Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere during the cold winter 2015/2016
Authors: Ziereis Helmut , Hoor Peter , Grooß Jens-Uwe , Zahn Andreas , Stratmann Greta , Stock Paul , Lichtenstern Michael , Krause Jens , Bense Vera , Afchine Armin , Rolf Christian , Woiwode Wolfgang , Braun Marleen , Ungermann Jörn , Marsing Andreas , Voigt Christiane , Engel Andreas , Sinnhuber Björn-Martin , Oelhaf Hermann 
Published: March 2022

Abstract. During winter 2015/2016, the Arctic stratosphere was characterized
by extraordinarily low temperatures in connection with a very strong polar
vortex and with the occurrence of extensive polar stratospheric clouds. From
mid-December 2015 until mid-March 2016, the German research aircraft
HALO (High Altitude and Long-Range Research Aircraft) was deployed to probe
the lowermost stratosphere in the Arctic region within the POLSTRACC (Polar
Stratosphere in a Changing Climate) mission. More than 20 flights have
been conducted out of Kiruna, Sweden, and Oberpfaffenhofen, Germany, covering
the whole winter period. Besides total reactive nitrogen (NOy),
observations of nitrous oxide, nitric acid, ozone, and water were used for
this study. Total reactive nitrogen and its partitioning between the gas and
particle phases are key parameters for understanding processes controlling
the ozone budget in the polar winter stratosphere. The vertical
redistribution of total reactive nitrogen was evaluated by using
tracer–tracer correlations (NOy–N2O and NOy–O3). The
trace gases are well correlated as long as the NOy distribution is
controlled by its gas-phase production from N2O. Deviations of the
observed NOy from this correlation indicate the influence of
heterogeneous processes. In early winter no such deviations have been
observed. In January, however, air masses with extensive nitrification were
encountered at altitudes between 12 and 15 km. The excess NOy amounted
to about 6 ppb. During several flights, along with gas-phase
nitrification, indications for extensive occurrence of nitric acid
containing particles at flight altitude were found. These observations
support the assumption of sedimentation and subsequent evaporation of nitric acid-containing particles, leading to redistribution of total reactive
nitrogen at lower altitudes. Remnants of nitrified air masses have been
observed until mid-March. Between the end of February and mid-March,
denitrified air masses have also been observed in connection with high
potential temperatures. This indicates the downward transport of air masses
that have been denitrified during the earlier winter phase. Using
tracer–tracer correlations, missing total reactive nitrogen was estimated
to amount to 6 ppb. Further, indications of transport and mixing of these
processed air masses outside the vortex have been found, contributing to the
chemical budget of the winter lowermost stratosphere. Observations within
POLSTRACC, at the bottom of the vortex, reflect heterogeneous processes from
the overlying Arctic winter stratosphere. The comparison of the observations
with CLaMS model simulations confirm and complete the picture arising from
the present measurements. The simulations confirm that the ensemble of all
observations is representative of the vortex-wide vertical
NOy redistribution.