ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-11451-2016Atmospheric lifetimes, infrared absorption spectra, radiative forcings and global warming potentials of NF3 and CF3CF2Cl (CFC-115)TotterdillAnnaKovácsTamásFengWuhuDhomseSandiphttps://orcid.org/0000-0003-3854-5383SmithChristopher J.Gómez-MartínJuan CarlosChipperfieldMartyn P.https://orcid.org/0000-0002-6803-4149ForsterPiers M.p.m.forster@leeds.ac.ukPlaneJohn M. C.https://orcid.org/0000-0003-3648-6893School of Chemistry, University of Leeds, Leeds, LS2 9JT, UKNCAS, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UKSchool of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UKEnergy Research Institute, School of Chemical and Process Engineering, University of Leeds, Leeds, LS2 9JT, UKPiers M. Forster (p.m.forster@leeds.ac.uk)14September20161617114511146316March201620April201623August201628August2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/11451/2016/acp-16-11451-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/11451/2016/acp-16-11451-2016.pdf
Fluorinated compounds such as NF3 and C2F5Cl (CFC-115) are
characterised by very large global warming potentials (GWPs), which result
from extremely long atmospheric lifetimes and strong infrared absorptions in
the atmospheric window. In this study we have experimentally determined the
infrared absorption cross sections of NF3 and CFC-115, calculated the
radiative forcing and efficiency using two radiative transfer models and
identified the effect of clouds and stratospheric adjustment. The infrared
cross sections are within 10 % of previous measurements for CFC-115 but are
found to be somewhat larger than previous estimates for NF3, leading to a
radiative efficiency for NF3 that is 25 % larger than that quoted in
the Intergovernmental Panel on Climate Change Fifth Assessment Report. A
whole atmosphere chemistry–climate model was used to determine the
atmospheric lifetimes of NF3 and CFC-115 to be (509 ± 21) years
and (492 ± 22) years, respectively. The GWPs for NF3 are estimated
to be 15 600, 19 700 and 19 700 over 20, 100 and 500 years, respectively.
Similarly, the GWPs for CFC-115 are 6030, 7570 and 7480 over 20, 100 and
500 years, respectively.
Introduction
Fluorinated compounds such as NF3 and CFC-115 are potentially important
for global warming (Myhre et al., 1998). The stability
and thermophysical properties of these gases have made them attractive
chemicals for use in many relatively modern industrial processes. NF3
is being used increasingly as a replacement for banned perfluorocarbons (PFCs)
which were utilised in processes such as chemical cleaning and
circuit etching. It is an extremely potent greenhouse gas with an estimated
100-year global warming potential (GWP) between 10 800 and 17 000
(Arnold et al., 2013; Robson et al., 2006; Weiss et al., 2008). Weiss et al. (2008)
reported a 2008 mean global tropospheric mixing ratio of 0.45 ppt increasing
at a rate of 0.053 ppt yr-1. The present-day mixing ratio will
therefore be close to 1 ppt, assuming no change in emission rate.
Arnold et al. (2013) found undetectable levels of
NF3 prior to 1975 in archived air samples and ice cores, indicating
that the major source of the gas is anthropogenic.
CFC-115 was introduced as refrigerant in the 1970s and prior to this was not
detected in the atmosphere. Following the phasing out of CFC-115 through the
1997 Montréal Protocol (Solomon et al., 2007), its
atmospheric concentration stabilised by 2005. A mixing ratio of 8.4 ppt was
reported in 2011, with a decreasing trend of 0.01 ppt yr-1
(Maione et al., 2013). Based on an atmospheric
lifetime of 1020 years, its 100-year GWP was estimated to be 7370
(Solomon et al., 2007).
The change in concentration of any trace gas depends, in part, on how its
emission evolves over time, but also on the rates of any chemical and
physical removal processes. The only important sinks for CFC-115 and
NF3 appear to be photolysis (Dillon et al., 2010; Totterdill et al., 2014, 2015) and
reaction with O(1D) (Baasandorj et al., 2013;
Dillon et al., 2011). Reaction with the meteoric metals Na and K is
a minor loss route (Totterdill et al., 2014, 2015).
Trace gas concentrations are dependent on the atmospheric lifetimes (τ)
of the species, defined as the ratio of the total
atmospheric burden to the total loss rate. Estimates of the atmospheric
lifetimes of NF3 and CFC-115 have recently been reported by
SPARC (2013). For NF3 the recommended value is 569
years, which was based on 2-D model calculations including loss due to
photolysis (71 % of total loss) and reaction with O(1D) (29 %).
This overall lifetime was slightly larger than the value of 500 years given in
the WMO (2011) assessment. For CFC-115, the recommended lifetime
is 540 years (37 % of loss due to photolysis, 63 % due to reaction with
O(1D)), which was much lower than the WMO (2011) value of
1020 years based on the previous O(1D) reactive yield from Sander et al. (2011).
Therefore, these fluorinated compounds are very potent global warming agents
because, in addition to having atmospheric lifetimes of many centuries, they
absorb infrared (IR) radiation strongly between 800 and 1200 cm-1. This
region of the electromagnetic spectrum is known as the “atmospheric window”
because of a pronounced minimum in atmospheric absorption by H2O,
CO2 and O3. Furthermore, the window overlaps with the peak in the
terrestrial infrared spectrum (500–1500 cm-1), making it a
particularly important region in the radiative balance of the atmosphere
(Pinnock et al., 1995).
However, it is difficult to quantify surface temperature changes resulting
from small perturbations due to climate variability and large uncertainties
in climate feedback mechanisms (Pinnock et
al., 1995). The historic effects of various drivers of climate change are
typically specified and compared in terms of their radiative forcings, a
measure of the perturbation to the Earth's energy budget. Various flavours
of radiative forcing exist (Myhre et al., 1998). The
effective radiative forcing measures top-of-atmosphere energy budget changes
following adjustments to the vertical temperature profile, clouds and
land-surface temperatures. The stratospheric adjusted radiative forcing
is defined as the change in net (i.e. down minus up) irradiance at the
tropopause (solar plus longwave, in W m-2) after allowing for
stratospheric temperatures to readjust to radiative equilibrium, but with
the surface and tropospheric temperatures and state held fixed at the
unperturbed values (Ramaswamy et al.,
2001). Note the instantaneous radiative forcing (IRF) can be obtained by not
applying the stratospheric adjustment. Although effective radiative forcing (ERF)
estimates are more representative of temperature changes, they are more
uncertain as they rely on climate model estimates of cloud response
(Sherwood et al., 2015). Further, climate model
radiation codes do not typically represent minor greenhouse gases (GHGs);
therefore it is not currently possible to estimate the ERF for the species
considered here. We have therefore estimated radiative forcing (RF) and IRF
using a line-by-line radiative transfer model (RTM). As the line-by-line RTM
only accounts for absorption, the extension to clouds and scattering
processes was performed by a secondary radiative transfer model using
atmospheric optical depth profiles generated by the RTM.
The purpose of this work was to update the values for the GWPs of NF3
and CFC-115, based on their cloudy-sky-adjusted radiative efficiencies (the
definition of the GWP is discussed in Section 6) and using a more
sophisticated atmospheric model. In order to achieve this, infrared
absorption cross sections for both NF3 and CFC-115 were measured and
then used as input into the Reference Forward Model (RFM)
(Dudhia, 2014) and library for radiative transfer
(libRadtran) (Mayer and Kylling, 2005), two radiative
transfer models used to calculate radiative forcings and efficiencies. Here,
radiative forcing refers to a perturbation of the modern-day concentration
of the compound against its pre-industrial concentration, and is given in
units of W m-2. Radiative efficiency refers to a perturbation of
0–1 ppb and is given in units of W m-2 ppbv-1. The sensitivity of
these determined forcings to a number of criteria including cloudiness and
stratospheric adjustment was also examined. The atmospheric concentrations
of NF3 and CFC-115 were then determined using the Whole Atmosphere
Community Climate Model (Garcia et al., 2007),
incorporating the chemical loss processes described in our recent papers
(Totterdill et al., 2014, 2015). The Whole Atmosphere Community Climate Model (WACCM) also produced estimates of the
atmospheric lifetimes of the two compounds.
Experimental
The IR spectrum of NF3 has been measured previously by
Robson et al. (2006) and Molina et al. (1995), and that of CFC-115 by
McDaniel et al. (1991). The cross sections
reported in the literature show some scatter, and few quantitative full
spectra measurements are available. This work was consequently carried out
in order to provide a more complete set of measurements and reduce
uncertainty in the published data.
Measurements were made in a 15.9 cm gas cell sealed with KBr windows, which
allowed transmission between 400 and 4000 cm-1. Spectra were recorded
with a Bruker Fourier transform spectrometer (model IFS/66) fitted with a
mid-infrared source and beam-splitting optics. Room temperature (296 ± 2 K)
measurements were carried out between 400 and 2000 cm-1 at a
spectral resolution of 0.1 cm-1. Absorption spectra were obtained by
averaging 128 scans at a scan rate of 1.6 kHz. Gas mixtures of different
strength were prepared to obtain absorbances with high signal-to-noise
ratio, while still being within the range of validity of the Beer–Lambert law.
Metal atom and O(1D) reactions with NF3 and CFC-115 added
to the WACCM chemistry module.
ReactionRate coefficient/cm3 molecule-1 s-1SourceO(1D) + NF3(2.0 ± 0.3) × 10-11Dillon et al. (2011)Na + NF36.0 × 10-10exp(-2240/T)Totterdill et al. (2014)+ 2.3 × 10-11exp(-589/T)K + NF316.0 × 10-10exp(-2297/T)Totterdill et al. (2014)+ 1.3 × 10-11exp(-866/T)O(1D) + CFC-115(6.5 ± 0.6) × 10-11exp(+30/T)Baasandorj et al. (2013)Na + CFC-1155.9 × 10-10exp(-4257/T)Totterdill et al. (2015)+ 1.8 × 10-11exp(-2093/T)K + CFC-1151.9 × 10-10exp(-1929/T)Totterdill et al. (2015)
Gas mixtures were made using between 12 and 307 Torr of NF3, and 6 and 77 Torr
of CFC-115, made up to 760 Torr with N2. Multiple mixtures were
made up so that the cross section could be obtained at a selected wavelength λ
by taking the slope of the linear regression of the corresponding
peak absorbance against concentration according to the Beer–Lambert law:
A(λ)=σ(λ)lc,
where A is the absorbance, σ is absorption
cross section in cm2, c is concentration in molecule cm-3
and l is path length in cm. Concentrations were determined from the
mixing ratios calculated from pressures measured with a capacitance
manometer (Baratron model 222 CA), calibrated with an oil manometer.
Uncertainties in concentrations are smaller than 1 % are reported in the
Results section. Analyte concentrations were selected so that A< 1,
to avoid deviation from the Beer–Lambert linear behaviour.
Baseline and background corrections (including removal of CO2 and
H2O) were performed after the experiments.
Reactant gas mixtures for the experiments were prepared on all-glass vacuum
lines. The gases N2 (99.9999 %, BOC) and NF3 (99.99 %, BOC) were
used without further purification. Samples of CFC-115, provided by Professor
William Sturges (University of East Anglia), were purified by
freeze–pump–thaw degassing on a glass vacuum line.
Atmospheric modelling
The atmospheric distributions of NF3 and CFC-115 were simulated using
the 3-D WACCM (Garcia et al., 2007). WACCM is a comprehensive
numerical model extending vertically from the ground up to the lower
thermosphere (∼ 140 km) and is part of the National Centre for
Atmospheric Research (NCAR) Community Earth System Model (CESM)
(Lamarque et al., 2012). WACCM
calculates the concentrations of atmospheric species by considering all
relevant chemical and dynamical processes. Here we have used a free running
version of WACCM 4 (Garcia et al., 2007), which has
66 levels from the surface to 5.96 × 10-6 Pa (∼ 140 km)
with a vertical resolution of 3.5 km scaled height in the mesosphere
and lower thermosphere (MLT) region and 1.9∘× 2.5∘
(latitude × longitude) horizontal resolution. The
model contains all the important details of the MLT processes including
radiative transfer, auroral processes, non-local thermodynamic equilibrium
and the molecular diffusion of constituents.
A series of NF3 and CFC-115 tracers were included in the model. For the
model simulations presented here, three loss processes for NF3 and
CFC-115 were included: reactions with O(1D); reactions with the
mesospheric metals (Na, K); and UV photolysis. For each compound, a set of
five tracers was used. One tracer was removed by all three processes; three
tracers were used to determine the individual impact of each loss process
acting alone; and the fifth was a passive (i.e. chemically inert) tracer. In
a recent paper, Papadimitriou et al. (2013) concluded that the
200–220 nm wavelength range is important for NF3 photolysis in the
stratosphere. The long-wavelength photolysis cross section also has a strong
temperature dependence. In the current study this wavelength region was also
included. Two model runs were carried out, without and with the
> 200 nm regime. For CFC-115 only one model run was carried out that included
both the short- and long-wavelength absorption cross sections. SPARC (2013)
reported that the 190–230 nm region accounts for 28 % of the total CFC-115 loss.
The relevant rate coefficients for O(1D) and the metal atom reactions
that were added to the chemistry module in WACCM are listed in Table 1. Note
that we have only included the reactions with Na and K, as the reactions
with the more abundant meteoric metals Fe and Mg are very slow at
temperatures below 300 K (Totterdill et al., 2014, 2015). The standard chemistry scheme in WACCM
contains 59 species and 217 gas-phase reactions (Kinnison et al., 2007).
O(1D) was determined from this scheme. Modules with a further
61 reactions describing the chemistry of Na and K were then added to this
scheme, along with the meteoric input functions required to simulate the Na
and K layers in the MLT (Marsh et al., 2013a; Plane et al., 2014).
The photolysis rates of NF3 and CFC-115 were calculated in WACCM using
the fitted expressions we determined previously for their absorption cross
sections as a function of wavelength between 121.6 and 200 nm
(Totterdill et al., 2014, 2015).
Following the study by Marsh et al. (2013b), the daily
solar spectral irradiances used in WACCM were specified from the model of
Lean et al. (2005), updated with the total solar
irradiance of Kopp and Lean (2011). The model was then
run from year 2000 for 13 years when the solar data are available and are
enough for the atmospheric lifetimes of NF3 and CFC-115 to reach a steady state. Although time-dependent solar forcing was used in the
simulations, it does not have any noticeable impact on the very long
lifetimes of these gases.
Radiative transfer modelling
Radiative forcing calculations were made using the Reference Forward Model (RFM)
(Dudhia, 2014). The RFM is a line-by-line radiative
transfer model based on the previous GENLN2 model (Edwards,
1987), and includes absorption cross sections for CFC-115 and NF3
derived from the HITRAN database (Rothman et al., 2013). In
addition to providing upwelling and downwelling longwave fluxes for
calculating the clear-sky forcing, the RFM was used to generate optical
depth profiles at a resolution of 1 cm-1 for input into the DISORT
radiative transfer solver as implemented in the libRadtran
(Mayer and Kylling, 2005). The clear-sky fluxes obtained
from the RFM were validated against results from libRadtran for the
cloudless, non-scattering case.
Calculations to obtain the IRF and RF were performed using the flux form of
the RFM at a spectral resolution of 0.1 cm-1, determined by the
resolution of the IR spectra measured in the present study (Sect. 2). The
radiative transfer calculation was performed on each spectral band between
550 and 2000 cm-1, and the irradiance flux was integrated over
the wavelength to obtain the net irradiance at each level in the model
atmosphere. For these calculations the Planck function was set to vary
linearly with optical depth. For the stratospheric adjustment, the
stratosphere temperatures were adjusted using an iterative process based on
heating rate changes that after 100 days have changed the stratospheric
temperatures and returned them to radiative equilibrium (see also in
Introduction). For both NF3 and CFC115 the temperature change at the
tropopause is ∼ 1.5 × 10-5 K for a perturbation
of 1 ppt (average tropospheric volume mixing ratio), and ∼ 0.01 K for a perturbation of 1 ppb.
Infrared absorption spectrum of NF3 at 295 ± 2 K on a
logarithmic (a) and linear (b) scale. The logarithmic plot
highlights the minor bands.
A compilation of line data for background species was obtained from HITRAN 2012
(Rothman et al., 2013), and absorption cross sections for
NF3 and CFC-115 were measured in the present study. The temperature,
pressure and mixing ratios of the major atmospheric constituents
CO2, H2O, CH4, N2O and O3, as well as NF3 and
CFC-115, were obtained from the WACCM output. The temperature dependence of
background species' absorption is automatically interpolated from HITRAN
data. Although the temperature dependence of the NF3 and CFC-115 cross
sections was not measured, we assume that it makes a negligible uncertainty
in the radiative forcing calculations.
The effect of seasonal and geographical variations on factors influencing
radiative forcing means that multiple averaged local radiative forcings
calculated across the location-time grid are needed to estimate global
forcing. The instantaneous and adjusted radiative forcings and efficiencies
were first calculated in the RFM for each month between -90 and 90∘,
at a 9∘ latitude and longitude resolution. We also created vertical profiles by averaging three latitudes
(representing the tropics, midlatitudes and high latitudes) for each month. The
area-averaged forcing from the three profiles was found to yield a forcing value
within 2 % of that obtained from the gridded data. Compared to using a
global annual mean profile, Freckleton et al. (1998) demonstrated that the method of averaging
a small number of latitudes is 5–10 % more accurate than use of the global
annual mean profile. Explanations for this difference in accuracy are
discussed in Sect. 5.3. The profiles representing the three latitudes for
each month were also used to calculate radiative forcings and efficiencies
in libRadtran. Clear-sky IRFs calculated in libRadtran were within 3 % of
those calculated by the RFM.
Integrated absorption cross sections of NF3 measured in the
present study at 296 ± 2 K (1σ uncertainty ±6 % for the
major bands), and compared to the previous studies of Robson et al. (2006) and
Molina et al. (1995).
Band limits/Integrated IR bandRatio toRatio tocm-1cross section/integratedintegrated10-18 cm2 molec-1 cm-1cross sectioncross sectionfrom Robsonfrom Molinaet al. (2006)et al. (1995)600–7000.411.061.20840–96065.031.031.79970–10855.881.111.391085–12000.100.660.681330–14400.084.354.761460–15800.211.541.591720–18700.710.971.091890–19700.651.011.06600–197073.501.041.72ResultsInfrared absorption spectra
Figure 1 illustrates the IR absorption cross section spectrum measured for
NF3 in the present study. The band strengths sections obtained from
this measured spectrum are listed in Table 2, which are also compared with the
integrated cross sections reported in the literature, where available.
Similarly, the CFC-115 spectrum is displayed in Fig. 2, and Table 3 lists
the corresponding integrated cross sections and comparison with the
literature. The uncertainties in the sample concentrations of NF3 and
CFC-115 were ±0.8 and 0.7 %, respectively. The average spectral
noise was ±5 × 10-21 cm2 molecule-1 per
1 cm-1 band. However, at wavenumbers < 550 cm-1, towards
the edge of the mid-IR, where the opacity of the beam-splitting filter
increased, this increased to ±1 × 10-20 cm2 molecule-1
per 1 cm-1 band. The average standard errors of the
slopes obtained from the regression of absorbance vs. concentration for
NF3 and CFC-115 at selected wavelengths (Eq. 1) were ±5 and
6 %, respectively. This results in an average overall spectral error
of ±6 % (1σ) in both cases. This is the average error of
the integrated band cross section and is fairly uniform for the major bands.
As shown in Table 2, the intensities of the two main absorptions bands
(840–960 and 970–1085 cm-1) of NF3 measured in the present work
are 3 and 11 % larger than those reported by Robson et al. (2006), with an average
deviation of 29 % over the minor bands and 4 % across the entire
spectrum. All differences excluding the minor bands in the region between
1085 and 1580 cm-1 are comfortably within the combined error of both
experiments. In contrast, the intensities of the two main absorption bands
are 79 and 39 % larger than those reported by Molina et al. (1995). The minor bands in
our spectrum are on average 32 % larger, and the absorption intensity is
42 % larger across the whole spectrum. These differences are greater than
the combined error from both experiments. Explanations for the lower values
reported by Molina et al. (1995) have been discussed by Robson et al. (2006).
Figure 3 shows that the intensity of the main CFC-115 absorption band
(1212–1265 cm-1) measured in the present work is 6 % smaller than
that reported by McDaniel et al. (1991). The
other significant bands at 946–1020, 1105–1150, 1160–1212 and 1326–1368 cm-1
are 9, 6 and 3 % smaller, respectively. These results are well within the
combined error of both experiments.
Integrated absorption cross sections of CFC-115 measured in the present
study at 296 ± 2 K (1σ uncertainty ±6 % for the major bands),
and compared to the previous study of McDaniel et al. (1991).
Band limits/Integrated IR bandRatio tocm-1cross section/integrated10-17 cm2 molec-1 cm-1cross sectionfrom McDanielet al. (1991)946–10202.5460.911105–11502.0150.941160–12121.3700.951212–12655.3810.941326–13680.6200.97
Infrared absorption spectrum of CFC-115 at 295 ± 2 K on a
logarithmic (a) and linear (b) scale. The logarithmic plot
highlights the minor bands.
(a) Globally averaged NF3 mixing ratio in January of
the thirteenth year of the WACCM simulation, illustrating the profiles of the
four individual tracers (passive, photolysis, metal reaction and reaction with
O(1D)). (c) The corresponding zonal mean NF3 mixing ratio (ppt)
as a function of altitude. (b) As (a) but for CFC-115.
(d) As (c) but for CFC-115.
Atmospheric loss rates of the total NF3 (left-hand panels) and
total CFC-115 (right-hand panels) showing the total loss rate, loss rates via
photolysis and reaction with O(1D), and the percentage contributions to the
total loss rates, in the thirteenth year of the simulation.
Atmospheric lifetimes
The monthly averaged NF3 and CFC-115 concentrations and loss rates in
each WACCM grid box were used to estimate the atmospheric lifetimes of
NF3 and CFC-115, which were computed by dividing the global atmospheric
burden of each compound by its integrated loss rate. The total loss rates
were obtained from the sum of the individual loss rates due to photolysis,
and reactions with mesospheric metals and with O(1D).
Figure 3a and b show the globally averaged profiles of the different modelled
NF3 and CFC-115 tracers, which illustrate the impact of the different
loss processes. The largest mixing ratio profiles for both NF3 and
CFC-115 are shown by the passive tracers as they are not subject to the
removal processes. Note that the decay of the passive tracer mixing ratios
above about 85 km is due to the very long timescale for the tracers to mix
vertically into this region due to the dominance of molecular diffusion.
Clearly, the reactions with atmospheric metals do not contribute to the
atmospheric removal of these gases (i.e. the metal loss tracers profiles are
almost identical to the passive profiles), and hence their impact on the
lifetimes is negligible. The reason is that their temperature-dependent rate
coefficients are much smaller than the rate coefficients of O(1D)
reactions and also, as is shown in Fig. 11 in Totterdill et al. (2014)
(NF3) and in Fig. 9a in Totterdill et al. (2015) (CFC-115), the
removal rates by mesospheric metals are 2 and 4 orders
of magnitudes smaller for NF3 and CFC-115, respectively, than the
removal rates by VUV (vacuum ultraviolet) photolysis, even at the peak of mesospheric metal layers
(90 km). In contrast, photolysis and the reactions with atmospheric
O(1D) are the dominant removal processes. Figure 3c and d are the
corresponding monthly averaged zonal mean profiles of NF3 and CFC-115
from the surface up to 140 km in January. The surface mixing ratios of
NF3 and CFC-115 are 1.05 and 7.9 pptv, respectively. Both tracers are
well mixed in the troposphere below 15 km. There are sharp decreases in the
tropical tropopause layer (TTL) (12–17 km). The mixing ratios of NF3
and CFC-115 are decreasing with increasing altitude from the lower stratosphere
to the mesosphere/lower thermosphere up to 90 km. Above 90 km, their mixing
ratios are quite small (< 0.1 ppt for both compounds). Around
40–50 km in the southern polar region, the low values of NF3 and CFC-115 are
caused by the removal of the reactions with O(1D).
Figure 4 shows the annual mean contributions of photolysis and reaction with
O(1D) to the loss of NF3 and CFC-115. The region of greatest loss
is in the tropics, while at high latitudes the removal rates are orders of
magnitude smaller. For both NF3 and CFC-115, the dominant region of
photolysis is in the stratosphere, below 50 km, although CFC-115 shows a
weak secondary peak in photolytic loss over 60–80 km, which is due to the
increased photolysis loss rate of 2 orders of magnitude over this range (see
Fig. 9b in Totterdill et al., 2014, 2015). Figure 5 is the annually averaged atmospheric lifetime for NF3
(for the run that does not include photolysis above 200 nm) and CFC-115 as a
function of simulation time. A steady state was assumed to be reached when the
lifetime did not change more than 1 % between consecutive years. The
length of the model run is sufficient for the tracers to mix below 85 km,
where the dominant loss processes occur (Fig. 4). When photolysis at
wavelengths > 200 nm is not included, the net lifetime is
(616 ± 34) years as shown in Fig. 5. However, inclusion of photolysis
above 200 nm reduces the mean lifetime of NF3 by 17 % to (509 ± 21) years.
This lifetime is 10 % shorter than the recently published value of
569 years (SPARC, 2013). The uncertainties were estimated
from the error bars of the removal rate coefficients only (see
Table 1), using the error propagation. Photolysis and reaction with O(1D) account
for 67.7 and 32.3 % of the global removal rate, respectively. The
corresponding percentages from SPARC (2013) are 71.3 and
28.7 %. In the case of CFC-115, the lifetime in the present study is
(492 ± 22) years, which is 9 % smaller than the value of 540 years from
SPARC (2013) and under half of that assumed in the GWP
calculations of IPCC AR4 and IPCC AR5 (Forster et al., 2007;
Myhre et al., 2013). Photolysis and reaction with O(1D)
account for 34.4 and 65.6 % of the global removal rate, respectively.
Again, these are close to the corresponding percentages from
SPARC (2013), which are 37.4 and 62.6 %. The
differences between our results and those of SPARC (2013)
likely arise from differences in the rates of atmospheric circulation
in the 3-D WACCM and the 2-D model used in the SPARC report, as well as the
implementation in WACCM of recently updated photolysis parameters
(Totterdill et al., 2014, 2015).
Instantaneous and stratosphere-adjusted radiative forcings of NF3
and CFC-115 in clear- and cloudy-sky conditions.
MoleculeInstantaneous Adjusted Clear,All-sky,Clear,All-sky,10-4 W m-210-4 W m-210-4 W m-210-4 W m-2NF33.662.304.182.61CFC-11527.7018.0929.7719.05
Annually averaged atmospheric lifetimes for NF3 (solid line) and
CFC-115 (dashed line) as a function of simulation time.
Contour plots for radiative forcing (W m -2) by latitude and
month for (a) instantaneous radiative forcing of NF3,
(b) stratospheric adjusted radiative forcing of NF3,
(c) instantaneous radiative forcing of CFC-115 and
(d) stratospheric adjusted radiative forcing of CFC-115. Note different
contour intervals between panels.
Variation of instantaneous radiative forcing for NF3 (W m-2)
with tropopause height for four profiles: thermal tropopause (red solid line),
average thermal tropopause (black dashed line), temperature minimum tropopause
(dash dot magenta line) and average temperature minimum tropopause (blue dotted line).
Calculation of radiative forcing and efficiencies
The calculations of radiative forcing and radiative efficiencies (RE) are
sensitive to several factors such as the choice of tropopause height and the
way clouds are included in the model. This section examines these
sensitivities and further examines the seasonal and latitudinal variations
in forcing.
Tropopause
The definition of tropopause height directly influences the calculation of
radiative forcing. There are three commonly used definitions: the thermal
tropopause (ThT), defined as the lowest level at which the temperature lapse
rate between this and all higher levels within 2 km falls below 2 K km-1
(WMO, 2007); the temperature minimum tropopause (TMT),
the base of the stratospheric temperature inversion; and a uniform pressure
level of 200 hPa as a proxy for the top of the convective level, where there
is a significant change in stability below the thermal tropopause
(Forster and Shine, 1997). Forster
and Shine (1997) identified the latter as the most appropriate for
radiative forcing calculations at high horizontal resolution. However, the
thermal tropopause (ThT) used in this study was found to generate results
which were accurate to within 0.5 % of those produced by the convective
tropopause (Freckleton et al., 1998). At lower horizontal resolution this
uncertainty is greater.
The temperature profile, and thus tropopause height, show a significant
spatial variation and are affected by profile averaging. This adds extra
inaccuracies when using a global annual mean profile, which does not account
for the variation in tropopause height. This effect was explored with
respect to the instantaneous radiative forcing of NF3. Figure 7 shows
the latitudinal variation of instantaneous radiative forcing with tropopause
height when using the ThT, TMT and globally averaged tropopause definitions.
The globally averaged thermal tropopause was found to be 12.8 km. When the
spatially varying thermal tropopause was applied to the forcing calculations
it yielded an average instantaneous radiative forcing 10 % lower than that
employing the ThT. The globally averaged TMT was found to be 14.9 km. When
this was applied it resulted in an average instantaneous radiative forcing
5 % higher than the ThT. The spatially averaged TMT was found to be 3 %
higher on average, compared to its global averaged profile.
The global mean TMT gives a significant overestimation of radiative forcing
from a high, unrepresentative tropopause of 14.9 km caused by temperature
variations being smoothed out through averaging. Additionally, the averaging
procedure affects parameters such as the H2O vapour and O3
profiles, so that averaging monthly profiles over latitudes representing the
topics and midlatitudes (rather than globally) gives a better
representation of these variables. Because both NF3 and CFC-115 are
well mixed in the stratosphere, they are potentially less affected by
tropopause height than species that decay strongly in the lower
stratosphere (Myhre et al., 1998). Consequently, the
spatially averaged ThT was selected for the calculations described below.
Seasonal–latitudinal variation
Figure 7 shows the large seasonal–latitudinal impact on variation in the IRF
and RF of NF3 and CFC-115 under clear-sky conditions. The latitudinal
variation in zonally averaged forcing in a single month can be a factor of 8;
the variation in monthly forcing for a single latitude is much smaller,
approximately a factor of 2 on average. The lack of uniformity across this
grid demonstrates the requirement for higher resolution calculations.
The variation of radiative forcing and efficiency as a function of latitude
is primarily due to changes in the Planck function caused by variation in
background temperature. Differences in cloudiness and H2O density
levels were also found to contribute. Forcings averaged across the Southern
Hemisphere were approximately 25 % lower than those averaged across the
Northern Hemisphere due to its average cooler surface temperature
(Prather and Hsu, 2008). The lowest radiative forcings
for each month are observed at the South Pole due to its cold surface
temperature, with the very lowest occurring at the winter Antarctic polar vortex.
Cloudiness
Because clouds absorb across the same spectral region as NF3 and
CFC-115, their presence will cause a reduction in radiative forcing.
Consideration of cloud coverage is therefore crucial to forcing
calculations. The treatment of clouds involves determination of cloud band
transmittance from user specified liquid water path, effective radius and
cloud fraction at each altitude level. The zonal mean coverage for a given
latitudinal band is obtained as monthly means from the International
Satellite Cloud Climatology Project (ISCCP) D2 dataset averaged from between 1983
and 2008 (Rossow et al., 1996). Results were
calculated from different, weighted combinations of clear sky plus various
configurations of cloud coverage using the independent pixel approximation
in libRadtran.
Instantaneous and stratosphere-adjusted radiative efficiencies of
NF3 and CFC-115 in clear- and cloudy-sky conditions.
Comparison of the 20-, 100- and 500-year global warming potentials for
NF3 and CFC-115 from this work, the IPCC AR4 (Forster et al., 2007) and the IPCC AR5
(Myhre et al., 2013).
This work IPCC AR4 IPCC AR5 MoleculeGWP20GWP100GWP500GWP20GWP100GWP500GWP20GWP100NF315 60019 70019 70012 300a17 200a20 700a12 800c16 100cCFC-1156030757074805310b7370b9990b5860d7670d
a Based on an atmospheric lifetime of 740 years.
b Based on an atmospheric lifetime of 1700 years.
c Based on an atmospheric lifetime of 500 years.
d Based on an atmospheric lifetime of 1020 years.
The IRF and RF in clear- and cloudy-sky conditions for NF3 and CFC-115
are given in Table 4, and the relative radiative efficiencies in Table 5.
Radiative forcings are shown in Fig. 6. For completeness, clouds and
stratospheric adjustment need to be included in the overall estimate of
radiative forcing and radiative efficiencies. Our best estimate of the all-sky
adjusted radiative efficiency for NF3 is 0.25 W m-2 ppb-1,
approximately 25 % larger than the 0.2 W m-2 ppb-1 quoted in
Myhre et al. (2013), and also larger than the 0.21 W m-2 ppb-1 estimated in Robson et al. (2006). These differences
can be attributed to the larger absorption cross section present in our
study (Table 2). The equivalent all-sky-adjusted RE for CFC-115
(0.21 W m-2 ppb-1) is very similar to that quoted in Myhre et
al. (2013). The globally averaged adjusted all-sky radiative forcings of
NF3 and CFC-115 are 2.6 × 10-4 W m-2 and 19 × 10-4 W m-2
respectively. Tables 4 and 5 also indicate that radiative
forcings and efficiencies are about 60 % larger if clouds are neglected.
This is because a cloud acts a black body, and so the contribution to the
total forcing from greenhouse gas changes situated below clouds is minimal.
Thus, the optical depth due to the greenhouse species, as well as
temperature difference between the emitting surface and the top of the
atmosphere, and the region in which the greenhouse species absorbs, are much
smaller where clouds are present (Myhre et al., 1998).
The impact of cloud on both NF3 and CFC-115 is similar; because they
are both very long-lived species, they are both well mixed in the
stratosphere, and so the magnitude of downward irradiance due to each species
at the tropopause is approximately the same.
Global warming potentials
Global warming potential is defined by the expression
GWP=∫0THaχ[χ(t)]dt∫0THar[r(t)]dt,
where TH is time horizon; aχ is radiative forcing due to a
unit increase in atmospheric abundance of the compound (W m-2 kg-1)
and [χ(t)] is its time-dependent decay in concentration
following its instantaneous release at time t= 0. The
denominator contains the corresponding quantities for CO2 as a
reference gas (Myhre et al., 2013). GWP is the most common metric
used by the WMO and IPCC to compare the potency of a greenhouse gas relative
to an equivalent emission of CO2 over a set time period. GWP takes into
account species lifetime. This means that a species with a very high radiative
forcing may still have a low GWP if it also has a short atmospheric
lifetime. Note that GWP is only one of a range of possible metrics and is
not necessarily representative of temperature changes or other climate
impacts, since it does not account for factors such as changes in emission
or the introduction of replacement species. Other criticisms are also
discussed in greater detail by Myhre et al. (2013).
Table 6 lists the 20-, 100- and 500-year GWPs based on cloudy-sky-adjusted
radiative efficiencies of NF3 and CFC-115 compared with the values
reported in IPCC AR4 and AR5 (Forster et al., 2007; Myhre
et al., 2013). The GWPs for NF3 are estimated to be 15 600, 19 700
and 19 700 over 20-, 100- and 500 years, respectively. These GWPs are considerably
larger than quoted in IPCC AR5 due to the stronger absorption cross section
and radiative efficiency of NF3. The GWPs for CFC-115 are 6030, 7570 and 7480
over 20-, 100- and 500 years, respectively. The forcing efficiencies determined in
the present study are much higher for NF3 but the same for CFC-115,
when compared to IPCC AR5 (Myhre et al., 2013). The effect of the
change in radiative efficiency is most obvious for the 20-year GWP, where,
because the atmospheric lifetimes of NF3 and CFC-115 are respectively
509 and 492 years, the species do not have time for significant loss to
occur. In contrast, the 500-year GWP is more representative of the impact of
the reduced lifetimes.
The trade-off between these competing effects is demonstrated in Table 6,
where NF3 and CFC-115 exhibit 20-year GWPs larger by 27 and 14 %,
respectively, than their IPCC AR4 determined values. These differences then
decrease for the 100-year GWP (14 and 2 %) and the 500-year GWP
(-4 and -25 %). Note that in the case of CFC-115, where the IPCC AR4
atmospheric lifetime used to define the GWP is 1700 years, which is over 3
times that of our value of 492 years, the 500-year GWPs from Forster
et al. (2007) are larger than our quoted values.
Summary and conclusions
In this study we have presented updated values for the IR absorption
cross sections and atmospheric lifetimes of NF3 and CFC-115, as well as
radiative forcing and radiative efficiencies, taking into account
stratospheric adjustment and cloudy skies. These values have then been used
to obtain values for the 20-, 100- and 500-year GWPs of both species. A
sensitivity analysis for the forcing calculations relating to tropopause
definition and grid resolution has also been provided.
The IR cross sections measured in the present study are larger than
previously reported for NF3. These larger cross sections and possible
differences in radiative transfer result in radiative efficiencies that are
approximately 25 % larger than those reported in IPCC AR5 for NF3.
Atmospheric lifetimes of (509 ± 21) years and (492 ± 22) years have
been determined for NF3 and CFC-115, respectively, using the Whole
Atmosphere Community Climate Model (WACCM). Model diagnostics confirm that
CFC-115 is removed faster than NF3 in the atmosphere, except in the
altitude region of 50–75 km, where photolysis is the dominant removal
process. Photolysis is the dominant loss process for NF3 loss
(67.7 %), while for CFC-115 the reaction with O(1D) dominates
(65.6 %). Our overall lifetimes for the two gases are similar (within
10 %) to those reported in the recent SPARC (2013)
assessment, but the contribution from the loss via photolysis is less in the
case of NF3. Photolysis at wavelengths > 200 nm causes
17 % reduction in the NF3 global lifetime.
Our model results show that omitting the stratospheric adjustment can result
in an underestimation of radiative forcing of around 5–35 % and
that omitting cloud can result in an overestimation of about 60 % (Tables 4 and 5).
These differences are a little higher than previous studies by
Pinnock et al. (1995), who found an overestimation of 25–50 % over several RF and IRF calculations for a
range of hydro-halocarbons. Our results also show a strong variation of
greenhouse gas forcings with season and latitude, varying by as much as
several orders of magnitude. The lifetime results and IR absorption cross
sections from the present study indicate global warming potentials (over a
500-year period) for NF3 and CFC-115 of 19 700 and 7480, respectively.
Data availability
WACCM (CESM) version 1.1.1 is available from http://www.cesm.ucar.edu/models/cesm1.1/
or from W. Feng upon request.
The Supplement related to this article is available online at doi:10.5194/acp-16-11451-2016-supplement.
Acknowledgements
This work was part of the MAPLE project (NE/J008621/1) from the UK Natural
Environment Research Council, which also provided a studentship for Anna Totterdill. We
thank William Sturges (University of East Anglia) for supplying a sample of
CFC-115 and Doug Kinnison (NCAR) for helping with WACCM simulations.
Edited by: J. B. Burkholder
Reviewed by: two anonymous referees
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