Ozone and UV Radiation

G. Bernhard¹, G. Manney^2,3, V. Fioletov⁴, J.-U. Grooß⁵,
A. Heikkilä⁶, B. Johnsen⁷, T. Koskela⁶, K. Lakkala⁸, R. Müller⁵,
C. Lund Myhre⁹, M. Rex¹⁰

¹Biospherical Instruments, San Diego, CA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
³New Mexico Institute of Mining and Technology, Socorro, NM
⁴Environment Canada, Toronto, Ontario, Canada
⁵Forschungszentrum Jülich, Jülich, Germany
⁶Finnish Meteorological Institute, Helsinki, Finland
⁷Norwegian Radiation Protection Authority, Østerås, Norway
⁸Finnish Meteorological Institute, Arctic Research Centre, Sodankylä, Finland
⁹Norwegian Institute for Air Research, Kjeller, Norway
¹⁰Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany

November 23, 2011

Highlights

• Ozone concentrations in the Arctic stratosphere during March 2011 were the lowest ever recorded during the period beginning in 1979; they were 6% below the previous record-low observed in 2000.
• The low levels of total ozone in March and April of 2011 led to elevated UV levels throughout the Arctic and sub-Arctic.

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Introduction

Ozone molecules in the Earth's atmosphere greatly attenuate the part of the Sun's ultraviolet (UV) radiation that is harmful to life. Reductions in the atmospheric ozone amount will always lead to increased UV levels, but other factors such as the height of the Sun above the horizon play an important role. This chapter discusses the large ozone loss observed over the Arctic in the spring of 2011 and the resulting increase in UV radiation. UV radiation at some Arctic locations spiked by more than 100% relative to the long-term mean; however, absolute changes remained modest because the ozone loss occurred early in spring when the Sun was still low in the sky.

Ozone

Stratospheric ozone loss in spring 2011 reached a level never observed before in the northern hemisphere (Schiermeier, 2011, Manney et al., 2011). The minimum total ozone column¹ for March 2011, averaged over the "equivalent latitude²" band 63°-90° N, was 297 Dobson Units (DU³). The previous record-low was 315 DU in 2000 (Fig. A8). The ozone loss in spring 2011 was comparable to that observed during the annually-recurring "ozone hole" over the Antarctic. The record loss was mostly caused by chemical destruction of ozone, attributed to the existing stratospheric burden of ozone-depleting halogens and favored by an unusually prolonged cold period in the lower stratosphere in 2011. These low temperatures facilitate the formation of polar stratospheric clouds (PSC), which provide surfaces for heterogeneous reactions that activate stratospheric chlorine. The activated chlorine, in turn, destroys ozone rapidly in catalytic cycles.

Fig. A8. Time series of minimum total ozone over for March in the Arctic, calculated as the minimum of daily average column ozone poleward of 63° equivalent latitude. Winters in which the vortex broke up before March (1987, 1999, 2001, 2006, and 2009) are shown as circles. Polar ozone in those years was relatively high because of mixing with air from lower latitudes. Figure adapted from Müller et al. (2008), updated using the combined total column ozone database of the New Zealand National Institute of Water and Atmospheric Research (NIWA), Version 2.7, provided by Hamish Chisholm of NIWA and Greg Bodeker of Bodeker Scientific.

The extraordinary situation in 2011 is further illustrated in Fig. A9, which compares satellite measurements of total ozone column on 3 April 1981 (a year with a long-lasting and cold Arctic vortex, and relatively low stratospheric chlorine concentrations), 3 April 2002 (long-lasting warm vortex, high total chlorine loading), and 3 April 2011 (long-lasting cold vortex, high chlorine). The figure emphasizes that chemical ozone loss resulting from chlorine activation is most effective in years such as 2011, when there was a long-lasting, cold vortex.

Fig. A9. Comparison of total ozone column measured by satellites on 3 April 1981, 2002 and 2011. Data are from the Total Ozone Mapping Spectrometer (TOMS) onboard the Nimbus-7 (1981) and Earth Probe (2002) satellites, and the Ozone Monitoring Instrument (OMI) onboard of the AURA spacecraft (2011).

The fraction of the Arctic vortex with total ozone below 275 DU is typically near zero for March, but reached nearly 45% in March 2011 (Fig. A10). Minimum total ozone in spring 2011 was continuously below 250 DU for about 27 days, with a maximum area below that level of about two million square kilometers. Values between 220 and 230 DU were reached for about one week in late March 2011.

Fig. A10. Fraction of vortex area with total ozone below 275 Dobson units (DU) in February-April in the Arctic (August-October in the Antarctic). Light gray shading shows the range of Arctic values for 1979-2010. Dark gray shading shows the range of Antarctic values for the same period. Because Antarctic ozone is so much lower, only the bottom of the range is apparent here; that is, the dark grey shading begins at the minimum value for that area in any of the years. Red, purple, blue and green lines show the 2010/2011, 1996/1997, 2004/2005 and 2007/2008 Arctic winters, respectively. 2005-2011 values are from Ozone Monitoring Instrument (OMI); earlier values are from the Total Ozone Mapping Spectrometer (TOMS) instruments, available at http://ozoneaq.gsfc.nasa.gov/.

Many aspects of the 2011 Arctic ozone depletion event resemble those of the annually reoccurring ozone hole over Antarctica. For example, ozone profiles in late March 2011 look more like typical Antarctic late-winter profiles than average Arctic profiles (Fig. A11).

Fig. A11. Comparison of vertical ozone profiles. Red: ozone profiles for a 4°×15° latitude×longitude box around 79°N, 12°E measured on 26 March 2011 by the Earth Observing System Microwave Limb Sounder (MLS); lavender: the MLS 7-day average for 2005-2010 centered on the same location and date; grey: average MLS profiles in a similar box around 79°S, 12°E measured on 26 September; black: a high-resolution ozonesonde profile measured at Ny Ålesund (79°N, 12°E) on 26-March 2011.

The low total ozone in March 2011 resulted primarily from chemical loss (Manney et al., 2011). Chemical destruction was severe between 16 and 22 km, with the largest loss exceeding 2.5 ppmv (parts per million by volume) by March 26. From about 18 to 20 km, over 80% of the ozone present in January had been chemically destroyed by late March (Manney et al., 2011).

Chemical destruction was facilitated by an unusually strong, cold and long-lasting stratospheric polar vortex. During February and March 2011, the vortex was the strongest throughout the lower stratosphere in either hemisphere in the last 30 years (Manney et al., 2011). The strong vortex led to the existence of PSCs over extended areas. Temperatures below the threshold temperature for PSC formation of about 196 K (-77°C) existed between December 2010 and early April 2011 (Fig. A12). Temperatures allowing PSC formation after early March were observed only once before (in 1997).

Fig. A12. Minimum temperatures at approximately 18 km altitude poleward of 40° latitude. Line colors and shading are as in Figure A10. The threshold temperature of about 196 K below which PSCs can exist is indicated by the horizontal line. Data are from the Goddard Earth Observing System Version 5.2.0 MERRA reanalyses.

PSCs are important because these clouds provide surfaces for heterogeneous conversion of chlorine into reactive (ozone-destroying) forms such as chlorine monoxide (ClO). ClO enhancement in March 2011 vastly exceeded the range from all previous Arctic winters (Manney et al., 2011). The volume of air, V_PSC, with temperatures below the PSC occurrence threshold is a good indicator of the potential for chlorine activation and ozone loss (Rex et al., 2004). V_PSC reached a value of 48 million km³ in 2011 (Fig. A13), similar to the previous records of 2000 and 2005. Since the polar vortex was smaller in 2011 compared to those years, the fraction of vortex air that had been exposed to PSC conditions was larger in 2011 than in any other year shown in Fig. A13. This, and the persistence of low temperatures, and hence the occurrence PSCs, into April 2011 (Fig. A12), led to much more chemical ozone loss in 2011.

Fig. A13. Evolution of the volume of air with temperatures below the PSC formation threshold (VPSC) for the Arctic. Data are from the European Centre for Medium-Range Weather Forecasts (ECMWF; solid black line) and Free University of Berlin (FUB; dashed line) data. Blue dots represent the maximum values of VPSC during five-year intervals. The gray shading represents the uncertainty of VPSC assuming a 1 K uncertainty in the long-term stability of radiosonde temperatures. Updated from Figure 3 of Rex et al. (2004).

UV Radiation

The low levels of total ozone in March and April of 2011 led to elevated UV levels throughout the Arctic and sub-Arctic, as shown in Fig. A14 and explained in more detail below. Note that the UV Index is a measure of the ability of UV radiation to cause erythema (sunburn) in human skin. It is calculated by weighting UV spectra with the CIE action spectrum for erythema (McKinlay and Diffey, 1987) and multiplying the result by 40 m²/W.

Fig. A14. Seasonal variation of the noontime UV Index for 12 Arctic and sub-Arctic sites measured by ground-based radiometers. The upper panel for each site compares the climatological average (blue line) with the measurements in 2010 (green dots) and 2011 (red dots), and historical minima and maxima (shaded range). The latter were calculated from measurements of the periods indicated in the top-right corner of the panel. The center panel shows the anomaly in the UV Index, calculated as the percentage departure from the climatological average. The numbers indicate the maximum anomalies for March and April 2011. The bottom panel shows a similar anomaly analysis for total ozone derived from measurements of the following satellites: TOMS/Nimbus7 (1991-1992), TOMS/Meteor3 (1993-1994), TOMS/EarthProbe (1996-2004), and OMI (2005-2011). The shaded range for the ozone data set is based on data for the years 1991-2009 (1996-2009 for Trondheim and Finse). Ozone data are available at http://toms.gsfc.nasa.gov and http://ozoneaq.gsfc.nasa.gov/. Vertical broken lines indicate the times of the vernal and autumnal equinoxes, and the summer solstice. (Enlarged figure in PDF)

In addition to atmospheric ozone concentrations, UV radiation is affected by the height of the Sun above the horizon (the solar elevation), clouds, aerosols (liquid and solid particles suspended in air), the reflectivity of the surface (high, when snow or ice covered), and other factors (Weatherhead et al., 2005). The main driver of the annual cycle is the solar elevation. Sites closest to the North Pole (Alert, Eureka and Ny-Alesund in Fig. A14) have the smallest peak radiation. Clouds lead to a large variability in UV levels on time scales from minutes to days, but their effect is largely reduced when the ground is covered by fresh snow. (Bernhard et al. 2008). Measurements at Barrow, and to a lesser extent at Alert and Eureka, show a large asymmetry between spring (low variability) and fall (high variability) because the surface at these sites is covered by snow until about June and free of snow thereafter until the beginning of winter. In particular, during summer and fall the variability introduced by clouds is substantially larger than that related to ozone variations (compare shaded ranges in center and bottom panels of Fig. A14).

The abnormally low stratospheric ozone concentrations in the spring 2011 led to substantially elevated UV Indices at all sites shown in Fig. A14. Changes in the UV Index anti-correlate with changes in total ozone (compare 2011 data in center and bottom panels of Fig. A14). Noontime UV Indices of March 2011 exceeded historical measurements for this month at all sites. Enhancements of the UV Index relative to the climatological average (the "UV anomaly" shown in the center panels of Fig. A14) were most pronounced at Andoya (122%), Jokioinen (120%) and Oesteraas (107%). The relative enhancement at the northernmost sites is somewhat smaller because changes in UV are less sensitive to changes in ozone when the solar elevation is small (Douglas et al., 2011). At all sites, the relative increase in the UV Index is outside the range defined by measurements of earlier years, and at many sites the anomaly is outside the envelope defined by the variability introduced by clouds. While these large relative increases are unprecedented, the absolute increases in UV levels were modest at all sites because the low-ozone event occurred early in spring when the solar elevation was still small. For example, at Andoya, the large relative increase of 122% changed the UV Index from 0.8 (average for 28-March) to 1.9. This value is less than half as large as the peak UV Index of 5.1 measured in July at this site. Although UV Indices below 2 are considered low (WHO, 2002), people involved in certain outdoor activities may receive higher-than-expected UV doses if their faces and eyes are oriented perpendicular to the low Sun or if they are exposed to UV radiation reflected off snow.

Larger absolute increases of UV Indices occurred at lower latitudes during excursions of the polar vortex in April. For example, on April 22, the clear-sky UV Index over parts of Mongolia (48°N, 98°E) estimated by TEMIS (Tropospheric Emission Monitoring Internet Service (TEMIS) at http://www.temis.nl/uvradiation/UVindex.html) was 8.6 when a lobe of the vortex extended to central Asia. The long-term average for this day at this location is 5.4 with a 1-σ standard deviation of 0.5, i.e., the anomaly was more than six standard deviations larger than the climatological mean. A similar situation occurred in central Europe on April 17 when a tongue of the vortex extended over the Alps. The noontime UV Index at Arosa, Switzerland (46.8°N, 9.7°E), was 7.4 on that day, which is four standard deviations larger than the long-term average of 5.3.

Figure A14 also highlights measurements made in 2010. In March 2010, UV Indices were abnormally small due to larger-than-normal stratospheric ozone concentrations in that year. The variability in the second half of 2010 was within the range of previous years.

¹Total ozone column is the height of a hypothetical layer which would result if all ozone molecules in a vertical column above the Earth's surface were brought to standard pressure (1013.25 hPa) and temperature (273.15 K).

²Equivalent latitude is a latitude-like coordinate aligned with the polar vortex (Butchart and Remsberg, 1986).

³Dobson Unit, the standard unit for measuring the total ozone column. 1 DU equals a column height of 0.01 mm and corresponds to
2.69×10¹⁶ molecules / cm².

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