L. P. Steele

Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn

A record of atmospheric CO2 mixing ratios from 1006 A.D. to 1978 A.D. has been produced by analysing the air enclosed in three ice cores from Law Dome, Antarctica. The enclosed air has unparalleled age resolution and extends into recent decades, because of the high rate of snow accumulation at the ice core sites. The CO2 data overlap with the record from direct atmospheric measurements for up to 20 years. The effects of diffusion in the firn on the CO2 mixing ratio and age of the ice core air were determined by analyzing air sampled from the surface down to the bubble close-off depth. The uncertainty of the ice core CO2 mixing ratios is 1.2 ppm (1 σ). Preindustrial CO2 mixing ratios were in the range 275–284 ppm, with the lower levels during 1550–1800 A.D., probably as a result of colder global climate. Natural CO2 variations of this magnitude make it inappropriate to refer to a single preindustrial CO2 level. Major CO2 growth occurred over the industrial period except during 1935–1945 A.D. when CO2 mixing ratios stabilized or decreased slightly, probably as a result of natural variations of the carbon cycle on a decadal timescale.

A history of chemically and radiatively important gases in air deduced from ALE/GAGE/AGAGE

We describe in detail the instrumentation and calibrations used in the Atmospheric Lifetime Experiment (ALE), the Global Atmospheric Gases Experiment (GAGE), and the Advanced Global Atmospheric Gases Experiment (AGAGE) and present a history of the majority of the anthropogenic ozone-depleting and climate-forcing gases in air based on these experiments. Beginning in 1978, these three successive automated high-frequency in situ experiments have documented the long-term behavior of the measured concentrations of these gases over the past 20 years, and show both the evolution of latitudinal gradients and the high-frequency variability due to sources and circulation. We provide estimates of the long-term trends in total chlorine contained in long-lived halocarbons involved in ozone depletion. We summarize interpretations of these measurements using inverse methods to determine trace gas lifetimes and emissions. Finally, we provide a combined observational and modeled reconstruction of the evolution of chlorocarbons by latitude in the atmosphere over the past 60 years which can be used as boundary conditions for interpreting trapped air in glaciers and oceanic measurements of chlorocarbon tracers of the deep oceanic circulation. Some specific conclusions are as follows: (1) International compliance with the Montreal Protocol is so far resulting in chlorofluorocarbon and chlorocarbon mole fractions comparable to target levels; (2) mole fractions of total chlorine contained in long-lived halocarbons (CCl 2 F 2 , CCl 3 F, CH 3 CCl 3 , CCl 4 , CHClF 2 , CCl 2 FCClF 2 , CH 3 Cl, CH 2 Cl 2 , CHCl 3 , CCl 2 =CCl 2 ) in the lower troposphere reached maximum values of about 3.6 ppb in 1993 and are beginning to slowly decrease in the global lower atmosphere; (3) the chlorofluorocarbons have atmospheric lifetimes consistent with destruction in the stratosphere being their principal removal mechanism; (4) multiannual variations in chlorofluorocarbon and chlorocarbon emissions deduced from ALE/GAGE/AGAGE data are consistent approximately with variations estimated independently from industrial production and sales data where available (CCl 2 F 2 (CFC-12) and CCl 2 FCClF 2 (CFC-113) show the greatest discrepancies); (5) the mole fractions of the hydrochlorofluorocarbons and hydrofluorocarbons, which are replacing the regulated halocarbons, are rising very rapidly in the atmosphere, but with the exception of the much longer manufactured CHClF 2 (HCFC-22), they are not yet at levels sufficient to contribute significantly to atmospheric chlorine loading. These replacement species could in the future provide independent estimates of the global weighted-average OH concentration provided their industrial emissions are accurately documented; (6) in the future, analysis of pollution events measured using high-frequency in situ measurements of chlorofluorocarbons and their replacements may enable emission estimates at the regional level, which, together with industrial end-use data, are of sufficient accuracy to be capable of identifying regional noncompliance with the Montreal Protocol.

The growth rate and distribution of atmospheric methane

Methane was measured in air samples collected approximately weekly from a globally distributed network of sites from 1983 to 1992. Sites range in latitude from 90°S to 82°N. All samples were analyzed by gas chromatography, with flame ionization detection at the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory in Boulder, Colorado, and the measurements were referenced against a single calibration scale. The estimated precision of the measurements is ±0.2%. Samples which had clear sampling or analytical errors, or which appeared to be contaminated by local CH4 sources, were identified and excluded from the data analysis. The data reveal a strong north-south gradient in methane with an annual mean difference of about 140 ppb between the northernmost and southernmost sampling sites. Methane time series from the high southern latitude sites have a relatively simple seasonal cycle with a minimum during late summer-early fall, almost certainly dominated by the seasonality in its photochemical destruction. Typical seasonal cycle amplitudes there are about 30 ppb. Seasonal cycles at sites in the northern hemisphere are complex when compared to sites in the southern hemisphere due to the interaction among CH4 sources and sinks, and atmospheric transport. Seasonal cycle amplitudes in the high north are about twice those observed in the high southern hemisphere. Annual mean methane mixing ratios were ∼1% lower at 3397 m than at sea level on the island of Hawaii. Trends were determined at each site in the network and globally. The average increase in the globally averaged methane mixing ratio over the period of these measurements is (11.1±0.2) ppb yr−1. Globally, the growth rate for methane decreased from approximately 13.5 ppb yr−1 in 1983 to about 9.3 ppb yr−1 in 1991. The growth rate of methane in the northern hemisphere during 1992 was near zero. Various possibilities for the long-term, slow decrease in the methane growth rate over the last decade and the rapid change in growth rate in the northern hemisphere in 1992 are given. The most likely explanation is a change in a methane source influenced directly by human activities, such as fossil fuel production.

Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability

Atmospheric methane mixing ratios from 1000 A.D. to present are measured in three Antarctic ice cores, two Greenland ice cores, the Antarctic firn layer, and archived air from Tasmania, Australia. The record is unified by using the same measurement procedure and calibration scale for all samples and by ensuring high age resolution and accuracy of the ice core and firn air. In this way, methane mixing ratios, growth rates, and interpolar differences are accurately determined. From 1000 to 1800 A.D. the global mean methane mixing ratio averaged 695 ppb and varied about 40 ppb, contemporaneous with climatic variations. Interpolar (N-S) differences varied between 24 and 58 ppb. The industrial period is marked by high methane growth rates from 1945 to 1990, peaking at about 17 ppb yr−1 in 1981 and decreasing significantly since. We calculate an average total methane source of 250 Tg yr−1 for 1000–1800 A.D., reaching near stabilization at about 560 Tg yr−1 in the 1980s and 1990s. The isotopic ratio, δ13CH4, measured in the archived air and firn air, increased since 1978 but the rate of increase slowed in the mid-1980s. The combined CH4 and δ13CH4 trends support the stabilization of the total CH4 source.

Distribution of halon‐1211 in the upper troposphere and lower stratosphere and the 1994 total bromine budget

We report here on the details of the first, in situ, real-time measurements of H-1211 (CBrClF2) and sulfur hexafluoride (SF6) mixing ratios in the stratosphere up to 20 km. Stratospheric air was analyzed for these gases and others with a new gas Chromatograph, flown aboard a National Aeronautics and Space Administration ER-2 aircraft as part of the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft mission conducted in 1994. The mixing ratio of SF6, with its nearly linear increase in the troposphere, was used to estimate the mean age of stratospheric air parcels along the ER-2 flight path. Measurements of H-1211 and mean age estimates were then combined with simultaneous measurements of CFC-11 (CCl3F), measurements of brominated compounds in stratospheric whole air samples, and records of tropospheric organic bromine mixing ratios to calculate the dry mixing ratio of total bromine in the lower stratosphere and its partitioning between organic and inorganic forms. We estimate that the organic bromine-containing species were almost completely photolyzed to inorganic species in the oldest air parcels sampled. Our results for inorganic bromine are consistent with those obtained from a photochemical, steady state model for stratospheric air parcels with CFC-11 mixing ratios greater than 150 ppt. For stratospheric air parcels with CFC-11 mixing ratios less than 50 ppt (mean age ≥5 years) we calculate inorganic bromine mixing ratios that are approximately 20% less than the photochemical, steady state model. There is a 20% reduction in calculated ozone loss resulting from bromine chemistry in old air relative to some previous estimates as a result of the lower bromine levels.

Modeling air movement and bubble trapping in firn

A finite difference model for gas diffusion and bubble trapping in firn is described. The model uses prescribed profiles of density, open and closed porosity, and diffusivity to determine the diffusion and trapping processes. The model is calibrated and tested by using measured air composition in the firn at the DE08-2 site on Law Dome, Antarctica. In particular, we focus on carbon dioxide (CO2), methane (CH4), and sulfur hexafluoride (SF6), which have well-determined atmospheric records (CO2 since 1958, CH4 since 1983, and SF6 since 1978). These trace gases are used to tune the diffusivity-porosity relationship, which is the most uncertain of the model inputs. Modeled trace gas profiles in the DE08-2 firn are improved if allowance is made for reduced diffusion through the most prominent DE08-2 melt layer from the summer of 1989/1990. The relatively rapid growth rate of SF6 in the atmosphere permits good definition of the diffusion reduction due to the melt layer (about 80%). The model quantifies the smoothing effect of the firn diffusion and bubble trapping on atmospheric signals. Gravitational separation in the firn is investigated by comparison of modeled δ15N2 with observations. The model is used to calculate the isotopic diffusion correction for δ13C02 and δ13CH4. This corrects for the fractionating effects of the firn diffusion process on the different isotopes. The diffusion and gravitational corrections are critical at the measurement precision currently being obtained; for δ13CO2 the diffusion correction is up to about 10 times the current measurement precision. The diffusion correction is even more significant for δ13CH4; at over 1‰ at the bottom of the firn it is more than double the change over the last decade. The fully corrected δ13C02 record from the DE08-2 firn is compared with the history of Cape Grim direct atmospheric measurements with excellent agreement.

Continuous high-frequency observations of hydrogen at the Mace Head baseline atmospheric monitoring station over the 1994–1998 period

Continuous high-frequency (every 40-min) automatic measurements of hydrogen have been made at the Mace Head atmospheric research station on the Atlantic Ocean coast of Ireland throughout 1994–1998. These observations represent one the most comprehensive in situ records of a trace gas that has received comparatively little attention. Individual measurements have been sorted by four independent methods to separate clean, maritime air masses from regionally polluted European air masses. Hydrogen concentrations in midlatitude Northern Hemisphere baseline air show a distinct seasonal cycle with highest concentrations during spring and lowest concentrations during late autumn, with a peak-to-trough amplitude of 38±6 ppb, averaged over the observed seasonal cycles from 1994 to 1998. The mean hydrogen concentration in midlatitude Northern Hemisphere baseline air on January 1, 1995, was estimated as 496.5 ppb with an upward trend of 1.2±0.8 ppb yr−1. Evidence has also been obtained for European pollution sources with source strength of about 0.8 Tg yr−1 and for deposition of hydrogen to soils. The observation of slightly elevated hydrogen concentrations relative to baseline levels in tropical maritime air masses points to a latitudinal gradient in hydrogen with higher concentrations in lower latitudes of the Northern Hemisphere and in the Southern Hemisphere. This is confirmed by comparable hydrogen observations at Cape Grim, Tasmania, which are consistently higher than measurements recorded at Mace Head. Mean hemispheric concentrations of 504 and 520 ppb have been estimated for the Northern and Southern Hemispheres, respectively, for January 1, 1996, corresponding to a total atmospheric hydrogen burden of 182 Tg.

A history of δ13C in atmospheric CH4 from the Cape Grim Air Archive and Antarctic firn air

Marine (baseline) air from Cape Grim (41°S), collected and archived in high-pressure metal containers, provides a history of δ13C in atmospheric methane from 1978. A similar history is obtained from air pumped from different layers of the firn at Law Dome, Antarctica, after correction for diffusion and gravitational settling effects in the firn. The archive records are linked to measurements since 1992 using 5-L glass flasks filled at Cape Grim, and compared to data since 1989 from a comparable site at Baring Head, New Zealand. Over 18 years the δ13C of atmospheric methane in the extratropical Southern Hemisphere has increased by ∼0.6‰ while the methane mixing ratio increased by ∼200 ppb. The δ13C growth rate decreases over the 18-year period, but by relatively less than the simultaneous decrease in mixing ratio growth rate. The overall increase in δ13C is significantly smaller than, and the recent slowing is in conflict with, previous estimates [Stevens and Engelkemeir, 1989]. The long-term trend in δ13C, and the different shape to the trend in mixing ratio, are shown to be consistent with constant global methane sources and sinks since 1982. The slower equilibration of observed δ13C, compared to that of the mixing ratio, is an example of an effect pointed out recently by Tans [1997]. The data presented here constrain changes in the relative mix of isotopically heavy and light sources to be small and suggest that there was little change in the ratio of anthropogenic to natural sources in the 1978 to 1995 period.

Correction to “Sulfuryl fluoride in the global atmosphere”

[1] In the paper ‘‘Sulfuryl fluoride in the global atmosphere’’ by J. Muhle et al. (Journal of Geophysical Research, 114, D05306, doi:10.1029/2008JD011162, 2009), the final two coauthors were omitted. P. B. Krummel and L. P. Steele are affiliated with Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D10303, doi:10.1029/2009JD012306, 2009