The atmosphere experienced a period of relatively low CO2 growth during 1992-1993, despite continuing emissions of CO2 from the burning of fossil fuels. This was followed by two years of above average growth. The most likely explanation is that the rate of respiration of plants and soils was somewhat lower than normal on the continents, which were cooler in the aftermath of the Pinatubo eruption due to resulting stratospheric aerosol. This diminished respiration allowed the buildup of slightly more young and fairly labile organic material, which decayed in the following years. This interpretation is strengthened by the time history of the 13C/12C isotopic ratio of CO2. The manufacture of organic material during photosynthesis is accompanied by isotopic fractionation. Photosynthate contains slightly less 13C than expected from the atmospheric composition. This leaves the CO2 remaining in the atmosphere slightly enriched in 13C.
The atmospheric 13C/12C ratio is expected to decrease every year because of the continuing addition of fossil fuel carbon depleted in 13C. In 1992-1993 the atmospheric 13C/12C did not decrease, and during 1994-1995 the 13C/12C decrease was more rapid than usual. Therefore, the CO2 and 13C/12C data are consistent with the net removal of a large amount of CO2 into organic material in 1992-1993, which was largely delivered back to the atmosphere in the following two years. The atmospheric CO2 growth rate during 1996-1997 was close to the average experienced during the decade of the 1980s.
The figure shows the rate of global CO2 production from fossil fuels, and the rate of CO2 uptake by the oceans and the terrestrial biosphere combined. The difference between the two curves equals the rate of atmospheric increase. The dashed curve is a 5-year moving average.
Results of the analysis of total column ozone data from the NOAA/CMDL Cooperative Dobson Observing Network were published in Geophysical Research Letters (Komhyr et al.,1997) Total column ozone at five U.S. mainland stations for the period 1979-1996 showed a significant decline of 3.3%/decade with the largest decreases of nearly 5%/decade taking place during spring months. At the tropical northern hemisphere site at Mauna Loa, a small, statistically insignificant ozone increase was noted while in the southern hemisphere a small but significant annual decline was detected. In Antarctica, ozone column amounts have declined in all seasons but losses of nearly 20%/decade in the October 15 – November 30 portion of the year, when the Antarctic ozone hole forms, led to the annual decline of 12%/decade.
The ground-based Dobson network provides a unique perspective on column ozone behavior prior to 1979 and the advent of regular satellite observations. Based on observations back to 1963 at several continental U.S. sites, ozone at the end of 1996 was 6.7% lower than in the mid-1960s. At South Pole for the month of October, which has had the most dramatic ozone loss, column ozone during the 1990s was about one-half of what it was in the 1960s.
Updated data through 1997 from the NOAA/CMDL Cooperative Dobson Network played a key role in the 1998 Ozone Assessment where the surface based observations were crucial in determining the global ozone trends. The NOAA/CMDL 16 station network represents about 20% of the global surface based measurement sites.
Reference: Komhyr, W.D., G.C. Reinsel, R.D. Evans, D.M. Quincy, R.D. Grass, and R.K. Leonard, Total ozone trends at sixteen NOAA/CMDL and cooperative Dobson spectrophotometer observatories during 1979-1996, Geophys. Res. Lett., 24, 3225-3228, 1997.
Methyl bromide (CH3Br) is of interest because of its involvement in the depletion of stratospheric ozone. Unlike the chlorofluorocarbons, which are entirely anthropogenic, CH3Br has both natural and anthropogenic sources. At approximately 10 parts per trillion in the troposphere, CH3Br is the single largest contributor of stratospheric bromine, which is about 50 times more effective in depleting stratospheric ozone than chlorine. Sinks for atmospheric CH3Br, once thought to include only photochemical, atmospheric reactions, now include photolysis, reaction with OH, uptake by the oceans, and loss to soils. One focus of study at CMDL in recent years has been the removal of atmospheric CH3Br by the oceans. Initial two-box model calculations showed that, owing to aquatic chemical degradation, the ocean was a significant sink for atmospheric CH3Br, regardless of its degree of saturation in seawater [Butler, J.H. Geophysical Research Letters 21, 185-188, 1994]. A global, ocean-atmosphere model later was developed to study the uptake of CH3Br by the oceans. Use of this model lowered the estimated lifetime of atmospheric CH3Br and significantly reduced the uncertainties in its partial atmospheric lifetime with respect to oceanic uptake [Yvon and Butler, Geophysical Research Letters, 23, 53-56, 1996]. Results also demonstrated that the distribution of oceanic degradation is important in calculating the partial atmospheric lifetime of a trace gas with respect to oceanic loss and, consequently, its total atmospheric lifetime (Red represents fast and blue represents slow in the above figure). Prompted by NOAA/CMDL findings in the Southern Ocean [Lobert, J.M. et al., Geophysical Research. Letter,. 24, 171-172 1997], as well as findings by others working in different parts of the ocean, that biological processes also contribute to the overall removal of CH3Br in seawater, we built a biological term into the model to determine the additional effect of oceanic biological degradation on the lifetime of atmospheric CH3Br [Yvon-Lewis and Butler, Geophysical Research Letters, 24, 1227-1230, 1997]. The atmospheric lifetime, once put at around 2 years, now stands at 0.7 (range of 0.5 to 1.2) years, with about 40% of the loss going to the ocean. Because the atmospheric lifetime of a compound is proportional to its ozone depletion potential (ODP), the lower atmospheric lifetime represents a lower, calculated ODP. The value determined here is that used in the 1998 WMO/UNEP Scientific Assessment of Ozone Depletion.
An important paper [Wamsley et al., Journal of Geophysical Research, 103, 1513-1526, 1998] was recently published that used ground-based trends and stratospheric observations of brominated compounds to describe the bromine loading of the stratosphere. Many of the stratospheric observations were made from the Airborne Chromatograph for Atmospheric Trace Species instrument (ACATS-IV) operated on the NASA ER-2 high-altitude aircraft. The instrument is the result of a collaborative effort between NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL) and Aeronomy Laboratory (AL). Bromine abundances are plotted in the figure as a function of stratospheric mixing ratios of chlorofluorocarbon-11 (CFC-11, CCl3F, used here as a proxy for altitude) and the mean age of sampled air parcels. Stratospheric mean age is determined from ACATS-IV observations of sulfur hexafluoride (SF6) and the SF6 trend obtained from the ground-based CMDL network. Total bromine (BrTOT, blue) in panel (a) is defined as the sum of bromine of all organic species that are present in the upper tropospheric air parcel at the time of entry into the tropical stratosphere. The increase in BrTOT with mean age demonstrates that bromine is still increasing throughout the atmosphere. Although production of the halons and CFCs in 1996 has ceased in developed countries under the auspices of the Montreal Protocol, atmospheric bromine loading continues to increase because of the release of stored halons in developed countries and the use of halons in developing countries. Total organic bromine (CBry, red) in the stratosphere is estimated from the ACATS-IV measurements of a major halon, H-1211 (CBrClF2), and modeled correlations of the unmeasured organic bromine species with CFC-11. The inorganic bromine (Bry) reservoir, containing the species BrO, BrONO2, Br, and HOBr, is produced by the destruction of CBry species in the stratosphere. The abundance of Bry (green) shown in panel (b) estimated as the difference of BrTOT (blue) and CBry (red), increases with mean stratospheric age. Many of the Bry species, along with reactive chlorine species, participate in the catalytic destruction of ozone throughout the stratosphere. Independent estimates for Bry (gold diamonds) based on observations of BrO made by scientists from Harvard University and model results for the remaining inorganic species in panel (b) show on average higher levels of Bry in the stratosphere. This discrepancy could be the result of larger uncertainties in the Harvard results (about ±50%), or an additional organic or inorganic compound entering the stratosphere from the troposphere. Thus, in contrast to the organic and inorganic budgets of chlorine which are in balance and slowly decreasing, the bromine budgets are still increasing in the atmosphere and may be out of balance. Understanding the bromine budget is essential because bromine is about 50 times more effective than chlorine in destroying stratospheric ozone. The results of this study are included in the recent 1998 World Meteorological Organization (WMO) United Nations Environmental Program (UNEP) Scientific Assessment of Ozone Depletion.
In a recent paper published in Geophysical Research Letters (Oltmans et al., 1998) we used a geographically disbursed set of surface ozone and ozone vertical profile measurements to document long-term ozone changes in the troposphere. In mid-latitudes of the northern hemisphere, where the influence of human-produced ozone precursor gasses is likely to have had the largest impact, there are significant regional differences. Over Europe ozone has increased dramatically throughout the troposphere (~15%/decade) over the past 25 years. In Japan an increase of nearly 10%/decade has occurred over this period, but this has taken place primarily in the lowest part of the troposphere. Over North America, on the other hand, the changes are small. This variation is somewhat surprising since it is expected that in all of these regions precursor emissions rose during the past 25 years. Most of the increase in Europe and Japan took place prior to the mid-1980s, with much smaller increases since then.
Although there are fewer data in other regions, several characteristics emerge. In tropical regions changes are generally small but at Mauna Loa, Hawaii there has been an increase of 3.5%/decade, but again with most of the change occurring before the early 1980s. At mid-latitudes of the southern hemisphere the records beginning in the early 1980s suggest a small but significant increase which is found primarily during the time of year when biomass burning in southern Africa and South America is most pronounced (July - October). The polar regions are unique in showing significant declines in tropospheric ozone amounts. Ozonesonde data from Canadian sites and the surface ozone record at South Pole show downward trends of about 7%/decade. The decline at sites in the polar regions appears to be linked to the significant decrease that has occurred in the lower stratosphere of these regions.
These results were used in both the 1998 WMO/UNEP Ozone Assessment and the recently published WMO/SPARC Assessment of the Trends in the Vertical Distribution of Ozone.
Reference: Oltmans, S.J. and 17 others, Trends of ozone in the troposphere, Geophys. Res. Lett., 25, 139-142, 1998.