Integrated Ozone in Dobson Units

Figure 1 (see color insert) Climntological distribution of tropospheric ozone derived from satellite measurements between 1979 and 2000 (from Fishman et al, 2002). Units of contours and Dobson Units (DU). Regions greater than 40 DU have been shaded. See ftp site for color image.

4 TROPOSPHERfC OZONE TRENDS IN NONURBAN TROPOSPHERE

4 TROPOSPHERfC OZONE TRENDS IN NONURBAN TROPOSPHERE

Figure 2 (a) (Upper left): monthly mean surface ozone at Barrow and the linear trend tor the entire data record. (Lower left): monthly mean surface ozone at Mauna Loa with the linear trend. (Upper right) annual mean ozone concentrations at Montsouris Observatory outside Paris (1876-1910) and Arkona. East Germany (1956-1984). The average ozone concentration at the beginning of the twentieth century near Paris was less than lOppb whereas in 1985 die typical ground-level concentrations in central Europe is approaching 3(1 ppb. implying an increase of about 200% during the century (from Volz and Kiev, 1988), (Lower right): ozone trend off coast of South America determined from analysis of satellite measurements of total ozone (from Jiang and Yung, 1996). See ftp site for color image.

Figure 2 (a) (Upper left): monthly mean surface ozone at Barrow and the linear trend tor the entire data record. (Lower left): monthly mean surface ozone at Mauna Loa with the linear trend. (Upper right) annual mean ozone concentrations at Montsouris Observatory outside Paris (1876-1910) and Arkona. East Germany (1956-1984). The average ozone concentration at the beginning of the twentieth century near Paris was less than lOppb whereas in 1985 die typical ground-level concentrations in central Europe is approaching 3(1 ppb. implying an increase of about 200% during the century (from Volz and Kiev, 1988), (Lower right): ozone trend off coast of South America determined from analysis of satellite measurements of total ozone (from Jiang and Yung, 1996). See ftp site for color image.

Northern Hemisphere

Barrow, Alaska

Whlteface Mountain, New York

Zugspitze, Germany

Barrow, Alaska

Zugspitze, Germany

Izana, Canary Islands

83 85 87 89 91 93 95

83 85 87 89 91 93 95

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Whlteface Mountain, New York

Izana, Canary Islands

35 30

75 77 79 81 83 85 87 Year

Southern Hemisphere

83 85 87 89 91 93 95

83 85 87 89 91 93 95

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91 93 95

American Samoa

25 20

ce 10

Cape Point, South Africa

75 77 79 81 83 85 87 89 91 93 95 Cape Grlm, Australia 41 °S

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Year

Year

Figure 2 (b) Annual average ozone mixing ratios (ppbv) for surface ozone measuring sites. The dashed line is the long-term average. The solid line is the linear least-squares fit to the plotted values. The linear trend and 95% confidence in percent per year is given with each location (from Oltmans et al„ 1998).

show that the long-term trend has a strong seasonal dependence; the increase during the summer is 1.73% per year whereas there is almost no trend (—0.07% per year) during the winter. Figure 2b summarizes a number of long-term measurements from the NOAA network as well as a few other stations where comparable data exists in the background atmosphere. Curiously, some stations such as American Samoa near the equator show a slight negative trend, whereas a significant negative trend exists at South Pole. The reason for these trend differences at these remote sites is not clear and is currently being studied.

Modern studies have reexamined the Schonbein paper ozone measurements from the late nineteenth century and early twentieth century to determine tropospheric ozone trends over longer time periods. These studies have carefully examined calibration procedures used last century and have determined that a significant increase in tropospheric ozone has occurred over the past century.

5 GLOBAL TROPOSPHERIC OZONE BUDGET 55

More than three decades of measurements using Schonbein's technique were obtained at the Montsouris Observatory outside Paris. The instrument used at this meteorological station was recalibrated and the observations were converted to standard units of measurement consistent with modern measurements. The results from this data set are compared with modern observations obtained in Germany and depicted in the upper right panel of Figure 2a. This and other analyses strongly suggest that ozone at the surface has risen from — 10 ppbv to more than 30ppbv in nonurban Europe and the eastern United States. Although ozone at the surface has likely increased significantly on the time scales of years and decades since the inception of the industrial era, tropospheric measurements above the surface are extremely scarce and difficult to interpret because of the different methods of measurement used since the 1960s. Most of the measurements are from ozonesondes (an ozone sensor placed on a balloon), but several types of sensors have been used and each type is susceptible to interference from other trace gases in the atmosphere. Despite the uncertainty in the measurements, it is generally believed that ozone has increased throughout the entire troposphere since the 1960s, when ozonesonde measurements started on a fairly regular basis.

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