Air sample collection at Mace Head is restricted to times when air masses
approach from the clean air sector and the data therefore represent a
baseline case for the time series of halogenated compounds. In contrast,
weekly air sample collection at Taunus Observatory is performed irrespective
of wind direction. Therefore mixing ratios at Taunus Observatory are supposed
to be higher than at the coastal site except for substances which are
strongly influenced by marine emissions, such as for example carbonyl sulfide
or iodomethane (not shown here). Because Taunus Observatory is located closer
to sources, not only higher absolute mixing ratios of most halocarbons are
expected to be measured, but also a higher atmospheric variability for
substances with ongoing emissions.
(Hydro)chlorofluorocarbons: CFC-11, CFC-12, and HCFC-22
The Montreal Protocol strictly regulates production and use of CFC-11 and
CFC-12. Due to their long total atmospheric lifetimes of 52 years (CFC-11)
and 102 years (CFC-12) , tropospheric mixing ratios
decrease slowly but continuously, which is evident at Taunus Observatory and
at Mace Head as shown in Fig. . Remarkable in the time series is
an
episode in the time series in September 2016 with exceptionally
low mixing ratios occurring at both Taunus Observatory and Mace Head, which
is apparent for CFC-11 and CFC-12 but is more pronounced for CFC-12. A CFC-12
mixing ratio of only 509.7 ppt was measured on 16 September 2016. Flasks
from the two sites for this period were analysed on different days, making a
measurement artefact unlikely.
Comparing the means of the detrended time series, baseline data from the two
sites for CFC-11 and CFC-12 agree within their respective standard deviation,
although mixing ratios at Taunus Observatory are on average higher by 1 ppt
(0.5 %) for CFC-11 but only 0.4 ppt (<0.1 %) higher for CFC-12.
Applying a linear fit function to the time series, baseline mixing ratios of
CFC-11 at Taunus Observatory decrease at a rate of
-1.2±0.1 ppt year-1, at MHD with a rate of -1.0±0.1 ppt year-1.
This result agrees with the global decrease rate of
-1.0±0.2 ppt year-1 determined for the period 2015–2017 at NOAA
background measurement sites .
In addition to the expected overall similar behaviour of CFC-11 and CFC-12,
also differences become apparent in the two gases' time series at Taunus
Observatory. For CFC-11, the outlier filter routine identifies four samples
in the time series at MHD as outliers, two below and two above the baseline
variability, the latter two occurring in winter 2017/2018. For CFC-12 no
outliers above the baseline occur at MHD. At Taunus Observatory, CFC-11 also
has a larger number of polluted outliers with exceptionally high mixing
ratios (18 %, 28 of 156 datapoints) than CFC-12 (4 %, 7 of 174
datapoints). Outliers in one of the two compounds are only in one case an
outlier in the other one. CFC-11 outliers above the baseline at Taunus
Observatory are more likely to correspond to outliers in HCFC-22 (11 pairs),
HFC-134a (19 pairs) or dichloromethane (16 pairs). This is similar to
observations of correlations between CFC-11 and HCFC-22 and dichloromethane
at Mauna Loa after 2012 by .
Another difference between CFC-11 and CFC-12 becomes apparent in the slope of
the resulting function when applying Eq. () to the
baseline time series. For CFC-12, mixing ratios steadily decrease. In
mathematical terms, the first derivative of the fit curve is negative at all
times. This is not the case for CFC-11, for which the first derivative of the
fit function periodically becomes positive, indicating short periods of
increasing mixing ratios. This occurs during summer months when mixing ratios
slightly increase after a spring minimum, and this behaviour points to
ongoing emissions of CFC-11.
Production and use of both CFCs are regulated, and their emissions should
slowly approach zero. Their seasonality should therefore be driven mainly by
transport patterns, in particular the intrusion of aged air with lower mixing
ratios from the lower stratosphere . To
assess the seasonality of mixing ratios, baseline time series were detrended
relative to January 2013 by subtracting the linear and quadratic term of
Eq. (). Figure shows the
resulting seasonal cycles for the two CFCs as differences to their respective
annual mean for Taunus Observatory and Mace Head.
Shown are monthly means and error bars indicate the error of the mean.
In Fig. , both CFCs show elevated mixing ratios in
winter with regard to the annual mean and reach minimum mixing ratios in
spring/summer. Commonly, such behaviour occurs for gases which are
predominantly removed from the atmosphere by reaction with OH or/and have
increased wintertime emissions, which in Europe is typically the case for
combustion products. CFC-11 and CFC-12 do not have an OH sink and their
emissions are not related to combustion processes. The seasonal cycle of CFCs
is driven by the seasonality of stratosphere–troposphere exchange, which in
the Northern Hemisphere maximises in late winter and
spring , and potential seasonal variation in emissions. If
remaining emission sources were regionally co-located, seasonality of
large-scale transport patterns should affect CFC-11 and CFC-12 similarly. In
Fig. , the cycle of CFC-11 is shifted forward by about 2
months in comparison to that of CFC-12. Another difference between the two
compounds with regard to seasonality is that, as mentioned above, the fit
curve of CFC-11 has a positive curvature in summer pointing to a small
periodic increase in mixing ratios, whereas for CFC-12 curvature is negative
at all times, which means that mixing ratios continuously decrease. This
holds for Mace Head and Taunus Observatory.
These observational differences between CFC-11 and CFC-12 are in agreement
with higher estimates of ongoing European emissions for CFC-11 than for
CFC-12 . Although this was derived for the year 2009, it is
likely that remaining emissions from banks still show this behaviour. In
particular, the high number of outliers is indicative of ongoing emissions of
CFC-11 in regions that influence trace gas mixing ratios at Taunus
Observatory. New significant sources of CFC-11 were only recently reported
for Asia by .
Seasonality of CFC-11 (a) and CFC-12 (b) at Taunus
Observatory (coloured symbols, baseline data only) and Mace Head (black
symbols). Plotted are monthly mean values as difference to the mean of the
total detrended data.
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As an example of the first-generation replacement substances,
Fig. shows time series and seasonal cycle of HCFC-22.
HCFC-22 is widely used as a cooling agent for refrigeration and air
conditioning as well as for foam blowing and for production of synthetic
polymers. Production and use of HCFC-22 have been regulated globally and are
projected to be almost completely phased out worldwide by 2030. EU regulation
has banned the use of fluorinated greenhouse gases with global warming
potentials higher than 150 even earlier, depending on the type of
application. Globally, HCFC-22 mixing ratios are still increasing, as evident
from ground-based measurements and also from MIPAS satellite
data . Already for the period 2005–2009 emissions had
been diagnosed to at best stagnate in some regions such as parts of North
America and Europe with no significant emissions changes, while still
increasing globally mainly due to rising emissions from Asia and
Africa . found European emissions to
decrease for the period 2002–2012. In this Bayesian inversion study using
data from Monte Cimone and from Mace Head, European emissions were estimated
on the national level, with emissions occurring all over Europe but
predominantly in western Europe.
The main removing process of HCFC-22 from the atmosphere is via reaction with
OH with a lifetime of 10.8 years . Thus, a seasonality of
atmospheric mixing ratios with a summer minimum and a winter maximum is
expected. However, for both observational sites, Taunus Observatory and Mace
Head, a semi-annual cycle adding higher harmonic terms to the fit equation
yields a better fit to the seasonal cycle derived from the detrended data as
shown in Fig. b (cf. Eq. ).
χ(t)=a+b⋅t+c⋅t2+d⋅sin(2πt)+e⋅cos(2πt)+f⋅sin(4πt)+g⋅cos(4πt).
This has been taken into account for flagging individual samples as outliers.
Of 168 valid data points, 27 are identified as outliers above the baseline
which, keeping in mind the limited statistics, occur most frequently and with
highest enhancements during the summer months. The outlier frequency of the
still used compound HCFC-22 is thus comparable to that of CFC-11 which should
have been phased out globally and therefore should exhibit fewer outliers.
HCFC-22 enhancements of up to approx. 20 ppt were measured (average 5.9 ppt
(2.3 %)), CFC-11 enhancements even reached 22 ppt (average 3.9 ppt
(1.6 %)). 11 outliers occurred simultaneously in HCFC-22 and in CFC-11,
while only one outlier sample was found to be enhanced in CFC-11 and CFC-12
and only four in CFC-12 and HCFC-22.
derived average annual growth rates of 6.9 and
7.0 ppt year-1 from high frequency measurements of HCFC-22 at Monte Cimone and
at Mace Head for the time period 2002–2012. Growth rates increased until
2008 and started to decline afterwards with values around 3 ppt year-1 in
2012. At Taunus Observatory, where measurements started in late 2013,
atmospheric mixing ratios steadily increase over the measurement period, with
the increase rate at Taunus Observatory slowing down from around
5 ppt year-1
in 2014 to around 3 ppt year-1 in 2017.
In a global inversion study using ground station data,
derived a seasonality of regional emissions of
HCFC-22 possibly arising from a seasonality in the use of air conditioning
and refrigeration devices. This was most pronounced in eastern Asia, the US,
and the Middle East.
In an inversion study based on data from NOAA and AGAGE station networks and
additionally constrained by data from airborne measurements from the
HIaper-Pole-to-Pole Observations (HIPPO) missions, could
reproduce the observed seasonalities in HCFC-22 mixing ratios only with a
seasonal adjustment to emissions with higher summertime emissions.
Observations at Taunus Observatory are consistent with the assumption of
seasonally varying emissions.
Time series (a) and seasonality (b) of HCFC-22 at
Taunus Observatory (blue) and at Mace Head (black). Open symbols in the time
series indicate samples flagged as outliers. Seasonality is shown as the
difference of the monthly mean to the mean of the full detrended baseline
time series.
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Long-lived hydrofluorocarbons: HFC-134a, HFC-245fa, and HFC-227ea
As replacement for the ozone-depleting CFCs and HCFCs, hydrofluorocarbons are
now commonly used. Not containing chlorine or bromine atoms, they are not
ozone depleting substances. However, long-lived HFCs are strong greenhouse
gases contributing to global warming (cf. Table ). To
reduce the adverse contribution of HFCs to future global warming, regulation
of their use and production has been added to the Montreal Protocol in 2016
in the Kigali amendment. Several HFCs are measured regularly at AGAGE and
NOAA sites and also at Taunus Observatory.
Figure a shows the time series of HFC-245fa measured with
the QP mass spectrometer. The QP dataset is used because of better data
coverage, although the TOF instrument yields a better precision (cf.
Table ). For the time period covered by the QP as well as
the TOF mass spectrometer, the correlation of the two datasets yields a slope
of 0.97±0.04 ppt ppt-1 (axis offset of 0.14±0.14 ppt),
taking into account the precision of both instruments in an orthogonal
data-fitting routine (cf. Fig ).
Time series of HFC-245fa (a) and of HFC-227ea
(b) at Taunus Observatory (coloured symbols) and Mace Head (black
symbols). HFC-227ea measurements were performed with the time-of-flight mass
spectrometer (coloured diamonds); for comparison, data from the quadrupole
mass spectrometer (coloured triangles) are shown in addition. Solid lines
represent a second-order polynomial harmonic fit to the Taunus Observatory
baseline dataset. Open symbols denote samples flagged as outliers at Taunus
Observatory.
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reported first measurements of HFC-245fa
(CHF2CH2CF3) in the atmosphere from Jungfraujoch (Switzerland) where
mixing ratios reached 0.68 ppt at the end of 2005. At Taunus Observatory,
baseline mixing ratios had climbed up to values above 4 ppt by the end of
2017. Mace Head data overall agree with the Taunus Observatory baseline.
Outliers at Taunus Observatory were few in 2014 but their number increases
through 2017. HFC-245fa exhibits a seasonal cycle with minimum mixing ratios
in summer, consistent with an OH sink, and a second, less pronounced minimum
in January, possibly related to transport of stratospheric air containing
lower amounts of HFC-245fa. Its total lifetime is estimated to 7.9 years,
dominated by loss through reaction with OH . Although its
stratospheric lifetime is much longer, aged stratospheric air contains lower
HFC-245fa mixing ratios due to its tropospheric trend.
Emissions of HFC-227ea started in the early 1990s from a range of
applications, such as from fire extinguishers replacing bromine-containing
halons. first reported HFC-227ea (CF3CHFCF3)
in atmospheric air samples and firn air samples. In 2009, atmospheric mixing
ratios were about 0.5–0.6 ppt, with higher values in the Northern
Hemisphere. Panel (b) of Fig. displays the time series
of HFC-227ea mixing ratios at Taunus Observatory and at Mace Head. HFC-227ea
was only added to the quadrupole measurements in 2016. For the time period
covered by both instruments, the two datasets agree with a linear correlation
coefficient of r2=0.96, and the orthogonal fitting procedure yields a
slope of 1.06±0.04 ppt ppt-1 (axis offset of
-0.13±0.06 ppt). This example highlights the potential of the TOF mass
spectrometer for retrospective analysis of non-target compounds.
In early 2014, background mixing ratios of HFC-227ea reached approx. 1 ppt
and atmospheric mixing ratios continue to increase at both sites, showing
almost no seasonality as expected from its long lifetime τOH=45.3 a . With the exception of a few outliers in early
2014, baseline mixing ratios at Mace Head are below those at Taunus
Observatory, which also exhibits a larger scatter. Both aspects are expected
for a widely used compound. HFC-227ea sticks out with a large frequency of
outliers: up to one-third of samples contain mixing ratios significantly
above the baseline variability. Outliers were particularly frequent and
strong in 2014 and in 2017.
While global regulation of production and use of long-lived HFCs according to
the Kigali amendment to the Montreal Protocol does not foresee a reduction
prior to 2029, the states of the European Union have adopted the so-called
F-gas directive (Regulation (EU) No 517/2014 on fluorinated greenhouse gases)
aiming at a reduction of European emissions of fluorinated greenhouse gases
by regulating their use after 2015. It is unclear whether these measures
already cause the observed slow-down of HFC-245fa, but considering its
lifetime with respect to reaction with OH of 7.8 years, it is a compound
expected to respond to regulation rather quickly. HFC-227ea with
τOH=45.3 a does not show a significant slow-down of the
increase rate.
Additional European regulation was implemented in the mobile air conditioning
directive (Directive 2006/40/EC relating to emissions from air-conditioning
systems in motor vehicles). This directive restricts type approvals of
vehicles fitted with an air conditioning system operating with fluorinated
greenhouse gases with a global warming potential higher than 150 after 2011.
Figure shows data for HFC-134a, a widely used compound
affected by this. The short-lived compound HFC-1234yf, which is already used
as a replacement for HFC-134a, was successfully identified among the
non-target species measured by our time-of-flight mass spectrometer.
Currently HFC-134a is the most prevalent HFC in the atmosphere, with baseline
mixing ratios at Taunus Observatory varying around 120 ppt in 2018 and a
lifetime of τOH=14.5 a. Mixing ratios at Taunus Observatory
are consistently approx. 10 ppt (≈10 %) above those at Mace Head
and exhibit a large variability and a large number of outliers above the
baseline. Baseline mixing ratios increase at a slightly accelerating rate of
around 6–7 ppt year-1. Inversion-based top–down estimates of
emissions yielded increasing global emissions for the period 2004–2012 with
a large discrepancy from emissions reported to the UNFCCC .
Time series (a) of HFC-134a at Taunus Observatory (blue)
and at Mace Head (black). Open symbols in the time series indicate samples
flagged as outliers.
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Time series (a) and seasonality (b) of
dichloromethane at Taunus Observatory (blue) and at Mace Head (black). Open
symbols in the time series indicate samples flagged as outliers. Seasonality
is shown as the difference of the monthly mean to the mean of the full
detrended baseline time series.
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found a similar result for HFC-134a as for HCFC-22, namely that
summertime emissions exceed emissions during winter. While this was reflected
in the seasonal cycle of HCFC-22 mixing ratios at Taunus Observatory,
HFC-134a baseline data at the site show only a weak seasonality due to the
large variability. Because of its use in mobile air conditioning, sources of
HFC-134a are ubiquitous in central Europe. Taunus Observatory therefore is
very close to emissions and the high variability in the dataset masks the
baseline. Mixing ratios at Mace Head are below those measured at Taunus
Observatory throughout the observation period and show a weak seasonality
similar to that of HCFC-22 (not shown). The subset of data identified by the
baseline detection algorithm therefore does not represent the European
background. Still, outliers can be statistically evaluated the same way as
for the other substances. The frequency of outliers above the baseline is
lowest during winter months (DJF) and has a maximum in summer (JJA).
Enhancement relative to the baseline also maximises during summer. This
behaviour is consistent with HFC-134a being predominantly used for air
conditioning and an enhanced leakage rate of air conditioning devices during
that time of the year.
Dichloromethane
As an exemplary substance with strong seasonality, Fig. a
shows mixing ratios of dichloromethane. Data have been fitted using
Eq. (), as including higher-order harmonics did not
improve the quality of the fit. Mace Head data represent a lower envelope to
the Taunus Observatory baseline time series, with the exception of one
outlier at Mace Head in September 2014. The seasonal cycle is mainly driven
by the reaction of dichloromethane with the OH radical with a lifetime of
approximately 5 months . At both observation
sites, seasonality shown in Fig. b exhibits an annual
minimum in September and maximum mixing ratios in early spring.
Outliers in the time series from Taunus Observatory occur mainly during
spring, but the highest enhancements in individual samples are observed in
summer and autumn, reaching values more than twice the respective baseline
averages. On average, enhancement of outliers at Taunus Observatory is
approx. 30 % above the baseline mixing ratio. Outliers with high mixing
ratios of dichloromethane were also outliers with regard to their CFC-11
mixing ratio in 16 cases; thus, in more than half of all observed CFC-11
outliers, also elevated mixing ratios of dichloromethane were also found.
In the time series shown in Fig. , mixing ratios were
relatively stable at both sites over the period 2014–2016. From 2016 to 2017
an increase of approximately 3.5 ppt is registered at Mace Head; at Taunus
Observatory, the baseline mixing ratios (annual means) increased by 2 ppt
from 2016 to 2017, which is less than the annual variability of the baseline
data (ca. 8.0–8.5 ppt).