Seasonal aspects of pollution transport . Field studies established that the Arctic haze phenomenon occurs regularly 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 equator-ward 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 vapour 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.