for changing liquid water contents and sub-zero temperatures, and indicates that dry conditions (<10 wt.% liquid) promote the strongly size-selective collection of sub-31 μm particles into aggregates (given by aggregation coefficients >1). These quantitative relationships may be used to predict the timescales and characteristics of aggregation, such as aggregate size spectra, densities and constituent particle size characteristics, when the initial size distribution and hydrometeor content of a volcanic cloud are known. The integration of numerical modeling, laboratory experimentation and field data lead to several key conclusions. (1) The importance of the microphysics of ash-water interactions in governing the eruption cloud structure, boosting the dispersal power of the cloud and controlling aggregate formation in response to differing water contents and eruption rates. (2) Recognition of the contrasting roles of differential aggregation versus cloud grain size in controlling the formation and nature of aggregate particles, notably those with characteristic ultrafine outer rims. (3) The importance of pyroclastic density currents as triggers for convection and aggregation processes in the eruption cloud.</p>

Dynamics of Large, Wet Volcanic Clouds : The 25.4 ka Oruanui Eruption of Taupo Volcano, New Zealand

<p>This work investigates the dynamics of large-scale, ‘wet’ volcanic eruption clouds generated by the interaction of silicic magma with external water. The primary case study draws from a detailed record of non-welded pyroclastic deposits from the ~25.4 ka Oruanui eruption of Taupo volcano, New Zealand, one of the largest phreatomagmatic eruptions documented worldwide. This research uses a three-pronged approach, integrating results from (i) field observations and textural data, (ii) mesoscale numerical modeling of volcanic plumes, and (iii) analogue laboratory experiments of volcanic ash aggregation. This interdisciplinary approach provides a new understanding of dynamic and microphysical interactions between collapsing and buoyant columns, and how this behavior controls the large-and small-scale nature of phreatoplinian eruption clouds. Stratigraphic field studies examine the styles of dispersal and emplacement of deposits from several phases of the Oruanui eruption (primarily phases 2, 3, 5, 6, 7 and 8). Detailed stratigraphic observations and laser diffraction particle size analysis of ash aggregates in these deposits clarify the evolution of aggregation mechanisms with time through the relevant eruption phase, and with distance from vent. Deposits of the wettest phase (3) show the key role of turbulent lofting induced by pyroclastic density currents in forming aggregates, particularly those with ultrafine ash rims (30-40 vol.% finer than 10 μm) which are uniquely formed in the ultrafine ash-dominated clouds above the currents. Drier deposits of phases 2 and 5, which also saw lower proportions of material emplaced by pyroclastic density currents, contain fewer aggregates that are related to low water contents in the medial to distal plume. Discovery and documentation of high concentrations of diatom flora in the Oruanui deposits indicates efficient fragmentation and incorporation of paleo-lake Taupo sediments during the eruption. This highlights the potential for incidental contamination of volcanic deposits with broader implications for correlation of distal tephras and possible contamination of paleoenvironmental records due to incorporation of diachronous populations of volcanically-dispersed diatoms. The impact of extensive surface water interaction on large-scale volcanic eruptions (>108 kg s-1 magma) is examined by employing the first 2-D large-eddy simulations of ‘wet’ volcanic plumes that incorporate the effects of microphysics. The cloud-resolving numerical model ATHAM was initialized with field-derived characteristics of the Oruanui case study. Surface water contents were varied from 0-40 wt.% for eruptions with equivalent magma eruption rates of c. 1.3 x108 and 1.1 x109 kg s-1. Results confirm that increased surface water has a pronounced impact on column stability, leading to unstable column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents (PDCs). Contrary to the suggestion of previous studies, however, abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds, owing to vigorous microphysics-assisted lofting of PDCs. Key processes influencing the aggregation of volcanic ash and hydrometeors (airborne water phases) are examined with a simple and reproducible experimental method employing vibratory pan agglomeration. Aggregation processes in the presence of hail and graupel, liquid water (<30 wt.%), and mixed water phases are investigated at temperatures from 18 to -20 °C. Observations from impregnated thin sections, SEM images and x-ray computed microtomography of these experimental aggregates closely match natural examples from phreatomagmatic phases of the ~25.4 ka Oruanui and Eyjafjallajökull (May 2010) eruptions. These experiments demonstrate that the formation of concentric, ultrafine rims comprising the outer layers of rim-type accretionary lapilli requires recycled exposure of moist, preexisting pellets to regions of volcanic clouds that are relatively dry and dominated by ultrafine (<31 μm) ash. This work presents the first experimentally-derived aggregation coefficients that account for changing liquid water contents and sub-zero temperatures, and indicates that dry conditions (<10 wt.% liquid) promote the strongly size-selective collection of sub-31 μm particles into aggregates (given by aggregation coefficients >1). These quantitative relationships may be used to predict the timescales and characteristics of aggregation, such as aggregate size spectra, densities and constituent particle size characteristics, when the initial size distribution and hydrometeor content of a volcanic cloud are known. The integration of numerical modeling, laboratory experimentation and field data lead to several key conclusions. (1) The importance of the microphysics of ash-water interactions in governing the eruption cloud structure, boosting the dispersal power of the cloud and controlling aggregate formation in response to differing water contents and eruption rates. (2) Recognition of the contrasting roles of differential aggregation versus cloud grain size in controlling the formation and nature of aggregate particles, notably those with characteristic ultrafine outer rims. (3) The importance of pyroclastic density currents as triggers for convection and aggregation processes in the eruption cloud.</p>

Improved estimation of volcanic SO₂ injections from satellite observations and Lagrangian transport simulations: the 2019 Raikoke eruption

Monitoring and modeling of volcanic plumes is important for understanding the impact of volcanic activity on climate and for practical concerns, such as aviation safety or public health. Here, we applied the Lagrangian transport model Massive-Parallel Trajectory Calculations (MPTRAC) to estimate the SO2 injections into the upper troposphere and lower stratosphere by the eruption of the Raikoke volcano (48.29° N, 153.25° E) in June 2019 and its subsequent long-range transport and dispersion. First, we used SO2 observations from the AIRS (Atmospheric Infrared Sounder) and TROPOMI (TROPOspheric Monitoring Instrument) satellite instruments together with a backward trajectory approach to estimate the altitude-resolved SO2 injection time series. Second, we applied a scaling factor to the initial estimate of the SO2 mass and added an exponential decay to simulate the time evolution of the total SO2 mass. By comparing the estimated SO2 mass and the observed mass from TROPOMI, we show that the volcano injected 2.1 ± 0.2 Tg SO2 and the e-folding lifetime of the SO2 was about 13 to 17 days. The reconstructed injection time series are consistent between the AIRS nighttime and the TROPOMI daytime measurements. Further, we compared forward transport simulations that were initialized by AIRS and TROPOMI satellite observations with a constant SO2 injection rate. The results show that the modeled SO2 change, driven by chemical reactions, captures the SO2 mass variations observed by TROPOMI. In addition, the forward simulations reproduce the SO2 distributions in the first ~10 days after the eruption. However, diffusion in the forward simulations is too strong to capture the internal structure of the SO2 clouds, which is further quantified in the simulation of the compact SO2 cloud from late July to early August. Our study demonstrates the potential of using combined nadir satellite observations and Lagrangian transport simulations to further improve SO2 time- and height-resolved injection estimates of volcanic eruptions.

The CAMS volcanic forecasting system utilizing near-real time data assimilation of S5P/TROPOMI SO₂ retrievals

The Copernicus Atmosphere Monitoring Service (CAMS), operated by the European Centre for Medium-Range Weather Forecasts on behalf of the European Commission, provides daily analyses and 5-day forecasts of atmospheric composition, including forecasts of volcanic sulphur dioxide (SO2) in near-real time. CAMS currently assimilates total column SO2 retrievals from the GOME-2 instruments on MetOp-B and -C and the TROPOMI instrument on Sentinel-5P which give information about the location and strength of volcanic plumes. However, the operational TROPOMI and GOME-2 retrievals do not provide any information about the height of the volcanic plumes and therefore some prior assumptions need to be made in the CAMS data assimilation system about where to place the resulting SO2 increments in the vertical. In the current operational CAMS configuration, the SO2 increments are placed in the mid-troposphere, around 550 hPa or 5 km. While this gives good results for the majority of volcanic emissions, it will clearly be wrong for eruptions that inject SO2 at very different altitudes, in particular exceptional events where part of the SO2 plume reaches the stratosphere. A new algorithm, developed by DLR for GOME-2 and TROPOMI and optimized in the frame of the ESA-funded Sentinel-5P Innovation–SO2 Layer Height Project, the Full-Physics Inverse Learning Machine (FP_ILM) algorithm, retrieves SO2 layer height from TROPOMI in NRT in addition to the SO2 column. CAMS is testing the assimilation of these data, making use of the NRT layer height information to place the SO2 increments at a retrieved altitude. Assimilation tests with the TROPOMI SO2 layer height data for the Raikoke eruption in June 2019 show that the resulting CAMS SO2 plume heights agree better with IASI plume height retrievals than operational CAMS runs without the TROPOMI SO2 layer height information and that making use of the additional layer height information leads to improved SO2 forecasts than when using the operational CAMS configuration. By assimilating the SO2 layer height data the CAMS system can predict the overall location of the Raikoke SO2 plume up to 5 days in advance for about 20 days after the initial eruption.

Observation and modelling of ozone-destructive halogen chemistry in a passively degassing volcanic plume

Volcanoes emit halogens into the atmosphere that undergo complex chemical cycling in plumes and cause destruction of ozone. We present a case study of the Mount Etna plume in the summer of 2012, when the volcano was passively degassing, using aircraft observations and numerical simulations with a new 3D model “WRF-Chem Volcano” (WCV), incorporating volcanic emissions and multi-phase halogen chemistry. Measurements of SO2 – an indicator of plume intensity – and ozone were made in the plume a few tens of kilometres from Etna, revealing a strong negative correlation between ozone and SO2 levels. From these observations, using SO2 as a tracer species, we estimate a mean in-plume ozone loss rate of 1.3×10−5 molecules of O3 per second per molecule of SO2. This value is similar to observation-based estimates reported very close to Etna's vents, indicating continual ozone loss in the plume up to at least tens of kilometres downwind. The WCV model is run with nested grids to simulate the plume close to the volcano at 1 km resolution. The focus is on the early evolution of passively degassing plumes aged less than 1 h and up to tens of kilometres downwind. The model is able to reproduce the so-called “bromine explosion”: the daytime conversion of HBr into bromine radicals that continuously cycle in the plume. These forms include the radical BrO, a species whose ratio with SO2 is commonly measured in volcanic plumes as an indicator of halogen ozone-destroying chemistry. The species BrO is produced in the ambient-temperature chemistry, with in-plume BrO / SO2 ratios on the order of 10−4 mol/mol, similar to those observed previously in Etna plumes. Wind speed and time of day are identified as non-linear controls on this ratio. Sensitivity simulations confirm the importance of near-vent radical products from high-temperature chemistry in initiating the ambient-temperature plume halogen cycling. Heterogeneous reactions that activate bromine also activate a small fraction of the emitted chlorine; the resulting production of chlorine radical Cl strongly enhances the methane oxidation and hence the formation of formaldehyde (HCHO) in the plume. Modelled rates of ozone depletion are found to be similar to those derived from aircraft observations. Ozone destruction in the model is controlled by the processes that recycle bromine, with about three-quarters of this recycling occurring via reactions between halogen oxide radicals. Through sensitivity simulations, a relationship between the magnitude of halogen emissions and ozone loss is established. Volcanic halogen cycling profoundly impacts the overall plume chemistry in the model, notably hydrogen oxide radicals (HOx), nitrogen oxides (NOx), sulfur, and mercury chemistry. In the model, it depletes HOx within the plume, increasing the lifetime of SO2 and hence slowing sulfate aerosol formation. Halogen chemistry also promotes the conversion of NOx into nitric acid (HNO3). This, along with the displacement of nitrate out of background aerosols in the plume, results in enhanced HNO3 levels and an almost total depletion of NOx in the plume. The halogen–mercury model scheme is simple but includes newly identified photo-reductions of mercury halides. With this set-up, the mercury oxidation is found to be slow and in near-balance with the photo-reduction of the plume. Overall, the model findings demonstrate that halogen chemistry has to be considered for a complete understanding of sulfur, HOx, reactive nitrogen, and mercury chemistry and of the formation of sulfate particles in volcanic plumes.

The Four Stage Development of Starting Turbulent Buoyant Plumes

Turbulent heated and buoyant plumes have important applications in the atmosphere such as wildland fire plumes, volcanic plumes, and chemical plumes. The purpose of the study is to analyze the turbulence structures, and to understand the stages of the development of the starting turbulent plumes. For this purpose, data generated from an in-house Weather Research Forecast model coupled with Large-eddy simulation (WRF-bLES) with two-way feedback between the buoyant plume and the atmosphere developed has been used. The release of both dense gases (Co2, So2) and, buoyant gases (He, NH3, heated air) from a circular source at the bottom of the domain have been investigated. The simulations of the axisymmetric plume were performed at a high Reynolds number of 108. Vortex Identification methods were used to extract the Coherent structures and the large-scale features of the flow. The results have demonstrated that both the dense and the buoyant heated plumes with different initial characters exhibited universal characteristics and the development of the starting plumes occurred in four characteristic stages: Stage 1 is the plume acceleration stage, followed by stage 2 which corresponds to the formation of the head of the plume which grows spatially. Stage 3 is when the plume head is fully formed and the flow transitions to quasi-steady-state behavior. The final stage is the fully developed plume. The identification of the four-stage development of the plume in the neutral environment is the first step in studying the turbulent heated and buoyant plumes development in order to characterize realistic plumes and to quantify the extent of mixing at each of these stages. This work has important contributions to fundamental fluid dynamics of buoyant plumes with implications on forecasting the plume trajectory of smoke, wildland fire, and volcanic plumes.