Antarctic Ozone Bulletin

Transcription

1 Antarctic Ozone Bulletin No 1 / Altitude [km] Temperature [ C] Neumayer 15 August Ozone Temperature Ozone sounding on 15 August from the German NDACC/GAW station Neumayer (70.65 S, 8.26 W). The sounding programme is carried out by the Alfred Wegener Institute for Polar and Marine Research. Data have been provided by Claudia Rudolph and Gert König-Langlo Ozone partial pressure [mpa] 18 G l o b a l A t m o s p h e r e Wa t c h 23 Aug.

2 Executive Summary During the April-late June time period, 50 hpa temperatures averaged over the S region have been close to, or somewhat below, the average. From late June until late July this mean temperature was above the average. Since late July, the mean temperature has been oscillating around the mean. During the last few weeks, the 50 hpa temperature in the S region has been approx. 2 K warmer than at the same time last year. Minimum temperatures in the vortex at 50 hpa are quite similar to those of 2006, but at 10 hpa the August minimum temperatures are lower this year than last year. Since mid-june, temperatures low enough for nitric acid trihydrate (NAT or PSC type I) formation have covered about 70-75% of the vortex area. This is less than in 2006, when the NAT area in late July corresponded to 90% of the vortex area. Since the onset of NAT temperatures in mid-may, the NAT area was larger than the average until late June. From early July to mid-july the NAT area was lower than the average, but after that, the NAT area has again been somewhat above the long-term mean. Compared to the four most recent winters, the NAT area so far in is relatively small. Until early July, the NAT area behaved quite similarly to these recent winters, but after that the NAT area has been lower than for any of the winters in the time period. The size of the vortex at the 460 K isentropic level has been higher than the average since early May. On certain days in July and August the vortex has been larger than the maximum for the period. It should be pointed out, however, that vortex size gives no direct indication of the degree of ozone loss that might occur later in the season. The longitudinally averaged heat flux between 45 S and 75 S is an indication of a disturbed stratosphere. From April to the end of June, the heat flux was close to or below the average. In July, the heat flux increased a lot and was, on some days, larger than the maximum. This is a sign of an unstable vortex. During August the heat flux has become smaller and is now approaching the average for the season. At the altitude of ~18 km the vortex is now almost entirely depleted of HCl, one of the reservoir gases that can be transformed to active chlorine. In the sunlit collar along the vortex edge there is ppb of active chlorine (ClO), and some first signs of ozone depletion is visible. The south polar vortex is less concentric in than in 2006, and this has led to a relatively early onset of ozone depletion. As the sun returns to Antarctica after the polar night, it is expected that ozone destruction will speed up. It is still too early to give a definitive statement about the development of this year's ozone hole and the degree of ozone loss that will occur. This will, to a large extent, depend on the meteorological conditions. The small NAT area observed so far could indicate that the ozone hole will be relatively small. WMO and the scientific community will use ozone observations from the ground, from balloons and from satellites together with meteorological data to keep a close eye on the development during the coming weeks and months.

3 Introduction The meteorological conditions in the Antarctic stratosphere found during the austral winter (June-August) set the stage for the annually recurring ozone hole. Low temperatures lead to the formation of clouds in the stratosphere, so-called polar stratospheric clouds (PSCs). The amount of water vapour in the stratosphere is very low, only 5 out of one million air molecules are water molecules. This means that under normal conditions there are no clouds in the stratosphere. However, when the temperature drops below -78 C, clouds that consist of a mixture of water and nitric acid start to form. These clouds are called PSCs of type I. On the surface of particles in the cloud, chemical reactions occur that transform passive and innocuous halogen compounds (e.g. HCl and HBr) into so-called active chlorine and bromine species (e.g. ClO and BrO). These active forms of chlorine and bromine cause rapid ozone loss in sun-lit conditions through catalytic cycles where one molecule of ClO can destroy thousands of ozone molecules before it is passivated through the reaction with nitrogen dioxide (NO 2 ). When temperatures drop below -85 C, clouds that consist of pure water ice will form. These ice clouds are called PSCs of type II. Particles in both cloud types can grow so large that they no longer float in the air but fall out of the stratosphere. In doing so they bring nitric acid with them. Nitric acid is a reservoir that liberates NO 2 under sunlit conditions. If NO 2 is physically removed from the stratosphere (a process called denitrification), active chlorine and bromine can destroy many more ozone molecules before they are passivated. The formation of ice clouds will lead to more severe ozone loss than that caused by PSC type I alone since halogen species are more effectively activated on the surfaces of the larger ice particles. The Antarctic polar vortex is a large low-pressure system where high velocity winds (polar jet) in the stratosphere circle the Antarctic continent. Figure 1 depicts the vortex on 19 August. The region poleward of the polar jet includes the lowest temperatures and the largest ozone losses that occur anywhere in the world. During early August, information on meteorological parameters and measurements from ground stations, balloon sondes and satellites of ozone and other constituents can provide some insight into the development of the polar vortex and hence the ozone hole later in the season. The situation with annually recurring Antarctic ozone holes is expected to continue as long as the stratosphere contains an excess of ozone depleting substances. As stated in the recently published Executive Summary of the 2006 edition of the WMO/UNEP Scientific Assessment of Ozone Depletion, severe Antarctic ozone holes are expected to form during the next couple of decades. For more information on the Antarctic ozone hole and ozone loss in general the reader is referred to the WMO ozone web page: ozone/index.html.

4 ECMWF Analysis of PV (10-6 Km 2 /kgs) Θ = 500 K 19 Aug 12 UT Day number 231 Figure 1. Polar orthographic map of potential vorticity at the potential temperature level of 500 K (ca. 20 km) over the south polar region for 19 August. The polar vortex is less concentric than at the same time in The plot is based on data from the European Centre for Medium range Weather Forecasts (ECMWF). Data extraction and plotting is done at the Norwegian Institute for Air Research (NILU). Plotted at NILU by t106glob

5 Meteorological conditions Temperatures Meteorological data from the National Center for Environmental Prediction (NCEP) in Maryland, USA, show that stratospheric temperatures over Antarctica have been below the PSC type I threshold of -78 C since mid May and below the PSC type II threshold of 85 C since early June, as shown in Figure 2. This figure also shows that the daily minimum temperatures at the 50 hpa level have been close to (mainly below) the average. The development of the minimum temperatures in is quite similar to the development in Data from NCEP, made available through the Ozonewatch web page of NASA (see section on Acknowledgements and links at the end of the Bulletin), show that during the Aprillate June time period, 50 hpa temperatures averaged over the S region have been close to, or somewhat below, the average. From late June until late July this mean temperature was above the long-term average. Since late July, this polar cap mean temperature has been oscillating around the mean. During the last few weeks, the 50 hpa temperature in the S region has been approx. 2 K warmer than at the same time last year. A similar development is also seen at the 30 and 70 hpa levels. Temperature [K] HNO 3 = 6 ppbv, H 2 O = 4 ppmv S Minimum Temperature 50 hpa Type I PSC Type II PSC % 30-70% Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 2. Time series of daily minimum temperatures at the 50 hpa isobaric level south of 50 S. The red curve shows (until 20 August). The blue line shows The average of the period is shown for comparison in black. The thin black lines represent the highest and lowest daily minimum temperatures in the time period. The light blue-green shaded area represents the 10th and 90th percentile values and the dark blue-green shaded area the 30th and 70th percentiles. The two horizontal green lines at 195 and 188 K show the thresholds for formation of PSCs of type I and type II, respectively. The plot is adapted from a plot downloaded from the Ozonewatch web site at NASA and based on data from NOAA/NCEP.

6 Meteorological conditions The mean temperature at 50 hpa in the S region was well below the average in May and June, but has been warmer than average on most days since early July. The development is similar at the 30 and 70 hpa levels, but at 30 hpa the temperatures are close to the average. At the 10 hpa level the temperature conditions are colder than the average and since early August minimum temperatures at 10 hpa are among the coldest recorded since 1979 and colder than in PSC Area and volume Since mid-june, temperatures low enough for nitric acid trihydrate (NAT or PSC type I) formation have covered an area of more than 20 million square kilometres, or about 70-75% of the vortex area. This is less than in 2006 when the NAT area in late July corresponded to approx. 90% of the vortex area. The temporal development of the NAT area is shown in Figure 3. Since the onset of NAT temperatures in mid-may the NAT area was larger than the average until late June. From early July to mid-july the NAT Area [10 6 km 2 ] Southern Hemisphere PSC NAT Area at Θ = 460 K % 30-70% Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 3. Time series of the area where temperatures are low enough for the formation of nitric acid trihydrate (NAT or PSCs of type I) at the 460 K isentropic level. This corresponds to an altitude of approximately 18 km. The red curve shows (until 20 August). The blue, green, orange and magenta curves represent 2006, 2005, 2004 and 2003, respectively. The average of the period is shown for comparison in black. The two thin black lines show the maximum and minimum PSC area during the time period for each date. The light blue-green shaded area represents the 10th and 90th percentile values and the dark blue-green shaded area the 30th and 70th percentiles. The plot is adapted from a plot downloaded from the Ozonewatch web site at NASA and based on data from NOAA/NCEP.

7 Meteorological conditions area was lower than the average, but after that, the NAT area has been somewhat above the long-term mean. Compared to the four most recent winters, the NAT area so far in is relatively small. Until early July, the NAT area behaved quite similarly to these recent winters, but after that the NAT area has been lower than for any of the winters in the time period. Rather than looking at the NAT area at one discrete level of the atmosphere it makes more sense to look at the volume of air with temperatures low enough for NAT formation. The so-called NAT volume is derived by integrating the NAT areas over a range of input levels. The daily progression of the NAT volume in is shown in Figure 4 in comparison to recent winters and long-term statistics. Also here it can be seen that the development during the early part of the winter was similar to recent winters, but since early July the NAT volume has been low compared to the winters. However, with the exception of a period in the first half of July, the NAT volume has been significantly larger Volume [10 6 km 3 ] Southern Hemisphere PSC NAT Volume % 30-70% Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 4. Time series of the volume of the region where temperatures are low enough for the formation of nitric acid trihydrate (NAT or PSCs of type I). The red curve shows (until 20 August). The blue, green, orange and magenta curves represent 2006, 2005, 2004 and 2003, respectively. The average of the period is shown for comparison in black. The two thin black lines show the maximum and minimum PSC area during the time period for each date. The light blue-green shaded area represents the 10th and 90th percentile values and the dark blue-green shaded area the 30th and 70th percentiles. The plot is adapted from plots downloaded from the Ozonewatch web site at NASA and based on data from NOAA/NCEP.

8 Meteorological conditions than the average, and the winter is so far among the 30% coldest. The area or volume with temperatures low enough for the existence of PSCs is directly linked to the amount of ozone loss that will occur later in the season, but the degree of ozone loss depends also on other factors, such as the amount of water vapour and HNO 3. Based upon the historical meteorological record it is expected that the extent and frequency of PSC occurrence will level off and begin to decrease now as the sun rises over Antarctica, whereas the vortex will gradually increase in size throughout most of August. Vortex size and stability Figure 1 shows a map of potential vorticity at the 500 K potential temperature level (~ 20 km). This picture indicates how isolated the polar air mass is from air masses outside the polar vortex. Yellow, orange and red colours depict regions where the air is particularly well isolated from the surroundings. Presently the vortex is less circular than at the same time last year. This has already led to an earlier onset of ozone depletion than last year. Figure 5 (next page) shows the daily geographical extent of the south polar vortex at the isentropic level of 460 K (~ 17 km) and it can be seen that the size of the vortex has been higher than the average since early May. On certain days in July and August the vortex has been larger than the maximum for the period. It should be pointed out, however, that vortex size gives no direct indication of the degree of ozone loss that might occur later in the season. The longitudinally averaged heat flux between 45 S and 75 S is an indication of how much the stratosphere is disturbed. From April to the end of June, the heat flux was close to or below the average. In July, the heat flux increased a lot and was, on some days, larger than the maximum. This is a sign of an unstable vortex. During August the heat flux has become smaller and is now approaching the average for the season. An updated plot of the heat flux can be found here: ftp://hyperion.gsfc.nasa. gov/pub/ftpmet/nmcdata/annual/vt45_75-45s_100_.gif

9 Meteorological conditions 60 Southern Hemisphere Vortex Area at 460K Vortex area [10 6 km 2 ] % 30-70% 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 5. Time series of the area of the south polar vortex at the isentropic level of 460 K (~17 km). The vortex is defined as the region inside the steepest gradient in potential vorticity. The red curve shows (until 20 August). The blue curve shows The average of the period is shown for comparison in black. The two thin black lines show the maximum and minimum vortex area during the time period for each date. The plot is adapted from a plot downloaded from the Ozonewatch web site at NASA and based on data from NOAA/NCEP.

10 Ozone observations Satellite observations Most of Antarctica still remains under winter darkness, so the average rate of ozone loss there remains relatively low. However, satellite data show that the depletion has started along the vortex edge. Figure 6 shows minimum ozone columns as measured by the SCIAMACHY instrument on board ENVISAT in comparison with data for the nine previ- ous years (SCIAMACHY and GOME). It can be seen that ozone depletion has started. Around the middle of August the minimum columns were lower than for any of the previous nine years for that time of the year. The last few days the minimum columns have been close to the average of the previous nine years. Ozone depletion has started earlier in than in 2006, but it should be pointed out that the depletion set in relatively late in 2006 due to a concentric vortex, something that led to less exposure to sunlight. Dobson Units Minimum Ozone Column in the Southern Hemisphere GOME / SCIAMACHY Assimilated Ozone KNMI / ESA 22 Aug Aug 01 Sep 01 Oct 01 Nov 01 Dec Dec Figure 6. Daily minimum total ozone columns in the Southern H e m i s p h e r e as observed by GOME, and SCIAMA- CHY. The red dots show the SCIAMACHY observations for. The data now show minimum ozone columns down to 175 DU. The plot is provided by the Netherlands Meteorological Institute (KNMI). 10

11 Ozone observations Figure 7 shows minimum ozone as measured with the OMI instrument on board the AURA satellite. These data confirm the findings from the SCIAMACHY measurements: Record low minimum ozone in mid August and near average minimum ozone around 20 August. Figure 8 (next page) shows satellite maps from OMI for 19 August for the years 2005, 2006 and. It can be seen that 2005 had more extensive ozone depletion on this date compared to 2006 and. On 19 August, there is a region in the Atlantic sector, off the coast of Antarctica, that has lower ozone than the rest of the vortex. Total Ozone [DU] Southern Hemisphere Minimum Ozone from OMI % Figure 7. Daily minimum total ozone columns in the Southern Hemisphere as observed by OMI and TOMS. The red curve show the OMI observations for. In mid August minimum ozone columns dropped to below 150 DU, which is the lowest ever for this time of the year. The plot is adapted from a plot downloaded from NASA s ozone watch web site % Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 11

12 Ozone observations 19 August August August Total ozone [Dobson Units] Figure 8. Total ozone map for 19 August 2005, 2006 and based on data from OMI on board the AURA satellite. The data are processed and mapped at NASA. 12

13 Ozone observations Balloon observations Neumayer Total ozone columns deduced from soundings carried out at the German NDACC/GAW station at Neumayer (70.65 S, 8.26 W) show some first signs of a decline, but substantial ozone loss is not observed yet. Total ozone is still above 200 DU. As seen from the satellite maps in Figure 8, the ozone depletion has so far occurred further north, where there is more sunlight to drive the ozone depleting reactions. South Pole Figure 10 shows the km partial column derived from ozonesonde observations carried out at the NDACC/GAW station on the South Pole. As of 16 August there are no signs of ozone depletion. Data from earlier years show that depletion sets in around late August/early September Total ozone from integrated ozonesonde profiles Neumayer South Pole km partial ozone column Integrated ozone [DU] May Jun Jul Aug Sep Oct Nov Dec Figure 9. Total ozone at the German NDACC/GAW station Neumayer (70.65 S, 8.26 W) as derived by integrating ozonesonde profiles. The total ozone values are taken from the provided data files. As the data might be recalculated, these values might change. The data should therefore be considered as preliminary. Figure 10. Partial column of ozone in the km height range for the NDACC/GAW station at the South Pole. The brown diamonds represent (until 16 August) and a selection of previous winters are shown for comparison. The km range has been chosen since this is the altitude range where ozone depletion is the most severe. 13

14 Chemical activation of the vortex Satellite observations The south polar vortex is now activated and primed for ozone depletion. As soon as the sun comes back after the polar winter, ozone depletion will set in. Figure 11 shows the extent of removal of hydrochloric acid (HCl), which is one of the reservoirs for active chlorine, for 15 August. As can be seen from the figure, HCl is almost completely removed inside the vortex at the 490 K isentropic level. Removal of HCl is an indicator for chemical activation of the vortex. Another indicator for vortex activation is the amount of chlorine monoxide (ClO). It should be noted, however, that ClO dimerises and forms (ClO) 2 in darkness. The dimer is easily cracked in the presence of sunlight. ClO will therefore be present in the sunlit parts of the vortex, whereas the dark areas will be filled with (ClO) 2, which is not observed by Aura-MLS. Figure 12 (next page) shows the amount of ClO on 15 August. One can see an area of elevated ClO that forms a collar along the vortex edge. This collar constitutes the sunlit part of the vortex. The sector between 0 and 60ºE has particularly high ClO, with certain areas exceeding 1.65 ppb. Figure 13 (next page) shows the ozone mixing ratio on the 490 K isentropic level on 15 August. No substantial ozone depletion can be seen yet, but one can discern regions around 65-70ºS, where there is somewhat less ozone than in surrounding areas. As the sun rises and the days get longer, the daily ozone loss rates will increase. HCl ppbv Aug Figure 11. Mixing ratio of HCl at the isentropic level of 490 K (~18 km). The white contours indicate isolines of scaled potential vorticity. The map is made at NASA's Jet Propulsion Laboratory and based on data from the Aura-MLS satellite instrument. 14

15 Chemical activation of the vortex ClO 15 Aug O 3 15 Aug black SZA=94 o ppbv ppmv Figure 12. Mixing ratio of ClO at the isentropic level of 490 K (~18 km). The white contours indicate isolines of scaled potential vorticity. The black contour line encircles the region where the solar zenith angle (SZA) is larger than 94º. The map is made at NASA's Jet Propulsion Laboratory and based on data from the Aura-MLS satellite instrument. Figure 13. Mixing ratio of ozone at the isentropic level of 490 K (~18 km). The white contours indicate isolines of scaled potential vorticity. The map is made at NASA's Jet Propulsion Laboratory and based on data from the Aura-MLS satellite instrument. 15

16 Ozone hole area and mass deficit The region where total ozone is less than 220 DU (ozone hole area) as deduced from the OMI instrument on AURA is shown in Figure 14. The area increased rapidly during the first half of August and reached a temporary maximum around 15 August. Figure 15 (next page) shows the ozone mass deficit as deduced from the GOME and SCIAMACHY satellite instruments. On 22 August, the mass deficit was close to 5 Megatonnes and similar to what was observed in 2000 and in Ozone Hole Area from TOMS/OMI Area [10 6 km 2 ] % 30-70% 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 14. Area (millions of km 2 ) where the total ozone column is less than 220 Dobson units. is showed in red (until 19 August) is shown in blue. The smooth black line is the average. The dark green-blue shaded area represents the 30th to 70th percentiles and the light green-blue shaded area represents the 10th and 90th percentiles for the time period The plot is adapted from a plot downloaded from the NASA Ozonewatch web site and is based on data from the OMI instrument on AURA and various TOMS instruments. 16

17 Ozone hole area and mass deficit Ozone mass deficit [megatonnes] Ozone Loss w.r.t. 220 DU in the Southern Hemisphere GOME / SCIAMACHY Assimilated Ozone 2000 KNMI / ESA Aug Aug 01 Sep 01 Oct 01 Nov 01 Dec 31 Dec Figure 15. Ozone mass deficit (megatonnes) for the years from 1998 to (black dots). The mass deficit is the amount of ozone that would have to be added to the ozone hole in order to bring the total column up to 220 DU in those regions where the total column is below this threshold. This plot is produced by KNMI and is based on data from the GOME and SCIAMACHY satellite instruments. 17

18 UV radiation UV radiation is measured by various networks covering the southern tip of South America and Antarctica. There are stations in Southern Chile (Punta Arenas), southern Argentina (Ushuaia) and in Antarctica (Belgrano, Marambio, Mc- Murdo, Palmer, South Pole). No station has yet measured a UV index superior to 2 so far during the current season. As the sun rises in the sky and ozone gets depleted, higher UV indices are expected. Links to sites with data and graphs on UV data are found in the Acknowledgements and Links section at the end of the Bulletin. Distribution of the bulletins The Secretariat of the World Meteorological Organization (WMO) distributes Bulletins providing current Antarctic ozone hole conditions beginning around 20 August of each year. The Bulletins are available through the Global Atmosphere Watch programme web page at wmo.int/pages/prog/arep/gaw/ozone/index.html. In addition to the National Meteorological Services, the information in these Bulletins is made available to the national bodies representing their countries with UNEP and that support or implement the Vienna Convention for the Protection of the Ozone Layer and its Montreal Protocol. Acknowledgements and links These Bulletins use provisional data from the WMO Global Atmosphere Watch (GAW) stations operated within or near Antarctica by: Argentina (Comodoro Rivadavia, San Martin, Ushuaia), Argentina/Finland (Marambio), Argentina/Italy/Spain (Belgrano), Australia (Macquarie Island and Davis), China/Australia (Zhong Shan), France (Dumont D Urville and Kerguelen Is), Germany (Neumayer), Japan (Syowa), New Zealand (Arrival Heights), Russia (Mirny and Novolazarevskaja), Ukraine (Vernadsky), UK (Halley, Rothera), Uruguay (Salto) and USA (McMurdo, South Pole). More detailed information on these sites can be found at the GAWSIS web site ( Satellite ozone data are provided by NASA ( NOAA/TOVS ( noaa.gov/products/stratosphere/tovsto/), NOAA/SBUV/2 ( and ESA/Sciamachy ( Satellite data on ozone, ClO, HCl and a number of other relevant parameters from the MLS instrument on the Aura satellite can be found here: Potential vorticity and temperature data are provided by the European Centre for Medium Range Weather Forecasts (ECMWF) and their daily T 106 meteorological fields are analysed and mapped by the Norwegian Institute for Air Research (NILU) Kjeller, Norway, to provide vortex extent, PSC area and extreme temperature information. Meteorological data from the US National Center for Environmental Prediction (NCEP) are also used to assess the extent of PSC temperatures and the size of the polar vortex ( shtml). NCEP meteorological analyses and climatological data for a number of parameters of relevance to ozone depletion can also be acquired through the Ozonewatch web site at NASA ( 18

19 Acknowledgements and links Ozone data analyses and maps are prepared by the World Ozone and UV Data Centre at Environment Canada ( by the Royal Netherlands Meteorological Institute ( nl/protocols/o3global.html) and by the University of Bremen ( UV data are provided by the U.S. National Science Foundation s (NSF) UV Monitoring Network ( UV indices based on the SCIAMACHY instrument on Envisat can be found here: Ultraviolet radiation data from the Dirección Meteorológica de Chile can be found here: Data on ozone and UV radiation from the Antarctic Network of NILU-UV radiometers can be found here: The Executive Summary of the 2006 WMO/UNEP Scientific Assessment of Ozone Depletion can be found here: Questions regarding the scientific content of this Bulletin should be addressed to Geir O. Braathen, mailto:gbraathen@wmo.int, tel: The next Antarctic Ozone Bulletin is planned for 6 September. 19