4.1.4. Surface and Tropospheric Ozone

The surface in situ ozone measurement program continued to operate in a configuration similar to previous years with some changes. Aging instrumentation continues to create data gaps at some of the stations, particularly the tropical sites at the Samoa Observatory, American Samoa (SMO) and at Barbados. After 2 years of intermittent performance, the SMO program is fully functional. A TEI Model 49 ozone monitor was installed in September 1997 and has been running in parallel to the Dasibi Model 1003AH monitor that has been running there for many years. At Barbados attempts to restart the program have not been successful and data are not available after May 1995. Intercomparisons with the network standard instrument were carried out in 1997 for all four of the CMDL baseline site ozone monitors. Based on the calibrations and reanalysis of earlier calibrations, revised surface ozone data for the Barrow Observatory, Barrow, Alaska (BRW), MLO, and the South Pole Observatory, Antarctica (SPO) are presented in Table 4.5 for 1992-1997, which updates a similar table in Hofmann et al. [1996].

TABLE 4.5. Monthly Mean Surface Ozone Mixing Ratios (ppbv)

In February 1997, in cooperation with the New Zealand antarctic program and the Lauder Observatory of the National Institute for Water and Atmosphere (NIWA), an ozone monitor was installed at Arrival Heights near Scott Base and McMurdo station on the coast of Antarctica. We hope to use this information to better understand several years of earlier data obtained by other investigators. Records of the calibration of the instruments used to obtain much of this data are not available or non-existent. In addition, there is renewed interest in the role of halogens in destroying ozone in the lower troposphere in a manner analogous to what is seen in the arctic.

Starting in June 1997 an ozone monitor was installed at a camp on the Greenland plateau. This station is to be operated for about 10 months with the primary purpose of understanding deposition to the snow surface. This will be the first time such a camp has been operated during the winter on the plateau. The ozone observations will add to the 2 months of data obtained in Greenland from late May to July 1993.

We have recently analyzed data from a number of the longer-term surface ozone observations and tropospheric information from selected ozonesonde sites [Oltmans et al., 1998] to develop a fairly comprehensive picture of tropospheric ozone changes. Since 1970 significant increases were found in Europe and Japan, and at the South Pole ozone has declined markedly. Since 1980 there have been relatively small changes at all sites except in the antarctic and arctic where significant declines are evident. At the three CDML sites with long-term records and data through 1997 (Figure 4.2), several of these features are evident. At BRW ozone mixing ratios were increasing, especially during the summer, through the 1980s. Very low values in 1992-1993 following the eruption of Mt. Pinatubo seem to reflect the very reduced amounts seen in the lower stratosphere. These reduced amounts were seen through the entire tropospheric column in the Canadian arctic ozonesonde data [Tarasick et al., 1995; Oltmans et al., 1998]. Over the entire period of the record, ozone has increased about 3% per decade with the largest increases during the summer (Table 4.6). For 1980-1997 there has been no change. Ozone con-centrations at MLO have also risen significantly since the early 1970s (Table 4.6) but have shown no growth since 1980. This result is affected somewhat by a couple of years of high amounts in the early 1980s. Four out of the last 5 years have had above-average concentrations (Figure 4.2). By contrast at SPO ozone amounts have declined by over 15% since the measurements began in 1975 (Table 4.6). This decline has taken place since the mid-1980s and seems to have paralleled the declines in the lower stratosphere [Oltmans et al., 1998].

TABLE 4.6. Annual and Seasonal Linear Trends of Surface Ozone Mixing Ratio in Percent per Year with 95% Confidence Limits

BRW: latitude, 71.3°N; longitude, 156.6°W; elevation, 10 m

MLO: latitude, 19.5°N; longitude, 155.6°W; elevation, 3380 m

SMO: latitude, 14.3°S; longitude, 170.6°W; elevation, 30 m

SPO: latitude, 90.0°S; elevation; elevation, 2838 m

Fig. 4.2. Annual mean surface ozone mixing ratios at BRW, MLO, and SPO. The solid line is a linear regression fit to the monthly mean values. Numerical trend information is in Table 4.6.

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