[1] The Lagrangian particle dispersion model FLEXPART was used to construct a global data set of 1.4 million continuous trajectories. At the model start, particles were distributed homogeneously in the atmosphere and were then transported for 5.5 years using both resolved winds from European Centre for Medium-Range Weather Forecasts analyses and parameterized turbulent and convective transport. On the basis of this data set, a climatology of transport in and to the Arctic was developed. It was found that the time air resides continuously north of 70N, called its Arctic age, is highest near the surface in the North American sector of the Arctic. North of 80N and near the surface, the mean Arctic age of air is about 1 week in winter and 2 weeks in summer. It decreases rapidly with altitude to about 3 days in the upper troposphere. In the most isolated regions of the Arctic, air is exposed to continuous darkness for, on average, 10–14 days in December. Transport from the stratosphere to the lower troposphere is much slower in the Arctic than in the middle latitudes. In the central Arctic, for instance, the probability that air near the surface was transported from the stratosphere within 10 days is only about 1 % in winter and 0.3 % in summer. Air pollution can be transported into the Arctic along three different

Abstract. Current Lagrangian particle dispersion models, used to simulate the dispersion of passive tracers in the turbulent planetary boundary layer (PBL), assume that the density is constant within the PBL. In deep PBLs, where the density at the boundary-layer top may be lower by more than 20% than at the surface, this assumption leads to errors in the tracer concentrations on the order of 10%. In the presence of a vertical wind shear, this also leads to inaccurate calculations of the horizontal tracer transport. To remove this deficiency, a Langevin equation is presented that contains a density correction term. The effect of the density correction is studied using data from a large-scale tracer experiment. It is found that for this experiment, the main effect of the density correction is an increase in the surface tracer concentrations, whereas the horizontal tracer transport patterns remain largely unaffected.

A diagnostic Lagrangian method to trace the budgets of freshwater fluxes, first described in Part I of this article, is used here to establish source–sink relationships of moisture between earth’s ocean basins and river catchments. Using the Lagrangian particle dispersion model FLEXPART, driven with meteorological analyses, 1.1 million particles, representing the mass of the atmosphere, were tracked over a period of 4 yr. Via diagnosis of the changes of specific humidity along the trajectories, budgets of evaporation minus precipitation (E P) were determined. For validation purposes, E P budgets were calculated for 39 river catchments and compared with climatological streamflow data for these rivers. Good agreement (explained variance 87%) was found between the two quantities. The E P budgets were then tracked forward from all of earth’s ocean basins and backward from the 39 major river catchments for a period of 10 days. As much previous work was done for the Mississippi basin, this basin was chosen for a detailed analysis. Moisture recycling over the continent and moisture transport from the Gulf of Mexico were identified as the major sources for precipitation over the Mississippi basin, in quantitative agreement with previous studies. In the remainder of the paper, global statistics for source–sink relationships of moisture between the ocean basins and river catchments are presented. They show, for instance, the evaporative capacity of monsoonal

This paper presents the revision and evaluation of the interface between the convective parameterization by Emanuel and Živković-Rothman and the Lagrangian particle dispersion model “FLEXPART ” based on meteorological data from the European Centre for Medium-Range Weather Forecasts (ECMWF). The convection scheme relies on the ECMWF grid-scale temperature and humidity and provides a matrix necessary for the vertical convective particle displacement. The benefits of the revised interface relative to its previous version are presented. It is shown that, apart from minor fluctuations caused by the stochastic convective redistribution of the particles, the well-mixed criterion is fulfilled in simulations that include convection. Although for technical reasons the calculation of the displacement matrix differs somewhat between the forward and the backward simulations in time, the mean relative difference between the convective mass fluxes in forward and backward simulations is below 3 % and can therefore be tolerated. A comparison of the convective mass fluxes and precipitation rates with those archived in the 40-yr ECMWF Reanalysis (ERA-40) data reveals that the convection scheme in FLEXPART produces upward mass fluxes and precipitation rates that are generally smaller by about 25 % than those from ERA-40. This result is interpreted as positive, because precipitation is known to be overestimated by the ECMWF model.

[1] We present a factor analysis-based method for differentiating air masses on the basis of source influence and apply the method to a broad suite of trace gas and aerosol measurements collected at Chebogue Point, Nova Scotia, during the summer of 2004 to characterize the chemical composition of atmospheric outflow from eastern North America. CO, ozone, and aerosol mass were elevated by 30%, 56%, and more than 300 % at Chebogue Point during U.S. outflow periods. Organic aerosol mass was highest during U.S. pollution events, but made up the largest fraction (70%) of the total aerosol during periods of primary and especially secondary biogenic influence, indicating the importance of both anthropogenic and biogenic organic aerosol. Anthropogenic and oxygenated volatile organic compounds account for the bulk of the gas-phase organic carbon under most conditions; however, biogenic compounds are important in terms of chemical reactivity. Biogenic emissions thus have a significant impact on the chemistry of air masses downwind of the polluted northeastern United States. Using output from a global 3-D model of atmospheric composition (GEOS-Chem), we estimate that CO directly emitted from U.S. pollution sources makes up 28 % of the total CO observed at Chebogue Point during U.S. outflow events and 19 % at other times, although more work is needed to improve U.S. emission estimates for CO and other pollutants. We conclude that the effects of North American pollution on the chemistry of the western North Atlantic boundary layer are pervasive and not restricted to particular events. Citation: Millet, D. B., et al. (2006), Chemical characteristics of North American surface layer outflow: Insights from Chebogue

The possibility to calculate linear-source receptor relationships for the transport of atmospheric trace substances with a Lagrangian particle dispersion model (LPDM) running in backward mode is shown and presented with many tests and examples. This mode requires only minor modifications of the forward LPDM. The derivation includes the action of sources and of any first-order processes (transformation with prescribed rates, dry and wet deposition, radioactive decay, etc.). The backward mode is computationally advantageous if the number of receptors is less than the number of sources considered. The combination of an LPDM with the backward (adjoint) methodology is especially attractive for the application to point measurements, which can be handled without artificial numerical diffusion. Practical hints are provided for source-receptor calculations with different settings, both in forward and backward mode. The equivalence of forward and backward calculations is shown in simple tests for release and sampling of particles, pure wet deposition, pure convective redistribution and realistic transport over a short distance. Furthermore, an application example explaining measurements of Cs-137 in Stockholm as transport from areas contaminated heavily in the Chernobyl disaster is included.

Offline atmospheric transport models are normally driven with meteorological analyses. However, subsequent analysis fields are dynamically not consistent with each other, because they are produced in independent data assimilation cycles that lack strong dynamical constraints between each other. In this paper, it is shown that when these data are used with Lagrangian transport models, spurious mixing results from the dynamic incon-sistencies. As a consequence, quantities such as potential vorticity or specific humidity that tend to be conserved along trajectories are found to be significantly less well conserved when analysis data are used than when forecast data are used for the trajectory calculations. This leads, for instance, to enhanced stratosphere–troposphere exchange. It is also shown that the dispersion of initially neighboring particles occurs more rapidly with the analysis than with the forecast data. It is therefore concluded that small-scale tracer structures develop too quickly in Lagrangian models, due to the inconsistencies between the driving wind fields. 1.

S tratosphere–troposphere exchange (STE) is im-portant for the chemical composition of both thelowermost stratosphere (LS) and the troposphere. Modifications thereof in a changing climate may sig-nificantly affect stratospheric ozone depletion (Butchart and Scaife 2001) and the oxidizing capac-ity of the troposphere (Lelieveld and Dentener 2000). However, STE is still poorly understood and inad-equately quantified, due to the involvement of physi-cal and dynamical processes on local to global scales (Holton et al. 1995), and conceptual problems. On a long-term and global scale, and in the zon-ally averaged sense (Brewer 1949), there is slow as-cent from the troposphere to the stratosphere in the Tropics (Plumb 1996; Mote et al. 1996), quasi-isen-tropic transport to the extratropics in the stratosphere (Waugh 1996), and downward flow from the strato-sphere to the troposphere in middle and higher lati-tudes (Fig. 1). This circulation is related to the dissi-pation of extratropical planetary and gravity waves in the stratosphere (Haynes et al. 1991). Based upon calculations of the monthly averaged hemispheric mass flux into the LS (Rosenlof 1995) and of the mass of the LS, the monthly mean net mass flux across the tropopause can be evaluated (Appenzeller et al. 1996). This two-dimensional picture may suggest that STE in the extratropics is a continuous downward flow. However, actual STE is highly episodic, associ-ated with strong mesoscale perturbations of the tropo-pause (Appenzeller and Davies 1992), and occurs in both directions. In situ and remote sensing observa-tions have produced evidence for the existence of lay-ers of originally tropospheric air in the midlatitude LS (Hintsa et al. 1998; Vaughan and Timmis 1998), and stratospheric intrusions into the troposphere

Vaccination with intestinal tract antigens does not induce protective immunity in a permissive model of filariasis

[1] In the Arctic, atmospheric concentrations of gaseous elemental mercury (GEM) can decrease strongly in spring when mercury is deposited to the snow. Some studies suggest mercury can accumulate in the snow while others suggest rapid reemission after atmospheric mercury depletion events. We have combined measurements of GEM at the Arctic site Zeppelin (Ny Ålesund, Spitsbergen) with the output of the Lagrangian particle dispersion model FLEXPART, for a statistical analysis of GEM source and sink regions. We find that the Arctic is a strong net sink region for GEM in April and May, suggesting that mercury accumulates in the Arctic snow pack. For summer, we find the Arctic to be a GEM source, indicating reemission of previously deposited mercury when the snow and/or ice melts, or evasion from the ocean through sea ice leads and polynyas. Our results are corroborated by a related analysis