Plume Dispersion Research Articles

On the Convergence and Capability of the Large-Eddy Simulation of Concentration Fluctuations in Passive Plumes for a Neutral Boundary Layer at Infinite Reynolds Number

Large-eddy simulation (LES) experiments have been performed using the Parallelized LES Model (PALM). A methodology for validating and understanding LES results for plume dispersion and concentration fluctuations in an atmospheric-like flow is presented. A wide range of grid resolutions is shown to be necessary for investigating the convergence of statistical characteristics of velocity and scalar fields. For the scalar, the statistical moments up to the fourth order and the shape of the concentration probability density function (p.d.f.) are examined. The mean concentration is influenced by grid resolution, with the highest resolution simulation showing a lower mean concentration, linked to larger turbulent structures. However, a clear tendency to convergence of the concentration variance is observed at the two higher resolutions. This behaviour is explained by showing that the mechanisms driving the mean and the variance are differently influenced by the grid resolution. The analysis of skewness and kurtosis allows also the obtaining of general results on plume concentration fluctuations. Irrespective of grid resolution, a family of Gamma p.d.f.s well represents the shape of the concentration p.d.f. but only beyond the peak of the concentration fluctuation intensity. In the early plume dispersion phases, the moments of the p.d.f. are in good agreement with those generated by a fluctuating plume model. To the best of our knowledge, our study demonstrates for the first time that, if resolution and averaging time are adequate, atmospheric LES provides a trustworthy representation of the high order moments of the concentration field, up to the fourth order, for a dispersing plume.

Quantifying gas emissions associated with the 2018 rift eruption of Kīlauea Volcano using ground-based DOAS measurements

Starting on 3 May 2018, a series of eruptive fissures opened in Kīlauea Volcano’s lower East Rift Zone (LERZ). Over the course of the next 3 months, intense degassing accompanied lava effusion from these fissures. Here, we report on ground-based observations of the gas emissions associated with Kīlauea’s 2018 eruption. Visual observations combined with radiative transfer modeling show that ultraviolet light could not efficiently penetrate the gas and aerosol plume in the LERZ, complicating SO2 measurements by differential optical absorption spectroscopy (DOAS). By applying a statistical method that integrates a radiative transfer model with the DOAS retrievals, we were able to calculate sulfur dioxide (SO2) emission rates along with estimates of their uncertainty. We find that sustained SO2 emissions were highest in June and early July, when approximately 200 kt SO2 were emitted daily. At the 68% confidence interval, we estimate that 7.1–13.6 Mt SO2 were released from the LERZ during the entire May to September eruptive episode. Scaling our results with in situ measurements of plume composition, we calculate that 11–21 Mt H2O and 1.5–2.8 Mt CO2 were also emitted. The gas and aerosol emissions caused hazardous conditions in areas proximal to the active vents, but plume dispersion modeling shows that the eruption also significantly impacted air quality hundreds of kilometers downwind. Combined with petrologic studies of the erupted lavas, our measurements indicate that 1.1–2.3 km3 dense-rock equivalent of lava were erupted from the LERZ, which is approximately twice the concomitant collapse volume of the volcano’s summit.

Historical smelting activities in Eastern Canada revealed by Pb concentrations and isotope ratios in tree rings of long-lived white cedars (Thuja occidentalis L.)

Mining activities at Duparquet in Western Quebec (Canada) have significantly affected the local environment and left behind significant amounts of metals. Monitoring this contamination is essential to infer its past and present impacts on environmental quality and to evaluate the resulting human exposure. In that context, we measured long time series of Pb concentrations and their corresponding stable isotope ratios in long-lived white cedars (Thuja occidentalis L.) growing at Duparquet Lake in order to evaluate potential time variations of the Pb environmental contamination as well as to identify the responsible source(s). Results show that before 1950, Pb at Duparquet is mostly terrigenous. Lead concentrations rapidly increase afterwards. A simultaneous shift to lower 206Pb/207Pb ratios identifies the smelting of Abitibi ores as the source of contamination. An isotope mass balance model evaluates at roughly 7.5–20%, 5–40%, 5–9% and <3% the Pb contributions from local smelters at distances of 3.6, 3.9, 7 and 9 km, respectively. The dispersion of the Pb contamination plume is possibly driven by the distance from the Beattie smelter. We finally estimated a delay of at least 13 years between atmospheric emissions from the Beattie smelting activities and the time they are recorded by tree rings. Ultimately, this study demonstrates that white cedar tree rings series provide reliable archives of past and present Pb atmospheric contamination.

Effective flow and transport properties of heterogeneous unsaturated soils

The heterogeneity of field scale soils poses a challenge to predictive large scale flow and transport modeling. The theory of effective macroscale parameters holds good and is applicable in dealing with such problems. But the validity of the analytic stochastic solutions obtained for randomly heterogeneous soils is debatable, as the test cases under which they are validated are of limited scope due to linearization and perturbation approximations. In this study, samples of heterogeneous soils are generated using sets of spatially correlated random field parameters that are either geometrically isotropic, or else, geometrically anisotropic with either horizontal or vertical stratification (perfect or imperfect). Several combinations of ratios of correlation length and capillary dispersion lengths are considered. Numerical simulations of unsaturated flow are performed on each randomly heterogeneous soil sample. The principal components K^ii(Ψ) of the macroscale effective unsaturated conductivity are then obtained as a function of the mean suction Ψ of the sample. They are compared to stochastic spectral perturbation theory, and to a probabilistic semi-empirical Power Average Model (PAM). They are also compared with arithmetic, geometric and harmonic mean conductivity-suction curves. The numerically upscaled principal conductivity curves match quite well the PAM, better than the classical means (Arithmetic, Geometric, Harmonic), and also somewhat better than the curves obtained from stochastic spectral perturbation theory. It is observed that the upscaled principal components Kii(ψ), obtained numerically and with the PAM along directions “i” orthogonal/parallel to perfect stratification coincide with the harmonic/arithmetic mean curves at low suctions (i.e., near saturation), but deviate from it and come closer to the geometric mean at higher suctions. The PAM appears suitable for generation of approximate upscaled conductivity curves, e.g., for obtaining the mesh-scale or block-scale conductivity curves in large scale simulation codes. Transient solute transport simulations are then performed on the detailed random velocity fields obtained from the steady state simulations of unsaturated flow in the randomly heterogeneous soil samples. Snapshots of solute concentration C(x,z,t) are taken at different times. The temporal evolution of spatial moments of concentration is analyzed in order to characterize the macroscale advection and dispersion of the unsaturated concentration plume, and in particular, its macro-dispersion coefficient (D) and dispersivity length scale (A). For the synthetic soil samples considered in this study, the macro-dispersive spreading of the solute is stronger for flow parallel to vertical stratification, compared to flow perpendicular to horizontal stratification, and also, compared to flow in statistically isotropic non-stratified soil.

Air Pollution Dispersion Modeling in Port Areas

For the last couple of decades, environmental protection awareness within port areas is gaining ever more importance. Ports can have a tremendous impact on the environment, especially in terms of air pollution. The main pollution sources are various port activities such as road and rail traffic, cargo handling and marine vessel operations. Air quality models can be of great help in estimating the effect on the ambient air quality from one or more sources emitting pollutants to the atmosphere. One of those models is the widely used Gaussian Plume dispersion approach. Based on existing measurements and port activity data, models can simulate the dispersion of air pollutants caused by activities and operations taking place within the port. By using historical data, they can simulate the current state of the air quality in the port and with the help of weather predictions simulate possible future situation. Simulations can assist the port manager/operator in the decision-making process in order to optimize various activities within the port and minimize their impact on the environment. One of the main objectives of the Horizon 2020 Project PIXEL (Port IoT for environmental leverage) is the deployment of environmental pollution models which can aid in the decision-making processes within the port domain. This paper reviews the current advances in the field of air pollution modelling with a special emphasis on port scenarios.

Investigating the Large-Scale Transport of a Volcanic Plume and the Impact on a Secondary Site

Volcanic plumes can be transported across vast distances and can have an impact on solar ultraviolet radiation (UVR) reaching the surface due to the scattering and absorption caused by aerosols. The dispersion of the volcanic plume from the Puyehue-Cordón Caulle volcanic complex (PCCVC) eruption was investigated to determine the effect on aerosol loading at Cape Point, South Africa. The eruption occurred on 4 June 2011 and resulted in a plume reaching a height of between 9 and 12 km and was dispersed across the Southern Hemisphere. Satellite sulphur dioxide (SO2) observations and a dispersion model showed low concentrations of SO2 at the secondary site. However, satellite observations of volcanic ash and ground-based aerosol measurements did show increases between 10 and 20 June 2011 at the secondary site. Furthermore, there was good agreement with the dispersion model results and observations from satellites with most of the plume located between latitudes 40°–60° South.

Long-term and comparative impacts of combined sewers and municipal effluents to freshwater mussels

Excess rainfall events could lead to overflows and combined sewer overflows, which could threaten local mussel populations. This study sought to compare the long- term effects of combined sewer overflows and treated municipal effluents in caged Elliptio complanata mussels. Mussels were caged at 2 overflow sites, one downstream site of a major municipal effluent dispersion plume and a reference upstream site for 3 months during the summer. At the end of the exposure period, mussels were collected, analyzed for municipal contaminants (including pharmaceuticals), and effects biomarkers based on endocrine disruption (vitellogenin expression), xenobiotic detoxification (glutathione S-transferase and metallothioneins), oxidative stress/inflammation (cyclooxygenase and lipid peroxidation) and DNA damage. The data revealed that surface waters contained less pharmaceutical products than the downstream site but atrazine and its metabolite were at higher levels in overflow sites. Mussels contained elevated amounts of total heterotrophic bacteria, caffeine, acebutolol and venlafaxine at the downstream site relative to the upstream site where caffeine was higher at one of the overflow site. The levels of vitellogenin gene expression were significantly increased in both sexes of mussels caged at the downstream site only. Multivariate analysis revealed that the biomarker responses were completely separated between upstream, overflow and downstream sites. The site discrimination was based on vitellogenin, metallothioneins, DNA damage in gonad and digestive gland, gonad lipids/proteins reserves, lipid peroxidation, gonado-somatic index and condition factor. Adverse outcome pathways analysis using the power law approach revealed that most changes in the biomarkers identified by discriminant function analysis were significantly scaled to gonad energy reserves, tissue/ mussel size and loss of weight following air emersion stress. In conclusion, the toxic effects of mussels caged at overflow sites generally displayed lower responses than mussels caged downstream a treated municipal effluent suggesting that these overflows pose a lower risk than the continuous exposure to treated municipal effluent.

Modeling lateral plume deflection in the wake of an elongated building.

The plume dispersion model AERMOD provides an efficient method for modeling ground-level pollutant concentrations in wakes of buildings. In recent years, several studies have shown that the downwash algorithms within AERMOD often perform poorly in certain applications. Some studies have proposed modifications to the downwash algorithm in AERMOD to bring the model closer to representing the underlying physical processes associated with building downwash and closer to more accurately modeling observed pollutant concentrations. One such study by Monbureau et al. (2018) made changes to the model that significantly improved its ability to model ground level concentrations for a simple case of a single rectangular building with an elevated, effluent-emitting stack experiencing winds perpendicular to the upwind side of the building. The present study introduces a simple algorithm to enhance AERMOD's ability to appropriately match the dispersion pattern in the complex flow case of non-orthogonal winds. This algorithm, which is based on a rich set of Large-Eddy Simulations (LES), applies to a variety of building dimensions, stack locations, and stack heights. A sensitivity analysis demonstrates how additional modifications to the downwash algorithm may further improve AERMOD in modeling the spatial location of observed ground-level effluent.

PRCI ambient NO2 AERMOD performance assessment and model improvement project: Modeled to observed comparison

ABSTRACT On January 22, 2010, the U.S. Environmental Protection Agency (EPA) established a new one-hour probabilistic National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide (NO2) at 100 parts per billion (ppb) or approximately 188 micrograms per cubic meter (µg/m3). The 1-hr NO2 NAAQS is considerably more stringent than the longstanding annual standard of 53 ppb or approximately 100 µg/m3. AERMOD is the EPA recommended dispersion modeling tool for predicting impacts from emission sources, including compressor station reciprocating engines. Existing datasets were previously reviewed to evaluate and improve AERMOD 1-hr NO2 model performance. However, these datasets were not representative due to either an absence of accurate concurrent hourly emission inventories, being associated with larger electric generating units, shorter duration tracer studies, or were too site-specific (e.g., external influences such as terrain). To address this data gap, an intensive 13-month field data program was undertaken by Pipeline Research Council International (PRCI) to collect a comprehensive dataset including concurrent emissions, meteorological, and ambient ozone and nitrogen oxide measurements. This paper describes the Balko, OK natural gas compressor station host site and the design of the meteorological, ambient NOx and O3 monitors and continuous Parametric Emissions Monitoring System (PEMS). Two comparisons are made between the monitored data and AERMOD results. First, the measured onsite data (ambient air, meteorological, and emissions) were used as input to AERMOD to make comparisons between measured and modeled NOx and NO2. These comparisons showed that NOx was both over and under predicted, sometimes beyond a factor-of-two. The second comparison uses the typical permitting approach where onsite data are not available to contrast how conservative input assumptions for emissions, background O3, NO2, and offsite meteorology cause significant over predictions especially for this site where engines have a low annual utilization rate and operate infrequently throughout the year. Implications: This project resulted following the promulgation of the new 1-hr NO2 NAAQS and the challenges that natural gas pipeline compressor stations have in modeling intermittent emissions using AERMOD to demonstrate compliance with this standard. AERMOD was developed and validated with a primary focus on large sources with tall stacks, such as electric utility boilers. A recently collected dataset at a gas compressor station in Balko, Oklahoma shows that the use of permitted allowable emission values compared to actual hourly emission values causes over predictions using typical regulatory methods. Comparison of model results using actual hourly emission values to coincident ambient air monitoring values at the site shows that AERMOD over predicts near-field impacts due to simplifying assumptions including building downwash, plume dispersion, chemistry, and 1-hr invariant meteorological conditions.

Methane emissions from the Munich Oktoberfest

Abstract. This study presents the first investigation of the methane (CH4) emissions of a large festival. Munich Oktoberfest, the world's largest folk festival, is a potential source of CH4 as a large amount of natural gas for cooking and heating is used. In 2018 we measured the CH4 emissions of Oktoberfest using in situ measurements combined with a Gaussian plume dispersion model. Measurements were taken while walking and biking around the perimeter of the Oktoberfest premises (Theresienwiese) at different times of the day, during the week and at the weekend. The measurements showed enhancements of up to 100 ppb compared to background values and measurements after Oktoberfest. The average emission flux of Oktoberfest is determined as (6.7±0.6) µg (m2 s)−1. Additional analyses, including the daily emission cycle and comparisons between emissions and the number of visitors, suggest that CH4 emissions of Oktoberfest are not due solely to the human biogenic emissions. Instead, fossil fuel CH4 emissions, such as incomplete combustion or loss in the gas appliances, appear to be the major contributors to Oktoberfest emissions. Our results can help to develop CH4 reduction policies and measures to reduce emissions at festivals and other major events in cities. Furthermore, events with a limited duration have not yet been included in the state-of-the-art emission inventories, such as TNO-MACC, EDGAR or IER. Our investigations show that these emissions are not negligible. Therefore, these events should be included in future emission inventories.

Measurement and modelling of deep sea sediment plumes and implications for deep sea mining

Deep sea mining concerns the extraction of poly-metallic nodules, cobalt-rich crusts and sulphide deposits from the ocean floor. The exploitation of these resources will result in adverse ecological effects arising from the direct removal of the substrate and, potentially, from the formation of sediment plumes that could result in deposition of fine sediment on sensitive species or entrainment of sediment, chemicals and nutrients into over-lying waters. Hence, identifying the behaviour of deep-sea sediment plumes is important in designing mining operations that are ecologically acceptable. Here, we present the results of novel in situ deep sea plume experiments undertaken on the Tropic seamount, 300 nautical miles SSW of the Canary Islands. These plume experiments were accompanied by hydrographic and oceanographic field surveys and supported by detailed numerical modelling and high resolution video settling velocity measurements of the in situ sediment undertaken in the laboratory. The plume experiments involved the controlled formation of benthic sediment plumes and measurement of the plume sediment concentration at a specially designed lander placed at set distances from the plume origin. The experiments were used as the basis for validation of a numerical dispersion model, which was then used to predict the dispersion of plumes generated by full-scale mining. The results highlight that the extent of dispersion of benthic sediment plumes, resulting from mining operations, is significantly reduced by the effects of flocculation, background turbidity and internal tides. These considerations must be taken into account when evaluating the impact and extent of benthic sediment plumes.

Plume dynamics and dispersion characteristics in oil horizontal release from damaged submarine pipeline

The dynamics model of plume for underwater oil spill is improved by considering the drag force of the plume. The comparisons between prediction and experiment show that the improved model is viable. The improved model combining the Lagrangian random walk method is used to simulate the entire oil transport process from release exit to water surface in underwater oil horizontal release and some new results are obtained. Firstly, the release rate, the current velocity and its direction have evident influence on plume dynamics. The plume length increases as release flow rate increases. For small oil release rate, the plume length is very short so that the plume stage can be ignored due to the small initial momentum. When the oil release in direction is the same as the current, the current will play a positive role in the plume motion and the plume will therefore extend a longer distance. In this case, the greater the current velocity is, the longer the plume extension length will be. And when the oil release in direction is opposite to the current, the plume first moves forward and then is reversed to move along the opposite direction. The position of the reversal point depends on the current velocity and the release rate. Secondly, the release rate and the current velocity also have a significant effect on oil droplet diffusion. For small oil release rate, the oil distribution area is narrow and the concentration near upper side of droplets moving path is relatively higher. As the release rate or current velocity increases, the oil droplet diffusion area expands away from the release point. Finally, the movement characteristics of oil droplets in the plume and diffusion stages will affect the time and location for oil droplets reaching the surface. In a small release rate as well as a small current velocity, the time for oil droplet reaching surface is very short and the horizontal position reaching surface is closer to release point. In this case, the time spent in the plume stage is extremely short compared to the time spent in the entire oil motion process. Therefore, the plume dynamics stage can be ignored in this case. With the increase of release rate or current velocity, the plume will be extended and the time spent in plume dynamics stage will also be longer, so the plume dynamics stage can no longer be ignored for larger release rate or current velocity. In the case of reverse current, as the plume momentum be partially consumed by the reverse current, the plume is reversed and decays quickly, the time taken in the plume dynamics stage is relatively shorter than that for positive current. Correspondingly, the total time is also somewhat shortened. The shape of the plume depends on the oil release rate, current velocity and direction.

Effects of exposing shrimp larvae (Pandalus borealis) to aquaculture pesticides at field relevant concentrations, with and without food limitation

Anti-parasitic drugs used in the aquaculture industry are discharged to the sea after treatment of salmon. In this study, the effects of azamethiphos (AZA) in the Salmosan® formulation and deltamethrin (DEL) in the Alpha Max® formulation, have been assessed in Northern shrimp larvae (Pandalus borealis) when administered both separately and in combination. The exposure concentrations were 100 ng/L for AZA and 2 ng/L for DEL, each representing a 1000-fold dilution of the prescribed concentrations for salmon. These two chemicals were combined at these concentrations to give a third treatment (AZA + DEL). When larvae were exposed for two hours on the first, second and third days post hatch (dph), significantly increased mortality and reduced swimming activity were observed for larvae from the DEL and combined AZA + DEL treatments 4 dph, though not in larvae from the AZA treatment. A single pulse exposure, delivered on the first day post hatch, caused similar effects on mortality and swimming activity 4 dph as the three-pulse exposure. Mortality was driven by the presence of DEL in both experiments, with no amplification or reduction of effects observed when DEL and AZA were combined. Larvae were observed for 13 days following the single pulse exposure, with food limitation introduced as an additional stressor on day 4. In the DEL and AZA + DEL treatments mortality continued to increase regardless of food level, with no larvae completing development to stage II. The overriding toxicity of DEL masked any potential effects the reduced food ration may have exerted. Swimming activity was lower for AZA treated larvae than Control larvae 13 dph, when both groups were fed daily, though no other significant changes to mortality, development to stage II, feeding rate or gene expression were observed. Food limited Control and AZA larvae had lower swimming activity and feeding rate than daily fed Control larvae, with expression of pyruvate kinase and myosin genes also downregulated. However, there was no negative effect on survival or successful development to stage II in these treatments. In addition, mesencephalic astrocyte-derived neurotropic factor was downregulated in food limited Control larvae when compared with the daily fed Controls.Results from this study together with reported estimates of dispersion plume concentrations of discharged pesticides indicate that toxic concentrations of deltamethrin could reach shrimp larvae several kilometers from a treated salmon farm.

Dispersion and Intersection of Hydrothermal Plumes in the Manus Back-Arc Basin, Western Pacific

The composition of hydrothermal plumes reflects the physical and chemical characteristics of seafloor hydrothermal fluids, which in turn reflects the host rock and subseafloor reaction conditions as well as the water column processes that act to alter the plumes as they disperse and age. Here, we show that the turbidity, current, pH value, dissolved Fe (dFe), and dissolved Mn (dMn) compositions of hydrothermal plumes can be used to understand the spatial distribution and source of hydrothermal systems in the submarine geological environment. Data were obtained from 18 hydrocast stations, among which the water column samples were collected at 8 stations during the MANUS cruise of R/V KEXUE in 2015. The results showed that the Satanic Mills plume and Fenway plume rose approximately 140 m and 220 m above the seafloor, respectively. In the Satanic Mills plume, dFe remained longer than dMn during lateral plume dispersal. There was a clear intersection of the Satanic Mills plume and Fenway plume between 1625 m and 1550 m in the PACMANUS hydrothermal field, and the varied dispersion trends of the mixed plumes were affected by current velocities at different depths. The physical and chemical properties of the seawater columns in the Manus Basin were affected by the input of high-Mn, high-Fe, and low-Mg vent fluids. The turbidity and dFe, dMn, and dissolved Mg concentrations in the sections of the plumes proximal to the Satanic Mills, Fenway, and Desmos vent sites were generally higher (turbidity, Mn, and Fe) and lower (Mg) than those in the sections of the plumes that were more distal from the vent sites. This implied that the plumes proximal to their vent fluid sources, which were interpreted to have relatively young ages, dispersed chemically over time, and their concentrations became more similar to those of the plumes that were more distal from their vent fluid sources.

Numerical simulations of the flow field and pollutant dispersion in an idealized urban area under different atmospheric stability conditions

This study simulated the flow and near-field plume dispersion in an urban-like environment under unstable, neutral and stable atmospheric stratification using the steady Reynolds-averaged Navier-Stokes (RANS) methodology. First, a validation study for two trials of the Mock Urban Setting Test (MUST) experiments is performed to examine the predictive performance of the computational fluid dynamics (CFD) model, Fluidyn-PANACHE. The effects of atmospheric stability on the flow structure in street canyons under perpendicular incident flow conditions are investigated. In addition, the patterns of urban dispersion in different cases of stability are also analysed under perpendicular and oblique wind direction conditions. The results show that in the urban environment, the influence of atmospheric stability on the canyon vortex intensity, flow structure and plume dispersion is apparent; intense thermal turbulence enhances the vortex intensity and plume dilution in the street canyon under unstable conditions; when the atmospheric conditions are stable, the vertical profile of the streamwise velocity is significantly decreased by the obstacles, and the concentration level and spread of pollutants increase in the street canyon due to relatively weak turbulent motions; plume deflection within the obstacle array is noteworthy when the incident flow is oblique; in particular, the transport of the plume is basically independent of the wind direction very near the ground.

Quantifying the Uncertainty of a Coupled Plume and Tephra Dispersal Model: PLUME‐MOM/HYSPLIT Simulations Applied to Andean Volcanoes

AbstractNumerical modeling of tephra dispersal and deposition is essential for evaluation of volcanic hazards. Many models consider reasonable physical approximations in order to reduce computational times, but this may introduce a certain degree of uncertainty in the simulation outputs. The important step of uncertainty quantification is dealt in this paper with respect to a coupled version of a plume model (PLUME‐MoM) and a tephra dispersal model (HYSPLIT). The performances of this model are evaluated through simulations of four past eruptions of different magnitudes and styles from three Andean volcanoes, and the uncertainty is quantified by evaluating the differences between modeled and observed data of plume height (at different time steps above the vent) as well as mass loading and grain size at given stratigraphic sections. Different meteorological data sets were also tested and had a sensible influence on the model outputs. Other results highlight that the model tends to underestimate plume heights while overestimating mass loading values, especially for higher‐magnitude eruptions. Moreover, the advective part of HYSPLIT seems to work more efficiently than the diffusive part. Finally, though the coupled PLUME‐MoM/HYSPLIT model generally is less efficient in reproducing deposit grain sizes, we propose that it may be used for hazard map production for higher‐magnitude eruptions (sub‐Plinian or Plinian) for what concern mass loading.

Ash sedimentation by fingering and sediment thermals from wind-affected volcanic plumes

Ash fallout and volcanic plume dispersion represent critical hazards for local and global human populations for minutes to years after the onset of an eruption. Understanding the key processes governing the sedimentation of ash particles is a major challenge in modern volcanology from modeling and risk management perspectives. Recent experiments predict that sedimentation from eruption clouds rich in fine-grained ash can be driven by convective phenomena in the form of ∼100 m to km-scale ash fingers related to the intermittent formation and detaching of ash-rich particle boundary layer. Remote sensing observations of mammatus and cloud veils from numerous eruptions over recent years as well as field observations of spatially discontinuous ash deposits are consistent with this prediction. Indeed, this mode of ash sedimentation is predicted to be the predominant mechanism of fine ash removal for a significant fraction of eruptions in the geological records. Here, we use a novel combination of 3 millimeter-wavelength Doppler radar observations and time series of optical disdrometer data to characterize for the first time in real-time the time-varying structure and sedimentation properties of volcanic ash plumes under steady wind conditions. As a case study, we apply this new method to weak short-lived plumes from Stromboli Volcano. 96% of the disdrometer proximal sedimentation data highlight pulsatory phases of increased sedimentation rate that are 20–60 s apart and characterized by particle size distribution variation with bulk concentrations up to 681 mg/m3. Radar data also record intermittent periods of higher reflectivity (i.e. a factor of 3 in mass concentration) inside the ash sedimentation interspersed by 30 to 50 s and interpreted as ash fingers crossing the radar beam. From time series of radar signals and ground-based disdrometer measurements, together with simple analog experiments, we develop a conceptual model for intermittent sedimentation from wind-affected ash plumes. In particular, we show that when wind speeds are comparable to or greater than ash settling velocities, the dynamics of wind-driven rolls in ash clouds can govern the production of gravitational instabilities and the occurrence and timing of ash fingers that form descending sediment thermals in turn. This study suggests that ambient wind should affect the maximum size and minimum concentration of ash particles needed to form fingers but also control where and when fingers form at the base of wind-drifted ash plumes. These novel predictions highlight the need for future work aimed at refining our understanding of ash finger formation under windy conditions from the perspectives of in-situ characterization, numerical modeling and risk assessment of fine ash dispersal from the most frequent style of eruptions on Earth.

Australian Lidar Measurements of Aerosol Layers Associated with the 2015 Calbuco Eruption

The Calbuco volcano in southern Chile (41.3° S, 72.6° W) underwent three separate eruptions on 22–23 April 2015. Following the eruptions, distinct layers of enhanced lidar backscatter at 532 nm were observed in the lower stratosphere above Buckland Park, South Australia (34.6° S, 138.5° E), and Kingston, Tasmania (43.0° S, 147.3° E), during a small set of observations in April–May 2015. Using atmospheric trajectory modelling and measurements from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) space-borne lidar and the Ozone Mapping Profiler Suite (OMPS) instrument on the Suomi National Polar-orbiting Partnership (NPP) satellite, we show that these layers were associated with the Calbuco eruptions. Buckland Park measurements on 30 April and 3 May detected discrete aerosol layers at and slightly above the tropopause, where the relative humidity was well below saturation. Stratospheric aerosol layers likely associated with the eruptions were observed at Kingston on 17 and 22 May in narrow discrete layers accompanied by weaker and more vertically extended backscatter. The measurements on 22 May provided a mean value of the particle linear depolarisation ratio within the main observed volcanic aerosol layer of 18.0 ± 3.0%, which was consistent with contemporaneous CALIOP measurements. The depolarisation measurements indicated that this layer consisted of a filament dominated by ash backscatter residing above a main region having likely more sulfate backscatter. Layer-average optical depths were estimated from the measurements. The mean lidar ratio for the volcanic aerosols on 22 May of 86 ± 37 sr is consistent with but generally higher than the mean for ground-based measurements for other volcanic events. The inferred optical depth for the main volcanic layer on 17 May was consistent with a value obtained from OMPS measurements, but a large difference on 22 May likely reflected the spatial inhomogeneity of the volcanic plume. Short-lived enhancements of backscatter near the tropopause of 17 May likely represented the formation cirrus that was aided by the presence of associated volcanic aerosols. We also provide evidence that gravity waves potentially influenced the layers, particularly in regard to the vertical motion observed in the strong layer on 22 May. Overall, these observations provide additional information on the dispersal and characteristics of the Calbuco aerosol plumes at higher southern latitudes than previously reported for ground-based lidar measurements.

Large-Eddy Simulations with an Immersed Boundary Method: Pollutant Dispersion over Urban Terrain

In urban canopies, the variability of pollution may be influenced by the presence of surface heterogeneities like orography and buildings. Using the Meso-NH model enhanced with an immersed boundary method (IBM) to represent accurately the impact of the 3D shape of buildings on the flow, large-eddy simulations are performed over city of Toulouse (France) with the dispersion of a plume following a plant explosion on 21 September 2001. The event is characterized by a large quantity of nitrogen dioxide released in a vertical column after the explosion, quickly dispersed by a moderate wind prevailing in the lower atmospheric layers. Assuming a passive pollutant, the model develops a realistic plume dispersion. A sensitivity analysis of the advection scheme to the spread is presented. The limited population’s exposure to pollution developed by the model appears in good agreement with previous health studies. Beyond this case, IBM is a promising way to represent flow interaction with buildings and orography in atmospheric models for urban applications.

Influence of forest fire episodes on the distribution of gaseous air pollutants over Uttarakhand, India

ABSTRACTWidespread forest fire events occurred in the foothills of North Western Himalaya during 24 April to 2 May 2016 (Event-1) and 20–30 May 2018 (Event-2). Their impacts were investigated on the distribution of pollutant gases ozone (O3), carbon monoxide (CO), and oxides of nitrogen (NOx) over Uttarakhand using simulations of Weather Research and Forecasting model coupled with chemistry (WRF-Chem) and in-situ observations of these gases over Dehradun, the capital of Uttarakhand. During Event-1, the observed CO mixing ratio over Dehradun increased from 25 April 2016 onwards, attained maximum (705.8 ± 258 ppbv) on 2 May 2016 and subsequently decreased. The rate of increase of daily baseline CO was 29 ppbv/day during HFAP (High Fire Activity Period). During Event-2, daily average concentrations of CO, O3, and NOx showed systematic increase over Dehradun during HFAP period. The rate of increase of CO was 9 ppbv/day, while it was very small for NOx and O3. To quantitatively estimate the influence of forest fire emissions, two WRF-Chem simulations were made: one with biomass burning (BB) emissions and other without BB emissions. These simulations showed 52% (34%) enhancement in CO, 52% (32%) enhancement in NOx, and 11% (9%) enhancement in O3 during HFAP for Event-1 (Event-2). A clear positive correlation (r = 0.89 for Event-1, r = 0.69 for Event-2) was found between ∆O3 (O3with BB minus O3without BB) and ∆CO (COwith BB minus COwithout BB), indicating rapid production of ozone in the fire plumes. For both the events, the vertical distribution of ∆O3, ∆CO, and ∆NOx showed that forest fire emissions influenced the air quality upto 6.5 km altitude. Peaks in ∆O3, ∆CO, and ∆NOx during different days suggested the role of varying dispersion and horizontal mixing of fire plumes.