Springtime Arctic aerosol: Smoke versus haze, a case study for March 2008
Highlights
► A case of each Arctic Haze and Arctic Smoke (biomass burning) is presented. ► Photometer and lidar data are analyzed from 2 Arctic sites. ► Microphysical aerosol properties are similar for both cases. ► The applicability and limitations of air trajectories are discussed. ► In the Arctic the driving meteorological field limits the trajectories’ precision.
Introduction
In the Arctic troposphere increased aerosol loads can occur during spring, a phenomenon which is called Arctic Haze (Quinn et al., 2007). The radiation impact of this aerosol is still poorly understood. Moreover, the Arctic environment is very vulnerable during that season, as aerosol deposition on snow or ice covered surfaces reduces the albedo and favors an earlier onset of the melting season (Flanner et al., 2007, Clarke and Noone, 2007, Stroeve et al., 2007). The direct forcing of aerosol depends, among other factors, on its soot content (Stone et al., 2008, Ramanathan and Carmichael, 2008) and surface albedo. For these reasons a modeling of the radiative impact of aerosol is still challenging and large regional deviations in temperature response, including both warming and cooling, must be considered (Rinke et al., 2004, Treffeisen et al., 2005).
While in earlier studies on Arctic Haze (Rahn, 1981, Barrie, 1986, Yamanouchi et al., 2005, Law and Stohl, 2007) an anthropogenic origin was already shown, several publications during recent years also revealed that biomass burning (as well forest fire as from agricultural origin) is one possibly important constituent of Arctic Haze as well (Warneke et al., 2009, Fu et al., 2009, Stohl et al., 2007). However, so far to our knowledge biomass burning aerosol has overwhelmingly been observed in summer over Spitsbergen (Stohl et al., 2006) or due to agricultural flaming in eastern Europe once in May 2006 (Stohl et al., 2007). The extremely large AOD of more than 0.5 at 500 nm in May 2006 was observed over Ny-Ålesund due to direct transport of polluted air masses caused by a strong Icelandic low in the lowest 3 km of the troposphere.
On the other hand, biomass burning events turned out to be the main source for air pollution in Alaska already in April 2008 (Warneke et al., 2009, Warneke et al., 2010). Generally air transport into the Arctic is facilitated if it occurs isentropically, along paths with constant potential temperature. This means that cold Eurasian sites should be the main source regions for short living pollutants (Barrie, 1986). The economical growth of East Asia may lead to increased pollution entry from this region (Koch and Hansen, 2005) but due to the temperature gradient to the Arctic this transport pattern will predominantly take place in the high troposphere.
There are indications that in Siberia the boreal vegetation is spreading North (Soja et al., 2007) and forest fires might have increased during the 20th century (Kasischke et al., 2004). Furthermore the climate predictions (as unsure as they are in the Arctic) indicate an increase of Siberian fire events for the 21st century (Malevsky-Malevich et al., 2008). Therefore it is possible that in the future the “Asian” components of aerosol might become more important than the “European” ones.
The pollution pathways for Arctic Haze can be quite complex which necessitates aerosol measurements, especially in the Russian part. In this paper we present remote sensing measurements of aerosol events at two different Arctic sites, from the Russian drifting ice-flow NP-35 and from Spitsbergen. An early season fire event and “classical” Arctic Haze have been measured and compared with photometer and lidar.
Section snippets
Instrumentation and measurement sites
The measurements were performed at the AWIPEV Research Base in Ny-Ålesund, Spitsbergen (78.9°N, 11.9°E, referred to as Ny-Ålesund) and at the 35. North Pole Drifting Station (referred to as NP-35). Ny-Ålesund is operated by the German Alfred Wegener Institute of Polar and Marine Research (AWI) and the French Institut polaire français Paul-Emile Victor IPEV. Ny-Ålesund provides a unique infrastructure for atmospheric research in the European Arctic with different scientific facilities, including
Biomass burning aerosol at NP-35 (17.03.2008)
Sun photometer measurements in March 2008 at the NP-35 showed two days (17./21.03.) with unusual high AOD values (τ500nm > 0.3, see Fig. 2 and Table 1). The trajectory calculations for March 17 in Fig. 3(a) shows the peninsula Kamchatka as source region. A slight difference in horizontal pathways can be seen between PEP-Tracer (NCEP), PEP-Tracer (ECMWF) and HYSPLIT. The horizontal and vertical spread of the trajectories is very low, which indicates stable atmospheric conditions. Warneke et al.
Aerosol event on March 21 and 23
The time series of sun photometer measurements in March 2008 in Fig. 2 shows additionally to the discussed March 17 two more days with high AOD values – March 21 at NP-35 and March 23 at Ny-Ålesund. The calculated trajectories for the March 21 and March 23 in Fig. 3(b) and Fig. 3(c) show for both days almost identical backward trajectories arriving from northwestern Siberia. For March 21 the output of all trajectory models is equal. Larger differences between PEP-Tracer (NCEP) and PEP-Tracer
Discussion
A summary of all photometer measurements during March 2008, both from NP-35 and Ny-Ålesund is given in Table 1. The separation in fine and coarse mode was calculated according to O’Neill et al. (2003). Persistently the AOD over NP-35 was larger, and the 3 days with highest AOD are shown in detail. The forest fire event (March 17) showed the highest AOD recorded during the campaign. Noticeable is also the low variability for that day, both in fine and coarse mode. This stability means that
Conclusion
The presented analysis clearly shows the importance of aerosol measurements in the Central and Russian Arctic to interpret observations in the European Arctic. Without photometer measurements at NP-35 the strong Khabarovsk forest fire event would have been completely unnoticed, despite its significant impact on AOD in the central Arctic.
During March 2008 the measured AOD was systematically higher over NP-35 than over Ny-Ålesund, even without the forest fire event. This result is astonishing if
Acknowledgments
We would like to acknowledge N. O’Neill for providing the spectral deconvolution code. Special thanks we would like to give to our Russian partner V. Sokolov from the Arctic and Antarctic Research Institute (AARI) in St. Petersburg. His cooperation and organization efforts where essential for the successful participation of J. Gräser (AWI) at NP-35.
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