Stratospheric Residence Time Research Articles

Unexpected self-lofting and dynamical confinement of volcanic plumes: the Raikoke 2019 case

Recent research has provided evidence of the self-lofting capacity of smoke aerosols in the stratosphere and their self-confinement by persistent anticyclones, which prolongs their atmospheric residence time and radiative effects. By contrast, the volcanic aerosols—composed mostly of non-absorptive sulphuric acid droplets—were never reported to be subject of dynamical confinement. Here we use high-resolution satellite observations to show that the eruption of Raikoke volcano in June 2019 produced a long-lived stratospheric anticyclone containing 24% of the total erupted mass of sulphur dioxide. The anticyclone persisted for more than 3 months, circumnavigated the globe three times, and ascended diabatically to 27 km altitude through radiative heating of volcanic ash contained by the plume. The mechanism of dynamical confinement has important implications for the planetary-scale transport of volcanic emissions, their stratospheric residence time, and atmospheric radiation balance. It also provides a challenge or “out of sample test” for weather and climate models that should be capable of reproducing similar structures.

Modeling the Sulfate Aerosol Evolution After Recent Moderate Volcanic Activity, 2008–2012

AbstractVolcanic activity is a major natural climate forcing and an accurate representation of volcanic aerosols in global climate models is essential. This is a complex task involving many uncertainties in the model design and setup and observations. We analyze the performance of the aerosol‐chemistry‐climate model SOCOL‐AERv2 for three medium‐sized volcanic eruptions. We focus on the impact of differences in volcanic plume height and SO2 estimates on the stratospheric aerosol burden. The influence of internal model variability and dynamics are addressed through an ensemble of free‐running and nudged simulations at different vertical resolutions. Comparing the modeled evolution of the stratospheric aerosol loading to satellite measurements reveals a good model performance. However, a conclusive validation is complicated by uncertainties in observations and emission estimates. The large spread in emitted sulfur amount and its vertical distribution consequently lead to differences in simulated aerosol burdens. Varying tropopause heights among free‐running simulations add to these differences, modulating the amount of sulfur injected into the stratosphere. In nudged mode, volcanic aerosol burden peaks are well reproduced, however changes in convection and clouds affect SO2 oxidation paths and cross‐tropopause transport, leading to increased background burdens compared to observations. This effect can be reduced by leaving temperatures unconstrained. A higher vertical resolution of 90 levels increases the stratospheric residence time of sulfate aerosol by reducing the diffusion out of the tropical reservoir. We conclude that the model set‐up (vertical resolution and free‐running vs. nudged) as well as forcing parameters (volcanic emission strength and plume height) contribute equally to the model uncertainties.

Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer

Abstract. Reduction of surface temperatures of the planet by injecting sulfate aerosols in the stratosphere has been suggested as an option to reduce the amount of human-induced climate warming. Several previous studies have shown that for a specified amount of injection, aerosols injected at a higher altitude in the stratosphere would produce more cooling because aerosol sedimentation would take longer. In this study, we isolate and assess the sensitivity of stratospheric aerosol radiative forcing and the resulting climate change to the altitude of the aerosol layer. We study this by prescribing a specified amount of sulfate aerosols, of a size typical of what is produced by volcanoes, distributed uniformly at different levels in the stratosphere. We find that stratospheric sulfate aerosols are more effective in cooling climate when they reside higher in the stratosphere. We explain this sensitivity in terms of effective radiative forcing: volcanic aerosols heat the stratospheric layers where they reside, altering stratospheric water vapor content, tropospheric stability, and clouds, and consequently the effective radiative forcing. We show that the magnitude of the effective radiative forcing is larger when aerosols are prescribed at higher altitudes and the differences in radiative forcing due to fast adjustment processes can account for a substantial part of the dependence of the amount of cooling on aerosol altitude. These altitude effects would be additional to dependences on aerosol microphysics, transport, and sedimentation, which are outside the scope of this study. The cooling effectiveness of stratospheric sulfate aerosols likely increases with the altitude of the aerosol layer both because aerosols higher in the stratosphere have larger effective radiative forcing and because they have higher stratospheric residence time; these two effects are likely to be of comparable importance.

Exploring accumulation-mode H<sub>2</sub>SO<sub>4</sub> versus SO<sub>2</sub> stratospheric sulfate geoengineering in a sectional aerosol–chemistry–climate model

Abstract. Stratospheric sulfate geoengineering (SSG) could contribute to avoiding some of the adverse impacts of climate change. We used the SOCOL-AER global aerosol–chemistry–climate model to investigate 21 different SSG scenarios, each with 1.83 Mt S yr−1 injected either in the form of accumulation-mode H2SO4 droplets (AM H2SO4), gas-phase SO2 or as combinations of both. For most scenarios, the sulfur was continuously emitted at an altitude of 50 hPa (≈20 km) in the tropics and subtropics. We assumed emissions to be zonally and latitudinally symmetric around the Equator. The spread of emissions ranged from 3.75∘ S–3.75∘ N to 30∘ S–30∘ N. In the SO2 emission scenarios, continuous production of tiny nucleation-mode particles results in increased coagulation, which together with gaseous H2SO4 condensation, produces coarse-mode particles. These large particles are less effective for backscattering solar radiation and have a shorter stratospheric residence time than AM H2SO4 particles. On average, the stratospheric aerosol burden and corresponding all-sky shortwave radiative forcing for the AM H2SO4 scenarios are about 37 % larger than for the SO2 scenarios. The simulated stratospheric aerosol burdens show a weak dependence on the latitudinal spread of emissions. Emitting at 30∘ N–30∘ S instead of 10∘ N–10∘ S only decreases stratospheric burdens by about 10 %. This is because a decrease in coagulation and the resulting smaller particle size is roughly balanced by faster removal through stratosphere-to-troposphere transport via tropopause folds. Increasing the injection altitude is also ineffective, although it generates a larger stratospheric burden, because enhanced condensation and/or coagulation leads to larger particles, which are less effective scatterers. In the case of gaseous SO2 emissions, limiting the sulfur injections spatially and temporally in the form of point and pulsed emissions reduces the total global annual nucleation, leading to less coagulation and thus smaller particles with increased stratospheric residence times. Pulse or point emissions of AM H2SO4 have the opposite effect: they decrease the stratospheric aerosol burden by increasing coagulation and only slightly decrease clear-sky radiative forcing. This study shows that direct emission of AM H2SO4 results in higher radiative forcing for the same sulfur equivalent mass injection strength than SO2 emissions, and that the sensitivity to different injection strategies varies for different forms of injected sulfur.

Baby, it's cold outside: Climate model simulations of the effects of the asteroid impact at the end of the Cretaceous

AbstractSixty‐six million years ago, the end‐Cretaceous mass extinction ended the reign of the dinosaurs. Flood basalt eruptions and an asteroid impact are widely discussed causes, yet their contributions remain debated. Modeling the environmental changes after the Chicxulub impact can shed light on this question. Existing studies, however, focused on the effect of dust or used one‐dimensional, noncoupled atmosphere models. Here we explore the longer‐lasting cooling due to sulfate aerosols using a coupled climate model. Depending on aerosol stratospheric residence time, global annual mean surface air temperature decreased by at least 26°C, with 3 to 16 years subfreezing temperatures and a recovery time larger than 30 years. The surface cooling triggered vigorous ocean mixing which could have resulted in a plankton bloom due to upwelling of nutrients. These dramatic environmental changes suggest a pivotal role of the impact in the end‐Cretaceous extinction.

Seasonal ventilation of the stratosphere: Robust diagnostics from one‐way flux distributions

We present an analysis of the seasonally varying ventilation of the stratosphere using one‐way flux distributions. Robust transport diagnostics are computed using GEOSCCM subject to fixed present‐day climate forcings. From the one‐way flux, we determine the mass of the stratosphere that is in transit since entry through the tropical tropopause to its exit back into the troposphere, partitioned according to stratospheric residence time and exit location. The seasonalities of all diagnostics are quantified with respect to the month of year (a) when air enters the stratosphere, (b) when the mass of the stratosphere is partitioned, and (c) when air exits back into the troposphere. We find that the return flux, within 3 months since entry, depends strongly on when entry occurred: (34±10)% more of the air entering the stratosphere in July leaves poleward of 45°N compared to air that enters in January. The month of year when the air mass is partitioned is also found to be important: The stratosphere contains about six times more air of tropical origin during late summer and early fall that will leave poleward of 45° within 6 months since entering the stratosphere compared to during late winter to late spring. When the entire mass of the air that entered the stratosphere at the tropics regardless of its residence time is considered, we find that (51±1)% and (39±2)% will leave poleward of 10° in the Nothern Hemisphere (NH) and Southern Hemisphere (SH), respectively.

Flux distributions as robust diagnostics of stratosphere‐troposphere exchange

We perform the first analysis of stratosphere‐troposphere exchange in terms of distributions that partition the one‐way flux across the thermal tropopause according to stratospheric residence time τ and the regions where air enters and exits the stratosphere. These distributions robustly quantify one‐way flux without being rendered ill defined by the short‐τ eddy‐diffusive singularity. Diagnostics are computed with an idealized circulation model with topography only in the Northern Hemisphere (NH) run under perpetual NH winter conditions. Suitable integrations of the flux distribution are used to determine the stratospheric mean residence time and the mass fraction of the stratosphere in any given residence time interval. We find that the largest mass fraction is destined for isentropic cross‐tropopause transport, with one‐way fluxes that are sustained over a broad range of residence times. Air exiting the stratosphere in the winter hemisphere has significantly longer mean residence times than air exiting in the summer hemisphere because the winter hemisphere has a deeper circulation and stronger eddy diffusion. We also explore the sensitivity of the stratosphere‐troposphere exchange to changes in the circulation by increasing the amplitude of the topography. The resulting more vigorous residual mean circulation dominates over increased eddy diffusion, leading to decreased except for air exiting at high NH latitudes, for which increases. These findings underline that the flux distributions diagnose the integrated advective‐diffusive tropopause‐to‐tropopause transport and not merely advection by the residual mean circulation.

Fate of volcanic ash: Aggregation and fallout

While suspended in Earth's atmosphere, fine ash influences radiative transfer, weather, and climate (e.g., [Newhall and Self, 1982][1]). Sulfate aerosol, also in volcanic ash clouds, dominates atmospheric effects, having stratospheric residence times of months to years ([Robock, 2000][2]), so large

Emission location dependent ozone depletion potentials for very short-lived halogenated species

Abstract. We present trajectory-based estimates of Ozone Depletion Potentials (ODPs) for very short-lived halogenated source gases as a function of surface emission location. The ODPs are determined by the fraction of source gas and its degradation products which reach the stratosphere, depending primarily on tropospheric transport and chemistry, and the effect of the resulting reactive halogen in the stratosphere, which is determined by stratospheric transport and chemistry, in particular by stratospheric residence time. Reflecting the different timescales and physico-chemical processes in the troposphere and stratosphere, the estimates are based on calculation of separate ensembles of trajectories for the troposphere and stratosphere. A methodology is described by which information from the two ensembles can be combined to give the ODPs. The ODP estimates for a species with a fixed 20 d lifetime, representing a compound like n-propyl bromide, are presented as an example. The estimated ODPs show strong geographical and seasonal variation, particularly within the tropics. The values of the ODPs are sensitive to the inclusion of a convective parametrization in the trajectory calculations, but the relative spatial and seasonal variation is not. The results imply that ODPs are largest for emissions from south and south-east Asia during Northern Hemisphere summer and from the western Pacific during Northern Hemisphere winter. Large ODPs are also estimated for emissions throughout the tropics with non-negligible values also extending into northern mid-latitudes, particularly in the summer. These first estimates, whilst made under some simplifying assumptions, show larger ODPs for certain emission regions, particularly south Asia in NH summer, than have typically been reported by previous studies which used emissions distributed evenly over land surfaces.

Evidence for ice particles in the tropical stratosphere from in-situ measurements

Abstract. In-situ ice crystal size distribution measurements are presented within the tropical troposphere and lower stratosphere. The measurements were performed using a combination of a Forward Scattering Spectrometer Probe (FSSP-100) and a Cloud Imaging Probe (CIP), which were installed on the Russian high altitude research aircraft M55 "Geophysica" during the SCOUT-O3 campaign in Darwin, Australia. One of the objectives of the campaign was to characterise the Hector convective system, which appears on an almost daily basis during the pre-monsoon season over the Tiwi Islands, north of Darwin. In total 90 encounters with ice clouds, between 10 and 19 km altitude were selected from the dataset and were analysed. Six of these encounters were observed in the lower stratosphere, up to 1.4 km above the local tropopause. Concurrent lidar measurements on board "Geophysica" indicate that these ice clouds were a result of overshooting convection. Large ice crystals, with a maximum dimension up to 400 μm, were observed in the stratosphere. The stratospheric ice clouds included an ice water content ranging from 7.7×10−5 to 8.5×10−4 g m−3 and were observed at ambient relative humidities (with respect to ice) between 75 and 157%. Three modal lognormal size distributions were fitted to the average size distributions for different potential temperature intervals, showing that the shape of the size distribution of the stratospheric ice clouds are similar to those observed in the upper troposphere. In the tropical troposphere the effective radius of the ice cloud particles decreases from 100 μm at about 10 km altitude, to 3 μm at the tropopause, while the ice water content decreases from 0.04 to 10−5 g m−3. No clear trend in the number concentration was observed with altitude, due to the thin and inhomogeneous characteristics of the observed cirrus clouds. The ice water content calculated from the observed ice crystal size distribution is compared to the ice water content derived from two hygrometer instruments. This independent measurement of the ice water content agrees within the combined uncertainty of the instruments for ice water contents exceeding 3×10−4g m−3. Stratospheric residence times, calculated based on gravitational settling, and evaporation rates show that the ice crystals observed in the stratosphere over the Hector storm system had a high potential of humidifying the stratosphere locally. Utilizing total aerosol number concentration measurements from a four channel condensation particle counter during two separate campaigns, it can be shown that the fraction of ice particles to the number of aerosol particles remaining ranges from 1:300 to 1:30 000 for tropical upper tropospheric ice clouds with ambient temperatures below −75°C.

Tritiated water vapor in the stratosphere: Vertical profiles and residence time

The historic and partly unpublished measurements of the tritium content in stratospheric water vapor made at the National Center for Atmospheric Research between 1975 and 1983 are reanalyzed. The resulting vertical profiles of the T content, mainly at 32°N latitude, show little variation with altitude above 20 km but a strong decay with time. This decay is a consequence of the large T injections into the stratosphere by the atmospheric tests of high‐yield thermonuclear devices prior to 1963 and seems to proceed with a single e‐fold time of 5.12 years. Correcting for the radioactive decay of HTO within the stratosphere and for a temporal increase in stratospheric H2O, we obtain a decay time for stratospheric HTO of 7.7 ± 2.0 years. This decay time, which is solely due to the transport of HTO into the troposphere, is much longer than the age of stratospheric air at these altitudes or the accepted values for stratospheric residence times. The differences are discussed and resolved by interpreting the HTO decay time as the Eigentime of the longest‐lived mode of the stratospheric transport equations. This Eigentime should provide a useful constraint in modeling stratospheric transport.

Changes of tracer distributions in the doubled CO2 climate

Changes in tracer distributions in the troposphere and stratosphere are calculated from a control and doubled CO2 climate simulation run with the Goddard Institute for Space Studies Global Climate Middle Atmosphere Model. Transport changes are assessed using seven on‐line tracers. Results show that interhemispheric transport is reduced by 5% along with a reduction in the Hadley circulation. Tropical transport from the troposphere into the stratosphere increases by some 30% associated with an increase in the stratospheric subtropical residual circulation. The tropical pipe becomes significantly more leaky, and greater transport into the lowermost stratosphere in the subtropics appears to be occurring, possibly in conjunction with a poleward shift in wave energy convergences. An increase in the high‐latitude lower stratosphere residual circulation reduces the stratospheric residence time of extratropical injections such as bomb 14C by 11%. Vertical mixing within the troposphere by convection increases, reducing low level concentrations of tracers. The Hadley cell change is affected by the latitudinal gradient of tropical warming. The high‐latitude lower stratosphere residual circulation change depends on the latitudinal gradient of the extratropical warming. Increased penetrating convection to the upper troposphere and the intensified residual circulation in the tropical upper troposphere/lower stratosphere appear to be the most robust of these results, with a magnitude that depends upon the degree of tropical warming. The consequence of this circulation change is to increase trace gas concentrations in the stratosphere and to decrease them in the troposphere for those species that have tropospheric sources.

Stratospheric residence time and its relationship to mean age

Transport inaccuracies in stratospheric models are a major source of uncertainty in predicting the environmental impact of trace gases with stratospheric source, such as emissions from high‐flying aircraft. Because there are no observed tracers of stratospheric aircraft emissions, a direct evaluation of the models' transport of the emissions is not possible. Here we examine the relationship between the stratospheric residence time τR for tracers of midlatitude stratosphere source and the mean age Γ, a transport diagnostic for which observations are available and have been used to evaluate stratospheric models. Employing a representation of the stratosphere that includes the basic kinematic features of the global circulation, we find that τR and Γ are correlated over a range of plausible circulations, but that the correlation is imperfect. A major (but not sole) limitation on Γ as a proxy for τR is the midlatitude tropopause height ZM. Elevating ZM reduces τR significantly, while Γ is only weakly affected. The relationship between τR and Γ seen among the participants of a recent stratospheric model intercomparison is consistent with this analysis.

Soot aerosol in the lower stratosphere: Pole‐to‐pole variability and contributions by aircraft

A NASA ER‐2 high‐altitude research aircraft intercepted the exhaust wake of a supersonic Concorde aircraft in the stratosphere near New Zealand on October 8, 1994. Black carbon (soot) aerosol (BCA) was sampled by wire impactors during the first five of 12 short‐duration wake intercepts. BCA concentration in Concorde exhaust at 16.3 km altitude was 0.2 particles cm−3, the size distribution peaked at a geometric mean radius of 0.09 μm, and the mass loading was 2.0±1.4 ng m−3. With a plume dilution factor (DF) of 1.0×10−5, determined by the ratio of CO2 measured in the plume (above the ambient stratospheric background level) to CO2 in the engine exhaust plane, the Concorde BCA emission index was EI(BCA)=0.07±0.05 g BCA per kg fuel burned. Applying this EI to estimates of aircraft fuel burned by the current subsonic fleet in the stratosphere yields average stratospheric BCA loadings of 0.5 ng m−3, commensurate with observations in the northern stratosphere. Applying the Concorde EI to fuel consumption by a projected future fleet suggests a twofold‐threefold increase of stratospheric BCA by the year 2015. A strong gradient in BCA concentration exists between the northern and the southern hemispheres, indicating interhemispheric mixing times longer than stratospheric residence times.

A 5‐year simulation of supersonic aircraft emission transport using a three‐dimensional model

A 5‐year simulation of supersonic aircraft exhaust using a three‐dimensional transport model has been completed using winds from the NASA/Goddard data assimilation system. A tracer based on emission rates of reactive nitrogen species (NOy) for all forecasted flight routes is continuously injected into the model. A parameterized upper stratospheric loss mechanism and a tropospheric sink due to rainout approximately balance the nitrogen emissions after several years of integration. Maximum values for exhaust NOy occur during the northern hemisphere (NH) summer months, and minimum values occur during winter. The pollutant is most zonally asymmetric during the NH summer. The peak values are never more than twice the zonal mean. This supports the use of zonally averaged two‐dimensional models to evaluate the impact of the exhaust on the lower stratospheric composition. Budget calculations from the transport model show that most exhaust released in the NH is transported downward into the troposphere, where it is destroyed. In the model, about 15–20% of exhaust released poleward of 30°N is transported into the tropics, where it is lofted. The stratospheric residence time for the exhaust is estimated to be 13 months.

Post‐entry and volcanic contaminant abundances of zinc, copper, selenium, germanium and gallium in stratospheric micrometeorites

Abstract— Some fraction of Zn, Cu, Se, Ga and Ge in chondritic interplanetary dust particles (IDPs) collected in the lower stratosphere between 1981 May and 1984 June has a volcanic origin. I present a method to evaluate the extent of this unavoidable type of stratospheric contamination for individual particles. The mass‐normalised abundances for Cu and Ge as a function of mass‐normalised stratospheric residence time show their time‐integrated stratospheric aerosol abundances. The Zn, Se and Ga abundances show a subdivision into two groups that span approximately two‐year periods following the eruptions of the Mount St. Helens (1980 May) and El Chichón (1982 April) volcanos. Elemental abundances in particles collected at the end of each two‐year period indicate low, but not necessarily ambient, volcanic stratospheric abundances. Using this time‐integrated baseline, I calculate the stratospheric contaminant fractions in nine IDPs and show that Zn, Se and Ga abundances in chondritic IDPs derive in part from stratospheric aerosol contaminants. Post‐entry elemental abundances (i.e., the amount that survived atmospheric entry heating of the IDP) show enrichments relative to the CI abundances but in a smaller number of particles than previously suggested.

Model study of atmospheric transport using carbon 14 and strontium 90 as inert tracers

The observed excess carbon 14 in the atmosphere from 1963 to 1970 provides unique, but limited, data up to an altitude of about 35 km for testing the air motions calculated by 11 multidimensional atmospheric models. Strontium 90 measurements in the atmosphere from 1964 to mid‐1967 provide data that have more latitude coverage than those of carbon 14 and are useful for testing combined models of air motions and aerosol settling. Model calculations for carbon 14 begin at October 1963, 9 months after the conclusion of the nuclear bomb tests; the initial conditions for the calculations are derived by three methods, each of which agrees fairly well with measured carbon 14 in October 1963 and each of which has widely different values in regions of the stratosphere where there were no carbon 14 measurements. The model results are compared to the stratospheric measurements, not as if the observed data were absolute standards, but in an effort to obtain new insight about the models and about the atmosphere. The measured carbon 14 vertical profiles at 31°N are qualitatively different from all of the models; the measured vertical profiles show a maximum mixing ratio in the altitude range of 20 to 25 km from October 1963 through July 1966, but all modeled profiles show mixing ratio maxima that increase in altitude from 20 km in October 1963 to greater than 40 km by April 1966. Both carbon 14 and strontium 90 data indicate that the models differ substantially among themselves with respect to stratosphere‐troposphere exchange rate, but the modeled carbon 14 stratospheric residence times indicate that differences among the models are small with respect to transport rate between the middle stratosphere and the lower stratosphere. Strontium 90 data indicate that aerosol settling is important up to at least 35 km altitude. Relative to the measurements, about three quarters of the models transport carbon 14 from the lower stratosphere to the troposphere too rapidly, and all models with all three sets of initial conditions appear to sweep carbon 14 out of the midstratosphere (above 28 km) much more slowly than was observed from 1963 to 1970. Atmospheric dynamicists should seriously consider what the carbon 14 measurements imply with respect to advection in the midstratosphere.

Possible effects of CO2 increase on the high‐speed civil transport impact on ozone

The role of heterogeneous chemistry on the potential impact on ozone of a commercial fleet of high speed civil transport aircraft (HSCT) has been recently studied with assessment models. Here an attempt is made to model the effects of the carbon dioxide increase which is predicted in the future atmosphere when HSCT should be operational. For this purpose we have first used a three‐dimensional model for the radiative and dynamical calculations and then a photochemical two‐dimensional model including an explicit gas‐particle interaction in the process of aerosol formation. The denoxification and denitrification associated with the formation of nitric acid trihydrate (NAT) aerosols is shown to significantly affect the partition of chemical families. The radiative perturbation introduced by the CO2 increase is shown to perturb the stratospheric dynamics in such a way that the lower stratospheric residual circulation is enhanced. This has the effect of reducing by about 15% the stratospheric residence time of odd nitrogen injected by the aircraft, so that the overall perturbation of stratospheric chemistry due to HSCT is mitigated with respect to the reference case in which CO2 is kept at the present level. Another effect is found to be produced by the stratospheric temperature cooling following the CO2 increase. Our model predicts a large enhancement of the surface area density of NAT aerosols in the arctic region, so that the additional denitrification produces a further decrease of the relative role of the NOx catalytic cycle for ozone destruction in the lower stratosphere. For this reason, the ClO and OH decreases associated with the HSCT‐induced NOx increase are found to be dominant in the ozone budget, thus producing a global ozone increase in the case of 500 ppmv CO2 (+0.27% for mach 2.4 and NOx emission index 15). The balance between the different tendencies of ClO, NOx, and OH cycles is found to be closer in the reference case of 335 ppmv CO2, where a small column ozone depletion by HSCT is predicted (−0.53% globally).

Cross‐tropopause transport of excess 14C in a two‐dimensional model

The processed excess 14C data are used to calibrate the cross‐tropopause transport in a two‐dimensional model. The model results are used to diagnose the mechanisms responsible for cross‐tropopause transport in the model and the sensitivity of the transport rate to the changes in the transport parameter. Although the total flux across the tropopause is dominated by the eddy diffusion flux along the isentropic surface due to the large values assigned to the diffusion coefficient Kyy at the boundary, the cross‐tropopause transport is more sensitive to changes in the circulation. Nitrogen oxides emitted by engines of high‐speed civil transport could cause ozone depletion in the lower stratosphere. The expected depletion is proportional to the amount of oxides of nitrogen retained in the stratosphere. For a typical fleet, lowering the tropopause height by 1.2 km can cause a 14–22% increase in the amount of reactive nitrogen retained in the stratosphere. Thus it is necessary to have a fine enough vertical resolution near the tropopause in the two‐dimensional model for determining the stratospheric residence time of the engine emissions deposited near the tropopause.

The bromine content of micrometeorites: Arguments for stratospheric contamination

Bromine‐salt nanocrystals are associated with a porous chondritic micrometeorite (W7029E5) that was collected in the lower stratosphere. These salt nanocrystals occur together with volcanic Na and K salt nanocrystals embedded in sulfuric acid droplets that were originally adhered to the particle. These materials were concentrated during hexane rinsing as part of routine curation procedures at the NASA Johnson Space Center Cosmic Dust Curatorial Facility. This observation is fortuitous to the extent that the concentration of nanocrystals and sulfuric acid is an experimental artifact of curation. If bromine is a stratospheric contaminant due to surface adsorption, there should be a positive linear relationship between the mass‐normalized residence time and bromine content of individual micrometeorites. I show that the predicted correlation exists using a new model to calculate the stratospheric residence time of individual nonspherical micrometeorites in the slow‐settling Wilson‐Huang regime of the stratosphere.