Eric A. Ray

Chemical depletion of Arctic ozone in winter 1999/2000

During Arctic winters with a cold, stable stratospheric circulation, reactions on the surface of polar stratospheric clouds (PSCs) lead to elevated abundances of chlorine monoxide (ClO) that, in the presence of sunlight, destroy ozone. Here we show that PSCs were more widespread during the 1999/2000 Arctic winter than for any other Arctic winter in the past two decades. We have used three fundamentally different approaches to derive the degree of chemical ozone loss from ozonesonde, balloon, aircraft, and satellite instruments. We show that the ozone losses derived from these different instruments and approaches agree very well, resulting in a high level of confidence in the results. Chemical processes led to a 70% reduction of ozone for a region ∼1 km thick of the lower stratosphere, the largest degree of local loss ever reported for the Arctic. The Match analysis of ozonesonde data shows that the accumulated chemical loss of ozone inside the Arctic vortex totaled 117 ± 14 Dobson units (DU) by the end of winter. This loss, combined with dynamical redistribution of air parcels, resulted in a 88 ± 13 DU reduction in total column ozone compared to the amount that would have been present in the absence of any chemical loss. The chemical loss of ozone throughout the winter was nearly balanced by dynamical resupply of ozone to the vortex, resulting in a relatively constant value of total ozone of 340 ± 50 DU between early January and late March. This observation of nearly constant total ozone in the Arctic vortex is in contrast to the increase of total column ozone between January and March that is observed during most years.

Severe chemical ozone loss inside the Arctic Polar Vortex during winter 1999–2000 Inferred from in situ airborne measurements

Lower stratospheric in situ observations are used to quantify both the accumulated ozone loss and the ozone chemical loss rates in the Arctic polar vortex during the 1999–2000 winter. Multiple long-lived trace gas correlations are used to identify parcels in the inner Arctic vortex whose chemical loss rates are unaffected by extra-vortex intrusions. Ozone-tracer correlations are then used to calculate ozone chemical loss rates. During the late winter the ozone chemical loss rate is found to be −46±6 (1σ) ppbv/day. By mid-March 2000, the accumulated ozone chemical loss is 58±4% in the lower stratosphere near 450 K potential temperature (∼19 km altitude).

Global‐scale seasonally resolved black carbon vertical profiles over the Pacific

[1] Black carbon (BC) aerosol loadings were measured during the High-performance Instrumented Airborne Platform for Environmental Research Pole-to-Pole Observations (HIPPO) campaign above the remote Pacific from 85°N to 67°S. Over 700 vertical profiles extending from near the surface to max ∼14 km altitude were obtained with a single-particle soot photometer between early 2009 and mid-2011. The data provides a climatology of BC in the remote regions that reveals gradients of BC concentration reflecting global-scale transport and removal of pollution. BC is identified as a sensitive tracer of extratropical mixing into the lower tropical tropopause layer and trends toward surprisingly uniform loadings in the lower stratosphere of ∼1 ng/kg. The climatology is compared to predictions from the AeroCom global model intercomparison initiative. The AeroCom model suite overestimates loads in the upper troposphere/lower stratosphere (∼10×) more severely than at lower altitudes (∼3×), with bias roughly independent of season or geographic location; these results indicate that it overestimates BC lifetime.

Evidence of the effect of summertime midlatitude convection on the subtropical lower stratosphere from CRYSTAL‐FACE tracer measurements

[1] Trace gas and particle measurements taken during the CRYSTAL-FACE mission are used to examine mixing in the summer subtropical lower stratosphere. Vigorous convection in the central and eastern United States injected a significant amount of tropospheric air into the lower stratosphere, which was subsequently advected over the region sampled during the CRYSTAL-FACE mission. Aerosols produced by biomass burning were observed over Florida during a time period with a large number of forest fires in the western United States and eastern Canada, providing evidence of convective injection of tropospheric air into the lower stratosphere. The circumstances of the large-scale flow pattern in the upper troposphere and lower stratosphere, vigorous summertime convection, abundant forest fires, and the downstream sampling allow a unique view of mixing in the lower stratosphere. We calculate the fractions of midlatitude tropospheric air in the sampled lower stratosphere and mixing rates on the basis of consistency between a number of tracer-tracer correlations. The tropospheric endpoints to the mixing estimates give an indication of midlatitude continental convective input into the lower stratosphere. We also discuss the possible impact of summertime midlatitude convection on the composition of the stratosphere as a whole.

An analysis of the structure and forcing of the equatorial semiannual oscillation in zonal wind

The low-frequency variability in the equatorial middle atmosphere is dominated by the semiannual oscillation (SAO) and quasi-biennial (QBO) oscillation. Although the general characteristics and forcing mechanisms of these oscillations are thought to be known, these oscillations are still not well reproduced in modeling experiments of the middle atmosphere. The relatively long period (1992–1996) of Upper Atmosphere Research Satellite (UARS) measurements and U.K. Meteorological Office (UKMO) assimilated model data have been used to examine the zonal wind SAO in the upper stratosphere. The SAO in zonal wind is found to be latitudinally asymmetric about the equator with strongest amplitudes in the southern hemisphere subtropics. The latitudinal asymmetry is diagnosed by using the TEM momentum equation and UKMO data. The asymmetry is mostly due to the different phasing of the peak advective forcing by the mean meridional circulation in each hemisphere. The contribution of gravity waves to the zonal wind SAO are also explored by using the forcing from a linear gravity wave model. Gravity waves are found to be the primary source of westerly momentum in each hemisphere during fall and contribute to the latitudinal asymmetry of the SAO by having more easterly forcing in the southern hemisphere during summer compared to in the northern hemisphere during summer.

Improving stratospheric transport trend analysis based on SF6 and CO2 measurements

In this study we reexamine nearly four decades of in situ balloon-based stratospheric observations of SF6 and CO2 with an idealized model and reanalysis products. We use new techniques to account for the spatial and temporal inhomogeneity of the sparse balloon profiles and to calculate stratospheric mean ages of air more consistently from the observations with the idealized model. By doing so we are able to more clearly show and account for the variability of mean age of air throughout the bulk of the depth of the stratosphere. From an idealized model guided by the observations, we identify variability in the mean age due to the seasonal cycle of stratospheric transport, the quasi-biennial oscillation in tropical zonal winds, major volcanic eruptions, and linear trends that vary significantly with altitude. We calculate a negative mean age trend in the lowest 5 km of the stratosphere that agrees within uncertainties with a trend calculated from a set of chemistry climate model mean ages in this layer. The mean age trends reverse sign in the middle and upper stratosphere and are in agreement with a previous positive trend estimate using the same observational data set, although we have substantially reduced the uncertainty on the trend. Our analysis shows that a long time series of in situ profile measurements of trace gases such as SF6 and CO2 can be a unique and useful indicator of stratospheric circulation variability on a range of time scales and an important contributor to help validate the stratospheric portion of global chemistry climate models. However, with only SF6 and CO2 measurements, the competing effects on mean age between mean circulation and mixing (tropical entrainment) are not uniquely separable.

Global tracer modeling during SOLVE: High‐latitude descent and mixing

[1] We compare tracer observations made during the northern winter of 1999/2000 with the results of simulations with a three-dimensional chemical transport model, driven by assimilated winds. During the course of the winter, very low concentrations of tracers of tropospheric origin (such as N2O) descend into the lower stratosphere within the polar vortex. The altitude of origin of this air has been a matter of debate in the literature; by midwinter, both observations and model results indicate a significant fraction of mesospheric air in the lower stratosphere. Observations from aircraft and balloon flights reveal markers of mesospheric air within the Arctic vortex in the lower and middle stratosphere. An artificial tracer introduced into the model mesosphere at the start of winter descends (being diluted as it does so) all the way down to the 450 K potential temperature surface by March. Modeled tracer-tracer relationships evolve through the winter in a way similar to observations, but the separation between vortex and extravortex curves is exaggerated, suggesting that the model exhibits excessive horizontal mixing within and into the vortex. The tracer-tracer relationships are used to identify partly mixed air as lying, in tracer-tracer space, in a region intermediate between the characteristic vortex and midlatitude relationships. Air lying in a collar region just inside the vortex edge is thus identified as being mixed, and this indicates excessive horizontal mixing in the model across the vortex edge. INDEX TERMS: 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); KEYWORDS: stratosphere, polar vortex, tracer modeling, stratospheric tracers

Quantification of the SF6 lifetime based on mesospheric loss measured in the stratospheric polar vortex

Sulfur hexafluoride (SF6) is a greenhouse gas with one of the highest radiative efficiencies in the atmosphere as well as an important indicator of transport time scales in the stratosphere. The current widely used estimate of the atmospheric lifetime of SF6 is 3200 years. In this study we use in situ measurements in the 2000 Arctic polar vortex that sampled air with up to 50% SF6 loss to calculate an SF6 lifetime. Comparison of these measurements with output from the Whole Atmosphere Community Climate Model (WACCM) shows that WACCM transport into the vortex is accurate and that an important SF6 loss mechanism, believed to be electron attachment, is missing in the model. Based on the measurements and estimates of the size of the vortex, we calculate an SF6 lifetime of 850 years with an uncertainty range of 580–1400 years. The amount of SF6 loss is shown to be consistent with that of HFC-227ea, which has a lifetime of 670–780 years, adding independent support to our new SF6 lifetime estimate. Based on the revised lifetime the global warming potential of SF6 will decrease only slightly for short time horizons (<100 years) but will decrease substantially for time horizons longer than 2000 years. Also, the use of SF6 measurements as an indicator of transport time scales in the stratosphere clearly must account for potential influence from polar vortex air.

An unexpected and persistent increase in global emissions of ozone-depleting CFC-11

The Montreal Protocol was designed to protect the stratospheric ozone layer by enabling reductions in the abundance of ozone-depleting substances such as chlorofluorocarbons (CFCs) in the atmosphere1–3. The reduction in the atmospheric concentration of trichlorofluoromethane (CFC-11) has made the second-largest contribution to the decline in the total atmospheric concentration of ozone-depleting chlorine since the 1990s1. However, CFC-11 still contributes one-quarter of all chlorine reaching the stratosphere, and a timely recovery of the stratospheric ozone layer depends on a sustained decline in CFC-11 concentrations1. Here we show that the rate of decline of atmospheric CFC-11 concentrations observed at remote measurement sites was constant from 2002 to 2012, and then slowed by about 50 per cent after 2012. The observed slowdown in the decline of CFC-11 concentration was concurrent with a 50 per cent increase in the mean concentration difference observed between the Northern and Southern Hemispheres, and also with the emergence of strong correlations at the Mauna Loa Observatory between concentrations of CFC-11 and other chemicals associated with anthropogenic emissions. A simple model analysis of our findings suggests an increase in CFC-11 emissions of 13 ± 5 gigagrams per year (25 ± 13 per cent) since 2012, despite reported production being close to zero4 since 2006. Our three-dimensional model simulations confirm the increase in CFC-11 emissions, but indicate that this increase may have been as much as 50 per cent smaller as a result of changes in stratospheric processes or dynamics. The increase in emission of CFC-11 appears unrelated to past production; this suggests unreported new production, which is inconsistent with the Montreal Protocol agreement to phase out global CFC production by 2010.Atmospheric CFC-11 concentrations have been declining less rapidly since 2012; evidence suggests that this finding is explained by an increase in the emission of CFC-11during these years.

A Cost-Effective Trace Gas Measurement Program for Long-Term Monitoring of the Stratospheric Circulation

A stratospheric trace gas measurement program using balloon-based sonde and AirCore sampler techniques is proposed as a way to monitor the strength of the stratospheric mean meridional or Brewer–Dobson circulation. Modeling work predicts a strengthening of the Brewer–Dobson circulation in response to increasing greenhouse gas concentrations; such a change will likely impact tropospheric climate. Because the strength of the Brewer–Dobson circulation is an unmeasureable quantity, trace gas measurements are used to infer characteristics of the circulation. At present, stratospheric trace gas measurements are sporadic in time and/or place, primarily associated with localized aircraft or balloon campaigns or relatively short-lived satellite instruments. This program would consist of regular trace gas profile measurements taken at multiple latitudes covering tropical, midlatitude, and polar regimes. The program would make use of the relatively low-cost AirCore and sonde techniques, allowing more frequent measur...