Ozone Trends

Most research reports on UV radiation effects assume continued depletion of ozone, (e.g., Ono et al., 2005). As recently as 2004, a report from the CDC alluded to prospects for additional increases in UV-B as a result of ozone depletion (Saraiya et al., 2004), although, in doing so, they cited Diffey (1991) and Koh et al. (1993), rather than more recent work on stratospheric ozone trends.

Ground level measurements of total column ozone (TOC, includes ozone in the lower atmosphere, the troposphere, and the stratosphere—the major part of total column ozone) are made at about 30 locations In the U.S., with UltraViolet MultiFilter Rotating-Shadowband Radiometer (UV-MFRSR) instruments (Slusser et al., 1999; Gao et al., 2001), which have the advantage of automatically measuring total column ozone under most cloud conditions. Some of these measurements were recorded over the course of a decade, which is sufficient for predicting ozone trends. An examination of operational ozone measurements by UV-MFRSRs at Beltsville and Queenstown, MD since 1999 (data from the USDA UV-B Monitoring and Research Program, http://uvb.nrel.colostate.edu/UVB/index.jsf) indicates that there are cycles of ± 10% in the differences in TOC (depth of all ozone in the atmosphere if it could be brought together in a layer, measured in Dobson Units (DU) = 0.001 cm) measured at the two UV-MFRSR sites, but that long-term averages between the ozone records are similar. The ratio of the Beltsville UV-MFRSR ozone to Brewer (Brewer Spectrophotometer, an instrument to measure TOC, SO2, and UV spectra) measurements of ozone at the National Atmospheric and Space Administration, Goddard Space Flight Center (NASA/GSFC) at nearby Greenbelt, MD (data from Alexander Cede and Gordon Labow) is 0.996 over about four years.

A cursory examination of the TOC records from the Earth Probe Total Ozone Mapping Spectrometer (EPTOMS) satellite for Washington, DC, the UV-MFRSR

sensors at Beltsville and Queenstown, MD, the Brewer sensor at GSFC, and the recent Ozone Monitoring Instrument (OMI, which continues the TOMS satellite ozone measurements) ozone measurements (data from Gordon Labow) suggests that ozone in this locale tended to level off between 2000 and 2005. This conclusion is reached even after making allowance for the observed error in TOMS ozone since 2002 (Gordon Labow, pers. comm.), the cycles in the UV-MFRSR measurements, and the preliminary nature of the OMI measurements. One impression from examining TOC records over a period of years is that variability is large over short periods, and that there is a great need for continued ozone and UV monitoring, improvement of monitoring instrumentation and systems, and characterization of extreme ozone and UV events. Grant and Slusser (2005) point out the importance of extreme ozone events for crops research; the same no doubt applies to human health.

The 1998 United Nations Environment Programme assessment of ozone depletion estimated that since the 1970s, the northern temperate regions have experienced increases in erythemal UV radiation of approximately 4% in summer and fall, and 7% in winter and spring (Madronich et al., 1998). In the Southern hemisphere, the corresponding increase was about 6% year-round. The 1998 assessment indicated that international agreements to limit emissions of ozone depleting substances were showing evidence of success in reducing most, but not all, types of the depleting chemicals, and it suggested that a turnaround in ozone depletion and high levels of UV-B radiation might begin about 2000. By 2003, there was some evidence that the trend of ozone reductions was at least slowing (Newchurch et al., 2003; Malanca et al., 2005). While these studies show a slowing trend in ozone reductions, they do not show ozone increases. The return to the ozone and UV-B conditions that existed prior to the 1980s is apparently still decades away (Weatherhead et al., 2000; McKenzie et al., 2003; World Meteorological Organization (WMO), 2003; Weatherhead and Andersen, 2006). The 2006 WMO assessment estimates that global (60°S - 60°N) ozone will return to pre-1980 levels around the middle of the 21st century, at or before the time when stratospheric abundances of ozone-depleting gases return to pre-1980 levels (WMO, 2007).

The possible effect of greenhouse gases and global temperature change on stratospheric ozone causes additional uncertainty in the recovery process; greenhouse gases may speed ozone recovery because the gases lead to cooling of the stratosphere, which reduces the efficiency of the catalytic ozone destruction processes (WMO, 2007; Dyominov and Zadorozhny, 2008). However, periods of unusually low stratospheric ozone at mid-latitudes correspond with the intrusion of sub-tropical air masses (Bojkov and Balis, 2001; Siani et al., 2002), which further underscores the need for continued monitoring. Climate change will also influence surface UV radiation through changes in cloud formations and the ability of the earth's surface to reflect light (WMO, 2007).

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