Geoscience Reference
In-Depth Information
However, there is still uncertainty regarding the relative contribution of different
geopolitical contributions to Arctic pollution. It has been suggested that Southeast
Asia may be a significant source of black carbon (soot) aerosols in the Arctic tropo-
sphere (Koch and Hansen,
2005
).
The peak in Arctic haze in late winter and spring reflects both the nature of the
atmospheric circulation that promotes transport from northern Eurasia and slow
removal processes. Concentrations are spatially and temporally heterogeneous. For
example, aerosol optical depth (a unitless measure of the cumulative depletion of
radiation through the atmosphere by aerosols; a larger value means more deple-
tion) measured from airborne sun photometer measurements during April 2009 in
the European to the Alaskan Arctic, from subarctic latitudes, and near the North
Pole ranged from about 0.12 to more than 0.35, with the aerosol haze concentrated
within and just above the surface-based temperature inversion layer (Stone et al.,
2010
). Based on continuous measurements at the Koldewey station in Spitzbergen
(79°N, 12°E), Arctic haze in spring was present 40 percent of the time, compared
to less than 4 percent in summer (Herber et al.,
2002
). The lower frequency of
haze events in summer reflects the greater efficiency of removal processes in this
season (Bourgeois and Bey,
2011
). Forest fires, which are more common in spring
and summer, can also lead to substantial concentrations of climate-relevant aerosol
species in the Arctic, including black carbon. C. Warneke and colleagues (
2010
)
find that forest fires in Russia during spring can double the high Arctic atmospheric
background aerosol concentrations that have built up during winter.
Globally, the direct climate effect of aerosols is cooling through the scattering of
solar radiation. However, direct effects depend on the type of aerosol. In contrast
to sulfate aerosols that cool, black carbon aerosols strongly absorb solar radiation
and hence have a warming effect on the atmosphere. D. Shindell and G. Faluvegi
(
2009
) argue that, because of a combination of cleaner combustion techniques in
the United States and Europe but growing anthropogenic emissions from Asia, the
Arctic now has more black carbon aerosols. Their modeling study suggests that this
change has substantially contributed to the observed Arctic warming over the past
three decades.
There are two known major aerosol indirect effects linked to cloud cover that
revolve around the role of aerosols as cloud condensation nuclei (CCN). The first,
known as the cloud-albedo or Twomey effect, says that an increase in CCN leads
to more cloud droplets of smaller size, manifested as an increase in cloud albedo.
The second is known as the life-time or Albrecht effect; that is, an increase in aero-
sol concentrations leads to more abundant and smaller droplets that take longer to
grow to precipitable sizes through collisions, increasing cloud lifetime. For both
of these indirect aerosol effects, impacts on cloud albedo compete with effects on
cloud longwave emissivity and hence the downward longwave flux to the surface.
C. Mauritzen et al. (
2011
) show that over the central Arctic when there are condi-
tions with low aerosol concentrations, a small increase in aerosol loading can sub-
stantially enhance cloudiness, which e warms the surface through enhancing the
downward longwave radiation flux.
