Vertical Transport Processes Research Articles

A numerical model for simulation of fluid mud with different rheological behaviors

In this paper, a three-dimensional isopycnal approach is presented to simulate the dynamics of fluid mud covering the formation, development, transport, and disappearance of fluid mud. The basic assumption is the assignment of the fluid’s density as the indicating parameter for the rheological behavior. Considering stable stratification, as is usually the case for fluid mud, layers of constant density discretize the vertical domain. The non-Newtonian dynamics of fluid mud is simulated by solving the Cauchy equations for general continuum dynamics. Instead of using a turbulent viscosity approach, the viscosity is allowed to vary according to the rheological behavior of mud suspensions. This apparent viscosity can be determined for different rheological formulations in dependence of the volume solid fraction and the shear rate. An existing three-dimensional isopycnal hydrodynamic model was extended for vertical mass transport processes and was applied on a schematic system with hindered settling. For including the rheological behavior of fluid mud, the Worrall–Tuliani approach was parameterized and implemented. The resulting flow behavior is shown on a model application of fluid mud layers moving down an inclined plane. With these changes, it is demonstrated that the isopycnal model is capable of simulating fluid mud dynamics.

The potential influence of Asian and African mineral dust on ice, mixed-phase and liquid water clouds

Abstract. This modelling study explores the availability of mineral dust particles as ice nuclei for interactions with ice, mixed-phase and liquid water clouds, also tracking the particles' history of cloud-processing. We performed 61 320 one-week forward trajectory calculations originating near the surface of major dust emitting regions in Africa and Asia using high-resolution meteorological analysis fields for the year 2007. Dust-bearing trajectories were assumed to be those coinciding with known dust emission seasons, without explicitly modelling dust emission and deposition processes. We found that dust emissions from Asian deserts lead to a higher potential for interactions with high ice clouds, despite being the climatologically much smaller dust emission source. This is due to Asian regions experiencing significantly more ascent than African regions, with strongest ascent in the Asian Taklimakan desert at ~25%, ~40% and 10% of trajectories ascending to 300 hPa in spring, summer and fall, respectively. The specific humidity at each trajectory's starting point was transported in a Lagrangian manner and relative humidities with respect to water and ice were calculated in 6-h steps downstream, allowing us to estimate the formation of liquid, mixed-phase and ice clouds. Downstream of the investigated dust sources, practically none of the simulated air parcels reached conditions of homogeneous ice nucleation (T≲−40 °C) along trajectories that have not experienced water saturation first. By far the largest fraction of cloud forming trajectories entered conditions of mixed-phase clouds, where mineral dust will potentially exert the biggest influence. The majority of trajectories also passed through atmospheric regions supersaturated with respect to ice but subsaturated with respect to water, where so-called "warm ice clouds" (T≳−40 °C) theoretically may form prior to supercooled water or mixed-phase clouds. The importance of "warm ice clouds" and the general influence of dust in the mixed-phase cloud region are highly uncertain due to both a considerable scatter in recent laboratory data from ice nucleation experiments, which we briefly review in this work, and due to uncertainties in sub-grid scale vertical transport processes unresolved by the present trajectory analysis. For "classical" cirrus-forming temperatures (T≲−40 °C), our results show that only mineral dust ice nuclei that underwent mixed-phase cloud-processing, most likely acquiring coatings of organic or inorganic material, are likely to be relevant. While the potential paucity of deposition ice nuclei shown in this work dimishes the possibility of deposition nucleation, the absence of liquid water droplets at T≲−40 °C makes the less explored contact freezing mechanism (involving droplet collisions with bare ice nuclei) highly inefficient. These factors together indicate the necessity of further systematic studies of immersion mode ice nucleation on mineral dust suspended in atmospherically relevant coatings.

Two-dimensional time-dependent model of the transport of minor species in the Venus night side upper atmosphere

We present a numerical tool developed to quantify the role of processes controlling the spatio-temporal distribution of the NO ultraviolet and O 2 ( Δ g 1 ) infrared nightglows in the Venus night side upper atmosphere, observed with the VIRTIS and SPICAV instruments on board Venus Express. This numerical tool consists in a two-dimensional chemical-transport time-dependent model which computes in a hypothetical rectangular solving domain the spatio-temporal distributions of the number densities of the four minor species at play in these two nightglow emissions. The coupled nonlinear system of the four partial differential equations, describing the spatio-temporal variations of the minor species, has been solved using a finite volume method with a forward Euler method for the time integration scheme. As an application, we have first simulated a time-constant supply of atoms through the upper boundary of the solving domain. The fluxes are inhomogeneous relative to its horizontal direction, in order to simulate regions of enhanced downward flow of oxygen and nitrogen giving rise to NO and O 2 brightening. Given that these two emissions show large time variations, we have also simulated a time-dependent downward flux of O and N atoms. It results from these simulations that the lack of correlation between the NO and O 2 ( Δ g 1 ) nightglows largely result from to the coupling between horizontal and vertical transport processes and the very different chemical lifetimes of the two species. In particular, we have quantified the role of each process generating spatio-temporal de-correlations between the NO and O 2 ( Δ g 1 ) nightglows.

Synthetic Convection Log — Characterization of vertical transport processes in fluid-filled boreholes

Two main types of vertical convective flows play an important role in transport along the fluid column: forced convective and free convective flows. Forced vertical convection in fluid-filled boreholes (short-circuit flow) can be detected by means of borehole measurements, e.g. different types of flowmeters, temperature logs, and fluid-logging. For detecting free vertical convection (natural convection), so far, no special logging device or interpretation algorithm was available. This paper presents a new synthetic borehole log, the so-called Synthetic Convection Log (SYNCO-Log). It enables in-situ detection and identification of free convective, including double-diffusive, flows using state-of-the-art geophysical borehole measurements. Vertical convection in fluid-filled boreholes is known to lead to transport of heat and mass. Thus, understanding free convective flow is crucial for geothermics, borehole geophysics, hydrological investigations, and meaningful fluid sampling. The SYNCO-Log is divided into two closely linked parts: (1) the cause-oriented approach compares the situation along the fluid column with critical thresholds for the onset of free convection and (2) the effect-oriented approach separates the anomalies and patterns in fluid quality that are induced by free convection. Inputs for the interpretation algorithm are simultaneously acquired temperature and mudresistivity (or fluid conductivity) logs, hydraulic pressure, and borehole diameter. Output of the algorithm is a computer generated, descriptive illustration of the results including a classified plot for delineating the type of flow. The reliability of the SYNCO-Log is high, as causes and effects, i.e. driving forces and resulting heat and mass transport, are simultaneously identified. Its applicability and the relevance of the results are shown on the example of borehole measurements from the KTB-MH deep crustal borehole, located in the Bavarian region of Germany.

Dioxins, PCBs, and HCB in soil and peat profiles from a pristine boreal catchment

The aim of this study was to explore how atmospherically derived soil pollution is affected by environmental processes at two typical boreal catchment landscape type settings: wetlands and forested areas. Measurements of hydrophobic organic compounds (HOCs) in forest soil and peat from an oligotrophic mire at various depths were performed at a remote boreal catchment in northern Sweden. HOCs in peat were evenly distributed throughout the body of the mire while levels of HOCs in the forest soil increased with increased amount of organic matter. Evaluation of HOC composition by principal component analysis (PCA) showed distinct differences between surface soils and deeper soil and peat samples. This was attributed to vertical transport, degradation and/or shifting sources over time. The calculated net vertical transport differed between surface layers (0.3%) and deeper soils (8.0%), suggesting that vertical transport conditions and processes differ in the deeper layers compared to the surface layers.

Understanding of regional air pollution over China using CMAQ, part II. Process analysis and sensitivity of ozone and particulate matter to precursor emissions

Following model evaluation in part I, this part II paper focuses on the process analysis and chemical regime analysis for the formation of ozone (O 3) and particulate matter with aerodynamic diameter less than or equal to 10 μm (PM 10) in China. The process analysis results show that horizontal transport is the main contributor to the accumulation of O 3 in Jan., Apr., and Oct., and gas-phase chemistry and vertical transport contribute to the production and accumulation of O 3 in Jul. Removal pathways of O 3 include vertical and horizontal transport, gas-phase chemistry, and cloud processes, depending on locations and seasons. PM 10 is mainly produced by primary emissions and aerosol processes and removed by horizontal transport. Cloud processes could either decrease or increase PM 10 concentrations, depending on locations and seasons. Among all indicators examined, the ratio of P HNO 3 / P H 2 O 2 provides the most robust indicator for O 3 chemistry, indicating a VOC-limited O 3 chemistry over most of the eastern China in Jan., NO x-limited in Jul., and either VOC- or NO x-limited in Apr. and Oct. O 3 chemistry is NO x-limited in most central and western China and VOC-limited in major cities throughout the year. The adjusted gas ratio, AdjGR, indicates that PM formation in the eastern China is most sensitive to the emissions of SO 2 and may be more sensitive to emission reductions in NO x than in NH 3. These results are fairly consistent with the responses of O 3 and PM 2.5 to the reductions of their precursor emissions predicted from sensitivity simulations. A 50% reduction of NO x or AVOC emissions leads to a reduction of O 3 over the eastern China. Unlike the reduction of emissions of SO 2, NO x, and NH 3 that leads to a decrease in PM 10, a 50% reduction of AVOC emissions increases PM 10 levels. Such results indicate the complexity of O 3 and PM chemistry and a need for an integrated, region-specific emission control strategy with seasonal variations to effectively control both O 3 and PM 2.5 pollution in China.

Evaluating the potential of IASI ozone observations to constrain simulated surface ozone concentrations

Abstract. A tracer study has been performed for two summers in 2003 and 2004 with a regional chemistry-transport model in order to evaluate the potential constraint that tropospheric ozone observations from nadir viewing infrared sounders like IASI or TES exert on modelled near surface ozone. As these instruments show high sensitivity in the free troposphere, but low sensitivity at ground, it is important to know how much of the information gained in the free troposphere is transferred to ground through vertical transport processes. Within the European model domain, and within a time span of 4 days, only ozone like tracers initialised in vertical layers above 500 hPa are transported to the surface. For a tracer initialised between 800 and 700 hPa, seven percent reaches the surface within one to three days, on the average over the European model domain but more than double over the Mediterranean Sea. For this region, trajectory analysis shows that this is related to strong subsident transport. These results are confirmed by a second tracer study taking into account averaging kernels related to IASI retrievals, indicating the potential of these measurements to efficiently constrain surface ozone values.

A review of phytoplankton ecology in estuarine and coastal waters around Ireland

A review of the distribution of phytoplankton in coastal, shelf and estuarine waters around Ireland was undertaken to consolidate our understanding of the ecology of these organisms in the region. In broad terms, the review has highlighted the relative importance of vertical water column stability and horizontal transport processes in influencing the abundance and composition of phytoplankton. In coastal waters, the seasonal stabilization and de-stabilization of the water column accounts for most of the natural variation in both phytoplankton species composition and biomass. Much of the remaining natural variability can be explained by the interaction of phytoplankton with a number of oceanographic features and processes such as the presence of tidal and thermohaline fronts, wind and topographically associated coastal upwelling, advection landward of offshore water masses and the flow of coastal and oceanic currents. In estuarine waters, the scenario is somewhat reversed, and although seasonality is important in broad terms, the structure of phytoplankton populations is determined more by local factors operating over much smaller time-scales in the order of days and weeks. The interplay of these factors, such as the periodic rise and fall of the tide and episodic changes in river flow, creates a broad range of conditions that result in greater variation in phytoplankton biomass, while at the same time selecting for a reduced number of species that are adapted to survive in this highly changeable environment.

Crustal noble gases in deep brines as natural tracers of vertical transport processes in the Michigan Basin

Noble gas concentrations and isotopic ratios are presented for 38 deep (∼0.5–3.6 km) brine samples in the Michigan Basin. These brine samples clearly show the presence of an important crustal component of 4He, 21Ne, 40Ar, and 136Xe. Both 40Arcrust and 136Xecrust display the presence of a strong vertical gradient along the sedimentary strata of the basin. We show that the in situ production for these two gases within the sedimentary strata is insufficient to account for the observed crustal component in the Michigan brines. These point to the presence of a deep, external source for crustal noble gases, likely the Precambrian crystalline basement beneath the Michigan Basin. Furthermore, observed elemental ratios of crustal noble gases (4He/40Ar, 21Ne/40Ar, 4He/136Xe, and 21Ne/136Xe) in these brines vary over several orders of magnitude with respect to the expected production ratios from the crystalline basement rocks and display a systematic pattern within the basin. Specifically, samples above the Salina Group (shallow formations) are relatively enriched in 4Hecrust and 21Necrust with respect to 40Arcrust and 136Xecrust, as opposed to those below the massive Salina evaporite layer (deeper formations) which exhibit complementary patterns. We show that such a general trend is best explained by a Rayleigh‐type elemental fractionation model involving upward transport of crustal noble gases and associated elemental fractionation processes, controlled by both diffusion‐ and solubility‐related mechanisms. As previously indicated by the mantle and atmospheric noble gas signatures in these same Michigan brine samples, release of deep crustal noble gases into the basin is yet another independent indicator pointing to the occurrence of a past thermal event in the basin. We suggest that recent reactivation of the ancient midcontinent rift system underneath the Michigan Basin is likely responsible for the upward transport of heat and loss of the atmospheric noble gas component, as well as release of crustal (still ongoing) and mantle noble gases into the basin via deep‐seated faults and fracture zones. Such a model also supports an internal heat source hypothesis as being largely responsible for the existence of past high temperatures in the basin without involvement of large‐scale brine migration from peripheral forming orogenic fold belts.

The variability of free tropospheric ozone over Beltsville, Maryland (39N, 77W) in the summers 2004–2007

Ozone profiles are often used to investigate day-to-day and year-to-year variability in origins of free tropospheric ozone. With this in mind, more than 50 ozonesonde launches were conducted in Beltsville, MD, during the summers of 2004 through 2007. Budgets of free tropospheric ozone were calculated for each ozone profile in the four summers using a laminar identification (LID) method and unusual episodes were analyzed with respect to meteorological variables. The laminar method showed that stratosphere-to-troposphere transport (ST) accounted for greater than 50% of the free tropospheric ozone column on 17% of days sampled, a more pronounced influence than regional convective and lightning (RCL) sources. The ST origins were confirmed with trajectories, and tracers (water vapor and potential vorticity). The amount of free tropospheric ozone from ST and RCL sources varied from year-to-year (up to 13%) and can be explained by differences in mean meteorological patterns. On average, almost 30% of the free tropospheric column was attributed to ST influence, about twice as much as RCL, although the LID method may not capture weeks-old lightning influences as in a chemical model. The prevalence of ST ozone in summertime Beltsville soundings was similar to six sounding sites in the IONS-04 campaign [Thompson, A.M., et al., 2007b. Intercontinental Transport Experiment Ozonesonde Network Study (IONS, 2004): 1. Summertime upper tropospheric/lower stratosphere ozone over northeastern North America. J. Geophys. Res. 112, D12S12; Thompson, A.M., et al., 2007c. Intercontinental Transport Experiment Ozonesonde Network Study (IONS, 2004): 2. Tropospheric ozone budgets and variability over northeastern North America. J. Geophys. Res. 112, D12S13.] and to statistics from a 30 year climatology of European soundings [Collette, A., Ancellet, G., 2005. Impact of vertical transport processes on the tropospheric ozone layering above Europe. Part II: Climatological analysis of the past 30 years. Atmos. Environ. 39, 5423–5435]. The Beltsville record also demonstrated the value of soundings for air quality forecasting in an urban area. The 22 nighttime soundings collected over Beltsville in 2004–2007 can be divided into distinct polluted and unpolluted subsets, the former 20 ppbv higher in residual layer ozone (1 km) than the latter. These distinctions propagated to daytime differences of 10 ppbv at the surface in the Washington, DC, area, with the high-ozone residual layers leading to non-attainment of the National Ambient Air Quality Standard for ozone. More frequent ozone observations aloft appear essential for better understanding ozone variability and for enabling air quality modelers to achieve more accurate ozone forecasts.

Interdependence of groundwater dynamics and land-energy feedbacks under climate change

Climate change will have a significant impact on the hydrologic cycle, creating changes in freshwater resources, land cover and land–atmosphere feedbacks. Simulations using a groundwater flow model with integrated overland flow and land-surface model processes show that groundwater depth, which results from lateral water flow at the surface and subsurface, determines the relative susceptibility of regions to changes in temperature and precipitation. Climate change will have a significant impact on the hydrologic cycle, creating changes in freshwater resources, land cover and land–atmosphere feedbacks. Recent studies have investigated the response of groundwater to climate change but do not account for energy feedbacks across the complete hydrologic cycle1,2. Although land-surface models have begun to include an operational groundwater-type component3,4,5, they do not include physically based lateral surface and subsurface flow and allow only for vertical transport processes. Here we use a variably saturated groundwater flow model with integrated overland flow and land-surface model processes6,7,8 to examine the interplay between water and energy flows in a changing climate for the southern Great Plains, USA, an important agricultural region that is susceptible to drought. We compare three scenario simulations with modified atmospheric forcing in terms of temperature and precipitation with a simulation of present-day climate. We find that groundwater depth, which results from lateral water flow at the surface and subsurface, determines the relative susceptibility of regions to changes in temperature and precipitation. This groundwater control is critical to understand processes of recharge and drought in a changing climate.

Variations of the residual circulation in the Northern Hemispheric winter

A multiyear time series of the vortex‐averaged diabatic descent for 47 Arctic winters from 1957/1958 until 2003/2004 is presented. The climatology of diabatic descent is based on trajectory calculations coupled with diabatic heating rate calculations carried out in the polar lower stratosphere of the Northern Hemisphere winters. We demonstrate the improved performance of the approach based on diabatic heating rates compared to the approach based on vertical winds from meteorological analysis. The time series of the vortex‐averaged diabatic descent gives a detailed picture of intensity and altitude dependence of the stratospheric vertical transport processes during the Arctic winter. In addition to the overall vortex‐averaged diabatic descent, the spatial structure of the descent is analyzed for two different Arctic winters. We demonstrate for this case study that not only the intensity but also the zonal structure of the diabatic descent depends on the meteorological conditions in the polar vortex. The climatology is characterized by very pronounced interannual variability which is linked to the variability of temperature anomalies and to the variability of Eliassen‐Palm (EP)‐flux anomalies, wherein strong planetary wave activity leads to strong diabatic descent and vice versa. The correlation between EP‐flux and descent shows that tropospheric dynamics have a strong influence on the strength of the polar branch of the residual circulation by means of the atmospheric wave activity.

Numerical simulation of sediment transport from Ba Lat Mouth and the process of coastal morphology

This paper presents an application of a 3D numerical model to simulate one vertical layer sediment transport and coastal morphodynamical process for the Hai Hau coastal area located in the north of Vietnam, where a very large amount of suspended sediment is carried into the sea from Ba Lat Mouth every year. Four simulations are based on the real data of waves supplied by the observation station close to Ba Lat Mouth. The conditions of wind and suspended sand concentration at Ba Lat Mouth are basically assumed from practice. The computed results show that the hydrodynamic factors strongly depend on the wind condition and these factors govern the direction and the range of suspended sand transport, especially in the shallow-water region. In the deep-water region this influence is not really clear when the wind force is not strong enough to modify the tidal current. In the area close to Ba Lat Mouth the flow velocity is very large with the maximum flood flow about 2.6 m s-1 and the maximum ebb flow about 1 m s-1 at the mouth, and this is one of the reasons for strong erosion. In the case of tidal flow only, the suspended sand concentration decreases resulting in local deposition. Therefore, the area influenced by suspended transport is small, about 12 km from the mouth. In the condition of wind and waves, the suspended sand transport reaches the end of the computation area within a few days, especially the cases with wind from the north-east-north. Through these simulation results, a common tendency of sediment movement from the north to the south is specified for the Hai Hau coastal area. In addition, the results also show that the coast suffers from strong erosion, especially the region near Ba Lat Mouth. From the simulation results it can be seen that the movement of the Red River sand along the Vietnamese coast is quite possible, which is an answer to a long-standing question. Furthermore, although the suspended sediment concentration is quite large, it is still not enough to compensate for the strong erosion of the Ba Lat Mouth region.

Design and Operability of a Large Weighable Fen Lysimeter

In principle, conventional lysimeters are suitable for the investigation of vertical water and solute fluxes. Lateral fluxes in water-saturated fen sites are characterized by heterogeneities and abnormities due to anisotropic layering. But due to lack of adequate monitoring techniques, these fluxes have been insufficiently analyzed. The newly developed large weighable fen lysimeter (LWFL) overcomes the limitations of conventional lysimetry and enables the measurement of vertical and horizontal transport processes in undisturbed large volume soil monoliths. The LWFL has a volume of 6 m3 (4 m length, 1 m width and 1.5 m depth) and was tested by filling the lysimeter with an undisturbed fen monolith. A special extraction procedure for the horizontal sliding of the lysimeter vessel through the natural fen was developed. In front of the vessel a converted cutting tool assisted in carving the soil monolith out of the peat, both vertically and horizontally. Inlet and outlet of the LWFL was constructed to allow the adjustment of a wide range of hydraulic gradients to depict natural occurring lateral transport processes. The LWFL including the measurement techniques was tested successfully for 3 years. On the basis of these tests, we conclude that complex physical and biogeochemical research problems involving lateral flows can be tackled now with multiphase observations and measurements at high spatial and temporal resolution, transdisciplinary data evaluation and numerical modelling approaches.

Detection of free vertical convection and double-diffusion in groundwater monitoring wells with geophysical borehole measurements

Two algorithms for in-situ detection and identification of vertical free convective and double-diffusive flows in groundwater monitoring wells or boreholes are proposed. With one algorithm the causes (driving forces) and with the other one the effects (convection or double-diffusion) of vertical transport processes can be detected based on geophysical borehole measurements in the water column. Five density-driven flow processes are identified: thermal, solutal, and thermosolutal convection leading to an equalization, as well as saltfingers and diffusive layering leading to an intensification of a vertical density gradient. The occurrence of density-driven transport processes could be proven in many groundwater monitoring wells and boreholes; especially shallow sections of boreholes or groundwater monitoring wells are affected dramatically by such vertical flows. Deep sections are also impaired as the critical threshold for the onset of a density-driven flow is considerably low. In monitoring wells or boreholes, several sections with different types of density-driven vertical flows may exist at the same time. Results from experimental investigations in a medium-scale testing facility with high aspect ratio (height/radius = 19) and from numerical modeling of a water column agree well with paramters of in-situ detected convection cells.

Ozone variation in the tropical tropopause layer as seen from ozonesonde data

Ozone variations with seasonal and intraseasonal timescales in the tropical tropopause layer (TTL) are investigated using a 5‐year tropical ozonesonde data set from the SHADOZ (Southern Hemisphere Additional Ozonesondes) archive. The longitudinal ozone distribution in the tropical upper troposphere (TUT) shows a zonal wave one structure with maxima around the Atlantic and Africa and minima around the western Pacific throughout the year, while the annual variation shows maxima during northern summer to autumn at most longitudes. We compare the ozone distribution with the vertical temperature structure and found that the lapse rate is gradual (steep) at the ozone‐enhanced (reduced) longitude and season. The east‐west temperature structure and ozone variation in the TUT may be explained by the longitudinal variation of the large‐scale atmospheric responses to the tropical heat source, which could govern both the temperature structure and the vertical transport processes. Ozone variability in the TUT is also large around the Atlantic and Africa and small around the western Pacific. However, the zonal wave one structure is not clear in the temperature variability and in the correlation coefficient between ozone and temperature, which can be related with wave activities around the tropopause. Remarkably large ozone variabilities with good correlation are observed in Africa during summer and in the central Pacific during autumn‐winter. These are associated with large‐scale equatorial waves, but the longitudinal variation of the wave activities does not seem to be an important factor in the zonal wave one structure of ozone.

Analyses and comparisons of variations of 7Be, 210Pb, and 7Be/210Pb with ozone observations at two Global Atmosphere Watch stations from high mountains

Simultaneous measurements of 7Be and 210Pb and the analyses of activity ratio, i.e., 7Be/210Pb, offer another useful analytical tool for improving our understanding of ozone variability occurring at high altitude because they can provide diagnostic indicators of horizontal and vertical transport processes. Generally, the increase (decrease) of 7Be with decrease (increase) of 210Pb in association with high (low) values of 7Be/210Pb indicates the mechanisms associated with vertical (for being favorable to the horizontal) transport processes. For the purpose of demonstrating these mechanisms for ozone transport, we have presented the distinct transport processes of ozone and background conditions for two Global Atmosphere Watch (GAW) stations: Mount Cimone station (44.18°N, 10.7°E, 2165 m asl) and Mount Waliguan station (36.29°N, 100.90°E, 3816 m asl), located high in the mountains of Italy and China, respectively. We have performed the frequency distribution analyses and various comparisons of variations of 7Be with 210Pb and variations of 7Be, 210Pb, and 7Be/210Pb with surface ozone. The maximum ozone levels occur in the summer, and minimum levels occur in the winter at these two high mountain stations. Our analyses suggest that the processes of the stratospheric intrusions/upper tropospheric transport combined with the long‐range transport from polluted areas over eastern central China are the mechanisms that cause the ozone summer peak at the Mount Waliguan station. The ozone summer peak at the Mount Cimone station is mainly the result of photochemical production in the lower atmosphere associated with the transport of polluted air masses on regional and continental scales. Frequency distributions of 7Be, 210Pb, 7Be/210Pb, and surface ozone can be well represented by the lognormal distributions. Strikingly, we have found that the similarities of bimodal distributions for 7Be/210Pb at these two stations are clearly exhibited except for the magnitude, although the lognormal distributions of 7Be and 210Pb, with the noteworthy difference in altitude of these two stations, are significantly different. The measurements of 7Be and 210Pb combined with the analyses using 7Be/210Pb activity ratio can enhance our understanding of the transport processes in the atmosphere.

Levels and vertical distribution of PCBs in agricultural and natural soils from Sweden

Soils represent an important reservoir for persistent organic pollutants (POPs) in the terrestrial environment and thus the fate of contaminants deposited to the surface soils is important to understand. Since only a limited number of studies of the vertical distribution of POPs are available in the literature, the purpose of this study was to collect and analyze PCB concentrations in different layers of soil cores taken at agricultural and non-agricultural sites in Sweden. PCB concentrations at the agricultural site were nearly uniform on a dry weight basis throughout the depth considered (0-12 cm) while a distinct gradient with depth was observed at the five non-agricultural sites. On an organic carbon normalized basis, the concentration gradient was maintained at three of the non-agricultural sites while a more uniform distribution was observed at the other two sites. A statistically significant relationship between dry weight soil concentrations and organic carbon content was determined for all PCB congeners in the surface layers (0-1 cm) but not in the deeper layers (4-5 cm, 9-10 cm). These results were interpreted in relation to vertical soil transport processes and then the implications for environmental fate models including soil compartments were discussed.

Impact of vertical transport processes on the tropospheric ozone layering above Europe. Part I: Study of air mass origin using multivariate analysis, clustering and trajectories

Abstract A new method is proposed to classify ozone-rich layers observed in tropospheric profiles in terms of their origin using multivariate analysis. We combine principal component and discriminant analyses to quantify the respective ability of 21 measured physical parameters to describe the layers. Agglomerative hierarchical clustering shows the existence of clusters of air masses with specific physical characteristics. Quadratic discriminant analysis allows the definition of multidimensional borders between these clusters. The geophysical characteristics of the clusters are discussed and related to the origins of the layers: recently transported from the stratosphere (ST) or from the boundary layer (BL) or transported over long distances in the free troposphere. This clustering is compared to the results of a Lagrangian particle dispersion model for a 2-year period. The proportions of layers originating either from the BL or from the ST are highly consistent using both methods as well as the respective contribution of each reservoir to the total ozone mass. About 10% of the ozone measured in the tropospheric layers was exported recently from the BL and one-fifth has a recent stratospheric origin. The remaining proportion could not be attributed to any recent transport pathway. Season-dependent criteria allow very satisfactory reproduction of the seasonal variability of the layering as seen by the Lagrangian model. Analysis of the geographical origin of BL air masses suggests that the statistical clustering underestimates long-range transport, especially in fall.

Impact of vertical transport processes on the tropospheric ozone layering above Europe.

Abstract Using the set of multivariate criteria described in a companion paper, ozone-rich layers detected in tropospheric soundings are clustered according to their stratospheric or boundary layer origin. An additional class for aged tropospheric air masses is also considered. This analysis is exclusively based on the measured physical properties of the layers. The database includes 27,000 ozone profiles collected above 11 European stations—two of which provide measurements since 1970. The seasonal cycle of the tropospheric ozone stratification exhibits a clear summer maximum. This increase is due to aged tropospheric air masses that are more frequently detected, suggesting an enhanced lifetime of layers in summer. In terms of ozone content, the relative impact of stratospheric ozone compared to the other sources is highest in winter while export from the boundary layer presents a uniform seasonal cycle. Altitude and thickness distributions of the layers are consistent with the dynamical processes involved in the layering. Northernmost and southernmost stations are more exposed to stratospheric air intrusions into the free troposphere. Long-term trends show that transport from the tropopause region has increased since the mid 1980s. This trend being concomitant with lower ozone content of such layers, a moderate trend of the transport efficiency from the stratosphere on total tropospheric ozone is observed. The increase of ozone detected in tropospheric layers since the mid 1980s cannot be attributed to any recent export process from either the stratosphere or the boundary layer but rather to enhanced photochemical production in aged air masses or to an increase in the lifetime of the layers.