[1] This study was part of the international field measurement Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006). We investigated a new particle formation event in a highly polluted air mass at a regional site south of the megacity Beijing and its impact on the abundance and properties of cloud condensation nuclei (CCN). During the 1-month observation, particle nucleation followed by significant particle growth on a regional scale was observed frequently (~30%), and we chose 23 August 2006 as a representative case study. Secondary aerosol mass was produced continuously, with sulfate, ammonium, and organics as major components. The aerosol mass growth rate was on average 19 μg m -3 h -1 during the late hours of the day. This growth rate was observed several times during the 1-month intensive measurements. The nucleation mode grew very quickly into the size range of CCN, and the CCN size distribution was dominated by the growing nucleation mode (up to 80% of the total CCN number concentration) and not as usual by the accumulation mode. At water vapor supersaturations of 0.07-0.86%, the CCN number concentrations reached maximum values of 4000-19,000 cm -3 only 6-14 h after the nucleation event. During particle formation and growth, the effective hygroscopicity parameter κ increased from about 0.1-0.3 to 0.35-0.5 for particles with diameters of 40-90 nm, but it remained nearly constant at ~0.45 for particles with diameters of ~190 nm. This result is consistent with aerosol chemical composition data, showing a pronounced increase of sulfate.

Rapid aerosol particle growth and increase of cloud condensation nucleus activity by secondary aerosol formation and condensation: A case study for regional air pollution in northeastern China

Annual review of earth and planetary sciences

Each model file is named "LLNL_G3Dv3.Interpolated.Layer{n}_{Layer Descriptor}.txt" where "n" is the layer number from the top of Earth's surface to the core-mantle boundary. The "Layer Descriptor" often contains just the name of the layer (for example: "Lower_Crust_Bottom"). Others will have the depth if the model were spherical (for example: “Lower_Mantle_1271km"). In addition, some descriptors will have the nominal depth preceded by the letter "a" (for example: "Transition_Zone_a660km_topside"). The "a" means "about" since these are actually undulating layers.

LLNL-G3Dv3: global P-wave tomography model for improved regional and teleseismic travel time prediction

As subducting plates reach the base of the upper mantle, some appear to flatten and stagnate, while others seemingly go through unimpeded. This variable resistance to slab sinking has been proposed to affect long-term thermal and chemical mantle circulation. A review of observational constraints and dynamic models highlights that neither the increase in viscosity between upper and lower mantle (likely by a factor 20–50) nor the coincident endothermic phase transition in the main mantle silicates (with a likely Clapeyron slope of –1 to –2 MPa/K) suffice to stagnate slabs. However, together the two provide enough resistance to temporarily stagnate subducting plates, if they subduct accompanied by significant trench retreat. Older, stronger plates are more capable of inducing trench retreat, explaining why backarc spreading and flat slabs tend to be associated with old-plate subduction. Slab viscosities that are ∼2 orders of magnitude higher than background mantle (effective yield stresses of 100–300 MPa) lead to similar styles of deformation as those revealed by seismic tomography and slab earthquakes. None of the current transition-zone slabs seem to have stagnated there more than 60 m.y. Since modeled slab destabilization takes more than 100 m.y., lower-mantle entry is apparently usually triggered (e.g., by changes in plate buoyancy). Many of the complex morphologies of lower-mantle slabs can be the result of sinking and subsequent deformation of originally stagnated slabs, which can retain flat morphologies in the top of the lower mantle, fold as they sink deeper, and eventually form bulky shapes in the deep mantle.

Subduction-transition zone interaction: A review

Reconstructing Greater India : Paleogeographic, kinematic, and geodynamic perspectives

Subducting-slab transition-zone interaction: Stagnation, penetration and mode switches

Seismic studies show that some subducting slabs penetrate straight into the lower mantle, whereas others seem to flatten near the base of the mantle transition zone. Slab stagnation is often attributed to an increase in viscosity and phase transformations in the olivine system. However, recent mineral physics studies showed that due to extremely low transformational diffusion rates, low-density metastable pyroxene may persist into the transition zone in cool slabs. Here we use a dynamically fully self-consistent subduction model to investigate the influence of metastable pyroxene on the dynamics of subducting oceanic lithosphere. Our results show that metastable pyroxene affects slab buoyancy at least as much as olivine metastability. However, unlike metastable olivine, which can inhibit slab penetration in the lower mantle only for cold, old, and fast slabs, metastable pyroxene is likely to also affect sinking of relatively young and slow slabs.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014GL062159

The effect of metastable pyroxene on the slab dynamics

The particle-in-cell method is generally considered a flexible and robust method to model the geodynamic problems with chemical heterogeneity. However, velocity interpolation from grid points to particle locations is often performed without considering the divergence of the velocity field, which can lead to significant particle dispersion or clustering if those particles move through regions of strong velocity gradients. This may ultimately result in cells void of particles, which, if left untreated, may, in turn, lead to numerical inaccuracies. Here we apply a two-dimensional conservative velocity interpolation (CVI) scheme to steady state and time-dependent flow fields with strong velocity gradients (e.g., due to large local viscosity variation) and derive and apply the three-dimensional equivalent. We show that the introduction of CVI significantly reduces the dispersion and clustering of particles in both steady state and time-dependent flow problems and maintains a locally steady number of particles, without the need for ad hoc remedies such as very high initial particle densities or reseeding during the calculation. We illustrate that this method provides a significant improvement to particle distributions in common geodynamic modeling problems such as subduction zones or lithosphere-asthenosphere boundary dynamics.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015GC005824

Advantages of a conservative velocity interpolation (CVI) scheme for particle‐in‐cell methods with application in geodynamic modeling

Two-dimensional simulations using a thermomechanical model based on a finite-difference method on a staggered grid and a marker in cell method are performed to study the plume-lithosphere interaction beneath moving plates. The plate and the convective mantle are modelled as a homogeneous peridotite with a Newtonian temperature- and pressure-dependent viscosity. A constant velocity, ranging from 5 to 12.5 cm yr−1, is imposed at the top of the plate. Plumes are generated by imposing a thermal anomaly of 150 to 350 K on a 50 km wide domain at the base of the model (700 km depth); the plate atop this thermal anomaly is 40 Myr old. We analyse (1) the kinematics of the plume as it impacts the moving plate, (2) the dynamics of time-dependent small-scale convection (SSC) instabilities developing in the low-viscosity layer formed by spreading of hot plume material at the base of the lithosphere and (3) the resulting thermal rejuvenation of the lithosphere. The spreading of the plume material at the base of the lithosphere, characterized by the ratio between the maximum down- and upstream horizontal (dimensionless) velocities in the plume-fed sublithospheric layer, Peup/Pedown depends on the ratio between the maximum plume upwelling velocity and the plate velocity, Peplume/Peplate. For fast plate velocities and sluggish plumes (low Peplume/Peplate), plate motion drags most plume material and downstream flow is dominant. As Peplume/Peplate increases, an increasing part of the plume material flows upstream. SSC systematically develops in the plume-fed sublithospheric layer, downstream from the plume. Onset time of SSC decreases with the Rayleigh number. For vigorous plumes, it does not depend on plate velocity. For more sluggish plumes, however, variations in the plume spreading behaviour at the base of the lithosphere result in a decrease in the onset time of SSCs with increasing plate velocity. In any case, SSC results in uplift of the isotherm 1573 K by up to 20 km relative to its initial equilibrium depth at the impact point.

https://hal.archives-ouvertes.fr/hal-00856763/document

Roberto Agrusta

Papers

Subduction-transition zone interaction: A review

Subducting-slab transition-zone interaction: Stagnation, penetration and mode switches

The effect of metastable pyroxene on the slab dynamics

Advantages of a conservative velocity interpolation (CVI) scheme for particle‐in‐cell methods with application in geodynamic modeling

Small-scale convection in a plume-fed low-viscosity layer beneath a moving plate