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 (< 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 .