Dana E. Hartley

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.

Atmospheric Trends and Lifetime of CH3CCI3 and Global OH Concentrations.

Determination of the atmospheric concentrations and lifetime of trichloroethane (CH3CCI3) is very important in the context of global change. This halocarbon is involved in depletion of ozone, and the hydroxyl radical (OH) concentrations determined from its lifetime provide estimates of the lifetimes of most other hydrogen-containing gases involved in the ozone layer and climate. Global measurements of trichloroethane indicate rising concentrations before and declining concentrations after late 1991. The lifetime of CH3CCI3 in the total atmosphere is 4.8 � 0.3 years, which is substantially lower than previously estimated. The deduced hydroxyl radical concentration, which measures the atmospheres oxidizing capability, shows little change from 1978 to 1994.

Global average concentration and trend for hydroxyl radicals deduced from ALE/GAGE trichloroethane (methyl chloroform) data for 1978–1990

Atmospheric measurements at several surface stations made between 1978 and 1990 of the anthropogenic chemical compound 1,1,1-trichloroethane (methyl chloroform, CH3CCl3) show it increasing at a global average rate of 4.4 ± 0.2% per year (1σ) over this time period. The measured trends combined with industrial emission estimates are used in an optimal estimation inversion scheme to deduce a globally averaged CH3CCl3 tropospheric (and total atmospheric) lifetime of 5.7 (+0.7, −0.6) years (1σ) and a weighted global average tropospheric hydroxyl radical (OH) concentration of (8.7 ± 1.0) × 105 radical cm−3 (1σ). Inclusion of a small loss rate to the ocean for CH3CCl3 of 1/85 year−1 does not affect the stated lifetime but lowers the stated OH concentration to (8.1 ± 0.9) × 105 radical cm−3 (1σ). The rate of change of the weighted global average OH concentration over this time period is determined to be 1.0 ± 0.8% per year (1σ) which has major implications for the oxidation capacity of the atmosphere and more specifically for methane (CH4), which like CH3CCl3 is destroyed primarily by OH radicals. Because the weighting strongly favors the tropical lower troposphere, this deduced positive OH trend is qualitatively consistent with hypothesized changes in tropical tropospheric OH and ozone concentrations driven by tropical urbanization, biomass burning, land use changes, and long-term warming. We caution, however, that our deduced rate of change in OH assumes that current industry estimates of anthropogenic emissions and our absolute calibration of CH3CCl3 are accurate. The CH3CCl3 measurements at our tropical South Pacific station (Samoa) show remarkable sensitivity to the El Nino-Southern Oscillation (ENSO), which we attribute to modulation of cross-equatorial transport during the northern hemisphere winter by the interannually varying upper tropospheric zonal winds in the equatorial Pacific. Thus measurements of this chemical compound have led to the discovery of a previously unappreciated aspect of tropical atmospheric tracer transport.

GAGE/AGAGE measurements indicating reductions in global emissions of CCl3F and CCl2F2 in 1992–1994

Global Atmospheric Gases Experiment/Advanced GAGE (GAGE/AGAGE) observations of CCl 3 F indicate that global concentrations of this compound reached a maximum in 1993 and decayed slightly in 1994; CCl 2 F 2 concentrations increased approximately 7 ppt in both 1993 and 1994. The observations suggest that world emissions in these two years were smaller than industry production figures would suggest and have decreased faster than expected under the Montreal Protocol and its amendments. An analysis of regional pollution events at the Mace Head site suggest that industry may be underestimating the decline of emissions in Europe. It is argued, however, that the decline in European emissions is not biasing the background Mace Head measurements (or the GAGE global averages). Combining the chlorofluorocarbon measurements, including CCl 2 FCClF 2 , with GAGE/AGAGE measured global decreases in CH 3 CCl 3 and CCl 4 after 1992 and with Cape Grim archived air measurements of CHClF 2 , the measurements suggest that anthropogenic atmospheric chlorine loading from these six gases maximized in 1992 at 2.95 ± 0.04 ppb and that it had decreased by 0.02 ± 0.01 ppb by the beginning of 1995.

A new perspective on the dynamical link between the stratosphere and troposphere

Atmospheric processes of tropospheric origin can perturb the stratosphere, but direct feedback in the opposite direction is usually assumed to be negligible, despite the tropospheres sensitivity to changes in the release of wave activity into the stratosphere. Here, however, we present evidence that such a feedback exists and can be significant. We find that if the wintertime Arctic polar stratospheric vortex is distorted, either by waves propagating upward from the troposphere or by eastward-travelling stratospheric waves,, then there is a concomitant redistribution of stratospheric potential vorticity which induces perturbations in keymeteorological fields in the upper troposphere. The feedback is large despite the much greater mass of the troposphere: it can account for up to half of the geopotential height anomaly at thetropopause. Although the relative strength ofthefeedback is partly due to a cancellation between contributions to these anomalies from lower altitudes, our results imply that stratospheric dynamics and its feedback on the troposphere are more significant for climate modelling and data assimilation than was previously assumed.

Feasibility of determining surface emissions of trace gases using an inverse method in a three-dimensional chemical transport model

We investigate the feasibility of using an inverse method based on a linear Kalman filter to determine regional surface fluxes through comparisons between observations and predictions in a three-dimensional atmospheric transport model. The ability of the present ALE/GAGE observation sites to quantify the regional fluxes of anthropogenic trace gases is studied also. These investigations are done in the low-resolution spectral model of Golombek and Prinn (1986) using CFCl3 as the test tracer since its sources are relatively well known. The first of these investigations is done with the transport model being perfect in the sense that the “observations” were produced by running the model with the CFCl3 emissions derived from industry data. The inverse method used is capable in this case of accurately determining regional surface fluxes using the present ALE/GAGE sites and to converge to the correct solution within a year or two even using initial conditions very different from the final solution. We also investigate how well the Kalman filter approach works with a less than perfect chemistry circulation model by using the ALE/GAGE observations Of CFCl3 for the inversion. The success of this inversion depends largely on the ability of the model circulation to predict observed concentrations of CFCl3 since its chemistry is reasonably well understood. The larger the difference between the model and the observed values using the real (industry) emissions then the larger the bias will be in the estimated emissions. Such studies can help to understand the inherent biases in the model when used in an inverse scheme before trying to use the model to estimate unknown surface fluxes such as those for methane, nitrous oxide, and carbon dioxide. We also investigate where additional observational stations could be placed to enhance the capability of the present ALE/GAGE network for determining regional net fluxes. If we are prepared to accept the circulation in the spectral low-resolution model as sufficiently realistic, it appears that Hateruma (24N, 123E) and to a lesser extent Kamchatka (51N, 156E) are very promising locations for new stations and that Hateruma is superior to the ALE/GAGE Oregon station in providing information about Asian sources. This type of analysis done with a realistic circulation model can aid the process of choosing observation sites by addressing how well each site contributes to the different goals for use of the data.

Examination of tracer transport in the NCAR CCM2 by comparison of CFCl3 simulations with ALE/GAGE observations

The latest version of the National Center for Atmospheric Research (NCAR) community climate model (CCM2) contains a semi-Lagrangian tracer transport scheme for the purpose of advecting water vapor and for including chemistry in the climate model. One way to diagnose the CCM2 transport is to simulate CFCl3 in the CCM2 since it has a well-known industry-based source distribution and a photochemical sink and to compare the model results to Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment ALE/GAGE observations around the globe. In this paper we focus on this comparison and discuss the synoptic scale issues of tracer transport where appropriate. We compare the model and observations on both 12-hour and monthly timescales. The higher-frequency events allow us to diagnose the synoptic scale transport in the CCM2 associated with the observational sites and to determine uncertainties in our high-resolution source distribution. We find that the CCM2 does simulate many of the key features such as pollution events and some seasonal transports, but there are still some dynamical features of tracer transport such as the storm track dynamics and cross-equatorial flow that merit further study in both the model and the real atmosphere.

On using inverse methods for resolving emissions with large spatial inhomogeneities

We apply an inverse method to estimate the carbon monoxide emissions in Atlanta, Georgia. The resultant carbon monoxide inventory is unrealistically characterized by temporal oscillations on the frequency of 1 hour. We suspect that the difficulty in deducing the emissions is due to inhomogeneities in the spatial distribution of the emissions. In several controlled experiments we reproduce the oscillations by introducing relatively small errors into the spatial distribution of the emission inventory. In similar experiments with isoprene in which the emissions are more homogeneous, we do not find this problem. These results are discussed in the context of previous inverse studies of carbon monoxide and isoprene.

Optimizing an inverse method to deduce time‐varying emissions of trace gases

In previous work, an inverse method based on the Kalman filter was used to deduce regional emissions for chlorofluorocarbons (CFC-11) in a global chemical transport model [Hartley, 1992; Hartley and Prinn, 1993]. CFC-11 has reasonably constant emissions over the years we addressed; however, most trace gases with poorly constrained global budgets (i.e. CO2, CH4, and N2O) have seasonally varying sources and sinks. The goal of this work is to explore various adaptations that exist in Kalman filter theory and to identify the optimum method for deducing time-varying sources/sinks. To make this study feasible in both computer time and cost, we utilize a simplified atmospheric chemical transport model to investigate the methodology. We test many available adaptations in Kalman filter theory and conclude that the most accurate method is to use an adaptive-iterative approach [Young, 1984; Sastri, 1985; Bellgardt et al., 1986].

Inverse modeling of biogenic isoprene emissions

Using current biogenic emission estimates, Urban Airshed Model simulations for Atlanta, Georgia substantially underpredict isoprene concentrations relative to observations. In this work, an inverse method is used to determine the biogenic isoprene emissions for Atlanta that minimize the difference between the model-simulated and the observed isoprene concentrations. The resulting isoprene emissions are 2 to 10 times higher than any of the accepted emission estimates. Overall, the concurrent ozone simulation improved when the biogenic isoprene emissions were increased. These higher isoprene emissions significantly increase the concentration of model-simulated ozone within the plumes of nitrogen oxides emitted from large point sources. These results should be considered when developing control strategies for urban ozone.