TABLE OF CONTENTS
1. Introduction
2. Overall Goals
3. Transport Processes
4. Aerosol Radiative Effects in the Arctic
5. Boreal Forest Fires and their Effects on the Arctic
6. Composition and Chemistry of the Arctic Troposphere
7. Use of Satellite Data during POLARCAT
8. Use of Models during POLARCAT
9. Overview of the Planned Activities
10. Project Management
11. List of Researchers and Institutes Involved in the IPY Expression of Intent
12. References

Climate change is proceeding fastest at the high latitudes of the Arctic. Surface air temperatures in the Arctic have increased more than the global average over the past few decades (Houghton et al., 2001). Precipitation and river discharges into the Arctic Ocean have also increased (Wu et al., 2005), whereas sea ice extent has dropped dramatically (Parkinson et al., 1999). The enhanced freshwater input is suspected to have already freshened the Arctic Ocean and deep North Atlantic Ocean (Dickson et al., 2002; Curry et al., 2003). If this trend continues long enough, the present thermohaline circulation could eventually collapse, with serious worldwide consequences. This makes the Arctic a region where a better understanding of the processes leading to climate change is most urgently needed. While some of the changes are related to the global increase of the long-lived greenhouse gases, many processes causing them are specific to the Arctic. This study aims to improve our understanding of the role that tropospheric chemistry, aerosols, and transport play in these processes.

Because of its remoteness, the Arctic troposphere was long believed to be extremely clean. However, just before the last International Geophysical Year in 1957/58, pilots flying over the Canadian and Alaskan Arctic discovered a strange haze (Greenaway, 1950; Mitchell, 1957), which significantly decreased visibility. This so-called Arctic Haze is a recurring phenomenon that since then has been observed every winter and spring. It is now known to be the re-sult of long-range transport of anthropogenic pollution mostly from Europe and western Asia. While it is clear that deposition of some species associated with Arctic Haze can significantly impact Arctic ecosystems (Macdonald et al., 2005), the climate impact of Arctic Haze is still under discussion. Radiative effects of aerosols, both direct and indirect, can be quite different in the Arctic compared to elsewhere. Due to the high surface albedo of snow and ice, even weakly absorbing aerosol layers can heat the Earth/atmosphere system (Pueschel and Kinne, 1995). Furthermore, infrared emissions from the haze can heat the surface during the polar night, and during spring when the solar zenith angle is still large (MacCracken et al., 1986). These effects clearly need further study.

In addition, satellite imagery (see title page) shows that the Arctic can also be affected by pollution transport in summer, when forest fires are prevalent in the boreal region and are a strong high-latitude source of black carbon (Lavoue et al., 2000). As the boreal zone is warming, the frequency of fires appears to be increasing (Stocks et al., 1998). Smoke from the fires can travel over continental (Wotawa and Trainer, 2000), intercontinental (Forster et al., 2001), and even hemispheric (Damoah et al., 2004) distances. It has also been found recently that boreal forest fire smoke can penetrate deeply into the stratosphere (Fromm et al., 2005), where residence times could be long enough to have a significant impact on polar strato-spheric ozone loss. Smoke aloft heats the atmosphere but cools the surface (Robock, 1991). However, black carbon particles can also be deposited and can significantly decrease the albedo of snow and ice surfaces (Hansen and Nazarenko, 2004). The enhanced absorption of solar energy could possibly contribute strongly to the melting of Arctic land and sea ice. However, no data exists yet to quantify this effect. A model study has also suggested that growing emissions in East Asia may increase the soot deposition in the Arctic (Koch and Hansen, 2005). This impact needs quantification based on observations and further model studies.

Several chemical phenomena were discovered recently that are unique to the Arctic troposphere. Both ozone and mercury can be almost instantaneously and completely removed near the time of polar sunrise (Oltmans, 1981; Barrie et al., 1988; Schroeder et al., 1998) as a result of catalytic bromine chemistry. Satellite measurements show the existence of high total columns of BrO at the time of the ozone and mercury depletion events (Wagner and Platt, 1998; Frieß et al., 2004). However, the origin of the bromine has not been clarified yet. Furthermore, it is not known whether the bromine is located exclusively near the surface, or whether it can also exist throughout the free troposphere, with possibly large consequences for the chemistry of the Arctic atmosphere. Another recent discovery is the flux of nitrogen oxides from the snow pack into the Arctic boundary layer (Honrath et al., 1999). A unique feature at the cold temperatures of the Arctic troposphere is that most of the reactive nitrogen is stored in organic forms (e.g., peroxy acetyl nitrate, PAN) (Singh et al., 1992). However, if exported to warmer regions of the troposphere, PAN is easily decomposed to produce nitrogen oxides and lead to ozone formation. All of the phenomena described above are strongly influenced by the coupling of surface exchange processes, vertical transport, unique Arctic air chemistry, and import from and export to midlatitude regions. This coupling cannot be studied at a single site or by a single platform but instead needs a broad approach using measurements at the surface, aboard ships and aircraft and from satellites, and models as integrative tools, such as suggested by POLARCAT.

POLARCAT will execute a series of aircraft experiments at different times of the year in order to follow pollution plumes of different origin as they are transported into the Arctic and observe the chemistry, aerosol processes, and radiation effects of these plumes. It will also observe the atmospheric composition in relatively cleaner regions outside major plumes. The experiments will also take advantage of the long residence times of pollutants in the stably stratified Arctic atmosphere to study ageing processes by targeting air masses that have spent considerable time in the Arctic. The Arctic will, thus, also serve as a natural laboratory for investigating processes that cannot be studied elsewhere in such isolation. Measurements performed on a ship will investigate processes occurring in the lowest part of the troposphere, such as spring-time tropospheric ozone depletion events. In addition to the aircraft and ship-board experiments, satellite remote sensing data and surface measurements in the Arctic will be utilized. The wide range of surface measurements and ground-based remote sensing meas-urements (e.g., Notholt et al., 1997) that will take place as part of POLARCAT will also provide important information on the seasonal evolution of trace gases, aerosols and soluble species in rain and snow over the IPY timeframe. The combined analysis of these longer-term datasets, many of which will continue after IPY, will allow the campaign data to be put into a wider context. The aircraft campaign data, through vertical profiling, will also aide the interpretation of surface observations of trace constituents and precipitation chemistry, as well as ice core and firn measurements, by linking the surface with the boundary layer and free troposphere. Likewise, aircraft data profiles or lidar data will be used to validate satellite data. Models of differing complexity will be used to test our understanding of Arctic processes against the measurement data sets. These range from box models up to global or regional scale chemistry-aerosol-climate models.

Some of the processes that will be studied by POLARCAT in the Arctic are also operating in the Antarctic. However, the Arctic is influenced to a much larger extent by anthropogenic and biomass burning pollution sources. Therefore, POLARCAT will concentrate its field activities in the Arctic. Model studies, though, will also consider the Antarctic, and will compare the situations in both polar regions.

After specifying POLARCAT's overall objectives, the scientific background for POLARCAT and open questions shall briefly be reviewed. Thereafter, the POLARCAT scientific activities shall be described, as they are currently planned.

The overall goal of POLARCAT is to study the role of long-range poleward transport of aerosols and trace gases for climate change in the Arctic. Particular objectives are to:

• better characterize the transport pathways of pollution into the Arctic free troposphere and boundary layer, including quantification of the residence times of polluted air masses in the polar dome.
• determine the vertical layering of Arctic pollution from different sources and the associated optical properties of Arctic aerosol particles.
• study the seasonal and interannual variability of transport pathways and removal processes for aerosols and pollutants, and to investigate likely changes in a future climate.
• characterize the direct radiative effects (solar and terrestrial) within pollution layers in the Arctic.
• investigate the interactions of aerosols with clouds (e.g., their role as ice nuclei), and their impact on radiative forcing.
• quantify the albedo changes of snow and ice surfaces and resulting radiative effects due to the deposition of black carbon from anthropogenic and biomass burning sources.
• study the impact of boreal forest fire emissions on the chemical composition of the tropo-sphere and on the deposition of soot in the Arctic.
• determine the fate and effects of aerosols and chemical compounds injected into the stratosphere by pyro-convection, including their role for ozone formation and ozone de-pletion in the polar stratosphere.
• improve our understanding of the O₃/NO_x/HO_x chemistry in the Arctic troposphere.
• investigation the role of halogen atom chemistry in the Arctic boundary layer and free troposphere.
• study the impact of snow pack emissions of NOx, OVOC, and halogens on the Arctic troposphere.
• validate aerosol, trace gas, and cloud products of space observations from polar orbital satellites.

Seasonal aspects of pollution transport. Field studies established that the Arctic haze phenomenon occurs regulary in winter and maximizes in early spring, with the number and depth of the haze layers increasing with the season (Scheuer et al, 2003). In the 1970s it became clear that the haze was of anthropogenic origin (Rahn et al., 1977), and in the 1980s the Arctic Haze was traced back to sources located predominantly in northern Eurasia (Barrie, 1986).

The haze phenomenon is a result of the special meteorological situation in the Arctic in the winter. During winter, air in the lower troposphere over the Arctic is partially isolated from the rest of the atmosphere by a transport barrier. Potential temperature at the ground becomes extremely low within the Arctic. This leads to an extremely stable vertical stratification (Bradley et al., 1992), which reduces turbulent exchange and, thus, dry deposition. The low water vapour content also makes wet removal very inefficient, leading to a very long atmospheric lifetime of aerosols and other pollutants in the Arctic. The low surface temperatures also enhance the latitudinal temperature contrast. This means that surfaces of constant potential temperature form closed domes over the Arctic bounded by the "polar front" (Carlson, 1981; Iversen, 1984; Barrie, 1986). Air can only cross these surfaces and escape the "polar dome" if there are waves on the polar front resulting in equatorward excursions of air and associated heating from the warmer underlying surface. Air can enter the polar cap where it experiences significant diabatic cooling close to the polar front (Klonecki et al, 2003). There is a preferred entry route into the polar cap from Europe, associated partly with the extreme sea-land temperature contrast on the western seaboard of Eurasia (Rahn, 1981). Furthermore, because Europe is located at relatively high latitudes, a significant fraction of the European emissions can actually be injected directly into the polar cap, especially when wave activity takes the polar front relatively far south over Europe. Therefore, near the surface, Arctic Haze is primarily caused by emissions in Europe and northwestern Asia (Eckhardt et al., 2003).

Since black carbon (BC) emissions in south Asia are increasing, Koch and Hansen (2005) suggested that nowadays emissions from South Asia, together with biomass burning emissions, are the dominant source of BC in the Arctic. Emissions in south Asia occur at much more southerly latitudes (and, thus, higher potential temperatures) and they tend to be lofted over the North Pacific stormtrack if transported north (Stohl, 2001; Stohl et al., 2002). Upon arrival in the Arctic, they should be located at relatively high altitudes. However, according to Koch and Hansen's (2005) model calculations, Asian BC rivals European BC even at the surface. But it is not entirely clear how, in their model, south Asian air masses are cooled sufficiently to reach the Arctic lower troposphere in winter, although there is generally cooling and slow sinking motion over the Arctic itself, and pollutant layers aloft are slowly entrained into the Arctic boundary layer.

During the breakdown of the polar cap as polar night ends, outbreaks of Arctic air are most common across the Labrador Sea and along the coasts of Greenland (Honrath et al, 1996). The pollution that has built up over winter is then released to the mid-latitudes (Penkett et al, 1993). It has been speculated that this pulsed release of ozone precursors from the Arctic could lead to the observed spring-time ozone maximum at middle latitudes (Penkett and Price, 1986). Arctic Haze can also be exported to the middle latitudes (Heintzenberg et al., 2003) and it is assumed that polar air masses influence mid-latitude particle formation (Nilsson et al., 2001; Kulmala et al., 2003).

Inter-annual variability of pollution transport pathways into the Arctic.In the NH, especially during the Arctic Haze season in winter and early spring, the most prominent and recurrent pattern of atmospheric variability is the North Atlantic Oscillation (NAO; the NAO is strongly correlated to the so-called Arctic Oscillation). The NAO is identified by variations in the NAO index, which is typically defined by the difference in surface pressure between Iceland and the Azores/Lisbon (Hurrell, 1995). Oscillations between high and low NAO phases produce large changes in the Arctic wind field, surface air temperature, precipitation, river runoff, ocean currents, sea ice, and biological responses (see Macdonald et al., 2005, for a recent review). Especially in the 1980s, there was a strong upward trend in the NAO, which was associated with a substantial decrease in the Arctic sea ice cover. Transport of anthropogenic emissions from Europe, North America and Asia into the Arctic is significantly enhanced under positive NAO conditions, as can be seen both in model calculations and in Arctic observations (Eckhardt et al., 2003; Duncan and Bey, 2004). For instance, there is a positive correlation between carbon monoxide concentrations at Arctic measurement stations and the NAO index (Table 1). Transport of emissions from Europe is particularly enhanced under positive NAO conditions. GOME tropospheric NO₂ columns, for instance, show much stronger transport of NO_x from European sources towards the Arctic (and reduced transport towards the Atlantic and south-central Asia) for high-NAO conditions (Figure 1).

Changes in transport pathways to the Arctic associated with climate change. In climate change simulations, many models indicate a poleward shift of the Atlantic stormtrack between the 2071-2100 and 1961-1990 climates. Over the last century poleward shifts in the stormtrack have been highly correlated with positive NAO phases. IPCC, 2001 (Chapter 9) states, "a few studies have shown increasingly positive trends in the indices of the NAO/AO in simulations with increased greenhouse gases, although this is not true in all models, and the magnitude and character of the changes varies across models." Ulbrich and Christoph (1999) examined the stormtrack and NAO in a 300 yr control run and 240 yr scenario run of a coupled model (ECHAM4+OPYC3). They found that stormtrack activity increased over northwestern Europe in the scenario run, but the increase in NAO index was barely significant. They showed that the two centers of action of the first empirical orthogonal function of mean sea level pressure (an alternative definition of the NAO) shifted downstream in the scenario run, such that the poleward center of action moved from the east coast of Greenland into the Norwegian Sea. The lack of trend in NAO index was attributed to the weak projection of the increase in European storm activity on the spatially-fixed NAO pattern derived from the recent climate. However the variability is defined, the implication is that the winter stormtrack is likely to shift polewards and downstream towards Scandinavia with ramifications for increased pollution transport into the Arctic. Also associated with this shift would be increased heat transport and decreased sea-ice extent. Possibilities exist for climate feedbacks involving sea-ice cover and albedo changes over ice/snowpack associated with enhanced deposition of black carbon.

Stratosphere-troposphere exchange. The low tropopause elevation, and its weak expression during the winter, combined with the large-scale downward transport in the stratosphere at high latitudes, suggest that the Arctic troposphere may be strongly influenced by injections of stratospheric air. Intensive sampling campaigns have documented distinct episodes of stratosphere-to-troposphere transport (STT), but mainly in the North American region (e.g. TOPSE-2000). Results indicate that STT is probably the dominant source of O₃ and HNO₃ in the winter, and remains significant through the year. However, the increase in O₃ in spring is likely caused by photochemical production (Browell et al., 2003). Model studies suggest photochemical production of O₃ becomes the major source in late spring and into summer (e.g., Mauzerall et al., 1996; Wang et al., 2003). A year-long study at Alert used the radionuclide tracers ⁷Be and ¹⁰Be to investigate the STT in the high Arctic (Dibb et al., 1994). This investigation confirmed that STT was significant even at the surface throughout the year. Careful quantitative assessment of this transport term remains mandatory for estimating the ozone winter depletion by halogen species and spring photochemical production from measurements of the ozone seasonal cycle. Using water vapor isotope measurements above the Arctic tropopause in Scandinavia, Zahn (2001) showed that there is a 1-2 km thick layer where upwelling air from the troposphere mixes with downwelling air from the stratospheric vortex. A better characterization of the chemical composition of this layer and its dynamical coupling with the troposphere or the stratosphere is still an open issue.

POLARCAT objectives related to transport processes.

• Quantification of the residence times of (polluted) air masses in the Arctic polar dome, and their seasonal dependence, using transport models validated with Lagrangian balloon data.
• Determination of the gradients in the chemical composition of the troposphere at the edge of the polar dome and how it relates to mixing with mid-latitude air masses.
• Study of the transport pathways of pollution from south Asia into the Arctic free troposphere and boundary layer, using models, satellite and aircraft measurements.
• Determination of the vertical layering of pollution from different source regions, including its seasonal variation.
• Exploration of the horizontal and vertical structure and extent of Arctic Haze, and how it changes seasonally, using aircraft and surface measurements.
• In-situ quantification of net heating rates in polluted air masses in the Arctic, using Lagrangian balloons (see Figure 2). Measurement of the vertical temperature stratification in the remote Arctic using repeated vertical soundings by Lagrangian balloons.
• Study of the interannual variability in pollution transport pathways and associated removal mechanisms (e.g., wet deposition) into the Arctic.
• Investigation of the likely change of transport processes in a future climate, using climate model predictions.
• Quantification of transport from the stratosphere on the tropospheric chemistry, including deposition of nitrogen species to the snowpack, using models and surface measurements of tracers

Occurrence and optical properties of Arctic aerosol and pollutant haze. As already described, each winter through spring a sulfate-rich, persistent haze is observed in the Arctic. During early February, significant enhancements in sulfate aerosol are confined near the surface (less than 2 km) as long-range transport from northern Eurasia occurs along low level, sinking isentropes (Klonecki et al. 2003). As the haze season progresses, enhanced sulfate occurs at higher altitudes (up to at least 8 km). Since vertical mixing is prohibited by the persistent low-level inversion (Kahl, 1990), the higher altitude haze layers are thought to be due to transport into the Arctic along vertically higher isentropes tracing back to increasingly warmer surface source regions in northern Eurasia. During early April, sulfate layers below 3 km begin to dissipate due to the beginning of solar heating and resulting mixing near the surface. However, more stable isentropic transport continues at higher altitudes. By the end of May, both the lower and higher altitude sulfate enhancements are significantly decreased due to the continued break-up of the inversion and return of wet deposition.

Pollutant particles within the Arctic Haze are well-aged with a mass median diameter of about 0.2 µm or less. This particle size range is very efficient at scattering solar radiation since the peak in the particle surface-area size distribution is near the maximum efficiency for Mie scattering. The haze also is weakly absorbing due to the presence of black carbon. The result of the strong scattering and weaker absorption is a noticeable reduction in visibility to a few kilometers or less. Model calculations suggest that the "weak" absorption has significant climatic influences when the dark colored haze spreads out over the highly reflecting snow and ice pack of the Arctic. The highly reflecting surface enhances aerosol-radiative interactions due to multiple scattering between the surface and the haze.

The seasonality and trends of Arctic Haze are clearly seen in time series data of light absorption and scattering by aerosols. Figure 3 shows ground-based measurements which depict the pronounced increase in light scattering during March and April. Both aerosol scattering and optical depth (AOD) measurements at Barrow showed a maximum in 1982 followed by a factor of two decrease between 1982 and 1992 (Bodhaine and Dutton, 1993). A combination of a reduction in the pollution aerosol output by Eastern Europe and the former Soviet Union, and stricter pollution controls in Western Europe most likely contributed to the decrease. However, from 1997 to 2005 there has been a significant (at the 95% confidence level) increasing trend (Quinn et al., 2005). Similarly, light absorption at Barrow indicates an overall decrease between 1988 and 2005 but an increase for both March and April between 1997 and 2005. These results support the hypothesis that increasing black carbon emissions from southern Asia may be impacting the Arctic (Koch and Hansen, 2004). AOD at Barrow and Ny Ålesund, Spitsbergen also appears to be increasing although the trends do not match in time (Herber et al., 2002). The apparent changing trends in aerosol burdens and associated optical properties in the Arctic provide impetus for further investigation into the causes and impacts.

While almost all trace atmospheric constituents in the Arctic boundary layer, including aerosol mass, reach minimum concentrations during summer, resulting also in the seasonal minimum of AOD, the number concentration of aerosol particles reaches a maximum that time of the year. Most likely, in situ formation of new particles causes this maximum; but the mechanism is an ongoing matter of discussion. Leck and Bigg (1999) hypothesized that organic films in the surface water of open ice leads provides a source of new particles in the Arctic summer atmosphere. Ström et al. (2003) showed that there is a very strong relation between the amount of solar radiation reaching the Arctic surface and the number density of aerosol particles, suggesting that photochemistry could be the key process in the formation of the new particles. Year-round aerosol size distribution measurements at Ny Ålesund revealed a prevailing accumulation mode (150 nm) in spring followed by a dominating nucleation mode (30 nm) in the summer (Ström et al., 2003). The transition between the two regimes occurred within a few days. This distinct seasonal change also is seen in the upper troposphere (Treffeisen et al., 2005).

Climate effects of pollutant haze - direct effects. Absorption and scattering of radiation by aerosols directly affect the radiation balance of the Arctic. This region is thought to be particularly sensitive to changes in radiative fluxes because of the small amount of solar energy normally absorbed in the polar regions. Arctic Haze is present as a layer of light absorbing material over a highly reflective ice/snow surface. Several early calculations using 1-D radiative transfer models estimated that the diurnally averaged atmospheric warming due to the aerosol layer ranged between 2 and 20 W/m² with a corresponding depletion of the solar flux at the surface of 0.2 to 6 W/m² (e.g., Leighton, 1983; Blanchet and List; 1987, Shaw et al., 1993). These estimates agreed with direct measurements from wideband sun photometers (Mendonca et al., 1981). Heating rates of about 0.1 to 0.2 K/day were measured by Valero et al. (1989) during AGASP (Arctic Gas and Aerosol Sampling Program) II and by Treffeisen et al. (2005) during the ASTAR 2000 campaign in Svalbard. The AASE (Airborne Arctic Stratospheric Expedition) II flights in winter of 1992 revealed soot contaminated Arctic aerosols at altitudes of 1.5 km. Pueschel and Kinne (1995) calculated that this layer of aerosols could heat the earth-atmosphere system above surfaces of high solar albedo (ice/snow) even for single scattering albedos as high as 0.98. Hence, a modest amount of black carbon in the haze layers can result in a measurable contribution to diabatic heating.

MacCracken et al. (1986) estimated that the cooling of the surface due to absorption of solar radiation by the haze layers could be compensated by infrared emission from the atmosphere to the surface. During the dark winter, infrared emissions from the haze may heat the surface if deliquescent sulfate salts grow and become cloud droplets or ice crystals thereby enhancing their impact in the longwave. In addition, since the haze is present throughout the Arctic night, the integrated effect may modify the radiative budget.

Climate effects of pollutant haze - indirect effects. The indirect effect of aerosol particles on irradiances in the Arctic results from the impact of aerosol particles on the microphysical properties of clouds. Enhanced aerosol particle concentrations increase solar cloud albedo due to increasing the number concentration and decreasing the average size of cloud droplets provided the liquid water content in the clouds remains constant (Twomey, 1977). An increase in the number concentration of pollution aerosol particles that act as cloud condensation nuclei (CCN) will affect Arctic stratus and stratocumulus by increasing the cloud droplet number concentration which results in more radiation being reflected back to space (Albrecht, 1989; Twomey, 1991). The relatively low aerosol number concentrations in the Arctic results in a large percentage of particles activating during cloud formation (e.g. Komppula et al., 2005). Hence, changes in aerosol properties are likely to have a significant impact on microphysical and optical cloud properties. As the cloud droplet number concentration increases, cloud droplet size decreases which reduces drizzle formation and increases cloud coverage and lifetime (Hobbs and Rangno, 1998). Garrett et al. (2004) showed that low-level Arctic clouds are highly sensitive to particles that undergo long range transport during the winter and early spring. The sensitivity was detected as higher cloud droplet number concentrations and smaller cloud droplet effective radii compared to summertime clouds exposed to particles nucleated in the Arctic from local biogenic sources. In addition, Arctic stratus appears to be more sensitive to pollutant particles than clouds outside of the Arctic. The most significant effect of the change in cloud properties due to Arctic Haze may be on cloud emissivity. A decrease in droplet effective radius in these optically thin clouds will increase the infrared optical depth and thus the infrared emissivity (Curry and Herman, 1985; Garrett et al., 2002). The result is expected to be an increase in downwelling infrared irradiances from the cloud and an increase in the rate of spring- time snow pack melting (Zhang et al., 1996).

According to observations during the SHEBA experiment, supercooled cloud droplets are common in the Arctic even at temperatures of -20°C or lower (Curry et al., 1996). The sulfate-containing pollution aerosol within Arctic Haze is thought to impact ice nucleation. Models estimate that aerosols containing sulfuric acid produce fewer ice nuclei than nearly insoluble aerosols (Blanchet and Girard, 1995). Measurements corroborate this finding. Borys (1989) reported that Arctic Haze aerosol had lower ice nuclei (IN) concentrations, a lower IN to total aerosol fraction, and slower ice nucleation rates than aerosol from the remote unpolluted troposphere. The reduction in ice nuclei leads to a decrease in the ice crystal number concentration and an increase in the mean size of ice crystals (Girard et al., 2005). As a result, the sedimentation and precipitation rates of ice crystals increase leading to an increase in the lower troposphere dehydration rate and a decrease in the downwelling infrared irradiances from the cloud. Using a 1-D simulation and observations from Alert, Girard et al. (2005) found that a cloud radiative forcing of -9 W/m² may occur locally from the enhanced dehydration rate produced by sulfate aerosol. The mechanism that decreases IN concentrations in the presence of sulfuric acid aerosol is unknown and warrants further research. If this mechanism applies to much of the Arctic, it could explain the cooling tendency in the eastern high Arctic during winter.

Because of the combination of the static stability of the Arctic atmosphere, the persistence of low level clouds, and the relatively long lifetime of aerosol particles during the haze season, the impact of aerosols on cloud microphysical and optical properties may be larger in the Arctic than elsewhere on Earth (Garrett et al., 2004). The winter/spring occurrence of Arctic Haze events allows the study of anthropogenic influences against a very clean atmospheric background. In other regions of the globe, a reliable distinction between natural and anthropogenic effects is more difficult. In this sense, the Arctic is a natural laboratory to study the anthropogenic portion of the aerosol-cloud-radiation interactions.

Climate effects of pollutant haze - surface. Surface albedo affects the magnitude and sign of climate forcing by aerosol particles. Absorbing soot deposited to the surface via wet and dry deposition impacts the surface radiation budget by enhancing absorption of solar radiation at the surface (Warren and Wiscombe, 1980). Clarke and Noone (1985) found a 1 to 3 % reduction in snow albedo due to deposited BC with another factor of 3 reduction as the snow ages and BC becomes more concentrated. Hansen and Nazarenko (2004) have estimated that soot contamination of snow in the Arctic and the corresponding decrease in surface albedo yields a positive hemispheric radiative forcing of +0.3 W/m². The resulting warming may lead to the melting of ice and may be contributing to earlier snowmelts on tundra in Siberia, Alaska, Canada, and Scandinavia (Foster et al., 1992). New techniques to measure surface albedo from airborne platforms have been developed recently (Wendisch et al., 2004).

Clearly, the radiative impacts of pollutant aerosol particles in the Arctic are quite complex. Multiple feedbacks between aerosols, clouds, radiation, sea ice, and vertical and horizontal transport processes complicate a comprehensive picture as do potentially competing effects of direct and indirect forcing. As a result, the magnitude and sign of the forcing are not yet well understood in this region. Technological advances made in the past decade have provided us with new tools to further improve our understanding of the Arctic Haze phenomenon. These advances include modern aircraft payloads, model calculations of long-range pollutant transport, and new spaceborne observational methods. With these new tools, we are well posed to re-visit the Arctic and address questions of aerosol effects that cover large horizontal and vertical scales.

POLARCAT objectives related to aerosol radiative effects in the Arctic.

• Improvement of knowledge on the sources, evolution and removal of the tropospheric aerosol particles in the Arctic Haze season.
• Determination of the vertical distribution of chemical, physical and optical properties of Arctic aerosol particles
• Characterization of direct radiative effects (solar and terrestrial) within pollution layers in the Arctic.
• Investigation of the interactions of aerosols with clouds and their impact on radiative forcing based on observations, experiments and model studies.
• Detailed in situ observations of microphysical and optical properties of Arctic clouds including particularly the ice phase (mixed-phase clouds).
• Determination of the role of aerosols as ice nuclei.
• Characterization of albedo changes of snow and ice surfaces and the resulting solar radiative effects due to the deposition of black carbon from anthropogenic and biomass burning sources.
• Validation of aerosol and cloud products of space observations from polar orbital satellites (Aqua-Train, i.e. CALIPSO, CloudSat, etc.)

Climate change and boreal forest fires. Climate change is an accepted reality, and observed and forecast impacts are greatest at northern latitudes and over land, particularly over the more continental regions of Canada, Russia and Alaska. These are areas where large fires have been common since the last Ice Age, and recent research (e.g. Stocks et al. 1998; Flannigan et al. 2003) indicates that more frequent and severe fires are expected as the climate changes. This will have a significant impact on the age class structure and carbon budget of the boreal/Arctic zone in particular and the globe in general. Boreal fires consume large quantities of fuel and spread quickly, creating high energy release rates that are often sustained for long burning periods. This frequently results in convection columns with strong vertical development that reach beyond the tropopause. Long-range smoke transport from large boreal fires is already common, with smoke loads from Siberian fires often reinforcing smoke from North American fires. This phenomenon is expected to become even more common with more frequent and severe fires in the future, increasing the likelihood that smoke from boreal fires will provide a positive feedback to climate change (Kurz et al. 1994).

Long-range transport of boreal forest fire emissions. Large amounts of smoke and trace gases emitted by boreal forest fires can be subject to considerable vertical and horizontal transport (e.g. Stocks and Flannigan, 1987; Siebert et al., 2000; Fromm et al., 2005). Fire emissions can travel over continental (Wotawa and Trainer, 2000), intercontinental (Forster et al., 2001; Honrath et al., 2004), and even hemispheric (Damoah et al., 2004) distances. Recent satellite observations and lidar measurements observed substantial amounts of forest fire smoke in the tropopause region and lower stratosphere at high latitudes and in the Arctic region (e.g. Waibel et al., 1999; Fromm et al., 2000; Damoah et al, 2004). Especially over snow/ice surfaces the short-wave reflectivity to space can be considerably reduced by forest fire smoke, which may have important implications for the radiative energy budget in the polar region (Hsu et al., 1999). Episodically, the fires also pollute large regions in the lower troposphere at high latitudes (Forster et al., 2001), but unfortunately few measurements in the Arctic free troposphere (e.g., during the ABLE 3A and 3B campaigns; Harriss et al., 1994, Shipham et al., 1992) and at the surface (e.g., Ily-Tuomi et al., 2003) exist. There is clear evidence for deposition of ammonia from biomass burning sources in Arctic ice core measurements (Whitlow et al., 1994). Presumably, the deposition of substances like soot from such fires may decrease the albedo of ice and snow and lead to enhanced melting of Arctic glaciers and sea ice (Kim et al., 2005). However, to date no data exist to reliably establish such a connection.

Pyro-convection. An exciting new development with respect to climate change is the recent discovery of transport of biomass burning emissions into the lower stratosphere through an explosive combination of intense forest fires and extreme convection (see Figure 4). This "pyroCb" source of stratospheric injection has been observed remotely (lidar, balloon sounding, and satellite solar occultation) (e.g. Fromm et al., 2000; Fromm and Servranckx, 2003) and in-situ (e.g. Jost et al., 2004). Although boreal fire research scientists have reported forest fire convection columns above 13 kilometres in height (e.g. Stocks and Flannigan 1987), recent publications and unpublished data paint an emerging picture of the pyroCb phenomenon as a recurring one, with hemispherical impact (e.g. Fromm et al., 2005). UTLS enhancements, attributable to pyroCb, of aerosols, carbon monoxide, ozone, and acetonitrile have all been observed. While observations clearly show that deep upward transport of biomass burning emissions into the upper troposphere and lower stratosphere is frequent (e.g., Nedelec et al., 2005), the mechanisms are poorly understood. Factors that could enhance convective uplift over the fires are the heat and water vapour released by the fire, microphysical cloud processes (Andreae et al., 2004), and probably radiation absorption by soot particles above the cloud tops and in the stratosphere.

The highest altitude where forest fire smoke was observed in situ was 17 km (remote sensing observations exist even at higher altitudes), several kilometers above the tropopause and at potential temperatures greater than 380 K (Jost et al., 2004), thus in a region that is commonly referred to as the stratospheric overworld. The chemical impact of the forest fire emissions at such high altitudes is unknown. Both efficient ozone formation as well as severe ozone destruction are possible scenarios. Furthermore, the stratospheric residence time of aerosols may in fact be long enough to affect stratospheric ozone depletion during the following winter/spring.

Due to their proximity to the Arctic, their large source strength, and the special processes accompanying them as described above, boreal forest fires need special attention during POLARCAT. For this, an integrated study using low- as well as high-flying aircraft, satellite measurements, and models is needed. The launches of AURA, CALIPSO and METOP come at an opportune point to observe these processes and also provide critical data needed for the flight planning. In addition, satellite data like TOMS aerosol index, solar/ lunar occultation profiles (e.g. POAM III, SAGE II and III, and GOMOS), MODIS imagery, MOPITT, IASI, ACE, NOAA POES and GOES imagers will be used to provide forecast guidance for potential fire blowup conditions worldwide, enabling also targeted operating modes for certain satellite instruments.

POLARCAT objectives related to boreal forest fires.

• Comprehensive study of the impact of boreal forest fire emissions on the chemical composition of the Arctic troposphere.
• Study of the pathways of boreal forest fire plumes into the Arctic, particularly regarding the plume altitudes.
• Quantification of the impact of the deposition of soot from forest fires on the surface albedo of snow and ice surfaces, and investigation of the link with Arctic sea ice and glacier retreat.
• Investigation of the contribution of pyroCb aerosol injections to the stratospheric background aerosol concentrations in the Arctic, in particular during volcanically-quiescent periods.
• Determination of the residence times of aerosols in the Arctic stratosphere, in particular whether forest fire aerosols can remain in the stratosphere long enough to play a role in winter/spring ozone depletion.
• Study of the fates and effects of chemical compounds injected into the stratosphere by pyroCbs, including their role for ozone formation and ozone depletion.

Background. The Arctic troposphere is a unique environment within the earth's atmospheric system. Its uniqueness stems from generally cold temperatures, a prolonged period of darkness followed by a period of continuous light, underlying snow and ice, and a low tropopause above. While there are virtually no anthropogenic pollution sources within the Arctic itself, it is impacted by emissions from many of the world's largest industrial regions (e.g., Rahn et al., 1977; Barrie, 1986). Initial research into Arctic Haze found that emissions from northern Eurasia were most significant, though recent evidence suggests that contributions from rapidly developing economies in eastern Asia are growing in importance (Koch and Hansen, 2005). In summer, the arctic free troposphere frequently receives "pollution" from boreal forest fires (e.g., Mauzerall et al., 1996; Dibb et al., 1996). The composition of the Arctic troposphere is further influenced by snow to air exchange of key trace chemicals followed by homogeneous and heterogeneous reactions (e.g., Honrath et al., 1999; Dibb and Arsenault, 2002; Dibb et al., 2002).

Atmospheric chemistry research in the Arctic has tended to come in waves targeting largely separate questions. The discovery of Arctic Haze in the late 1950s led to the development of a network of surface observatories. Between 1983 and 1991 four AGASP airborne campaigns complemented the surface network by documenting the vertical and horizontal distribution and composition of haze in the lower troposphere over the western sector of the Arctic in late winter and spring. Sampling at Alert and Barrow in support of haze investigations discovered severe ozone depletion events (ODEs) in the boundary layer over the Arctic Ocean at the time of polar sunrise (Oltmans, 1981; Barrie et al., 1988). This lead to a series of increasingly intensive ground based campaigns in which the importance of halogen chemistry and the link to mercury depletion events were discovered (e.g., Schroeder et al., 1998; Bottenheim et al., 2002). The TOPSE airborne sampling campaign in 2000 helped to establish the vertical and geographic extent of ODEs in the Canadian Arctic, though the primary focus of this mission was to establish the cause of the springtime maximum in ozone in the Arctic mid troposphere (Atlas et al., 2003). The discovery of the stratospheric ozone hole over Antarctica led to a series of airborne campaigns probing also the Arctic stratosphere between 1989 and 2005. Unfortunately these missions devoted very few flight hours to sampling in the Arctic troposphere. The ABLE 3 summer campaigns in 1988 and 1990 did sample the North American free troposphere, but platform limitations and a focus on constraining surface fluxes resulted in much of the sampling at low levels (Harriss et al, 1992, 1994).

Recent years have seen tremendous advances in in-situ measurement capability and satellite observations of tropospheric composition. IPY offers a unique and timely opportunity for a coordinated and integrated international experiment to explore the chemistry of the entire arctic troposphere and its impacts on global chemistry and climate. Several unique phenomena have been identified in the arctic troposphere that need further systematic and coordinated investigation. Salient among these are: (1) causes of surface ozone and mercury depletion events; (2) the likely presence and role of halogen free radicals; (3) presence of atmospheric reservoirs of reactive nitrogen and their influence on ozone chemistry; (4) emissions of OVOC and NOx from ice surfaces; (5) influences of stratospheric intrusions; and (6) investigations of glacial ice cores to understand past atmospheric composition and recent human impacts.

Surface ozone and mercury depletion. As noted above, the discovery of ODEs in the Arctic atmospheric boundary layer near the time of polar sunrise (Oltmans, 1981, Barrie et al., 1988) sparked one of the waves of interest in Arctic tropospheric chemistry. An equally surprising discovery was that gaseous elementary mercury (Hg) appeared to undergo depletion in concert with ozone/HO_x/NO_x raising the specter of a potential major contamination of the Arctic biosphere (Schroeder et al., 1998). Figure 5 and Figure 6 show examples of such coincident depletions and their vertical extent.

A mechanism suggested to cause the observed sudden BrO enhancements in the marine boundary layer is the autocatalytic release of BrO involving heterogeneous reactions on sea-salt surfaces (Tang and McConnel, 1996; Vogt et al., 1996). Substantial concentrations of BrO have been observed in both the Arctic and the Antarctic (Wagner and Platt, 1998, Frieß et al., 2004). The efficiency of this cycle is limited by conversion of Br to the non-radical reservoir species. Models have been developed to explain the role of bromine and iodine chemistry in ozone and Hg depletions (Calvert and Lindberg, 2003).

More recently, studies have pointed out the importance of polar snow as a source of ambient nitrogen oxides (NO_x) (Honrath et al., 1999) and of precursors of hydrogen oxide radicals (HO_x) such as HCHO during spring (Sumner and Shepson, 1999). Snowpack photochemistry has been identified as a likely cause of large interstitial-air and ambient HCHO concentrations, and was shown to result in NO_x concentrations in interstitial-air up to an order of magnitude larger than ambient levels, consistent with the presence of an unexpected diurnal cycle in ambient-air NO_x.

In short, the Arctic boundary layer in spring time is influenced by industrial pollution, halogen chemistry, and ice driven NO_x and OVOC intrusions at the same time. To date, attempts to develop a coherent mechanistic explanation for these depletion processes on the basis of known gas-phase chemistry has not been successful and heterogeneous processes involving snow and aerosol particles are likely implicated. Observational data are critically needed to analyze the coupled evolution of BrO_x/ClO_x/ IO-NO_x-HO_x-O₃ chemistry during O₃ and Hg depletion events in the Arctic spring.

Halogens in the free troposphere. Little attention has been paid to the possibility that reactive halogens may have a significant impact in the Arctic free troposphere. Recent field campaigns at Summit, Greenland (72ºN, 38ºW, 3.1 km asl) suggest that this may indeed be the case. High levels of peroxy radicals (HO₂+RO₂) were measured consistent with photochemical theory given observed mixing ratios of precursors. However, OH levels were significantly elevated compared to steady state model simulations and previous measurements at South Pole (Huey et al., 2004a, 2004b; Sjosted et al., 2005). Observed values of the (HO₂+RO₂)/ OH ratio were generally 4-5 times lower than expected from theory. This disagreement was greatly accentuated during periods of high wind, when observed values of the (HO₂ +RO₂)/OH ratio were more than an order of magnitude lower than model estimates. These observations lead to the hypothesis that halogen chemistry may be responsible for much of the observed disturbance in HO_x partitioning at Summit.

Satellite observations (GOME and SCHIMACHY) suggest that the atmospheric column of BrO above central Greenland during summer is often on the order of 3-5 x 10¹³ mol cm^-2 (Figure 7) which would yield mixing ratios near 20 ppt if most of the BrO were in a 1 km deep boundary layer above the ice sheet (Richter et al., 1998; Wagner and Platt, 1998). Although substantial concentrations of BrO may be near the surface there is reason to believe that this BrO is distributed throughout the Arctic troposphere. Measurements of large perturbations in hydrocarbon ratios, enhanced soluble gas phase bromine (Dibb, unpublished data; Evans et al., 2003; Ridley et al., 2003) and ozone depletion (Peterson and Honrath, 2001; Helmig et al., 2005) in air filling the pore spaces of the snowpack indicate that halogen activation may proceed via heterogeneous reactions on ice crystal surfaces similar to those observed during polar sunrise at lower elevations in the Arctic. A small reactive halogen flux from the snow pack into overlying air could account for persistently elevated OH throughout summer. The impact on photochemistry in the free troposphere above sunlit snow may be significant if snow-impacted boundary-layer air is vertically mixed upward.

Reactive nitrogen, hydrogen, and ozone in the free troposphere. As has been stated above, the composition of the Arctic free troposphere and its linkages with the surface below and the stratosphere above have not been extensively studied. The ABLE-3A and 3B (Harris et al., 1992; 1994) campaigns did study O₃ chemistry but were restricted to middle troposphere altitudes due to platform limitations and were performed at a time when suitable instrumentation to measure many key species (e. g. free radicals) was unavailable. As noted earlier, one of the primary ABLE 3 objectives was to constrain biosphere/atmosphere exchange, hence there was a lower troposphere focus. A more recent effort was in TOPSE during Feb-March 2000 with an altitude limitation similar to the ABLE 3 experiments but improved instrumentation (Atlas et al., 2003). In this campaign, determining the impact of stratosphere-to-troposphere transport (STT) on the oxidative capacity of the troposphere was one of the key objectives. It should be noted that STT was found to be significant during both the summertime and the spring. However, model calculations constrained by the ABLE 3 and TOPSE data sets found that photochemical production appeared to be the dominant source of O₃ in the Arctic troposphere from late spring into summer.

The free troposphere of the Arctic is greatly influenced by fires in summer and Eurasian outflow in spring. Figure 8 shows an example of the role of PAN in the free troposphere where 80% of all reactive nitrogen is tied in this form. This is a unique feature of a very cold atmosphere where the organic forms of reactive nitrogen are highly stabilized (Singh et al., 1992). However, PAN is easily decomposed to produce NO_x and influence O₃ chemistry in other regions of the troposphere. There is a clear need to perform field missions that can cover the entire Arctic troposphere in at least two seasons with a capability to measure the O₃-HO_x-NO_x-halogen cycle including precursors and aerosols.

POLARCAT objectives related to tropospheric composition and chemistry.

The objective is to investigate the composition and chemistry of the entire Arctic troposphere in two seasons (spring and summer) with the goal of understanding the reactive nitrogen, reactive hydrogen, reactive halogen, and ozone cycles. An integrated approach that links surface, free tropospheric, and satellite observations with models of chemistry and climate is envisioned. The intensive sampling will quantify the relative importance of transport to and from the free troposphere, impact of halogen/OVOC/NO_x formation in the snowpack on the free troposphere, and the role of Arctic reservoir species on the global troposphere. Specific objectives are:

• Determination of the chemical composition of the entire Arctic troposphere in two seasons (spring and summer) using airborne, satellite, and surface platforms at a level of detail not hitherto possible.
• Improved understanding of the O₃/NO_x/HO_x chemistry in the Arctic troposphere.
• Investigation of the role of halogen atom chemistry in the Arctic boundary layer and the free troposphere.
• Improved understanding of the impact of snow pack emissions of NO_x, OVOC, and halogens on the Arctic troposphere.
• Validation of satellite observations of tropospheric composition.
• Improved knowledge of sources impacting the Arctic troposphere.
• An assessment of the impact of pollution transport to the Arctic on chemistry and climate based on an integrated analysis of data collected during IPY 2007/8 campaigns using 3-D models.

The Arctic is an ideal region for analyzing satellite measurements. Most of the instruments suitable for the characterization of aerosols and trace gases in the troposphere are carried on polar orbiting satellites and their tracks are most densely packed at the poles. Therefore, overpasses over a fixed location are much more frequent close to the poles than at lower latitudes (see Figure 9, for an example), providing excellent opportunities for comparisons with ground-based or aircraft measurements in the Arctic. The Arctic is also special because of the high albedo of ice and snow surfaces, which can enhance signals in DOAS-type retrievals of trace gas columns, but also makes detection of light scattering by aerosols more difficult.

Meteorological satellite data. Detection of cloud, water vapor and surface features at high temporal resolution (less than 1 hour) in the mid-latitudes at all longitudes is made possible by six geostationary satellites positioned around the equator. However, north of the Arctic Circle the geostationary images are distorted by the curvature of the Earth's surface. Therefore, at high latitudes polar orbiting weather satellites must be relied upon to provide images of clouds and surface features at high temporal resolution. While each of the four NOAA Polar-orbiting Operational Environmental Satellites (POES) passes over the Arctic region once every 102 minutes, their inclined orbit, coupled with the rotation of the Earth means the portion of the Arctic viewed by a particular satellite changes with each orbit. The inconsistency of the viewable region of the Arctic makes it difficult to track meteorological features associated with air pollution transport. For POLARCAT, merged visible, IR, and water vapor products using images from the Terra and Aqua satellites, all four NOAA/POES satellites and geostationary satellites will be created.

Aerosol and trace gas satellite measurements. A wide range of satellite data on the composition of both the troposphere and the stratosphere will be available for POLARCAT. Table 2 has been compiled from information given by people involved in the POLARCAT consortium.

Numerical models of atmospheric composition will play an important role in POLARCAT, both in addressing the key scientific issues of the project using the measurements made, and in operational support of the field campaign periods. A hierarchy of models will be used, ranging from process-based photochemical box models capable of detailed representation of reactive radical chemistry and aerosol microphysics to 3-D Chemical Transport Models (CTMs) and General Circulation Models (GCMs) able to resolve the interaction of aerosol, chemical and dynamical processes and to determine the impacts on regional or global climate.

The Arctic presents a unique photochemical environment characterized by low UV intensities, cold temperatures, halogen radical chemistry, and ice-covered surfaces, and strongly stratified conditions lead to aging times of several weeks. The dynamical and chemical conditions here are challenging to model, and the current generation of global CTMs show widely divergent behaviour over polar regions, as seen in recent studies of surface ozone (Stevenson et al., 2005). POLARCAT observations will provide a valuable test of the ability of CTMs to simulate the chemical and microphysical evolution of air masses in the region, and in combination with more detailed process-based box model analysis will contribute to improved treatments of stratification, slow and/or novel chemistry and surface processes. Reducing uncertainties in modelling the Arctic region is an important goal of the project and will contribute to an improved understanding of the impacts on regional chemistry and climate.

Analysis of POLARCAT observations will make use of box models, trajectory models and CTMs. Photochemical box (0-D) models including detailed representation of chemical processes (Crawford et al., 1999; Evans et al., 2003) will be used to interpret aircraft observations in terms of radical chemistry in the Arctic with particular attention to processes involving halogen radicals and heterogeneous reactions. Another class of box models including detailed representation of aerosol microphysics will be needed to describe the unique Arctic environment for nucleation and growth of particles. 3-D particle dispersion models will be used to derive flow climatologies for the Arctic region, and to determine source-receptor relationships to aid in interpretation of aircraft observations (Stohl et al., 2003). Photochemical box models following air mass trajectories will be used to trace the chemical evolution of air masses entering and leaving the region (e.g., Methven et al., 2003). Finally, 3-D chemical transport models (CTMs) will integrate the information from the surface, aircraft, and satellite platforms in terms of the constraints that they provide on source regions affecting the Arctic atmosphere, transport between mid-latitudes and the Arctic, large-scale vertical motions, and the chemical and aerosol evolution coupled with these dynamical processes. The CTMs will need to be at least hemispheric in scale to describe the range of motions affecting Arctic atmospheric composition.

Beyond their value for post-mission data analysis, the CTMs will be of critical importance for the planning and execution phases of the POLARCAT field missions. Model simulations conducted before the mission using hindcast meteorological fields, and evaluated with pre-existing surface and satellite observations in the Arctic, will provide critical input for selecting optimal mission time windows, bases of operations, and flight regions. The hindcast simulations will be used to develop a menu of flights to guide mission execution. During the execution phase of the missions, the same CTMs driven by meteorological forecasts will provide chemical forecasts to guide the aircraft on a day-to-day basis. These forecasting activities will involve a number of CTMs to provide different perspectives and to address the broad range of mission objectives. This hindcast-forecast methodology has been applied very successfully in a number of recent aircraft measurement campaigns including TRACE-P (Jacob et al., 2003; Kiley et al., 2003), ITCT-2K2 (Parrish et al., 2004; Forster et al., 2004) and the recent ICARTT/INTEX campaign.

In the 20th century, the anthropogenic emissions resulted in a general increase of the aerosol load (sulfate as well as carbonaceous) in North America, Europe, and East Asia. In the first part of the 21st century, according to the IPCC SRES scenario A1B, the most polluted regions are found at lower latitudes (Brazil, Africa, the Arabian Peninsula, India and China) whilst sulphate and carbonaceous aerosols have both decreased in North America and Europe. To study scenarios of possible future climate conditions and emission distributions, fully coupled aerosol-chemistry-climate models are needed. Transient as well as time-slice simulations using these models will be performed to study how Arctic Haze will develop in the future. Furthermore, these simulations will be used to identify possibly important feedback processes in the climate system involving aerosol and pollution transport at high latitudes.

POLARCAT will bring together intensive aircraft experiments, research ship cruises, monitoring activities at surface stations, ground-based remote sensing, balloon releases, satellite measurements, and a range of different models. In order to achieve its overall goals, POLARCAT will closely co-ordinate these different activities. For instance, the aircraft and shipboard experiments will be supported by forecasts from meteorological and chemical models, satellite observations, surface networks and enhanced ozone sonde releases. In some cases, pathfinder aircraft carrying remote sensing instrumentation will be used to guide other aircraft carrying in-situ instrumentation into pollution layers.

Aircraft measurements will also be closely co-ordinated with each other and with the releases of Lagrangian balloons, in order to sample the same polluted air masses repeatedly. Such a Lagrangian approach will allow constraining the overall chemical budget in an air parcel between individual observations.

The airborne measurements will be coordinated with satellite overpasses, especially of Aura, Aqua, Terra, Envisat, and Calipso. Validation of the satellite observations of tropospheric composition and aerosol parameters will receive a high priority. Vertical aircraft profiling will also be done above surface stations and the ship, in order to characterize the vertical (and horizontal) extent of phenomena observed at the stations. POLARCAT will also work together closely with other IPY core activities. For instance, it is planned that aircraft perform overflights over the icebreakers used in other programs (e.g., OASIS).

POLARCAT is a bottom-up project that will remain open for others to join. Therefore, the following description of activities reflects the current stage of planning. Additional activities can be suggested by others and added at any time. Figure 10 shows the location of the most important POLARCAT surface sites, and Figure 11 shows an overview of the major field activities using mobile platforms planned for spring and summer 2008. Note that activities are also planned during other periods, but these will be the periods with most concentrated efforts. In the following, short descriptions of all planned activities (aircraft experiments, ship cruises, balloon flights, surface stations, remote sensing, models) will be given. Table 3 gives an overview of the time schedule for the various activities.