Air Tracer Research Articles

Methodology to obtain highly resolved SO<sub>2</sub> vertical profiles for representation of volcanic emissions in climate models

Abstract. In this study we describe a methodology to create high-vertical-resolution SO2 profiles from volcanic emissions. We demonstrate the method's performance for the volcanic clouds following the eruption of Sarychev in June 2009. The resulting profiles are based on a combination of satellite SO2 and aerosol retrievals together with trajectory modelling. We use satellite-based measurements, namely lidar backscattering profiles from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite instrument, to create vertical profiles for SO2 swaths from the Atmospheric Infrared Sounder (AIRS) aboard the Aqua satellite. Vertical profiles are created by transporting the air containing volcanic aerosol seen in CALIOP observations using the FLEXible PARTicle dispersion model (FLEXPART) while preserving the high vertical resolution using the potential temperatures from the MERRA-2 (Modern-Era Retrospective analysis for Research and Application) meteorological data for the original CALIOP swaths. For the Sarychev eruption, air tracers from 75 CALIOP swaths within 9 d after the eruption are transported forwards and backwards and then combined at a point in time when AIRS swaths cover the complete volcanic SO2 cloud. Our method creates vertical distributions for column density observations of SO2 for individual AIRS swaths, using height information from multiple CALIOP swaths. The resulting dataset gives insight into the height distribution in the different sub-clouds of SO2 within the stratosphere. We have compiled a gridded high-vertical-resolution SO2 inventory that can be used in Earth system models, with a vertical resolution of 1 K in potential temperature, 61 ± 56 m, or 1.8 ± 2.9 mbar.

Characterization, sources and reactivity of volatile organic compounds (VOCs) in Seoul and surrounding regions during KORUS-AQ

The Korea-United States Air Quality Study (KORUS-AQ) took place in spring 2016 to better understand air pollution in Korea. In support of KORUS-AQ, 2554 whole air samples (WAS) were collected aboard the NASA DC-8 research aircraft and analyzed for 82 C1–C10 volatile organic compounds (VOCs) using multi-column gas chromatography. Together with fast-response measurements from other groups, the air samples were used to characterize the VOC composition in Seoul and surrounding regions, determine which VOCs are major ozone precursors in Seoul, and identify the sources of these reactive VOCs. (1) The WAS VOCs showed distinct signatures depending on their source origins. Air collected over Seoul had abundant ethane, propane, toluene and n-butane while plumes from the Daesan petrochemical complex were rich in ethene, C2–C6 alkanes and benzene. Carbonyl sulfide (COS), CFC-113, CFC-114, carbon tetrachloride (CCl4) and 1,2-dichloroethane were good tracers of air originating from China. CFC-11 was also elevated in air from China but was surprisingly more elevated in air over Seoul. (2) Methanol, isoprene, toluene, xylenes and ethene were strong individual contributors to OH reactivity in Seoul. However methanol contributed less to ozone formation based on photochemical box modeling, which better accounts for radical chemistry. (3) Positive Matrix Factorization (PMF) and other techniques indicated a mix of VOC source influences in Seoul, including solvents, traffic, biogenic, and long-range transport. The solvent and traffic sources were roughly equal using PMF, and the solvents source was stronger in the KORUS-AQ emission inventory. Based on PMF, ethene and propene were primarily associated with traffic, and toluene, ethylbenzene and xylenes with solvents, especially non-paint solvents for toluene and paint solvents for ethylbenzene and xylenes. This suggests that VOC control strategies in Seoul could continue to target vehicle exhaust and paint solvents, with additional regulations to limit the VOC content in a variety of non-paint solvents.

Reconnection scaling in quantum fluids

Fundamental to classical and quantum vortices, superconductors, magnetic flux tubes, liquid crystals, cosmic strings, and DNA is the phenomenon of reconnection of line-like singularities. We visualize reconnection of quantum vortices in superfluid 4He, using submicrometer frozen air tracers. Compared with previous work, the fluid was almost at rest, leading to fewer, straighter, and slower-moving vortices. For distances that are large compared with vortex diameter but small compared with those from other nonparticipating vortices and solid boundaries (called here the intermediate asymptotic region), we find a robust 1/2-power scaling of the intervortex separation with time and characterize the influence of the intervortex angle on the evolution of the recoiling vortices. The agreement of the experimental data with the analytical and numerical models suggests that the dynamics of reconnection of long straight vortices can be described by self-similar solutions of the local induction approximation or Biot-Savart equations. Reconnection dynamics for straight vortices in the intermediate asymptotic region are substantially different from those in a vortex tangle or on distances of the order of the vortex diameter.

Evaluation of stratospheric age of air from CF<sub>4</sub>, C<sub>2</sub>F<sub>6</sub>, C<sub>3</sub>F<sub>8</sub>, CHF<sub>3</sub>, HFC-125, HFC-227ea and SF<sub>6</sub>; implications for the calculations of halocarbon lifetimes, fractional release factors and ozone depletion potentials

Abstract. In a changing climate, potential stratospheric circulation changes require long-term monitoring. Stratospheric trace gas measurements are often used as a proxy for stratospheric circulation changes via the mean age of air values derived from them. In this study, we investigated five potential age of air tracers – the perfluorocarbons CF4, C2F6 and C3F8 and the hydrofluorocarbons CHF3 (HFC-23) and HFC-125 – and compare them to the traditional tracer SF6 and a (relatively) shorter-lived species, HFC-227ea. A detailed uncertainty analysis was performed on mean ages derived from these new tracers to allow us to confidently compare their efficacy as age tracers to the existing tracer, SF6. Our results showed that uncertainties associated with the mean age derived from these new age tracers are similar to those derived from SF6, suggesting that these alternative compounds are suitable in this respect for use as age tracers. Independent verification of the suitability of these age tracers is provided by a comparison between samples analysed at the University of East Anglia and the Scripps Institution of Oceanography. All five tracers give younger mean ages than SF6, a discrepancy that increases with increasing mean age. Our findings qualitatively support recent work that suggests that the stratospheric lifetime of SF6 is significantly less than the previous estimate of 3200 years. The impact of these younger mean ages on three policy-relevant parameters – stratospheric lifetimes, fractional release factors (FRFs) and ozone depletion potentials – is investigated in combination with a recently improved methodology to calculate FRFs. Updates to previous estimations for these parameters are provided.

Assumptions about footprint layer heights influence the quantification of emission sources: a case study for Cyprus

Abstract. Lagrangian particle dispersion models (LPDMs) in backward mode are widely used to quantify the impact of transboundary pollution on downwind sites. Most LPDM applications count particles with a technique that introduces a so-called footprint layer (FL) with constant height, in which passing air tracer particles are assumed to be affected by surface emissions. The mixing layer dynamics are represented by the underlying meteorological model. This particle counting technique implicitly assumes that the atmosphere is well mixed in the FL. We have performed backward trajectory simulations with the FLEXPART model starting at Cyprus to calculate the sensitivity to emissions of upwind pollution sources. The emission sensitivity is used to quantify source contributions at the receptor and support the interpretation of ground measurements carried out during the CYPHEX campaign in July 2014. Here we analyse the effects of different constant and dynamic FL height assumptions. The results show that calculations with FL heights of 100 and 300 m yield similar but still discernible results. Comparison of calculations with FL heights constant at 300 m and dynamically following the planetary boundary layer (PBL) height exhibits systematic differences, with daytime and night-time sensitivity differences compensating for each other. The differences at daytime when a well-mixed PBL can be assumed indicate that residual inaccuracies in the representation of the mixing layer dynamics in the trajectories may introduce errors in the impact assessment on downwind sites. Emissions from vegetation fires are mixed up by pyrogenic convection which is not represented in FLEXPART. Neglecting this convection may lead to severe over- or underestimations of the downwind smoke concentrations. Introducing an extreme fire source from a different year in our study period and using fire-observation-based plume heights as reference, we find an overestimation of more than 60 % by the constant FL height assumptions used for surface emissions. Assuming a FL that follows the PBL may reproduce the peak of the smoke plume passing through but erroneously elevates the background for shallow stable PBL heights. It might thus be a reasonable assumption for open biomass burning emissions wherever observation-based injection heights are not available.

Helium-filled soap bubbles for vortex core velocimetry

Velocity measurements within the core of high-swirl vortices are often hampered by heavier-than-air particle tracers being centrifuged outside the vortex core region. The use of neutrally buoyant and lighter-than-air tracers is investigated to aim at homogeneous tracers concentration in air flow experiments dealing with high-swirl vortices using particle image velocimetry. Helium-filled soap bubbles (HFSB) of sub-millimeter diameter are employed as flow tracers. Their density is controlled varying the relative amount of helium and soap solution composing the bubbles. The dynamics of HFSB and micro-size droplets is modeled within a Lamb–Oseen vortex to retrieve the order of magnitude of the tracers slip velocity. A positive radial drift for heavier-than-air tracers leads to an empty vortex core. In contrast, the concentration at the vortex axis is expected to increase for lighter than air tracers. Experiments are conducted on a sharp-edged slender delta wing at 20° incidence. At chosen chord-based Reynolds numbers of 2 × 105 and 6 × 105, a stable laminar vortex is formed above the delta wing. Laser sheet visualization is used to inspect the spatial concentration of tracers. A comparison is made between micron-sized fog droplets and HFSB tracers in the nearly neutrally buoyant condition. Stereo-PIV measurements with fog droplets return a systematically underestimated axial velocity distribution within the vortex core due to drop-out of image cross-correlation signal. The nearly neutrally buoyant HFSB tracers appear to maintain a homogeneous spatial concentration and yield cross-correlation signal up to the vortex axis. The resulting velocity measurements are in good agreement with literature data.

Frequency and Impact of Summertime Stratospheric Intrusions over Maryland during DISCOVER-AQ (2011): New Evidence from NASA's GEOS-5 Simulations.

Aircraft observations and ozonesonde profiles collected on July 14 and 27, 2011, during the Maryland month-long DISCOVER-AQ campaign, indicate the presence of stratospheric air just above the planetary boundary layer (PBL). This raises the question of whether summer stratospheric intrusions (SIs) elevate surface ozone levels and to what degree they influence background ozone levels and contribute to ozone production. We used idealized stratospheric air tracers, along with observations, to determine the frequency and extent of SIs in Maryland during July 2011. On 4 of 14 flight days, SIs were detected in layers that the aircraft encountered above the PBL from the coincidence of enhanced ozone, moderate CO, and low moisture. Satellite observations of lower tropospheric humidity confirmed the occurrence of synoptic scale influence of SIs as do simulations with the GEOS-5 Atmospheric General Circulation Model. The evolution of GEOS-5 stratospheric air tracers agree with the timing and location of observed stratospheric influence and indicate that more than 50% of air in SI layers above the PBL had resided in the stratosphere within the previous 14 days. Despite having a strong influence in the lower free troposphere, these events did not significantly affect surface ozone, which remained low on intrusion days. The model indicates similar frequencies of stratospheric influence during all summers from 2009-2013. GEOS-5 results suggest that, over Maryland, the strong inversion capping the summer PBL limits downward mixing of stratospheric air during much of the day, helping to preserve low surface ozone associated with frontal passages that precede SIs.

Sub-micron solid air tracers for quantum vortices and liquid helium flows.

The dynamics of quantized vortices in superfluids has received increased attention recently because of novel techniques developed to visualize them directly. One of these techniques [G. P. Bewley et al., Nature 441, 588 (2006)] visualized quantized vortices and their reconnections in superfluid flows of (4)He by using solid hydrogen tracers of micron-size or larger. The present work improves upon the previous technique by using substantially smaller particles created by injecting atmospheric air diluted in helium gas. These smaller particles are detectable thanks to the higher index of refraction of nitrogen compared to hydrogen and thanks to an improved visualization setup. The optical counting estimate, which agrees with terminal velocity estimates, suggests that the tracer diameter is typically 400 ± 200 nm and could be as small as 200 nm; being smaller, but not so small as to be influenced by thermal motion, the particles get trapped on the vortices faster, perturb the vortices less, possess smaller Stokes drag, and stay trapped on fast-moving vortices, as also on vortices generated closer to the superfluid transition temperature. Unlike the past, the ability to create particles in the superfluid state directly (instead of creating them above the λ-point and cooling the fluid subsequently), ensures greater temperature stability for longer periods, and enables the tracking of long and isolated vortices. These advantages have also led to the direct visualization of Kelvin waves. The use of other seed gases could lead to the visualization of even smaller tracers for quantized vortices. We discuss the visualization setup and provide suggestions for further improvement.

Characterizing the lifetime and occurrence of stratospheric‐tropospheric exchange events in the rocky mountain region using high‐resolution ozone measurements

AbstractThe evolution of a Stratospheric‐Tropospheric Exchange (STE) event from 4 to 8 August 2014 at Fort Collins, Colorado, is described. The event is characterized with observations from the Goddard Space Flight Center TROPospheric OZone (TROPOZ) Differential Absorption Lidar, the University of Wisconsin High Spectral Resolution Lidar, and multiple ozonesondes during NASA's Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality and the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) campaigns. Based on the extended TROPOZ observations throughout the entire campaign, it was found that STE events have largely contributed to an additional 10–30 ppbv of ozone at Fort Collins. Additional measurements of ozone and relative humidity from the Atmospheric Infrared Sounder are characterize the transport of the intrusion. The Real‐time Air Quality Modeling System simulated ozone agrees well with the TROPOZ ozone concentrations and altitude during the STE event. To extend the analysis into other seasons and years, the modeled ozone to potential vorticity ratio is used as a tracer for stratospheric air residing below the tropopause. It is found that at Fort Collins, CO, and depending on season from 2012 to 2014, between 18 and 31% of tropospheric ozone corresponds to stratospheric air. A relationship to determine the lifetime of stratospheric air below the tropopause is derived using the simulated ratio tracer. Results indicate that throughout summer 2014, 43% of stratospheric air resided below the tropopause for less than 12 h. However, nearly 39% persisted below the tropopause for 12–48 h and likely penetrated deeper in the troposphere.

Modelling the stratospheric budget of beryllium isotopes

A global 2-D (latitude–altitude) model describing both the stratospheric circulation and the detailed formation and growth of stratospheric aerosols is used to simulate the stratospheric cycle of cosmogenic isotopes of beryllium, Be-7 and Be-10. These isotopes have been extensively used as tracers of stratospheric air, as well as of solar variability. The simulation of these isotopes is used to quantify the relative importance of transport, microphysic processes related to aerosols, and radioactive decay, on their concentrations. Calculations of model budget show that the vertical transfer of these isotopes due to the aerosol sedimentation contributes to about half of the stratospheric Be-10 flux into the troposphere, but is negligible for the Be-7 budget. The simulated residence time of these isotopes in the lower stratosphere is monotonically related to the age of air; however, this relationship is neither linear nor uniform, which biases the age of air inferred from these isotopes.

Evidence of mixing between polluted convective outflow and stratospheric air in the upper troposphere during DC3

AbstractAircraft measurements, including non‐methane hydrocarbons (NMHCs), long‐lived halocarbons, carbon monoxide (CO), and ozone (O3) collected on board the NASA DC‐8 during the Deep Convection, Clouds, and Chemistry (DC3) field campaign (May – June 2012), were used to investigate interactions and mixing between stratospheric intrusions and polluted air masses. Stratospherically influenced air masses were detected using a suite of long‐lived halocarbons, including chlorofluorocarbons (CFCs) and HCFCs, as a tracer for stratospheric air. A large number of stratospherically influenced samples were found to have reduced levels of O3 and elevated levels of CO (both relative to background stratospheric air), indicative of mixing with anthropogenically influenced air. Using n‐butane and propane as further tracers of anthropogenically influenced air, we show that this type of mixing was present both at low altitudes and in the upper troposphere (UT). At low altitudes, this mixing resulted in O3 enhancements consistent with those reported at surface sites during deep stratospheric intrusions, while in the UT, two case studies were performed to identify the process by which this mixing occurs. In the first case study, stratospheric air was found to be mixed with aged outflow from a convective storm, while in the second case study, stratospheric air was found to have mixed with outflow from an active storm occurring in the vicinity of a stratospheric intrusion. From these analyses, we conclude that deep convective events may facilitate the mixing between stratospheric air and polluted boundary layer air in the UT. Throughout the entire DC3 study region, this mixing was found to be prevalent: 72% of all samples that involve stratosphere‐troposphere mixing show influence of polluted air. Applying a simple chemical kinetics analysis to these data, we show that during DC3, the instantaneous production of hydroxyl radical (OH) in these mixed stratospheric‐polluted air masses was 11 ± 8 times higher than that of stratospheric air, and 4.2 ± 1.8 times higher than that of background upper tropospheric air.

Sources of increase in lowermost stratospheric sulphurous and carbonaceous aerosol background concentrations during 1999–2008 derived from CARIBIC flights

This study focuses on sulphurous and carbonaceous aerosol, the major constituents of particulate matter in the lowermost stratosphere (LMS), based on in situ measurements from 1999 to 2008. Aerosol particles in the size range of 0.08–2 µm were collected monthly during intercontinental flights with the CARIBIC passenger aircraft, presenting the first long-term study on carbonaceous aerosol in the LMS. Elemental concentrations were derived via subsequent laboratory-based ion beam analysis. The stoichiometry indicates that the sulphurous fraction is sulphate, while an O/C ratio of 0.2 indicates that the carbonaceous aerosol is organic. The concentration of the carbonaceous component corresponded on average to approximately 25% of that of the sulphurous, and could not be explained by forest fires or biomass burning, since the average mass ratio of Fe to K was 16 times higher than typical ratios in effluents from biomass burning. The data reveal increasing concentrations of particulate sulphur and carbon with a doubling of particulate sulphur from 1999 to 2008 in the northern hemisphere LMS. Periods of elevated concentrations of particulate sulphur in the LMS are linked to downward transport of aerosol from higher altitudes, using ozone as a tracer for stratospheric air. Tropical volcanic eruptions penetrating the tropical tropopause are identified as the likely cause of the particulate sulphur and carbon increase in the LMS, where entrainment of lower tropospheric air into volcanic jets and plumes could be the cause of the carbon increase.

Study on Atmospheric Diffusion in a Street-Canyon by Air Tracer Dispersion Experiment (I) On the Distribution of the Tracer Gas Concentration at the Roadsides in KandaJimbocho Area

Study on Atmospheric Diffusion in a Street-Canyon by Air Tracer Dispersion Experiment (I) On the Distribution of the Tracer Gas Concentration at the Roadsides in KandaJimbocho Area

Impacts of boundary layer mixing on pollutant vertical profiles in the lower troposphere: Implications to satellite remote sensing

Mixing in the planetary boundary layer (PBL) affects vertical distributions of air tracers in the lower troposphere. An accurate representation of PBL mixing is critical for chemical-transport models (CTMs) for applications sensitive to simulations of the vertical profiles of tracers. The full mixing assumption in the widely used global CTM GEOS-Chem has recently been supplemented with a non-local PBL scheme. This study analyzes the impact of the non-local scheme on model representation of PBL mixing, consequences for simulations of vertical profiles of air tracers and surface air pollution, and implications for model applications to the interpretation of data retrieved from satellite remote sensing. The non-local scheme significantly improves simulations of the vertical distributions for NO 2 and O 3, as evaluated using aircraft measurements in summer 2004. It also reduces model biases over the U.S. by more than 10 ppb for surface ozone concentrations at night and by 2–5 ppb for peak ozone in the afternoon, as evaluated using ground observations. The application to inverse modeling of anthropogenic NO x emissions for East China using satellite retrievals of NO 2 from OMI and GOME-2 suggests that the full mixing assumption results in 3–14% differences in top–down emission budgets as compared to the non-local scheme. The top–down estimate combining the non-local scheme and the Lin et al. inverse modeling approach suggests a magnitude of 6.6 TgN yr −1 for emissions of NO x over East China in July 2008 and 8.0 TgN yr −1 for January 2009, with the magnitude and seasonality in good agreement with bottom–up estimates.

On the relationship between the strength of the Brewer‐Dobson circulation and the age of stratospheric air

The strength of the Brewer‐Dobson circulation is computed for multi‐decadal simulations of a coupled chemistry‐climate model covering the period 1960 to 2100. The circulation strength, as computed from the tropical mass upwelling, generally increases throughout the simulations. The model also includes an age of air tracer which generally decreases during the simulations. The two different transport concepts of mass upwelling and reciprocal of the age of air are investigated empirically from the model simulations. The results indicate that the variables are linearly related in the model but with a change of gradient some time near 2005. Possible reasons for the change of gradient are discussed.

Tropical and extratropical tropospheric air in the lowermost stratosphere over Europe: A CO‐based budget

We present a new approach to calculate a trace gas budget for the lowermost stratosphere based on airborne CO measurements. The measurements were obtained during the SPURT‐project (SPURT in German: Trace gas transport in the tropopause region) and covered the tropopause region over Europe from 35°–75°N during different seasons. Using CO as a tracer for tropospheric air we quantified the contribution of three source regions to the composition of the lowermost stratosphere. Influence from the extratropical troposphere mainly contributes to the mixing layer above the local tropopause, where it decreases rapidly to less than 25% within a potential temperature range of 25 K above the local tropopause. With increasing distance from the tropopause mixing layer, the tropospheric fraction is dominated by influence from the tropics. During winter and spring the latter contributes about 35% to the chemical composition of the lowermost stratosphere over Europe, increasing to about 55% in summer and autumn.

Study on Atmospheric Diffusion in a Street-Canyon by Air Tracer Dispersion Experiment (III)

Study on Atmospheric Diffusion in a Street-Canyon by Air Tracer Dispersion Experiment (III)

Descent and mixing in the 1999–2000 northern polar vortex inferred from in situ tracer measurements

In situ measurements from the Lightweight Airborne Chromatograph Experiment (LACE) and the Airborne Chromatograph for Atmospheric Trace Species (ACATS‐IV) taken during the SAGE III Ozone Loss and Validation Experiment (SOLVE) are used to examine the descent and mixing in the 1999–2000 northern stratospheric polar vortex. LACE was flown twice on an in situ balloon platform, in November 1999 just after the vortex formed and in March 2000 near the end of the vortex lifetime. The aim of this paper is to use simple models of vortex transport to try to explain the changes seen in the long‐lived tracers measured by LACE between the two vortex flights. It is the high precision and long‐term accuracy of the observations that allow small differences in the data from a number of species obtained on different flights to be used to infer transport processes. Changes in tracer profiles as a function of height or potential temperature can be attributed, to first order, to descent in the vortex. A calculation which used six tracers shows that the total descent was a strong function of potential temperature, from 200 K in the middle stratosphere to 50 K in the lower stratosphere over the nearly 4‐month period between flights. Observed changes in long‐lived tracer‐tracer correlations require a mixing process to have occurred since descent alone cannot change the correlations. Two simple models of mixing are used to examine the tracer correlation changes. One model simulates entrainment of midlatitude air across the vortex edge with relatively efficient mixing within the vortex. A second model simulates the effect of differential descent and subsequent efficient mixing within a vortex that is entirely isolated from the midlatitudes. It is shown that the differential descent and mixing calculation produces results which are much more consistent among all the tracer correlations compared to the results from midlatitude entrainment. The unique sensitivity of SF6 as a tracer of mesospheric air makes it an outlier in both the descent and mixing calculations. The inclusion of a small fraction of air from the mesosphere is necessary to bring the SF6 mixing and descent calculations into agreement with the other tracers. The conclusions regarding descent and descent rates in the vortex are consistent with other modeling studies. The results indicate that mixing of midlatitude air into the winter vortex is not a significant contributor to the observed ozone changes in the 1999/2000 season.

Computational analysis of a human inhalation test chamber for dosimetry-and-health effect studies.

Proper air flow and tracer gas distribution or contaminant ventilation are of great importance in biomedical test chambers or industrial workrooms. The focus is on mass transfer in an inhalation test chamber with a breathing subject on a bike exposed to a tracer gas environment (e.g., carbon monoxide). This is an environmentally realistic setup for dosimetry-and-health effect studies, which require controlled, near-uniform pollutant concentrations. However, unmodified test chambers exhibit a strong single vortex in the larger breathing zone, which, depending upon the subject's location, implies possible trace gas depletion during inhalation, foreign particle entrainment, excessive air velocities, and so on. Employing a commercial finite-volume code with user-enhanced Fortran programs, the transient three-dimensional turbulent momentum, mass, and heat transfer equations have been solved and the configurations of a suitable flow redirection device, different man-machine locations, and thermal effects have been analyzed. As a result, the best air flow device configuration and man-machine orientation have been determined to achieve high and consistent trace gas concentrations inhaled by the subject, for example, 96 percent of the CO concentration at the chamber inlet is inhaled by the subject for the optimal scenario.

Temporal development of total chlorine in the high‐latitude stratosphere based on reference distributions of mean age derived from CO2 and SF6

We present balloon‐borne observations of CO2 and SF6 and derived vertical profiles of mean age for polar winter (inside vortex) and for midlatitude (nonwinter) conditions. For SF6‐derived mean ages above 5 years a mesospheric SF6 sink may lead to an overestimation of mean age, while for younger mean ages (below 2 years) the seasonal cycle of CO2 may influence the mean age determination based on CO2. We suggest that SF6 be used as an age tracer in the lower part of the stratosphere (i.e., for low mean age), whereas CO2 will be the better age tracer for older air. The mean age distributions together with an estimate of the width of the age spectrum are used to estimate the stratospheric chlorine loading. On the basis of an emission scenario and the lifetimes of chlorine‐containing compounds we estimate the future stratospheric chlorine loading. Inside the polar vortex, at an altitude of 20 km, we expect total chlorine to return to the values present in this region in 1980, when the ozone hole first appeared, around the year 2060. This estimate is based on the assumption that the general transport characteristics will not change over the period of the analysis and can thus be regarded as a best estimate based on stationary transport, the assumed set of lifetimes and emissions, and the mean age distribution derived from our measurements. As other factors influencing ozone may undergo temporal changes, this does not mean that ozone will necessarily recover at this time. While total chlorine is probably a good proxy for inorganic chlorine at altitudes above 20 km inside the polar vortex, the changing halocarbon mix may actually lead to an earlier return of inorganic chlorine to pre‐ozone‐hole values at lower altitudes.