Detection of nitric acid (HNO<Subscript>3</Subscript>) in the atmosphere using the LOPAP technique

A new instrument (LOPAP: LOng Path liquid Absorption Photometer) for the sensitive detection of nitric acid (HNO₃) in the atmosphere is described. HNO₃ is sampled in a temperature controlled stripping coil mounted in an external sampling module to minimize sampling artefacts in sampling lines. After conversion into a strongly absorbing dye, HNO₃ is detected in long path absorption in special Teflon® AF 2400 tubes used as liquid core wave guides. For the correction of some interferences, due to for example HONO and particle nitrate, two channels are used in series. The interferences from several potential interfering compounds including particle nitrate were quantified in the laboratory and in a large outdoor simulation chamber. With the exception of the interference caused by N₂O₅, which is quantitatively measured by the instrument, all tested interferences can be corrected under atmospheric conditions. Thus, in the instrument only the sum of N(V) from HNO₃ and N₂O₅ is determined, which is expected to be a common problem of wet chemical HNO₃ instruments. The instrument has a detection limit of 5–30 pptv for a time response of 6–2 min, respectively and was validated against the FTIR technique in a large outdoor simulation chamber. In addition, the applicability of the instrument was demonstrated in a field campaign.     

Large eddy simulation of stratified mixing in a three-dimensional lock-exchange system

Accurate and computationally-efficient modeling of stratified mixing processes are of paramount importance in both coastal and large-scale ocean circulation. In this study, our main objective is to investigate the feasibility and accuracy of large eddy simulation (LES) as a possible tool to study small-scale oceanic processes. To this end, LES is evaluated in a 3D lock-exchange problem, which contains shear-driven mixing, internal waves, interactions with boundaries and convective motions, while having a simple domain, initial and boundary conditions, and forcing.Two general classes of LES models are tested, namely eddy viscosity (EV) models based on constant–coefficient and dynamic Smagorinsky models, and an approximate deconvolution (AD) model. By noting that the dynamic Smagorinsky and AD models have different strengths in that the former is good in providing appropriate dissipation while the latter in preserving the detail of coherent structures on coarse resolution meshes, a hybrid approach combining EV and AD models is also evaluated. A direct numerical simulation (DNS) is performed as the benchmark solution, and all LES models are tested on three coarse meshes. The main measure of mixing is taken as the temporal evolution of background potential energy.It is found that constant-coefficient Smagorinsky models can only provide a marginal improvement over under-resolved simulations, while both dynamic Smagorinsky and AD models lead to significant improvements in mixing accuracy. The primary accomplishment of this study is that it is shown that the hybrid approach attains the best agreement with the mixing curve from DNS, while being computationally approximately a thousand times faster.     

Messinian sea level fall in the Dacic Basin (Eastern Paratethys): palaeogeographical implications from seismic sequence stratigraphy

The signature of the Mediterranean Messinian Salinity Crisis (MSC) in the Paratethys has received wide attention because of the inferred changes in connectivity and base level. In this article, we present sequence stratigraphic interpretations on a seismic transect across the western part of the semi-isolated Late Miocene–Pliocene Dacic Basin (Eastern Paratethys, Romania), chronologically constrained by biostratigraphic field observations and well data. They reveal significant sea level changes during the middle Pontian that are coeval with the MSC. These changes were most likely transmitted to the western Dacic Basin from the downstream Black Sea and controlled by the sill height of the interconnecting gateway. During the middle Pontian lowstand of the western Dacian Basin, sedimentation continued in a remnant ∼300 m deep lake with a positive water balance. Our observations show that the evolution of semi-isolated sedimentary basins is strongly dependent on the communication with other depositional realms through its control on base level and sediment supply.     

Reynolds number dependence of mixing in a lock-exchange system from direct numerical and large eddy simulations

Turbulent mixing of water masses of different temperatures and salinities is an important process for both coastal and large-scale ocean circulation. It is, however, difficult to capture computationally. One of the reasons is that mixing in the ocean occurs at a wide range of complexity, with the Reynolds number reaching , or even higher.In this study, we continue to investigate whether large eddy simulation (LES) can be a reliable computational tool for stratified mixing in turbulent oceanic flows. LES is attractive because it can be times faster than a direct numerical simulation (DNS) of stratified mixing in turbulent flows. Before using the LES methodology to compute mixing in realistic oceanic flows, however, a careful assessment of the LES sensitivity with respect to Re needs to be performed first. The main objectives of this study are: (i) to investigate the performance of different LES models at high Re, such as those encountered in oceanic flows; and (ii) to study how mixing varies as a function of Re. To this end, as a benchmark we use the lock-exchange problem, which is described by unambigous and simple initial and boundary conditions. The background potential energy, which accurately quantifies irreversible mixing in an enclosed system, is used as the main criterion in a posteriori testing of LES.This study has two main achievements. The first is that we investigate the accuracy of six combinations of two different classes of LES models, namely eddy-viscosity and approximate deconvolution types, for 3×10³Re3×10⁴, for which DNS data is computed. We find that all LES models almost always provide significantly more accurate results than cases without LES models. Nevertheless, no single LES model that is persistently superior to others over this Re range could be identified. Then, an ensemble of the four best performing LES models is selected in order to estimate mixing taking place in this system at Re=10⁵ and 10⁶, for which DNS is presently not feasible. Thus the second achievement of this study is to quantify mixing taking place in this system over an Re range that changes by three orders of magnitude. We find that the background potential energy increases by about 67% when Re is increased from Re=10³ to Re=10⁶, within the computation period, with the most significant increase taking place from Re=3×10³ to Re=10⁵.