ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-6057-2018Mixing and ageing in the polar lower stratosphere in winter 2015–2016Mixing and ageing in the polar lower stratosphere in winter 2015–2016KrauseJenskrauseje@uni-mainz.dehttps://orcid.org/0000-0002-2136-1038HoorPeterEngelAndreashttps://orcid.org/0000-0003-0557-3935PlögerFelixGrooßJens-Uwehttps://orcid.org/0000-0002-9485-866XBönischHaraldhttps://orcid.org/0000-0002-1004-0861KeberTimoSinnhuberBjörn-Martinhttps://orcid.org/0000-0001-9608-7320WoiwodeWolfgangOelhafHermannInstitute for Atmospheric Physics, Johannes Gutenberg-University of Mainz, Mainz, GermanyInstitute for Atmospheric and Environmental Sciences, University of Frankfurt, Frankfurt, GermanyInstitute of Energy and Climate Research (IEK-7), FZ Jülich, Jülich, GermanyInstitute of Meteorology and Climate Research (IMK), Karlsruhe Institute of Technology (KIT), Karlsruhe, GermanyJens Krause (krauseje@uni-mainz.de)2May20181886057607312October201726October201723February20181April2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/6057/2018/acp-18-6057-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/6057/2018/acp-18-6057-2018.pdf
We present data from winter 2015–2016, which were measured during the
POLSTRACC (The Polar Stratosphere in a Changing Climate) aircraft campaign
between December 2015 and March 2016 in the Arctic upper troposphere and
lower stratosphere (UTLS). The focus of this work is on the role of transport
and mixing between aged and potentially chemically processed air masses from
the stratosphere which have midlatitude and low-latitude air mass fractions
with small transit times originating at the tropical lower stratosphere. By
combining measurements of CO, N2O and SF6 we estimate the evolution
of the relative contributions of transport and mixing to the UTLS composition
over the course of the winter.
We find an increasing influence of aged stratospheric air partly from the
vortex as indicated by decreasing N2O and SF6
values over the course of the winter in the extratropical lower and lowermost
stratosphere between Θ=360 K and Θ=410 K over the North
Atlantic and the European Arctic. Surprisingly we also found a mean increase in CO of (3.00 ± 1.64) ppbV from January to March
relative to N2O in the lower stratosphere. We show that this
increase in CO is consistent with an increased mixing of tropospheric air as
part of the fast transport mechanism in the lower stratosphere surf zone. The
analysed air masses were partly affected by air masses which originated at
the tropical tropopause and were quasi-horizontally mixed into higher
latitudes.
This increase in the tropospheric air fraction partly compensates for ageing
of the UTLS due to the diabatic descent of air masses from the vortex by
horizontally mixed, tropospheric-influenced air masses. This is consistent
with simulated age spectra from the Chemical Lagrangian Model of the
Stratosphere (CLaMS), which show a respective fractional increase in
tropospheric air with transit times under 6 months and a
simultaneous increase in aged air from upper stratospheric and vortex regions
with transit times longer than 2 years.
We thus conclude that the lowermost stratosphere in winter 2015–2016 was
affected by aged air from the upper stratosphere and vortex region. These air
masses were significantly affected by increased mixing from the lower
latitudes, which led to a simultaneous increase in the fraction of young air
in the lowermost Arctic stratosphere by 6 % from January to March 2016.
Introduction
Uncertainties in the description of mixing introduce large uncertainties to
quantitative estimates of radiative forcing which are of the order of 0.5 W m-2. Therefore it is important to quantify the
contribution of the dynamical processes which act on the distribution of
tracers. The Arctic UTLS during winter is affected by diabatic descent from
the stratosphere and quasi-horizontal mixing by the shallow branch of the
Brewer–Dobson circulation, which connects the tropical tropopause region with
the high Arctic e.g.. We present data from
winter 2015–2016, which were measured during the POLSTRACC (The Polar Stratosphere in a Changing Climate) aircraft campaign between December 2015
and March 2016 in the Arctic upper troposphere and lower stratosphere (UTLS)
(see Fig. ).
During winter the UTLS region (Fig. ) at high latitudes is strongly affected by the evolution of
the polar vortex. Diabatic descent in the polar stratosphere, which is
strongest inside the polar vortex, results as part of the Brewer–Dobson
circulation at midlatitudes and high latitudes as
a response to the breaking of planetary and gravity waves
in the upper stratosphere and
mesosphere. This downwelling leads to an increasing contribution of
stratospheric air masses from the overworld (defined as the region where
isentropes are entirely located in the stratosphere; ).
Over the course of the winter they contribute to the composition of the lower
overworld (Θ<420 K), where our measurements took place, and the
lowermost stratosphere (LMS) (defined as the region
bounded by the 380 K isentrope and the extratropical tropopause;
).
Air masses descending from the upper stratosphere and mesosphere
differ chemically from the composition of the LMS, since they are potentially affected
by ozone-depleting catalytic cycles . Since the air inside
the polar vortex is largely isolated and exhibits a strong diabatic descent
due to radiative cooling and the wave-driven Brewer–Dobson circulation, this
leads to an increased fraction of air masses with a high mean age of air in
the UTLS of high latitudes e.g..
The mean age of air is defined as the first moment of the transit time
distribution (or the age spectrum) . Mean
age can be determined from the observation of long-lived tracers, which
ideally have no sources or sinks in the stratosphere and of which the
temporal evolution of the mixing ratio at the tropical tropopause is well
known . Notably, the mean age is a bad descriptor for the
full age spectrum, which is highly skewed e.g. and
sometimes even multimodal . To estimate the
potential chemical impact of species, particularly with lifetimes of the order
of weeks to only a few months, the mean age is insufficient and the full
spectrum is needed , which is, however, only available
under very idealised conditions .
Cross section of the Northern Hemispheric UTLS (upper troposphere –
lower stratosphere) region, adapted from and
. The thermal tropopause is denoted by the thick black
solid line. The measurement region is depicted as a blue box subdivided into
the extratropical tropopause layer (ExTL), the lowermost stratosphere (LMS)
and the lower stratosphere (LS). LMS and LS are separated by the 380 K
isentrope (green dashed line). Transport pathways of air masses are denoted
by coloured, thick arrows from the tropical tropopause layer (TTL) (red) and
the polar Arctic upper stratosphere (blue) with respective mean age Γ
and trace-gas volume mixing ratio χ. Quasi-horizontal mixing is
represented by wavy double side arrows, indicating no net mass transport of
air masses. Dotted lines are isentropes in K. The solid dark blue line
indicates the 7 PVU contour, which is used to separate the regime of the
ExTL from the LMS and LS (for details see text). Thin orange contour lines
depict the zonal view of the jet stream.
Observations of SF6, N2O and CO2 from
the ER-2 aircraft show that the mean age at northern high latitudes at an
altitude of 20 km is of the order of 4–6 years .
Satellite observations of SF6 confirm this and show further a strong
interannual variability in the mean age in northern high latitudes
. The observations also indicate a
potential transport of mesospheric air to lower altitudes
, which, however, strongly depends on the strength
and persistence of the Arctic polar vortex during the individual winters.
In addition to diabatic descent inside and outside the polar vortex,
quasi-isentropic mixing from lower latitudes leads to a contribution of
relatively young air to the UTLS. As a result, a seasonal cycle of the
chemical composition of the UTLS up to Θ=430 K establishes a
relatively tropospheric character during northern summer/autumn and a more
stratospheric characteristic in late winter/spring . The
chemical composition and age structure of the extratropical UTLS (ExUTLS) are
affected by the competing diabatic downwelling of aged air and rapid quasi-isentropic mixing down to the tropopause
. The region between
Θ=380 K and the bottom of the tropical pipe around Θ=450 K
is a key region for the transition between these
transport regimes. The Θ=380 K isentrope coincides with the tropical
tropopause and is therefore directly affected by diabatic vertical transport
of tropospheric air through the tropical transition layer (TTL)
into the stratosphere. Above Θ=380 K these air
masses, which ascended through the TTL are rapidly mixed quasi-horizontally
by breaking planetary waves with air from high latitudes in addition to the
shallow branch of the Brewer–Dobson circulation
. This rapid transport particularly modifies the abundance of
water vapour and ozone in this region, which have seasonally
varying isentropic gradients
.
In our study we focus on the transition of the tracer composition in the
vortex-affected UTLS region up to Θ=410 K during winter 2015–2016. We
will quantify the effects of quasi-isentropic mixing from the tropics and
diabatic downwelling and its effect on the chemical composition as well as
the evolution of the age spectrum and the mean age in this region.
Meteorological conditions during winter 2015–2016
The early Arctic winter of 2015–2016 (November–December) was among the coldest
winters in the lower stratosphere (LS) after 1948. These extremely cold
conditions existed due to a strong and cold Arctic polar vortex which
developed in November 2015 due to very low planetary wave activity in the
stratosphere . From late December 2015 to early February
2016 the temperatures at Θ=490 K decreased to below 189 K. Therefore
strong dehydration and denitrification were seen in low H2O
and HNO3 volume mixing ratios, which finally led to a strong
chlorine activation in early winter. Using MLS data the chemical influence of
the vortex could be observed on isentropes below Θ=400 K
.
The major final warming (MFW) occurred on 5 March 2016,
which led to the vortex splitting 1 week later. This early final warming
was unusual, as only five other MFWs since 1958 have appeared before the middle of
March. Due to this, early warming air masses in the polar lower stratosphere
were mixed with non-vortex air and prevented chemical ozone depletion,
reaching record low values during winter 2015–2016 and references
therein.
The winter of 2015–2016 was characterised by an unprecedented anomaly of the
quasi-biannual oscillation (QBO) with a westerly jet that formed within the easterly
phase in the lower stratosphere . Since the QBO
affects the zonal wind direction in the tropical lower stratosphere
its strength and phase is crucial for stratospheric
transport processes and westerly phases are related to a
strong and cold polar Arctic vortex .
Further the winter of 2015–2016 was also affected by a strong warm phase of the
El Niño Southern Oscillation (ENSO) .
argue that the strong El Niño weakened the 2016 Arctic
vortex, while found a connection of this ENSO event to
the strong polar vortex and the easterly MFW.
Project overview and measurements
This work will address the evolution of composition, age structure and the
influence of transport and mixing of air masses in the lower stratosphere.
The composition of air masses inside the LS, which is affected by diabatic
descent of upper stratospheric air masses irreversibly mixed with younger
air from the TTL, is analysed by combining measurements of in situ data with
model calculations of the Chemical Lagrangian Model of the Stratosphere
(CLaMS) .
The POLSTRACC campaign 2015–2016
The data presented in this study were obtained during the POLSTRACC (The Polar Stratosphere in a Changing Climate) mission, which was part of the combined
PGS (POLSTRACC/GW-LCYCLE/SALSA) framework. The main objectives of the
POLSTRACC mission were the investigation of structure, composition and
dynamics of the Arctic LMS and processes involving chemical ozone depletion
and polar stratospheric clouds in the Arctic winter UTLS. In total there were
17 scientific flights from December 2015 to the end of March
2016 on board the new German research aircraft HALO (High Altitude Long
Range) from Oberpfaffenhofen, Germany (48.05∘ N,
11.16∘ E) and Kiruna, Sweden (67.49∘ N,
20.19∘ E), covering the region from 25 to 87∘ N and
24∘ E to 80∘ W (Fig. ). Typical flight
altitudes ranged from 10 to 14.5 km a.s.l.
above sea level.
,
corresponding to potential temperatures in the stratosphere from
Θ=320 K up to Θ=410 K. The total flight time was about
157 h, of which 19 h were in December 2015, 62 h were in
January–February and 76 in February–March. For this
study we focus on Arctic measurements starting from Kiruna, which took place
during two campaign phases, representing flights from 12 January 2016 to
2 February 2016 (phase 1) and from 26 February 2016 to 18 March 2016
(phase 2). For this work we use approximately 50 h of measurements of those
flights which were conducted to probe air masses above the extratropical
transition region (ExTL) and underneath the polar vortex above
PV = 7 PVU (1 PVU = 10-6 m2 s-1 K kg-1).
The research aircraft HALO is a modified business jet type Gulfstream G-550.
It has a maximum range of 12 500 km with a maximum altitude of 15.5 km and can
carry up to 3 t of scientific payload. The payload was a combination of
different remote sensing (e.g. WALES lidar; ;
Väisälä RD 49 dropsondes and GLORIA limb sounder;
) and in situ instruments that measured trace
gases with different lifetimes, sources and sinks.
Flight tracks during the POLSTRACC campaign. Blue colours indicate
flights during phase 1 (12 January–2 February), red colours indicate flights
during phase 2 (26 February–18 March). Flights that were used for the
analysis are shown as solid lines. Dotted lines denote flight legs with
PV < 7 PVU. For details see Sect. .
In situ trace-gas measurements
In this study we analyse measurements of N2O and CO, which
were measured with the TRIHOP instrument and
SF6 by the GhOST-MS instrument . For our
analysis the data are synchronised to a common time resolution of 0.1 Hz,
corresponding to a horizontal resolution of 2.5 km at typical HALO flight
speeds. GhOST data are available with a resolution of 60 s at an
integration time of 1 s which leads to a horizontal resolution of 15 km.
The TRIHOP instrument
The TRIHOP instrument is an infrared absorption laser
spectrometer with three quantum cascade lasers (QCL) operating between
wave numbers 1269 and 2184 cm-1 and measures CO,
N2O and CH4. The instrument uses a multipass
white cell with a constant pressure of 30 hPa to minimise pressure broadening
of the absorption lines. The three species are subsequently measured with an
integration time of 1.5 s per species during a full cycle which finally leads
to a time resolution of 7 s due to additional latency times when the
channels are switched. In-flight calibration is performed against compressed
ambient air standards that were calibrated against primary standards before
and after the campaign. The primary standards are traceable to the World
Meteorological Organisation Global Atmosphere Watch Central Calibration
Laboratory (WMO GAW CCL) scale (X2007) for greenhouse gases. During POLSTRACC
it was possible to achieve (2σ) precisions for CO, N2O
and CH4 of 1.15, 1.84 and 9.46 ppbV.
GhOST-MS in situ measurements
The GHOST-MS instrument is a two-channel gas chromatograph for airborne
measurements of trace gases. One channel uses a mass spectrometer (Agilent
MSD 5975) for the detection of atmospheric trace gases at a time resolution
of 4 min. This channel uses negative ion chemical ionisation as
described in to measure brominated hydrocarbons. The other
channel measures SF6 and CFC-12 using an ECD (electron
capture detector) with a time resolution of 1 min. For the POLSTRACC
campaign the precision for SF6 was 0.6 % and the precision
for CFC-12 was 0.2 %.
The mean age of air is inferred from SF6 measurements
. Due to its much higher atmospheric mixing ratio, the
precision of CFC-12 measurements is better than that of SF6
measurements. Prior to calculating mean age, the SF6 time
series has therefore been smoothed using the CFC-12, by applying a local (10 min of data before and after the time of measurement) fit between CFC-12
and SF6. This procedure removes parts of the instrumental
scatter but retains the local information, particularly keeping the atmospheric
variability (unlike averaging) and does not introduce any offset to the mean
age values. Mean age derived in this way has an overall precision of better
than 0.3 years and an estimated accuracy of 0.6 years, as explained in
. Both SF6 and CFC-12 are reported on the
SIO-2005 scale.
The Chemical Lagrangian Model of the Stratosphere (CLaMS)
The analysis of trace-gas measurements is complemented by simulations with
the Chemical Lagrangian Model of the Stratosphere CLaMS
. CLaMS is a Lagrangian chemistry transport
model, based on forward-trajectory calculations and a parameterisation of
small-scale atmospheric mixing which depends on the deformation rate of the
large-scale flow. The model simulation is driven with meteorological data
(e.g. horizontal wind fields) from European Centre of Medium Range Weather
Forecasts (ECMWF) ERA-Interim reanalysis and covers the
period 1979–2017. The model uses an isentropic vertical coordinate throughout
the stratosphere and the vertical velocity is deduced from the reanalysis
total diabatic heating rate. Further details about the model set-up and the
included chemical reactions (relevant species here are CO and
N2O) are given in . This long-term CLaMS
simulation has been shown to reliably represent transport processes in the
lower stratosphere for the relevant trace-gas species CO and
N2O as well as for mean age of air
.
Recently, a method for calculating the age of air spectrum has been implemented
in CLaMS , which will be used in the following analysis.
The age spectrum is the transit time distribution of air masses for transport
from a control surface (usually taken as the tropical tropopause or the
Earth's surface) to a given location in the stratosphere
e.g. and can be related to the Green's function
of the transport equation. The calculation method in CLaMS is based on inert
tracer pulses, with different tracers released every other month at the
surface in the tropics. This method allows the calculation of time-dependent
age spectra for the non-stationary atmospheric flow at any location and time
in the model domain (see for further details). Mean age
in CLaMS is calculated from an inert model “clock tracer” with a linear
increasing mixing ratio at the surface . The resulting mean
ages are fully consistent with mean age calculated as the first moment of the
CLaMS age spectrum .
A CLaMS simulation with full stratospheric chemistry was integrated as
described by . The upper boundary is set to Θ= 900 K
potential temperature, where tracers like O3,
N2O and CO are constrained by MLS satellite observations. Due
to its Lagrangian formulation, a box-trajectory model set-up is also possible
in which the identical chemistry scheme is used along single air mass
trajectories. This set-up is also used here to diagnose chemical pathways and
chemical conversion rates. This box model set-up is also used here to estimate
CO production and loss rates.
Results
As shown in rapid and frequent mixing with tropospheric air
mainly affects the region of PV < 7 PVU. To exclude mixing with air masses of
recent tropospheric origin or from the exTL (extratropical tropopause layer)
we only selected data above this level of potential vorticity. Therefore the
composition of analysed data is mainly affected by isentropically,
irreversibly mixed air mass signatures originating out of the tropics and
diabatically descended air masses from the upper stratosphere in the polar
region. In this analysis we further excluded flights which were dedicated to
the observation of gravity waves.
Tracer distributions and mean age
Figures , and show tracer
distributions as a function of equivalent latitude and potential temperature
θ. Equivalent latitude is
directly linked to the potential vorticity, which is conserved under
adiabatic processes . Therefore, these coordinates are
suitable for accounting for reversible adiabatic tracer transport.
Age of air
An air parcel in the stratosphere is a mixture of fractions of air with
different histories, transport pathways and individual transit times. The
transport pathways create an age spectrum or transit time distribution. The
age spectrum can be obtained by calculation of the Green's function of the
tracer continuity equation for a conserved and passive species
.
The mean age is defined as the first moment of the transit time distribution.
To determine the mean age from long-lived tracer measurements, the tracer
must have a well known source distribution at the tropical tropopause and a
well defined vertical gradient in the stratosphere . Since
SF6 is a long-lived inert trace gas with a well known
increase in its mean surface mixing ratio, it is commonly used for
calculations of mean age . The sink of SF6 is
in the mesosphere, where it is destroyed by shortwave UV radiation. The
lifetime of SF6 is assumed to be 3200 years, but recent
studies indicate a significantly shorter lifetime of about 850 years
. This implies that the mean age derived from SF6
may be too old. Models and observations both show a high bias of up to 1
year in the polar vortex or even more in mesospheric air
. Since we focus on the lower stratosphere far below the
mesospheric loss region for SF6 and we further focus on age changes, our data
are not affected by this fact.
Distributions of mean age from SF6 measurements in potential
temperature – equivalent latitude coordinates for PV > 7 PVU.
Panel (a) shows data for phase 1, (b) for phase 2 and
panel (c) shows their absolute difference (phase 2 - phase 1).
The colour code represents the mean age. Blue colours in panel (c)
indicate an increase in mean age in the subvortex region from January to
March. Only bins with more than 10 data points are shown.
Figure a and b show the distribution of mean age calculated
from SF6 measurements for phase 1 (January) and phase 2
(February–March). Panel (a) shows that the LS during phase 1 is dominated by
air masses of mean ages from 0.5 years to less than 3 years at
maximum. The oldest air masses with mean ages above than 2 years were
encountered furthest away from the tropopause and potential
temperatures ranged from Θ=360 K to Θ=380 K. In contrast,
during phase 2 (panel b) in general much older air masses, up to 5 years,
were found at potential temperatures of Θ=410 K. These higher
potential temperatures at flight altitude are the result of the diabatic
descent over the course of the winter and indicate an increasing influence of air
masses originating deeper in the stratosphere or from the Arctic polar
vortex. To directly compare the temporal evolution of the age of air in the
lower stratosphere, panel (c) shows the difference in age of air between both
phases and shows that the bulk of the air inside the LS is ageing, to between
Θ=330 K and Θ=380 K. The mean increase is 0.29 years,
indicating diabatic downwelling due to the evolution of the polar vortex and
thus an increased mean age in late winter.
Nitrous oxide
Nitrous oxide (N2O) has a lifetime of 123 years
and is released at the surface with no chemical sources in
the atmosphere . As a result, N2O has a near-constant tropospheric value of 329.3 ppbV (winter 2015–2016
according to ), which makes stratospheric influence identifiable
. The mean annual tropospheric increase is currently
approximately 0.78 ppbV per year .
The main sink reactions of N2O are due to photolysis in the
UV band (190 nm ≤λ≤220 nm) and the reaction with
O(1D), which only occurs within the upper stratosphere .
Thus, N2O above the tropopause shows a weak negative vertical
gradient which maximises during winter and spring due to the diabatic
downwelling by the Brewer–Dobson circulation.
As in Fig. but for N2O. Negative (blue)
values in panel (c) indicate an overall decrease in N2O in the
measurement region in accordance with increasing mean age
(Fig. c).
Figure a and b show N2O values between 276
and 325 ppbV measured during phase 1 and values below 200 ppbV measured during phase 2 above
Θ= 400 K. Figure c shows an overall decrease in N2O in the
polar lower stratosphere due to diabatic descent during winter, consistent
with mean age changes (Fig. c).
As in Figs. and but for CO. Note the
positive difference in CO in panel (c), indicating an increase in CO
in the measurement region.
Carbon monoxide
Carbon monoxide (CO) is released to the atmosphere mainly through incomplete
combustion processes and methane oxidation as the only significant in situ
source. Due to the high variability in anthropogenic surface emissions, CO
mixing ratios in the Northern Hemisphere vary from 70 to
200 ppbV and the CO lifetime is of the order of
weeks. In the lower stratosphere the main source of CO is methane
oxidation with the OH radical. The main sink is oxidation by OH. The CO
lifetime during polar night is a few months. In the stratosphere CO is
controlled by production from methane oxidation and CO degradation. In the
absence of transport from the troposphere this leads to an equilibrium
between production and destruction of CO. We found an equilibrium value of
10–15 ppbV in winter 2015–2016, depending on the integrated
temperature history of the respective air mass in agreement with previous
studies .
The reaction of CH4 with Cl is an insignificant source of CO
in the lower stratosphere . Transport from the mesosphere,
where CO is produced from the photolysis of CO2, also
provides a potential source of CO via strong diabatic descent during winter
under persistent polar vortices . These potential
influences are discussed in Sect. .
Figure shows the distribution of CO. During phase 2 (panel b)
the lowest mixing ratios of 15 ppbV were found at potential
temperatures between Θ=380 K and Θ=410 K and equivalent
latitudes > 60∘ N. As can be seen by the vertical branch of the
CO–N2O correlation (Figs. and
), this value is the stratospheric equilibrium during late
winter. Phase 1 (panel a) values ranged between 60 and
17 ppbV; hence the stratospheric background value was not
measured in January 2016. A strong tropospheric influence is evident below
Θ=340 K, with CO values up to 57 ppbV at phase 1 and 47 ppbV at phase 2. Hence the overall distribution of carbon
monoxide in the UTLS during the individual phases (Fig. a and
b) seems to be consistent with N2O and mean age obtained from
SF6 measurements, despite its much shorter lifetime compared
to the other species.
Sketch of tracer–tracer correlations with different lifetimes. Note
that the N2O axis is reversed. Panel (a) shows the L-shaped
structure with an air parcel (red box) on a straight mixing line for fast
mixing timescales. The horizontal red line represents the tropospheric
N2O background, the blue vertical line the stratospheric CO equilibrium.
Panel (b) shows the resulting curve in case of inefficient mixing
compared to the chemical lifetime. Panel (c) shows the change in
curvature depending on the strength of mixing and panel (d) shows
the influence of the mesosphere on the correlation.
However, when comparing the differences in the respective phases (panel c),
we see their behaviour is different to N2O and
SF6. We encountered an increase in carbon monoxide mixing
ratios over the course of the winter, which is at a first glance in contradiction
to the distributions of mean age and N2O. While the
distributions of long-lived tracers SF6 and
N2O indicate an ageing of air masses, the increase in
short-lived CO indicates a source of CO either from the troposphere or the
stratosphere. Note that the increase is observed above Θ=360 K and
50∘ N equivalent latitude. Below Θ=360 K decreasing values
are encountered. We will analyse the potential sources of CO in the following
and suggest that CO increases due to an enhancement of mixing tropospheric
air from the tropical lower stratosphere over the course of the winter
without an increase in the upper tropospheric mixing ratios, which are
affected by the surface emissions.
N2O–CO correlation for POLSTRACC flights with PV > 7 PVU.
The blue curve represents phase 1, the red curve represents phase 2. Data are
binned in steps of 5 ppbV N2O. The variability in each bin
is given by the vertical and horizontal lines.
Analysis
We found a decrease in the long-lived species SF6 and
N2O with their lowest values far above the local troposphere
in late winter, which fits well in the general picture of enhanced
downwelling of the Brewer–Dobson circulation in late winter–spring. The
unexpected, simultaneous increase in the short-lived CO over the course of the winter could indicate a strengthening of tropospheric transport by enhanced
mixing with air from the tropical lower stratosphere. In the following we
will discuss this hypothesis and also other potential sources for the
additional CO mixing ratios.
Identification of mixing on the basis of tracer–tracer correlations
To identify mixing processes across the tropopause CO-O3
correlations have been widely used
. Since ozone is
affected by chemical processes, particularly in the vortex region, we use
N2O as a stratospheric tracer instead of ozone. Carbon
monoxide, used here as a tropospheric tracer, also has sources in the
mesosphere and via chlorine chemistry in the stratosphere. In the LS the
influence of chlorine is small compared to the reaction with the hydroxyl
radical; therefore we investigated the influence of chlorine chemistry
regarding methane which leads to the formation of CO. This influence will be
discussed in detail later.
To analyse the effects of transport and mixing on the evolution of the UTLS
composition we used the N2O–CO relation as shown in Fig. . Tropospheric data have high N2O values
(> 328 ppbV) and are accompanied by high CO values, while
stratospheric data have N2O < 328 ppbV. Due
to the tropospheric background value of N2O and the
stratospheric equilibrium of CO, the troposphere can be identified as
the horizontal (high amount of N2O, variable CO) branch and the
stratosphere, free of tropospheric influence can be identified as the vertical
branch (low amount of CO, variable N2O) of the correlation.
Without any recent mixing, the tracer–tracer correlation of
N2O and CO would form an L-shaped structure
. In the presence of rapid mixing a straight mixing line
between two end members of the correlation is established (panel a)
. As stratospheric CO will relax towards its
stratospheric equilibrium value while N2O is long-lived in
the lower stratosphere, the initial linear correlation will become curved
with time in the case of inefficient mixing when the chemical lifetime is shorter
than the timescale of mixing (panel b). Depending on the strength of mixing
relative to the chemical CO sink the curvature will change and is less
pronounced as the mixing becomes more efficient (panel c). It is important to
note that the change in CO relative to a given N2O value can
only be explained by a change in the ratio between mixing and chemical timescales. Mixing alone acts on both tracers N2O and CO.
Therefore a change in the shape of the curve is a direct result of the
increased mixing relative to the chemical timescale, which is less efficient
when mixing becomes stronger. Figure d shows additionally
the correlation under mesospheric-influenced conditions. In this
case the correlation would rise to higher CO mixing ratios and lower
N2O mixing ratios, since N2O gets destroyed
and CO is produced in the mesosphere.
Figure shows the N2O–CO correlation for
POLSTRACC separated for phase 1 and phase 2, binned in intervals
of 5 ppbV N2O. It is evident that
tropospheric and stratospheric air masses are mixed in both phases. During phase 1 (blue curve) CO ranges between 20 ppbV and 60 ppbV at N2O values between
323 and 270 ppbV. Notably the red curve
(phase 2) shows a steeper gradient with CO values between 43 and 15 ppbV at N2O
values between 323 and 180 ppbV. There
are higher CO mixing ratios for N2O values lower than 310 ppbV in phase 2 of the measurements. Additionally
the red curve tends towards a CO equilibrium value of 15.67 ppbV for N2O values in the range of 220 to 180 ppbV.
Most importantly, there is an increase in CO on N2O isopleths
between 313 and 273 ppbV
N2O over the course of the winter. This is a remarkable result
since we expect that due to the ageing of air inside the lower stratosphere
in winter, the CO mixing ratio decreases with time. It is important to note
that the correlation along the mixing line, which connects tropospheric values
with the stratosphere, shows higher CO relative to N2O in
phase 2. As indicated in Fig. this is a clear indication of
enhanced mixing of tropospheric air masses for N2O values > 273 ppbV. Furthermore phase 1 shows higher CO values relative
to N2O compared to phase 2 for N2O values
larger than 313 ppbV. Therefore we can conclude that
regarding the CO–N2O correlation the tropospheric impact on
short timescales through the ExTL was greater in phase 1 than in phase 2,
indicating enhanced mixing with tropospheric-influenced air originating in
the TTL region during phase 2.
A potential mesospheric impact is highly unlikely due to the fact that during
phase 2 the N2O–CO correlation tends towards the equilibrium
value in the region of lower N2O values. This influence will
be discussed later in detail.
During both phases the UTLS between Θ=340 K and Θ=380 K was
covered by our measurements. Therefore we assume the TTL
region (Fig. ), where most of the tropospheric air masses are
transported into the stratosphere and references
therein is the main source for the enhanced CO values (Fig. c). Further on, rapid eddy mixing of air from the TTL
leads to an increase in tropospheric tracer signatures in the Arctic region
.
To quantify the increasing influence from tropospheric air masses in the
lower stratosphere, we applied a simple mass balance approach to quantify the
composition of the lower stratosphere. Therefore, we assume an air parcel in
the lower stratosphere may consist of either upper stratospheric or
tropospheric origin (Fig. ). This mass balance system is solved
to get the amount of tropospheric fraction ftrop of the measured air.
For a mixing ratio χ on a specific isentrope θ we assume
χ(θ)=ftrop⋅χtrop+fstrat⋅χstrat
and
ftrop+fstrat=1
which leads to the tropospheric fraction ftrop based on CO
measurements
ftrop=χCO,m-χCO,stratχCO,trop-χCO,strat
with χCO,m the measured CO mixing ratio,
χCO,strat the stratospheric CO background which was set
to 15.7 ppbV as mean of the vertical branch of the
CO–N2O correlation and χCO,trop the
tropospheric CO entry value in the TTL.
In situ measurements have shown that CO mixing ratios above the tropical
tropopause are at levels between 50 and 60 ppbV.
Figure shows the difference in the calculated tropospheric
fraction ftrop between phase 2 and phase 1 as a function of
N2O, which acts as a quasi-vertical coordinate. The CO
increase over the course of the winter corresponds to an increase in ftrop
of (6.8±3.7) % between 313 and 273 ppbV N2O by assuming 60 ppbV
of CO at the tropical tropopause as provided by in situ aircraft data from
and . Using COtrop= 80 ppbV as indicated by MLS at 100 hPa one obtains 32 % lower
values for ftrop, which is still a significant increase in tropospheric
air masses. Note that additionally the tropospheric fraction decreases
towards more tropospheric N2O values from phase 1 to phase 2.
This is clear evidence that an increase in the CO mixing ratio at the
tropopause is not the cause of the observed lower stratospheric CO increase.
This would be consistent with an increase in the fraction of young air of
tropospheric origin and more efficient mixing as indicated in Fig. .
(a) Tropospheric CO fraction from the mass balance equation
as a function of N2O showing the difference between phase 2 and phase 1.
(b) The same as panel (a) but as a distribution over
equivalent latitude. Red colours indicate an increase in the tropospheric CO
fraction.
Panel (b) shows the distribution against equivalent latitude. Note that the
observed increase is most prominent above Θ=360 K. This is a clear
indication that mixing at Θ<360 K is suppressed due to the strong
subtropical jet, which acts as a barrier for mixing and
would be consistent with enhanced mixing out of the TTL region.
Age spectra analysis
For further analysis of the relationship between diabatically descended, aged
air with longer transit times and potentially mixed with young tropospheric
air with shorter transit times we use age spectrum calculations of the CLaMS
(Chemical Lagrangian Model of the Stratosphere)
model, which gives information on
the full transit time distribution. Notably we have the age spectral
information for each individual data point along the flight track and
therefore can directly compare our measurements with the spectrum.
To test whether the model is able to reproduce the observations of tracers we
compared CO and N2O from CLaMS with the measurements (Fig. ). Model output is available along the flight track with a
time resolution of 10 s. Figure shows the
N2O–CO scatter plot for each data point. Panel (a) shows the
correlation measured with the TRIHOP instrument, panel (b) shows the
correlation calculated out of the CLaMS model. As is evident, CLaMS correctly
represents the increase in CO relative to N2O from phase 1 to
phase 2. Also, the separate branches of the two phases are reproduced and the
crossing of the correlation at 40 ppbV CO and 310 ppbV N2O is consistently simulated.
This remarkable agreement between model and observations further motivates
the usage of CLaMS for age analysis of our measurements.
As mentioned before, CLaMS is able to calculate the full transit time
distribution of analysed air masses for each individual data point along the
flight track. Figure shows the averaged age spectra of the
CLaMS model for the respective phase (panel a) and their difference (panel b). Vertical solid lines represent the mean age of the respective phase
(blue and red) calculated by the CLaMS model, the dashed vertical lines
separate young air masses with a mean age lower than 0.5 years from old air
masses with mean age larger than 2 years. Since we have the full transit time
distribution of each data point, we can compare this relation between the
different parts of the age spectrum. An increase in the tropospheric fraction
would be linked to an increase in the part of the age spectrum with low
transit times as indicated by the observed increase in CO relative to
N2O.
N2O–CO scatter plot measured by the TRIHOP
instrument (a) and CLaMS model output (b). Phase 1 coloured
in blue, phase 2 coloured in red. The model output is available along the
flight track with a time resolution of 10 s.
(a) Averaged age spectra simulated by CLaMS for phase 1
(blue) and phase 2 (red). These spectra represent the mean of the individual
age spectra available for each data point along the flight track. The mean
age is indicated by the respective coloured vertical lines (phase 1:
1.71 years, phase 2: 1.98 years). The difference between the spectra is
given in panel (b), showing an enhancement of young and old air
masses from phase 1 to phase 2. Vertical dashed lines indicate the transit
times of 6 months and 24 months (see next figures). The bin
size of a data point is 1 month.
Figure shows an absolute increase in air masses older than
2 years up to 0.3 % month-1. For air masses younger than 6 months,
there is also an increase in the age spectrum between phase 2 and phase 1, with
maximum values up to 0.9 % month-1, which is larger than the increase in the
old air masses. The relative change in air masses younger than 6 months is 19.5 % and for air masses older than 2 years it is 76.4 %. The
increase in the young fraction is in agreement with the observed CO increase,
indicating increased mixing with air from the TTL at the end of winter.
Since the mean age is calculated as the first moment of the distribution, its
value is most sensitive to changes in the old part (the so-called “old tail”) of the distribution
. Therefore the mean age rises by 0.27 years, from 1.71
to 1.98 years, as a result of the increase in the age spectrum distribution
for air masses older than 2 years. This matches the mean age increase in
SF6 and indicates, in agreement with the decrease in
N2O, the overall ageing in the lower and lowermost
stratosphere over the course of the winter. Since the integral over the Green's
function is normalised to one, the increases of air masses older than 2
years and younger than 6 months must result in a relative decrease in
between. Therefore air masses with mean ages between 0.5 and 2 years
are more enhanced in phase 1 than in phase 2, which is evident by the change in the transit time distribution up to -1.9 % month-1.
To further investigate the relationship of young versus aged air, we
calculated the accumulated fraction of air masses with transit times lower
than 6 months and older than 2 years for each data point. Figure shows the binned fraction of air masses with transit
times under 6 months versus the modelled mean age.
The comparison of the scatter plot for different times (phase 1 and phase 2)
shows that for a given mean age a significant increase in the young
tropospheric contribution is evident. Thus, according to the model and in
agreement with the observed increase in CO, the late winter LS is more
affected by tropospheric young air. Therefore our results demonstrate that
the mean age is an incomplete descriptor when referring to chemical
properties of air masses involving different chemical lifetimes of species.
Since the mean age is just a single number it might be insensitive to changes
in the processes and timescales contributing to the mean, which, however,
affect the chemical properties of the air parcel, e.g. by enhanced mixing of
short-lived species. Therefore it is important to account for the full
spectral shape when referring to chemical properties of an air mass rather
than only the mean age.
Mean age versus air fractions with transit times < 6 months from
the age spectra simulated by CLaMS for phase 1 (blue) and phase 2 (red). Each
data point is binned in steps of 5 ppbV N2O. The
variability in each bin is given by the vertical and horizontal lines.
During winter 2015–2016 CO mixing ratios in the LS increased from January to
March while long-lived trace gases denote ageing of the LS. The analysis
of CO–N2O correlations, the mass balance equation of
irreversible mixing and transport pathways in the LS and model
simulations points towards an increased influence of tropospheric air masses
from the tropical lower stratosphere. Additional potential sources of CO in
the LS are discussed in the following.
Discussion
Since there are different sources for CO at different locations in the
atmosphere, an increase in carbon monoxide mixing ratios can be due to (i) an
increase in isentropic mixing out of the TTL, (ii) an increase in the
tropospheric source strength, (iii) a potential influence of the mesosphere
and (iv) a change in chemical reaction cycles due to higher amounts of
reactive chlorine in the stratosphere. As already discussed the increase in
enhanced tropospheric source emissions (ii) is highly unlikely (see Fig. ). Since our analysis points to an increase in isentropic
mixing out of the TTL (i), the possible influence of points (iii) and (iv)
have to be further discussed.
Carbon monoxide is produced in the mesosphere due to the photo-dissociation
of carbon dioxide. Therefore the composition of mesospheric air masses is
clearly distinct from air mass composition of the stratosphere.
found increased CO mixing ratios up to
90 ppbV at altitudes around 25 km or Θ=630–670 K and
found CO values of 600 ppbV at an altitude of
32 km. Both studies show very low N2O mixing ratios
(< 50 ppbV). Although the authors found layers of mesospheric
air descending down to 22 km, this is not evident for the Arctic winter of
2015–2016, and the lowest N2O mixing ratios are found to be of the
order of 200 ppbV. This is reflected in the MLS observations
that determine the CLaMS upper boundary at Θ=900 K potential
temperature (Fig. ). The simulation indicates the expected
downward transport of mesospheric-influenced air, but down to Θ=600 K
at the end of March 2016 in agreement with our observations. CO values
minimise at the highest flight levels and equivalent latitudes.
Furthermore, an additional influence of descended mesospheric air into the
lower stratosphere would lead to mixing lines very strongly differing from
the observed relationship (see Fig. ), which is not observed
in agreement with the CLaMS N2O–CO scatter plot (Fig. ).
Temporal evolution of zonal mean CO for equivalent latitudes
> 70∘ N simulated by CLaMS for winter 2015–2016.
Net change in CO from January to March for air masses in the Arctic
vortex (equivalent latitude > 65∘ N) due to chemical reactions in
the stratosphere and mesosphere calculated by CLaMS. The blue line represents
the statistical mean, the dashed lines represent the 1σ standard deviation.
Zonal mean of air mass fractions (colour code) with transit times
< 0.5 years for January (a, c) and March (b, d) from
2004 to 2016 climatology (a, b) and for 2016 only (c, d)
against potential temperature. The contour lines show mean age in years and
the thick black line shows the WMO tropopause.
In general, another important source of carbon monoxide in the atmosphere is
the reaction of methane with reactive chlorine, which is not significant in
the lower stratosphere . However, air masses enriched in
reactive chlorine could have been transported downwards, providing potential
reactants for the chemical production of CO. Therefore, we simulated the CO
yield from the reactions of CH4 with chlorine, OH and
O(1D) using CLaMS simulations in the box model mode. A large number of air
parcel backward trajectories were calculated starting on 15 March from
locations within the vortex core (equivalent latitude > 65∘ N;
potential temperature between Θ=350 K and Θ=500 K).
The trajectories ended on 15 January and the chemical composition changes were calculated using the
CLaMS chemistry module running forward in time for a subset of the
trajectories with equivalent latitudes greater 50∘ N on 15 January
(21 480 trajectories). Figure shows the statistical evaluation
of the net CO change due to chemistry over the period as a function of
potential temperature on 15 March. The blue line represents the statistical
mean and the dashed lines represent the 1σ standard deviation. The mean overall
change is even negative over the entire profile, which is due to the
oxidation of the produced CO by the reaction with OH. Therefore we conclude
that the observed increase in CO in phase 2 is not due to the additional
chemical source reaction. The significant increase in air masses younger than
6 months (Fig. ) also indicates a strong contribution
of young rather than mesospheric air.
Figure shows the statistical evaluation of the net CO change
due to chemistry over the period as function of potential temperature on 15 March. The blue line represents the statistical mean and the dashed lines represent the
1σ standard deviation. As is evident the mean overall change is even
negative over the entire profile, which is due to the oxidation of the
produced CO by the reaction with OH. Therefore we conclude that the observed
increase in CO in phase 2 is not due to the additional chemical source
reaction.
To investigate whether transport and increased mixing of air mass fractions with
transit times under 6 months in winter 2015–2016 was special compared
to other years we analysed the climatology of these fractions from 2004 to
2016 and compared it to the calculated fractions in winter 2016. Both are from
the CLaMS model (Fig. ). The colour code represents
the fractions of air masses with transit times under 6 months, the
contour lines represent the mean age and the thick black line indicates the
WMO tropopause.
The climatology shows that the largest fraction of air masses with transit
times under 6 months exceeding 73 % is found between the equator and
30∘ N up to Θ=430 K. In January this strong signal has a
sharp gradient at Θ=450 K. These air fractions are transported from
January to March to the poles. As a result, northward of 70∘ N the
fraction of air masses with transit times under 6 months increases
by 5 % between Θ=360 K and Θ=420 K. From January 2016 to
March 2016 this transport is even stronger than in the climatology, as shown
by the different horizontal gradients in Fig. . The
mean age in March compared to January at Θ=400 K shows a higher value
in both the climatology and the winter of 2015–2016, whereas the
structure of the mean age contours show a more horizontal meridional gradient
in the winter 2016 compared to the climatology.
Finally, these findings highlight the role of mixing of young air in the
lower stratosphere of the polar Arctic region with an underlying increase in
mean age of air, indicating downward transported air masses of older air
fractions. The enhanced transport of young air is evident from the
climatology and turns out to be particularly strong in winter 2016.
Summary
We present tracer measurements of CO and N2O measured during
the POLSTRACC campaign in winter 2015–2016 on board the German HALO research
aircraft. The winter of 2015–2016 was characterised by an extremely cold and stable
polar vortex which broke up due to an MFW on 5 March 2016. In combination
with measurements of SF6 and model simulations by the CLaMS
model it was possible to analyse the contributions of diabatic transport and
isentropic mixing in the UTLS region. The mixing ratios of the long-lived
trace gases N2O and SF6 decreased over the
course of the winter and therefore denoted an overall ageing due to subsiding air
masses in the Arctic polar lower stratosphere. The calculated mean age based
on measured SF6 shows an ageing of 0.29 years (see Fig. c) and for CLaMS of 0.27 years (see Fig. a). Remarkably, the short-lived species CO increases at the
same time. Since mixing can be identified by tracer–tracer correlations we
used CO–N2O correlations to quantify the relation between
transport and chemistry. Our analysis shows an increase of 3.7 ppbV CO relative to N2O, which can be linked
to an increase of 6.8 % of mixed air masses out of the TTL region. The
comparison with the CLaMS model shows a very good agreement between
measurements and model calculations. The CO–N2O correlation
is well reproduced by the model. Analysis of the averaged age spectrum for
the respective phase shows that there is a simultaneous increase in fractions
of air with transit times longer than 2 years and fractions of air with
transit times smaller than 6 months. Since the mean age itself is most
sensitive to changes on the old tail of the age spectrum, the ageing of air
masses in the LS over the course of the winter can be explained by the increase in old air masses, characterised by low N2O and
SF6 measurements. Increased mixing of young air masses adds
to this and leads to an increased fraction of the younger part of the age
spectrum, which is consistent with the observed increase in CO. It is evident that
this enhancement is due to stronger mixing processes out of the TTL region,
where fresh tropospheric air is mixed into the polar lower stratosphere.
Other potential sources of CO like mesospheric air and the chemical reaction of
CH4 with chlorine are unlikely to have caused the observed
increase in CO.
Therefore we conclude that the Arctic lower stratosphere in March was
strongly affected by mixing with young tropospheric air, which partly
compensates for the overall ageing. These aged air masses are isentropically
mixed with younger air masses out of the TTL region. The observations are
in line with the climatology of mixing from 2004 to 2015 on the basis of
ERA-Interim by the CLaMS model and highlight the importance of horizontal
mixing from the tropics for the Arctic winter UTLS.
The observational data used in this study can be downloaded
from the HALO database (10.17616/R39Q0T; ) at
https://halo-db.pa.op.dlr.de/.
JK carried out the measurements and analysed the
data with the help of PH. FP and JUG did the
model simulations with the CLaMS model. AE, HB and
TK provided the measurement data of SF6 and mean age.
PH, AE, FP and JUG provided helpful
discussions and comments. JK and PH wrote the manuscript.
HO, BMS and WW coordinated the
POLSTRACC project.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “The Polar
Stratosphere in a Changing Climate (POLSTRACC) (ACP/AMT inter-journal SI)”.
It is not associated with a conference.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft (DFG, FKZ EN
367/13-1 and EN 367/11) and the Johannes Gutenberg-University Mainz (FKZ
8585084).
Jens Krause was partly funded under DFG grant HO 4225/7-1.
AGAGE is supported principally by NASA (USA) grants to MIT and SIO, and also
by DECC (UK) and NOAA (USA) grants to Bristol University, CSIRO and BoM
(Australia), FOEN grants to Empa (Switzerland), NILU (Norway), SNU (Korea),
CMA (China), NIES (Japan) and Urbino University (Italy).
Edited by: Martyn Chipperfield
Reviewed by: Eric Ray and two anonymous referees
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