[1] This paper presents results of a 1-year simulation of the transport of six passive tracers, released over the continents according to an emission inventory for carbon monoxide (CO). Lagrangian concepts are introduced to derive age spectra of the tracer concentrations on a global grid in order to determine the timescales and pathways of pollution export from the continents. Calculating these age spectra is equivalent to simulating many (quasi continuous) plumes, each starting at a different time, which are subsequently merged. Movies of the tracer dispersion have been made available on an Internet website. It is found that emissions from Asia experience the fastest vertical transport, whereas European emissions have the strongest tendency to remain in the lower troposphere. European emissions are transported primarily into the Arctic and appear to be the major contributor to the Arctic haze problem. Tracers from an upwind continent first arrive over a receptor continent in the upper troposphere, typically after some 4 days. Only later foreign tracers also arrive in the lower troposphere. Assuming a 2-day lifetime, the domestic tracers dominate total tracer columns over all continents except over Australia where foreign tracers account for 20% of the tracer mass. In contrast, for a 20-day lifetime even continents with high domestic emissions receive more than half of their tracer burden from foreign continents. Three special regions were identified where tracers are transported to, and tracer dilution is slow. Future field studies therefore should be deployed in the following regions: (1) In the winter, the Asia tracer accumulates over Indonesia and the Indian Ocean, a region speculated to be a stratospheric fountain.

This study presents the first climatology of so-called warm conveyor belts (WCBs), strongly ascending moist airstreams in extratropical cyclones that, on the time scale of 2 days, rise from the boundary layer to the upper troposphere. The climatology was constructed by using 15 yr (1979-93) of reanalysis data and calculating 355 million trajectories starting daily from a 1Њ ϫ 1Њ global grid at 500 m above ground level (AGL). WCBs were defined as those trajectories that, during a period of 2 days, traveled northeastward and ascended by at least 60% of the zonally and climatologically averaged tropopause height. The mean specific humidity at WCB starting points in different regions varies from 7 to 12 g kg Ϫ1. This moisture is almost entirely precipitated out, leading to an increase of potential temperature of 15-22 K along a WCB trajectory. Over the course of 3 days, a WCB trajectory produces, on average, about four (six) times as much precipitation as a global (extratropical) average trajectory starting from 500 m AGL. WCB starting points are most frequently located between approximately 25Њ and 45ЊN and between about 20Њ and 45ЊS. In the Northern Hemisphere (NH), there are two distinct frequency maxima east of North America and east of Asia, whereas there is much less zonal variability in the Southern Hemisphere (SH). In the NH, WCBs are almost an order of magnitude more frequent in January than in July, whereas in the SH the seasonal variation is much weaker. In order to study the relationship between WCBs and cyclones, an independent cyclone climatology was used. Most of the WCBs were found in the vicinity of a cyclone center, whereas the reverse comparison revealed that cyclones are normally accompanied by a strong WCB only in the NH winter. In the SH, this is not the case throughout the year. Particularly around Antarctica, where cyclones are globally most frequent, practically no strong WCBs are found. These cyclones are less influenced by diabatic processes and, thus, they are associated with fewer high clouds and less precipitation than cyclones in other regions. In winter, there is a highly significant correlation between the North Atlantic Oscillation (NAO) and the WCB distribution in the North Atlantic: In months with a high NAO index, WCBs are about 12% more frequent and their outflow occurs about 10Њ latitude farther north and 20Њ longitude farther east than in months with a low NAO index. The differences in the WCB inflow regions are relatively small between the two NAO phases. During high phases of the Southern Oscillation, WCBs occur more (less) frequent around Australia (in the South Atlantic).

[1] In this paper we present simulations with a Lagrangian particle dispersion model to study the intercontinental transport of pollution from North America during an aircraft measurement campaign over Europe. The model was used for both the flight planning and a detailed source analysis after the campaign, which is described here with examples from two episodes. Forward calculations of emission tracers from North America, Europe, and Asia were made in order to understand the transport processes. Both episodes were preceded by stagnant conditions over North America, leading to the accumulation of pollutants in the North American boundary layer. Both anthropogenic sources and, to a lesser extent, forest fire emissions contributed to this pollution, which was then exported by warm conveyor belts to the middle and upper troposphere, where it was transported rapidly to Europe. Concentrations of many trace gases (CO, NO y , CO 2 , acetone, and several volatile organic compounds; O 3 in one case) and of ambient atmospheric ions measured aboard the research aircraft were clearly enhanced in the pollution plumes compared to the conditions outside the plumes. Backward simulations with the particle model were introduced as an indispensable tool for a more detailed analysis of the plume's source region. They make trajectory analyses (which, to date, were mainly used to interpret aircraft measurement data) obsolete. Using an emission inventory, we could decompose the tracer mixing ratios at the receptors (i.e., along the flight tracks) into contributions from every grid cell of the inventory. For both plumes we found that emission sources contributing to the tracer concentrations over Europe were distributed over large areas in North America. In one case, sources in California, Texas, and Florida contributed almost equally, and smaller contributions were also made by other sources located between the Yucatan Peninsula and Canada. In the other case, sources in eastern North America, including moderate contributions from forest fires, were most important. The plume's maximum was mainly caused by anthropogenic emissions from the New York area. To our knowledge, this is the first case reported where a pollution plume from a megacity was reliably detected over another continent.

Abstract. In May 2003, severe forest fires in southeast Russia resulted in smoke plumes extending widely across the Northern Hemisphere. This study combines satellite data from a variety of platforms (Moderate Resolution Imaging Spectroradiometer (MODIS), Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Earth Probe Total Ozone Mapping Spectrometer (TOMS) and Global Ozone Monitoring Experiment (GOME)) and vertical aerosol profiles derived with Raman lidar measurements with results from a Lagrangian particle dispersion model to understand the transport processes that led to the large haze plumes observed over North America and Europe. The satellite images provided a unique opportunity for validating model simulations of tropospheric transport on a truly hemispheric scale. Transport of the smoke occurred in two directions: Smoke travelling northwestwards towards Scandinavia was lifted over the Urals and arrived over the Norwegian Sea. Smoke travelling eastwards to the Okhotsk Sea was also lifted, it then crossed the Bering Sea to Alaska from where it proceeded to Canada and was later even observed over Scandinavia and Eastern Europe on its way back to Russia. Not many events of this kind, if any, have been observed, documented and simulated with a transport model comprehensively. The total transport time was about 17 days. We compared transport model simulations using meteorological analysis data from both the European Centre for Medium-Range Weather Forecast (ECMWF) and the National Center for Environmental Prediction (NCEP) in order to find out how well this event could be simulated using these two datasets. Although differences between the two simulations are found on small scales, both agree remarkably well with each other and with the observations on large scales. On the basis of the available observations, it cannot be decided which simulation was more realistic.

[1] During summer of 2004, about 2.7 million hectare of boreal forest burned in Alaska, the largest annual area burned on record, and another 3.1 million hectare burned in Canada. This study explores the impact of emissions from these fires on light absorbing aerosol concentration levels, aerosol optical depths (AOD), and albedo at the Arctic stations Barrow (Alaska), Alert (Canada), Summit (Greenland), and Zeppelin/Ny Å lesund on Spitsbergen (Norway). The Lagrangian particle dispersion model FLEXPART was run backward from these sites to identify periods that were influenced by forest fire pollution plumes. It is shown that the fires led to enhanced values of particle light absorption coefficients (s ap ) at all of these sites. Barrow, about 1000 km away from the fires, was affected by several fire pollution plumes, one leading to spectacularly high 3-hour mean s ap values of up to 32 Mm À1 , more than the highest values measured in Arctic Haze. AOD measurements for a wavelength of 500 nm saturated but were estimated at above 4-5 units, unprecedented in the station records. Fire plumes were transported through the atmospheric column over Summit continuously for 2 months, during which all measured AOD values were enhanced, with maxima up to 0.4-0.5 units. Equivalent black carbon concentrations at the surface at Summit were up to 600 ng m À3 during two major episodes, and Alert saw at least one event with enhanced s ap values. FLEXPART results show that Zeppelin was located in a relatively unaffected part of the Arctic. Nevertheless, there was a 4-day period with daily mean s ap > 0.3 Mm À1 , the strongest episode of the summer half year, and enhanced AOD values. Elevated concentrations of the highly source-specific compound levoglucosan positively confirmed that biomass burning was the source of the aerosols at Zeppelin. In summary, this paper shows that boreal forest fires can lead to elevated concentrations of light absorbing aerosols throughout the entire Arctic. Enhanced AOD values suggest a substantial impact of these plumes on radiation transmission in the Arctic atmosphere. During the passage of the largest fire plume, a pronounced drop of the albedo of the snow was observed at Summit. We suggest that this is due to the deposition of light absorbing particles on the snow, with further potentially important consequences for the Arctic radiation budget.