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 result 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 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 stratospheric 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 mid-latitude 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 shipboard 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 measurements (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.