High Arctic aircraft measurements characterising black carbon vertical variability in spring and summer
Authors: Schulz Hannes , Zanatta Marco , Bozem Heiko , Leaitch W. Richard , Herber Andreas B. , Burkart Julia , Willis Megan D. , Kunkel Daniel , Hoor Peter M. , Abbatt Jonathan P. D. , Gerdes Rüdiger 
Published: February 2019

Abstract. The vertical distribution of black carbon (BC) particles
in the Arctic atmosphere is one of the key parameters controlling their
radiative forcing and thus role in Arctic climate change. This work
investigates the presence and properties of these light-absorbing aerosols
over the High Canadian Arctic (>70∘ N). Airborne campaigns were
performed as part of the NETCARE project (Network on Climate and Aerosols:
Addressing Key Uncertainties in Remote Canadian Environments) and provided
insights into the variability of the vertical distributions of BC particles
in summer 2014 and spring 2015. The observation periods covered evolutions of
cyclonic disturbances at the polar front, which favoured the transport of air
pollution into the High Canadian Arctic, as otherwise this boundary between
the air masses largely impedes entrainment of pollution from lower latitudes.
A total of 48 vertical profiles of refractory BC (rBC) mass concentration and
particle size, extending from 0.1 to 5.5 km altitude were obtained with a
Single-Particle Soot Photometer (SP2). Generally, the rBC mass concentration decreased from spring to summer by a
factor of 10. Such depletion was associated with a decrease in the mean rBC
particle diameter, from approximately 200 to 130 nm at low altitude. Due to
the very low number fraction, rBC particles did not substantially contribute
to the total aerosol population in summer. The analysis of profiles with potential temperature as vertical coordinate
revealed characteristic variability patterns within specific levels of the
cold and stably stratified, dome-like, atmosphere over the polar region. The
associated history of transport trajectories into each of these levels showed
that the variability was induced by changing rates and efficiencies of rBC import.
Generally, the source areas affecting the polar dome extended southward with increasing
potential temperature (i.e. altitude) level in the dome. While the lower dome was mostly
only influenced by low-level transport from sources within the cold central and marginal
Arctic, for the mid-dome and upper dome during spring it was found that a cold
air outbreak
over eastern Europe caused intensified northward transport of air from a corridor over
western Russia to central Asia. This sector was affected by emissions from gas flaring,
industrial activity and wildfires. The development of transport caused rBC concentrations in the second lowest
level to gradually increase from 32 to 49 ng m−3. In the third level this caused the initially low rBC concentration to increase from
<15 to 150 ng m−3. A shift in rBC mass-mean diameter, from above 200 nm in the lower polar dome dominated by low-level
transport to <190 nm at higher levels, may
indicate that rBC was affected by wet removal mechanisms preferential to
larger particle diameters when lifting processes were involved during
transport. The summer polar dome had limited exchange with the
mid-latitudes. Air pollution was supplied from sources within the marginal
Arctic as well as by long-range transport, but in both cases rBC was largely
depleted in absolute and relative concentrations. Near the surface, rBC
concentrations were <2 ng m−3, while concentrations increased to
<10 ng m−3 towards the upper boundary of the polar dome. The
mass mean particle diameter of 132 nm was smaller than in spring;
nonetheless the summer mean mass size distribution resembled the spring
distribution from higher levels, with depletion of particles >300 nm. Our work provides vertical, spatial and seasonal information of rBC
characteristics in the polar dome over the High Canadian Arctic, offering a
more extensive dataset for evaluation of chemical transport models and for
radiative forcing assessments than those obtained before by other Arctic aircraft
campaigns.