Gravitational Separation in the Stratosphere

EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Open Access Open Access Atmos. Chem. Phys. Discuss., 13, 4839–4861,Atmospheric 2013 Atmospheric www.atmos-chem-phys-discuss.net/13/4839/2013/Chemistry Chemistry doi:10.5194/acpd-13-4839-2013 ACPD and Physics and Physics © Author(s) 2013. CC Attribution 3.0 License. 13, 4839–4861, 2013 Discussions Open Access Open Access Atmospheric Atmospheric This discussion paper is/hasMeasurement been under review for the journal AtmosphericMeasurement Chemistry Gravitational and Physics (ACP). Please referTechniques to the corresponding ﬁnal paper in ACP ifTechniques available. separation in the Discussions stratosphere Open Access Open Access S. Ishidoya et al. GravitationalBiogeosciences separation in theBiogeosciences stratosphere – a new indicator of Discussions Open Access Open Access Title Page atmospheric circulationClimate Climate Abstract Introduction of the Past of the Past 1 2 3 4 4 Discussions 5 S. Ishidoya , S. Sugawara , S. Morimoto , S. Aoki , T. Nakazawa , H. Honda , Conclusions References 1 Open Access and S. Murayama Open Access Earth System Tables Figures 1 Earth System National Institute of Advanced Industrial Science and Technology (AIST),Dynamics Tsukuba 305–8569, Japan Dynamics Discussions J I 2Miyagi University of Education, Sendai 980–0845, Japan 3 Open Access National Institute of Polar Research,Geoscientific Tokyo 190-8518, Japan Geoscientific Open Access J I 4Center for Atmospheric andInstrumentation Oceanic Studies, Tohoku University, SendaiInstrumentation 980–8578, Japan Back Close 5Institute of Space and AstronauticalMethods Science and (ISAS), Japan AerospaceMethods Exploration and Agency (JAXA), Sagamihara 252-5210,Data Japan Systems Data Systems Full Screen / Esc Discussions Open Access Received: 30 December 2012 – Accepted: 12Open Access February 2013 – Published: 20 February 2013 Geoscientific Geoscientific Printer-friendly Version Correspondence to: S. Ishidoya ([email protected]) Model Development Model Development Published by Copernicus Publications on behalf of the European GeosciencesDiscussions Union. Interactive Discussion Open Access Open Access Hydrology and Hydrology and Earth System 4839 Earth System Sciences Sciences Discussions Open Access Open Access Ocean Science Ocean Science Discussions Open Access Open Access Solid Earth Solid Earth Discussions Open Access Open Access

The Cryosphere The Cryosphere Discussions icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Abstract ACPD As a basic understanding of the dynamics of the atmospheric circulation, it has been believed that the gravitational separation of atmospheric components is observable 13, 4839–4861, 2013 only in the atmosphere above the turbopause. However, we found, from our high- 5 precision measurements of not only the isotopic ratios of N2,O2 and Ar but also the Gravitational concentration of Ar, that the gravitational separation occurs even in the stratosphere separation in the below the turbopause; their observed vertical profiles are in good agreement with stratosphere those expected theoretically from molecular mass differences. The O2/N2 ratio observed in the middle stratosphere, corrected for the gravitational separation, showed S. Ishidoya et al. 10 the same mean air age as estimated from the CO2 concentration. Simulations with a 2- dimensional model of the middle atmosphere indicated that a relationship between the gravitational separation and the air age in the stratosphere would be significantly af- Title Page

fected if the Brewer-Dobson circulation is enhanced due to global warming. Therefore, Abstract Introduction the gravitational separation is usable as a new indicator of changes in the atmospheric 15 circulation in the stratosphere. Conclusions References

Tables Figures 1 Introduction J I The gravity of the earth prevents the atmosphere from dissipating into space. It is also known that the gravity causes atmospheric molecules to separate depending on their J I

molar masses, which is called “gravitational separation”. It is widely recognized that the Back Close 20 gravitational separation of atmospheric components occurs in the atmosphere above the turbopause (Lewis and Prinn, 1984; Jacobs, 1999). On the other hand, we have Full Screen / Esc suggested from our measurements of the stable isotopic ratios of major atmospheric 15 18 components (δ( N) of N2 and δ( O) of O2) (Ishidoya et al., 2006; Ishidoya et al., Printer-friendly Version 2008a, b) that the gravitational separation exists even in the stratosphere. However, in Interactive Discussion 25 our earlier studies, we were not able to exclude the possibility that some other factors affect our measurement results. 4840 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion In this paper, we present a new concrete evidence for the existence of the gravitational separation in the stratosphere. We also show a significant effect of the gravi- ACPD tational separation on the O concentration (δ(O /N )) value measured in the strato- 2 2 2 13, 4839–4861, 2013 sphere. Furthermore, we propose to use the gravitational separation as a new indica- 5 tor to detect a change in the Brewer-Dobson Circulation (BDC) (Brewer, 1949) in the stratosphere. Atmospheric general circulation models have shown that the mean age Gravitational of stratospheric air decreases as the BDC is enhanced under global warming (Austin separation in the and Li, 2006; Li et al., 2008), but a significant change of the air age has not been ob- stratosphere served so far (Engel et al., 2009). Since the gravitational separation occurs physically S. Ishidoya et al. 10 due to gravity, its effect is expected to become more pronounced as the stratospheric air is slowly transported poleward after its intrusion from the troposphere in the tropics. Therefore, in this conceptual framework, by measuring the gravitational separation, Title Page it would be possible to obtain information about changes in the stratospheric circulation. To confirm this supposition, we simulate the stratospheric gravitational separation Abstract Introduction 15 using a 2-dimensional model of the middle atmosphere (SOCRATES) (Huang et al., 1998), and then examine the relationship between the gravitational separation and the Conclusions References

stratospheric circulation. Tables Figures

2 Stratospheric air collection and sample analyses J I

The collection of stratospheric air has been carried out using a balloon-borne cryo- J I ◦ ◦ ◦ ◦ 20 genic air sampler over Sanriku (39 N, 142 E) and Taiki (43 N, 143 E), Japan since Back Close 1985 (e.g. Nakazawa et al., 1995; Aoki et al., 2003). Our air sampler consists mainly of 12 stainless-steel sample containers, a liquid helium dewar, a receiver, a transmitter, Full Screen / Esc a control unit and batteries; all these components are housed in a water- and pressure- proof aluminum chamber (Honda, 1990; Honda et al., 1996). The volume of each sam- Printer-friendly Version 25 ple container is about 760 mL, and its inner wall is electrically polished. A motor-driven Interactive Discussion metal-to-metal seal valve is attached to each sample container. The other end of the motor-driven valve is connected to a sample intake, located 3.5 m below the bottom of 4841 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion the aluminum chamber, through a manifold and a stainless-steel bellows tube with an inner diameter of 15 mm reinforced with mesh. Before sampling, all the sample con- ACPD tainers were evacuated with heating and then cooled by filling the dewar with liquid 13, 4839–4861, 2013 He. Then, the cryogenic air sampler was connected to a large balloon, launched from 5 the balloon center at Sanriku or Taiki early in the morning and recovered from the Pacific Ocean around noon. Air samples were collected in the containers at assigned Gravitational altitudes by opening and closing the motor-driven valves using a telecommand system. separation in the The flow rates of sample air at the respective altitudes were 50–100 Lmin−1 at ambient stratosphere pressures, and typical amounts of air samples collected were about 25 L at standard ◦ S. Ishidoya et al. 10 temperature (0 C) and pressure (1013.25 hPa). 15 To detect the gravitational separation in the stratosphere, we measured δ( N) of N2, δ(18O) of O and δ(O /N ) of the air samples collected over Sanriku on 31 May 1999, 2 2 2 Title Page 28 August 2000, 30 May 2001, 4 September 2002, 6 September 2004, 3 June 2006 and 4 June 2007, as well as of those over Taiki on 22 August 2010, using a mass spec- Abstract Introduction 15 18 15 trometer (Finnigan MAT-252) (Ishidoya et al., 2003). δ( N) of N2 and δ( O) of O2 were also measured for the samples collected over Sanriku on 8 June 1995. Furthermore, Conclusions References 15 18 the air samples over Sanriku on 4 June 2007 were analyzed for δ( N) of N2, δ( O) Tables Figures 40 of O2, δ(O2/N2), δ(Ar/N2) and δ( Ar) using a new mass spectrometer (Thermo Sci- 15 18 entific DELTA-V). In this study, δ( N) of N2, δ( O) of O2, δ(O2/N2), δ(Ar/N2) and J I 40 −6 20 δ( Ar) are reported in per meg (one per meg is equal to 1 × 10 ), J I h 15 14 14 14 i n( N N)/n( N N) Back Close sample δ 15 − ( N) = h i 1, (1a) n(15N14N)/n(14N14N) Full Screen / Esc reference h 18 16 16 16 i n( O O)/n( O O) Printer-friendly Version sample δ(18O) = − 1, (1b) h i Interactive Discussion n(18O16O)/n(16O16O) reference

4842 h i | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion n(16O16O)/n(15N14N) sample δ / − ACPD (O2 N2) = h i 1, (1c) n(16O16O)/n(15N14N) 13, 4839–4861, 2013 reference h i n(40Ar)/n(14N14N) sample δ / − Gravitational (Ar N2) = h i 1, (1d) n(40Ar)/n(14N14N) separation in the reference stratosphere

and S. Ishidoya et al. h i n(40Ar)/n(36Ar) 40 sample 5 δ( Ar) − 1. (1e) = h i Title Page n(40Ar)/n(36Ar) reference Abstract Introduction Here, n means the amount of each substance. The measurement precision of Finni- 15 18 Conclusions References gan MAT-252 for δ(O2N2), δ( N) and δ( O) of the stratospheric air samples were estimated to be ±30–40, ±12 and ±26 per meg, respectively, by repeating the analysis Tables Figures of the same air sample (Ishidoya et al., 2006). On the other hand, Thermo Scientiﬁc 15 10 DELTA-V showed the respective precision of ±5, ±7, ±35 and ±22 per meg for δ( N) J I 18 40 of N2, δ( O) of O2, δ(Ar/N2) and δ( Ar). J I

3 Results and discussion Back Close Full Screen / Esc 3.1 Vertical proﬁles of the isotopic ratios of N2,O2 and Ar, and the Ar/N2 ratio

Printer-friendly Version Figure 1 shows the vertical profiles of the isotopic ratios of N2,O2, and Ar, and the 15 Ar/N2 ratio over Sanriku on 4 July 2007. Since the differences in mass number (∆m) Interactive Discussion 18 40 for δ( O), δ(Ar/N2) and δ( Ar) are 2, 12 and 4, respectively, the measured values of each variable plotted in this figure are normalized to the values corresponding to 4843 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 18 40 ∆m =1, i.e. δ( O)/2, δ(Ar/N2)/12 and δ( Ar)/4. As seen in Fig. 1, all the vertical profiles show a gradual decrease with height, their average difference between the ACPD lowermost part of the stratosphere and 32 km being about 45 per meg, and the fluc- 13, 4839–4861, 2013 tuations of their profiles are correlated well with each other. A vertical profile calcu- 5 lated using a 1-dimensional steady state eddy-diffusion/molecular-diffusion model for ∆m = 1 is also shown in Fig. 1. The model is based on Eq. (33) in Lettau (1951). The Gravitational vertical eddy-diffusion and molecular-diffusion coefficients for the model were taken separation in the from Massie and Hunten (1981) and Lettau (1951), respectively. Atmospheric temper- stratosphere atures and pressures of the model were given by the meteorological data obtained S. Ishidoya et al. 10 from rawinsonde observations. The calculated vertical profile is highly consistent with the observational results, suggesting that the gravitational separation due to molecular

diffusion can occur even in the stratosphere. However, as pointed out in our previous Title Page studies (Ishidoya et al., 2006; Ishidoya et al., 2008a), the air intake of our cryogenic air sampler is gradually heated by the solar radiation during ascent, possibly causing Abstract Introduction 15 lighter molecules to enter the sample air preferentially at high altitudes by a process Conclusions References called thermal diffusion (Blaine et al., 2006). If this is the case, such an effect would yield a vertical profile similar to that expected from the gravitational separation. There- Tables Figures fore, for providing a definitive evidence of the existence of the gravitational separation

in the stratosphere, it is indispensable to quantitatively evaluate the magnitude of the J I 20 thermal diffusion effect. J I 3.2 Laboratory experiments on the thermal diffusion effect Back Close

To examine the thermal diffusion effect at the air intake, laboratory experiments were Full Screen / Esc carried out. Dry natural air was first filled into two 2700 mL Pyrex glass flasks connected in series at an over-pressure of 0.07 MPa, and then one was gradually cooled ◦ ◦ Printer-friendly Version 25 to −57 C, while the other was kept at room temperature (25 C). During this period, the air in the flask with room temperature was introduced into an inlet system of the mass Interactive Discussion spectrometer (Thermo Scientific DELTA-V). The air introduced was exhausted from the

4844 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion inlet system with a flow rate of 4 mL min−1 and only a smidgen of air was transferred to an ion source of the mass spectrometer through a thermally-insulated fused silica cap- ACPD 15 18 40 illary, to continuously measure δ( N), δ( O), δ(O2/N2), δ(Ar/N2) and δ( Ar). The 13, 4839–4861, 2013 reproducibility of our laboratory experiments were estimated to be ±8, ±20, ±5, ±10 15 18 40 5 and ±70 per meg for δ( N), δ( O), δ(O2/N2), δ(Ar/N2) and δ( Ar), respectively. Details of our measurements of these variables will be described somewhere (Ishidoya Gravitational and Murayama, 2013, manuscript in preparation). separation in the In general, the gravitational separation is completely mass-dependent (proportional stratosphere to the difference between the mass numbers of related molecules), while the fractiona- S. Ishidoya et al. 10 tion of molecules due to thermal diffusion is slightly mass independent (Severinghaus 18 40 15 et al., 2001). The relationships of δ( O)/2, δ(Ar/N2)/12 and δ( Ar)/4 with δ( N) of N2, obtained from our laboratory experiments, are shown in Fig. 2, together with those Title Page observed in the stratosphere. The experimentally determined ratios of δ(18O)/δ(15N), 15 40 15 Abstract Introduction δ(Ar/N2)/δ( N) and δ( Ar)/δ( N) are (1.55±0.02), (16.2±0.1) and (2.75±0.05) per −1 15 meg per meg , respectively. These ratios are clearly different from the corresponding Conclusions References values of (2.1 ± 0.2), (11.9 ± 1.4) and (4.2 ± 0.6) per meg per meg−1 derived from the observational results shown in Fig. 2, as well as of 2, 12 and 4 expected from the Tables Figures gravitational separation. These statistically significant differences clearly show that the effect of thermal diffusion on our observation data is negligibly small. Previous stud- J I 20 ies have reported the respective thermal diffusion factors’ ratios of approximately 1.6 (Grew and Ibbs, 1952; Severinghaus et al., 2001) and 2.6 (Severinghaus et al., 2001) J I 18 15 40 15 for δ( O)/δ( N) and δ( Ar)/δ( N), which are consistent with our experimental re- Back Close sults. Therefore, we conclude that the gravitational separation of the major atmospheric components is clearly observable, not only above the turbopause but also in the strato- Full Screen / Esc 25 sphere. Printer-friendly Version

Interactive Discussion

In order to determine the magnitude of the gravitational separation in the stratosphere, 13, 4839–4861, 2013 15 18 we define a parameter “δ” as an average value of δ( N) of N2, δ( O)/2 of O2, 40 5 δ(Ar/N2)/12 and δ( Ar)/4 for each collected air sample. From its vertical profile, Gravitational we are able to evaluate the effects of the gravitational separation on the concentra- separation in the tion (∆Cgrav) and the isotopic ratio (∆Rgrav) of a specified atmospheric component at stratosphere a certain altitude using the respective equations, S. Ishidoya et al. ∆C = [X ] × (m − m ) × ∆δ (ppm), (2) 10 grav air

and Title Page

∆Rgrav = ∆m × ∆δ (permeg). (3) Abstract Introduction

Here, [X ] is the mole fraction of the specified component, m and mair denote the rela- Conclusions References 15 tive molecular masses of the specified component and air, respectively, and ∆δ is the difference in the measured δ value from its tropospheric value. Tables Figures The data of δ(O2/N2) and CO2 concentration taken at altitudes above 18–25 km since 1999 were corrected for the gravitational separation using the above-mentioned J I equations, and their averages for each year are shown in Fig. 3. As seen in the fig- J I 20 ure, the corrected values of δ(O2/N2) are significantly higher than the observed val- Back Close ues. In addition, variability in the δ(O2/N2) value, represented by the error bars in Fig. 3, is much reduced by applying the gravitational separation correction. On the Full Screen / Esc other hand, the difference between the CO2 concentrations with and without the correction is quite small. These results imply that the gravitational separation correction is Printer-friendly Version 25 necessary when extremely small variations of abundant atmospheric component are discussed. Interactive Discussion It is also obvious in Fig. 3 that the stratospheric δ(O2/N2) value, corrected for the gravitational separation, decreases secularly following a similar temporal change in the 4846 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion troposphere (Ishidoya et al., 2012). The mean age of the stratospheric air has usu- ally been calculated from the stratospheric CO2 and SF6 concentrations by comparing ACPD them with their concentration histories in the equatorial troposphere (Waugh and Hall, 13, 4839–4861, 2013 2002). To validate our gravitational separation correction, we estimated the mean age 5 of the stratospheric air independently using the corrected middle stratospheric and upper tropospheric δ(O2/N2) values shown in Fig. 3. The average “δ(O2/N2) age” ob- Gravitational tained is (3.9±0.9) yr, which is consistent with the “CO2 age” of (4.0±0.4) yr calculated separation in the by the same procedure. stratosphere

3.4 Numerical simulation of the gravitational separation S. Ishidoya et al.

10 Existence of the gravitational separation in the stratosphere was also confirmed theoretically by numerical model simulations. As a first attempt, a 2-dimensional model of Title Page the middle atmosphere (SOCRATES) developed by NCAR was used to evaluate the Abstract Introduction basic structure of the gravitational separation in the stratosphere. In this model, mass transport processes caused by molecular diffusion were originally taken into account Conclusions References 15 only above the mesosphere, since the molecular diffusion effect was thought to be negligibly small in the stratosphere, compared with the eddy diffusion effect. In this study, Tables Figures we simply lowered its vertical domain to the tropopause for the calculation of molecular diffusion. Molecular diffusion theory included in SOCRATES is based on Banks and J I Kockarts (1973), J I ∂ni ni ni ∂T Back Close 20 fi = −Ki + + (1 + αTi ) , (4) ∂z Hi T ∂z Full Screen / Esc

where fi , Ki , ni , Hi , αTi are the vertical flux by molecular diffusion, the molecular diffusion coefficient, the number density, the scale height and the thermal diffusion factor for Printer-friendly Version species i, respectively. We assumed αTi to be zero, since the thermal diffusion effect Interactive Discussion 25 would be of no importance in the stratosphere. The molecular diffusion coefficient is

where mair has the same meaning as above, mi and N are the mean relative molecular Gravitational 5 mass of species i and the number density of air, respectively. The coeﬃcient of 1.52 × separation in the / / 1018 has the dimension of “cm−1 s−1 kg−1 2 K−1 2”. Based on this model set-up, we stratosphere 44 45 calculated height-latitude distributions of CO2 and CO2 concentrations, and then derived the isotopic ratio (per meg), S. Ishidoya et al.

h 45 . 44 i n CO2 n CO2 45 strat Title Page δ CO2 = − 1. (6) h 45 . 44 i n CO2 n CO2 Abstract Introduction trop

Conclusions References 10 Here, subscripts “trop” and “strat” denote the tropospheric and stratospheric values, 45 44 respectively. In the actual atmosphere, the CO2/ CO2 ratio has been decreasing Tables Figures secularly due to the emissions of isotopically light CO2 by human activities. However, in this study, we assumed the 45CO /44CO ratio to be constant in the lowermost layer 2 2 J I 44 of the model, since our aim is to simulate the gravitational separation on CO2 and 45 J I 15 CO2 molecules. The boundary conditions and other input data for the present simulations were the same as SOCRATES baseline-atmosphere run. At first, a 20-yr spin-up Back Close calculation was carried out with no CO2 increase in the troposphere, and then a 30- Full Screen / Esc yr simulation was performed with monotonically increasing CO2 at the model surface, 44 45 to evaluate both the CO2 age and the gravitational separation on CO2 and CO2. Printer-friendly Version 20 The CO2 age was simply defined as a time difference between the appearance of the same CO concentration in the stratosphere and the troposphere. Since the CO 2 2 Interactive Discussion age obtained from the model simulation was obviously underestimated compared with the observational result, probably due to atmospheric circulation moving too fast, the 4848 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion meridional mass transport was arbitrarily suppressed by changing the meridional mass stream function in this study so that the model-calculated CO2 ages at northern mid- ACPD latitudes were close to the results observed over Japan. The results obtained under 13, 4839–4861, 2013 this condition are referred to as “Control Run”. The annual mean distributions of the δ 5 value and the CO2 age calculated using SOCRATES are shown in Fig. 4a, b, respectively. The height-latitude distribution of the CO2 age is close to those summarized by Gravitational Waugh and Hall (2002) on the basis of the observations and model simulations. Sim- separation in the ulated vertical distributions of the δ value and the CO2 age at northern mid-latitudes stratosphere are almost consistent with the observational results. The model result predicts that S. Ishidoya et al. 10 the vertical gradient of the δ value increases with increasing height especially at mid- and high latitudes and that the differences between low and high latitudes will be also observable. The basic structure of the vertical-meridional distribution of the δ value re- Title Page produced by model calculations can be interpreted as a result of a balance between the mass-dependent molecular diffusions and the mass-independent transport processes Abstract Introduction 15 in the stratosphere. Conclusions References 3.5 Gravitational separation as a new indicator of atmospheric circulation Tables Figures Mean vertical δ profiles always show a negative gradient due to the gravitational separation effect, but its magnitude depends on the observation. To examine the temporal J I variations of the vertical gradient, we calculated an average vertical profile of δ using J I 20 all the data obtained in this study and then obtained deviations of the respective δ values from the average profile. The average of the deviation values of δ above 18–25 km Back Close for each observation is shown in Fig. 5 as δ anomaly, together with the CO age calcu- 2 Full Screen / Esc lated from our stratospheric and equatorial tropospheric concentration data (Nakazawa et al., 1997). The results indicate that the δ anomaly is negatively correlated with the Printer-friendly Version 25 CO2 age, which means that the gravitational separation becomes stronger when the relevant stratospheric air becomes older. Interactive Discussion As described in the previous section, the magnitude of the gravitational separation at a certain altitude in the stratosphere is basically determined by the molecular diffusion 4849 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion and the mass-independent atmospheric transport. Therefore, the CO2 age and the δ value would be affected to some extent by changes in the stratospheric circulation, ACPD such as an enhanced BDC due to global warming. To examine how the CO age and 2 13, 4839–4861, 2013 the δ value are influenced by changes in the stratospheric circulation, model simula- 5 tions were further made by arbitrarily changing the meridional mass stream function so that the CO2 age decreased to about 80 % at 30 km over northern mid-latitudes, in Gravitational accordance with the prediction made by a past study (Austin and Li, 2006) for poten- separation in the tial global warming effect over the period 1960–2100, in which the BDC is accelerated stratosphere due to global warming. The results obtained under this condition are referred to as S. Ishidoya et al. 10 “Enhanced BDC”. The annual mean distributions of the δ value and the CO2 age calculated using SOCRATES for Enhanced BDC are shown in Fig. 4c, d, respectively. The annual mean relationships between the CO2 age and the δ value obtained from Title Page the Control Run and Enhanced BDC simulations for northern mid-latitudes are also shown in Fig. 6, together with the observational results over Japan. Abstract Introduction 15 As seen in Fig. 6, the relationships between the CO2 age and the δ value for Control Run at northern mid-latitudes are fairly close to the observational results over Japan, Conclusions References

which implies that both the CO2 age and the δ value can be almost reproduced by Tables Figures SOCRATES. However, the relationships for Enhanced BDC are clearly different from those of Control Run, indicating that the CO2 age and the δ value respond differently J I 20 to changes in the stratospheric transport, i.e. the gravitational separation for the air molecules with the same age is enhanced when the BDC is accelerated. For example, J I the δ value of about −50 per meg and the CO2 age of about 5.0 yr are found at 30 km Back Close over the northern mid-latitudes for Control Run, while Enhanced BDC shows −100 per meg for the δ value and 5.0 yr for the CO2 age at 40 km over the same latitude region. Full Screen / Esc 25 This phenomenon is caused by a strong height dependency of the gravitational separation due to the fact that the molecular diffusion coefficient increases with increasing Printer-friendly Version height. Interactive Discussion It is difficult to detect a long-term change in the BDC only from the CO2 and SF6 ages because of their variability (Engel et al., 2009). But by using the definite relationship

4850 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion between the δ value and the air age found in this study, such a long-term change would be detectable. However, it is interesting to note that the observational results ACPD shown in Fig. 6 indicate the gravitational separation for the air with the same age slightly 13, 4839–4861, 2013 weakening with time for the period 1995–2010. This tendency is just the opposite of that 5 expected from the Enhanced BDC simulation. Balloon and satellite observations (Engel et al., 2009; Stiller et al., 2012) reported that the CO2 and SF6 ages in the stratosphere Gravitational over northern mid-latitudes showed no significant trend over the last 30 yr, while the separation in the satellite measurements indicate that the SF6 age might have increased for the period stratosphere 2002–2010. Our long-term record of the middle stratospheric CO2 concentration over S. Ishidoya et al. 10 Japan for the period 1985–2010 also shows a slight secular increase in the CO2 age (our unpublished data, but the CO2 age values for a limited time period of 1986–2001 are available from Engel et al. (2009)). These observational results on the gravitational Title Page separation and the air age could imply that the BDC has not changed significantly or weakened slightly over the past 10–30 yr, in conflict with the model prediction of an Abstract Introduction 15 enhancement of the BDC due to global warming (Austin and Li, 2006; Li et al., 2008). With respect to this interpretation, it may be worthwhile to describe the results of Conclusions References

simulations made by Ray et al. (2010) using a tropical leaky pipe (TLP) model. Ray Tables Figures et al. (2010) reported that a weakening of the mean circulation in the middle and upper stratosphere is of importance for obtaining an agreement with the observed mean age J I 20 over the past 3 decades. Their ﬁnding would strongly support the above-mentioned observational results. However, they also pointed out that the mean age trend is sensitive J I to the horizontal mixing into the tropics. Therefore, it is further necessary to examine Back Close how the horizontal mixing aﬀects not only on the mean age but also on the gravitational separation using stratospheric transport models. Full Screen / Esc

Printer-friendly Version 25 4 Conclusions Interactive Discussion To detect the gravitational separation in the stratosphere, the stratospheric air samples 15 18 collected over Japan were analyzed for δ( N) of N2, δ( O) of O2, δ(O2/N2), δ(Ar/N2) 4851 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion 40 18 40 15 and δ( Ar). The vertical profiles of δ( O)/2, δ(Ar/N2)/12, δ( Ar)/4 and δ( N) of N2 showed a gradual decrease with height, their average difference between the low- ACPD ermost part of the stratosphere and 32 km being about 45 per meg. The relationships 18 40 15 13, 4839–4861, 2013 of δ( O), δ(Ar/N2) and δ( Ar) with δ( N) of N2 were (2.1 ± 0.2), (11.9 ± 1.4) and −1 5 (4.2 ± 0.6) per meg per meg , respectively. These ratios are consistent with the corresponding values of 2, 12 and 4 per meg per meg−1 expected from the gravitational Gravitational separation, but are clearly different from (1.55 ± 0.02), (16.2 ± 0.1) and (2.75 ± 0.05) separation in the per meg per meg−1 determined experimentally for a possible thermal diffusion effect stratosphere at the air intake of the cryogenic air sampler. This fact indicates that the gravitational S. Ishidoya et al. 10 separation of the major atmospheric components is clearly observable even in the stratosphere. 15 18 By using a parameter “δ” defined as an average of δ( N) of N2, δ( O)/2 of O2, Title Page 40 δ(Ar/N2)/12 and δ( Ar)/4 for each collected air sample, we corrected the values Abstract Introduction of δ(O2/N2) observed in the middle stratosphere for the gravitational separation. The 15 δ(O2/N2) values, thus corrected, were found to be significantly higher than the ob- Conclusions References served values. This implies that the gravitational separation correction is necessary when extremely small variations of abundant atmospheric component are discussed. It Tables Figures was also found that the corrected values of δ(O2/N2) show a secular decrease similar to temporal change in the troposphere, and that the mean age of the middle strato- J I 20 spheric air calculated from δ(O2/N2) is consistent with the CO2 age. J I To examine how the CO2 age and the δ value are influenced by changes in the stratospheric circulation, we made numerical simulations using the SOCRATES model. Back Close It was found from the simulations that the CO2 age and the δ value respond differently to changes in the stratospheric transport. The simulation results also indicate that the Full Screen / Esc 25 gravitational separation for the air with the same age is strengthened if the BDC is enhanced due to global warming, which is just the opposite of our observational result Printer-friendly Version for the period 1995–2010. Interactive Discussion The δ value is a new, unique and excellent indicator to represent how much the gravitational separation occurs in the stratosphere. Therefore, analyses of the δ value, 4852 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion in addition to the CO2 and SF6 ages, would provide us with useful information about the stratospheric circulation. However, observations of the gravitational separation are ACPD still sparse. By taking longer records of the δ value we can detect a change in the BDC 13, 4839–4861, 2013 due to global warming, a feat that has not been achieved so far. Knowledge obtained 5 from observations of the gravitational separation also contributes to further progression of atmospheric science, especially for the middle atmosphere. Gravitational separation in the Acknowledgements. We deeply thank the balloon engineering group of the Institute of stratosphere Space and Astronautical Science, JAXA for their cooperation in our stratospheric air sampling. This work is partly supported by the Grants-in-Aid for Creative Scientific Research S. Ishidoya et al. 10 (2005/17GS0203), the Grants-in-Aid for Scientific Research (2006/18710004) and the subsi- dized project “Formation of a virtual laboratory for diagnosing the earth’s climate system” of the Ministry of Education, Science, Sports and Culture, Japan. Title Page

Abstract Introduction References Conclusions References Aoki, S., Nakazawa, T., Machida, T., Sugawara, S., Morimoto, S., Hashida, G., Yamanouchi, T., 15 Kawamura, K., and Honda, H.: Carbon dioxide variations in the stratosphere over Japan, Tables Figures Scandinavia and Antarctica, Tellus B, 55, 178–186, 2003. Austin, J. and Li, F.: On the relationship between the strength of the Brewer– Dobson circulation and the age of stratospheric air, Geophys. Res. Lett., 33, L17807, J I doi:10.1029/2006GL026867, 2006. J I 20 Banks, P. M. and Kockarts, G.: Aeronomy, Parts A and B, Academic Press, Inc., New York, 1973. Back Close Blaine, T. W., Keeling, R. F., and Paplawsky, W. J.: An improved inlet for precisely measuring the Full Screen / Esc atmospheric Ar/N2 ratio, Atmos. Chem. Phys., 6, 1181–1184, doi:10.5194/acp-6-1181-2006, 2006. 25 Brewer, A.: Evidence for a world circulation provided by the measurement of helium and water Printer-friendly Version vapor in the stratosphere, Q. J. Roy. Meteor. Soc., 75, 351–363, 1949. Engel, A., Mobius,¨ T., Bonisch,¨ H., Schmidt, U., Heinz, R., Levin, I., Atlas, E., Aoki, S., Interactive Discussion Nakazawa, T., Sugawara, S., Moore, F., Hurst, D., Elkins, J., Schauﬄer, S., Andrews, A., and

4853 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Boering, K.: Age of stratospheric air unchanged within uncertainties over the past 30 years, Nat. Geosci., 2, 28–31, doi:10.1038/Ngeo388, 2009. ACPD Grew, K. E. and Ibbs, L. T.: Thermal Diffusion in Gases, Cambridge Univ. Press., New York, 1952. 13, 4839–4861, 2013 5 Honda, H.: Balloon-borne cryogenic whole air sampling system, The Institute of Space and Astronautical Science, Sagamihara, Japan, 1990. Honda, H., Aoki, S., Nakazawa, T., Morimoto, S., and Yajima, N.: Cryogenic air sampling system Gravitational for measurements of the concentrations of stratospheric trace gases and their isotopic ratios separation in the over Antarctica, J. Geomagn. Geoelectr., 48, 1145–1155, 1996. stratosphere 10 Huang, T. Walters, S., Brasseur, G., Hauglustaine, D., and Wu, W.: Description of SOCRATES – a chemical dynamical radiative two-dimensional model, NCAR/TN-440+EDD NCAR Tech- S. Ishidoya et al. nical Note, http://nldr.library.ucar.edu/repository/collections/TECH-NOTE-000-000-000-383, 1998. Ishidoya, S., Aoki, S., and Nakazawa, T.: High precision measurements of the atmospheric Title Page 15 O2/N2 ratio on a mass spectrometer, J. Meteorol. Soc. Jpn., 81, 127–140, 2003. Ishidoya, S., Sugawara, S., Hashida, G., Morimoto, S., Aoki, S., Nakazawa, T., and Ya- Abstract Introduction manouchi, T.: Vertical profiles of the O2/N2 ratio in the stratosphere over Japan and Antarc- tica, Geophys. Res. Lett., 33, L13701, doi:10.1029/2006GL025886, 2006. Conclusions References Ishidoya, S., Sugawara, S., Morimoto, S., Aoki, S., and Nakazawa, T.: Gravitational separation Tables Figures 20 of major atmospheric components of nitrogen and oxygen in the stratosphere, Geophys. Res. Lett., 35, L03811, doi:10.1029/2007GL030456, 2008a. Ishidoya, S., Morimoto, S., Sugawara, S., Watai, T., Machida, T. Aoki, S., Nakazawa, T., and J I 15 18 Yamanouchi, T.: Gravitational separation suggested by O2/N2, δ N of N2, δ O of O2, Ar/N2 observed in the lowermost part of the stratosphere at northern middle and high latitudes J I 25 in the early spring of 2002, Geophys. Res. Lett., 35, L03812, doi:10.1029/2007GL031526, Back Close 2008b. Ishidoya, S., Aoki, S. Goto, D., Nakazawa, T., Taguchi, S., and Patra, P. K.: Time and space Full Screen / Esc variations of the O2/N2 ratio in the troposphere over Japan and estimation of global CO2 budget, Tellus B, 64, 18964, doi:10.3402/tellusb.v64i0.18964, 2012. Printer-friendly Version 30 Jacob, D. J.: Introduction to Atmospheric Chemistry, Princeton University Press, 1999. Lettau, H.: Diffusion in the upper atmosphere, Compendium of Meteorology American Meteo- Interactive Discussion rological Society, Boston, 1951.

4854 icsinPpr|Dsuso ae icsinPpr|Dsuso ae | Paper Discussion | Paper Discussion | Paper Discussion | Paper Discussion Lewis, J. S. and Prinn R. G.: Planets and Their Atmosphere: origin and evolution, International Geophysics Series, 33, Academic Press Inc., 1984. ACPD Li, F., Austin, J., and Wilson, J.: The Strength of the Brewer–Dobson Circulation in a changing climate: Coupled chemistry–climate model simulations, J. Climate, 21, 40–57, 13, 4839–4861, 2013 5 doi:10.1175/2007JCLI1663.1, 2008. Massie, S. T. and Hunten, D. M.: Stratospheric eddy diffusion coefficients from tracer data, J. Geophys. Res., 86, 9859–9868, 1981. Gravitational Nakazawa, T., Machida, T., Sugawara, S., Murayama, S., Morimoto, S., Hashida, G., separation in the Honda, H., and Itoh, T.: Measurements of the stratospheric carbon dioxide concentra- stratosphere 10 tion over Japan using a Balloon-borne cryogenic sampler, Geophys. Res. Lett., 22, 1229, doi:10.1029/95GL01188, 1995. S. Ishidoya et al. Nakazawa, T., Morimoto, S., Aoki, S., and Tanaka, M.: Temporal and spatial variations of the carbon isotopic ratio of atmospheric carbon dioxide in the western Pacific region, J. Geophys. Res., 102, 1271–1285, 1997. Title Page 15 Ray, E. A., Moore, F. L., Rosenlof, K. H., Davis, S. M., Boenisch, H., Morgenstern, O., Smale, D., Rozanov, E., Hegglin, M., Pitari, G., Mancini, E., Braesicke, P., Butchart, N., Hardiman, S., Abstract Introduction Li, F., Shibata, K., and Plummer, D. A.: Evidence for changes in stratospheric transport and mixing over the past three decades based on multiple data sets and tropical leaky pipe Conclusions References analysis, J. Geophys. Res., 115, D21304, doi:10.1029/2010JD014206, 2010. Tables Figures 20 Severinghaus, J. P., Grachev, A., and Battle, M.: Thermal fractionation of air in polar firn by seasonal temperature gradients, Geochem. Geophy. Geosy., 2, 1048, doi:10.1029/2000GC000146, 2001. J I Stiller, G. P., von Clarmann, T., Haenel, F., Funke, B., Glatthor, N., Grabowski, U., Kellmann, S., Kiefer, M., Linden, A., Lossow, S., and Lopez-Puertas,´ M.: Observed temporal evolution of J I 25 global mean age of stratospheric air for the 2002 to 2010 period, Atmos. Chem. Phys., 12, Back Close 3311–3331, doi:10.5194/acp-12-3311-2012, 2012. Waugh, D. W. and Hall, T. M.: Age of stratospheric air: theory, observations, and models, Rev. Full Screen / Esc Geophys., 40, 1010, doi:10.1029/2000RG000101, 2002.

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ACPD 13, 4839–4861, 2013 Delta-V δ(15N) 30 δ(18O)/2 Gravitational δ(Ar/N2) separation in the /12 stratosphere δ(40Ar)/4 S. Ishidoya et al. 25 MAT-252 δ(15N) 18 δ( O)/2 Title Page

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-40 -20 0 20 J I Normalized δ (per meg) Back Close 1 15 18 40 2 15Figure 1. δ(O182/N2), δ( N) of N2, δ( O)/2 of40 O2, δ(Ar/N2)/12 and δ( Ar)/4 observed Full Screen / Esc Fig. 1. δ( N) of N2, δ( O)/2 of O2, δ(Ar/N2)/12 and δ( Ar)/4 observed over Sanriku, Japan on 43 July over 2007. Sanriku, Also shown Japan is theon Julyvertical 4, profile 2007. of Alsoδ value shown calculated is the vertical using a profile 1-dimensional of δ value steady4 statecalculated eddy-di ffusingusion/molecular-di a 1-dimensionalffusion steady model state for eddy the mass-diffusion/molecular number difference-diffusion of 1. The model Printer-friendly Version tropopause5 for height the mass on 4 July number 2007 difference was estimated of 1. to The be about tropopause 12 km. height on July 4, 2007 was 6 estimated to be about 12 km. Interactive Discussion 7

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Sanriku, 2007 Title Page δ(18O)/2 δ(Ar/N )/12 -100 2 Abstract Introduction δ(40Ar)/4 Normalized Thermal diffusion Conclusions References δ(18O)/2 δ(Ar/N )/12 Tables Figures -150 2 δ(40Ar)/4 J I 0 -50 -100 -150 15 δ( N) of N2 (per meg) J I 1 2 18 40 15 Figure18 2. Plots of δ( O)/2, δ(Ar/N2)/1240 and δ( Ar)/4 against15 δ( N) for stratospheric air Back Close Fig. 2. Plots3 ofsampleδ( O)s collected/2, δ(Ar / overN2 )/ Sanriku,12 and Japanδ( Ar) on/ 4 July against 4, 2007,δ( andN) for those stratospheric for laboratory air samples collected4 experiments over Sanriku, to examine Japan a on possible 4 July effect 2007, of thermal and those diffusion for at laboratory the air intake experiments of the to Full Screen / Esc examine a5 possiblesampler e ﬀ onect the of collected thermal airdiﬀ usion samples at (see the text).air intake Blackof dashed the sampler line represents on the the collected air samples6 (seerelationship text). Black expected dashed from the line gravitational represents separation. the relationship expected from the gravitational separation.7 Printer-friendly Version 8 Interactive Discussion

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middle stratosphere 2 390 conc. (ppmv) upper troposphere Abstract Introduction 380 Conclusions References 370 Tables Figures 360 2000 2002 2004 2006 2008 2010 J I Year J I 1 Back Close 2Fig. Figure 3. Average 3. Average values values of δ(O 2of/ Nδ2(O) and2/N2 the) and CO the2 concentration CO2 concentration observed observed above above 18–25 18 km-25 over 3Sanriku km over and Sanriku Taiki, Japan and Taiki, for the Japan period for 1999–2010,the period 1999 after-2010, (black after open (black circles) open andcircles) before and (red 4triangles) before correcting(red triangles) for the correcting gravitational for the separation gravitational eﬀect. separation Asterisks effect denote. Asterisks the annual denote mean Full Screen / Esc 5values the of annualδ(O / meanN ) and values the COof δ(Oconcentration2/N2) and the observed CO2 concentration in the upper observed troposphere in the over upper Japan 6 2 2 2 for thetroposphere period 2000–2010. over Japan for the period 2000-2010. 7 Printer-friendly Version Interactive Discussion

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2 Full Screen / Esc 3 Fig. 4. Annual mean height-latitude distributions of (a) δ value (per meg) and (b) CO2 age 4 (years) calculated using the SOCRATES model for Control Run, and the corresponding results Printer-friendly Version 5 Figurefor Enhanced 4. Annual BDC mea (c andn heightd). The-latitudeδ values distributions lower than of− 500(a) δ per value meg (per are meg) shown and by (b) gray CO shades.2 6 age (years) calculated using the SOCRATES model for Control Run, and the Interactive Discussion 7 corresponding results for Enhanced BDC ((c) and (d)). The δ values lower than -500 per 8 meg are shown by gray shades. 9 4859

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Year Back Close 1 Fig.2 5.FigureAverage 5. A valuesverage of values the deviations of the deviations of δ from of its δ mean from verticalits mean proﬁle vertical over profile the observation over the Full Screen / Esc period (upper) and those of the CO ages (lower) at heights above 18–25 km for the respective 3 observation period (upper) and2 those of the CO2 ages (lower) at heights above 18-25 km years. 4 for the respective years. Printer-friendly Version 5 Interactive Discussion

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01 10 95 07 06 Gravitational -40 02 Year separation in the 04 2010 stratosphere 2008 -60 2006 S. Ishidoya et al. (per meg) 00 2004 δ Solid: Ctl. run 99 2002 Dashed: Enh. BDC 2000 30°N 1998 -80 35°N Title Page 40°N 1996 45°N Abstract Introduction 50°N -100 Conclusions References 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Tables Figures CO2 age (years)

Fig. 6. Plots of the δ value at 29 km against the average values of CO2 ages at heights above J I 18–25 km for the respective observations over Sanriku and Taiki, Japan (closed circles). Color bar and Arabic numerals near the symbols indicate the observation years. The results calcu- J I lated using the SOCRATES model for Control Run (solid lines) and Enhanced BDC (dashed Back Close lines) are also shown. Blue and red dotted lines represent the results obtained by applying a linear regression analysis to the data for the respective periods 1995–2001 and 2004–2010. Full Screen / Esc It is noted that the result for 2002 is not used in the regression analysis, since it was found to deviate signiﬁcantly from those of the other years, due to large variability in the vertical CO2 proﬁle observed in that year. It is also noted that the observed δ values plotted are the values Printer-friendly Version obtained by linearly interpolating the measured δ values of the corresponding observations for Interactive Discussion 29 km, which is approximately the highest altitude covered by all our observations.

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