ATM 507 Meeting 6 Text reading Section 5.7 Problem Set # 4 due Oct. 11 Today s topics Polar Stratospheric Chemistry and the Ozone Hole, Global Ozone Trends Required reading: 20 Questions and Answers from the 2006 WMO report on Ozone Depletion: ftp://ftp.nilu.no/pub/nilu/geir/assessment-2006/10%20q&aschapter.pdf http://www.esrl.noaa.gov/csd/assessments/ozone/2006/chapters/twentyquestions.pdf 1
Ozone Depletion over South Pole Balloon borne ozone profiles measured at South Pole: blue is the average of several profiles measured in September and October during 1967-1971 before the Antarctic ozone hole; red is on the day of the maximum ozone loss in 2001; green is the lowest total ozone recorded in 1986, the first year of CMDL's sounding program at the South Pole. Total column ozone is given in Dobson Units (DU) for each of the profiles. 2
Airplane missions ER-2 range up to ~ 20 km. Real-time measurements of O 3, NO, NO 2, NO y, ClO, BrO, N 2 O, H 2 O, CN (particles), and more. DC-8 Flew at lower altitudes but included some additional instruments like FT interferometer, LIDAR, and whole air sampler. ER-2 measurements provided the smoking gun. 3
Smoking Gun Measurements Outside vortex Inside vortex Outside vortex Inside vortex 4
What the measurements show Huge increases of active chlorine inside the vortex (100-500 times previously observed mid-latitude values at these altitudes). In late August, ozone inside and outside the vortex are essentially the same. Boundary of the vortex moves around some, and can get a little ragged. After 3 weeks of reaction time, ozone is down by a factor of two (or more) inside the vortex only. At the same time active chlorine (ClO) is up another 50% inside the vortex. Note the very strong anti-correlation between O 3 and ClO near the edge of the vortex on the second flight. 5
Why so different than the mid-latitudes? This phenomenon was a huge surprise to the scientific community. As it turns out, this was the first example of significant perturbation of atmospheric chemistry due to heterogeneous chemistry, in this case the chemistry occurring on/in ice clouds. For our purposes: Homogeneous chemistry: gas phase reactions only; Heterogeneous chemistry: gas/solid, gas/liquid, and/or gas/liquid/solid phase reactions all considered. Current understanding of polar ozone depletion centers around Polar Stratospheric Clouds or PSCs. Since there is very little H 2 O in the stratosphere (a few ppmv), very low temperatures are needed to form PSCs. 6
Recap Reservoir species and holding cycle reactions tend to keep the stratospheric ozone in balance. There is potential for large ozone destruction if the balance is disrupted. The disruption can occur if there are large unchecked emissions of the source gas in one chemical family (i.e., CFCs); or large removal of the radicals in one chemical family (i.e., NO x removal). Antarctic Ozone Hole appears around 1980. Polar night, polar votex, cold temperatures Spring time only (return of sun triggers loss) Occurs in 12 22 km altitude range New chemistry required to explain What is the link with Polar Stratospheric Clouds? 7
PSC Formation (Polar Stratospheric Clouds) In the Antarctic polar night the temperatures do routinely go below 190 K (and stay there). This is cold enough to force condensation. Step 1. As temperatures drop below 220-230 K, water condenses on the existing background aerosol particles, which are predominantly concentrated H 2 SO 4 droplets (~75% H 2 SO 4, ~25% H 2 O). As temperatures drop further, these particles take up additional water, become larger and more dilute. (Recall, mr of water in the stratosphere is ~ 3-5 ppmv, so VERY cold temperatures needed to condense the vapor.) 8
Simple Picture of PSC Formation 9
PSC Formation (cont.) Step 2. As the temperature approaches 195 K, small crystals of HNO 3 3(H 2 O) {NAT} form. These are called Type I PSCs. (size ~ 1 µm diameter) Step 3. As the temperature decreases below 190 K, crystals get larger and more dilute, with diameters of 10 µm and larger. These Type II PSCs can settle out of the stratosphere (due to gravity), removing NO x and H 2 O from the surrounding air. The Denitrification and Dehydration provide the strong perturbation required to disrupt the standard chemistry, i.e., the ozone balance. 10
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Temperature and PSC Formation Temperature (white bars) and PSC observations (black bars) For the given temperature bin, ratio of number of PSC observations to number of temperature observations (i.e., for low enough temperatures, PSCs are common.) 12
Polar Stratospheric Cloud, seen from the NASA DC-8 on 4 February 2003 (photograph by Mark Schoeberl, GSFC). This cloud was formed due to turbulent air motion over Iceland (mountain waves). Lidar measurements revealed the primary cloud was composed of ice (Type 2 PSC). The photograph was taken just at sunrise for the lower atmosphere. Under these conditions, the polar stratospheric cloud is illuminated directly by the sun while the lower atmosphere is still shaded from the 13 direct sunlight, accounting for the brilliant appearance of this cloud.
Polar Stratospheric Cloud, seen from the NASA DC-8 on 14 January 2003 (photograph by Paul Newman, GSFC). These are naturally occurring clouds composed of small nitric acid and water particles. The clouds have a colorful appearance because they contain similar sized, small particles each of which refracts sunlight in a similar manner. This image is notable for illustrating the high altitude (approximately 70,000 ft) of the polar stratospheric clouds, well above the cirrus cloud deck that lies below the DC-8 airplane. Reactions that occur on the surface of these clouds convert chlorine, most of which is due to human sources, from relatively inert forms to compounds that lead to rapid ozone destruction. 14
PSC Processing of Air 1. Most of the HNO 3 (the reservoir for NO x radicals) is tied up in PSC ice crystals and removed from the air mass. 2. HCl (one of the reservoirs for ClO X ) is also taken up by the ice crystals, but unlike HNO 3, it undergoes a chemical reaction before the crystals settle out: I. ClNO 3 + HCl(s) Cl 2 + HNO 3 (s) Note that ClNO 3 and HCl contain most of the Cl that has been liberated from the chlorine sources. 15
Unperturbed partitioning of chlorine species between sources, reservoirs, and active forms Note: ClONO 2 and ClNO 3 are the same thing. 16
Textbook Figure 5.14 Another picture of the unperturbed distribution of chlorine species in the stratosphere. Note the dominance of HCl and the importance of ClONO 2 (once the Cl is liberated from the CCl y source gases). 17
Chlorine liberation and Ozone destruction I. ClNO 3 + HCl(s) Cl 2 + HNO 3 (s) The ClNO 3 + HCl can proceed as long as the PSCs persist (days weeks). When sunlight returns to the region II. Cl 2 + hv Cl + Cl (very fast) III. 2[Cl + O 3 ClO + O 2 ] IV. 2[ClO + NO 2 + M ClNO 3 + M ] NET: HCl + 2 O 3 + 2 NO 2 ClNO 3 + 2O 2 + HNO 3 (s) These reactions (I IV) continue until either: HCl(s) is gone (used up) pre-1979 NO 2 is all converted (via ClNO 3 ) to HNO 3, at which point the ClO builds up and the Cl species are dramatically repartitioned. 18
Polar Ozone Chemistry (cont.) KEY IDEA: Inactive forms of Cl (HCl, ClNO 3 ) are transformed into active forms of Cl (Cl 2, then Cl and ClO) through PSC processing. In the simple reaction scheme shown above, if complete processing is assumed (i.e., reaction I fast), then large amounts of active chlorine (as ClO) are built up if: [HCl] i > ½ [NO X ] i Summarizing the necessary conditions: Denitrification required (to lower [NO x ]) [HCl] must be above some minimum level 19
Other heterogeneous reactions ClNO 3 + H 2 O(s) HOCl + HNO 3 (s) HOCl + HCl(s) Cl 2 + H 2 O(s) Ties up NO x Liberates active Cl Liberates active Cl N 2 O 5 + HCl(s) ClNO 2 + HNO 3 (s) N 2 O 5 + H 2 O(s) 2HNO 3 (s) Ties up NO x Liberates active Cl Ties up NO x These reactions also pave the way for chemical loss of O 3. 20
Chemical Loss of O 3 (review) ClO dimer mechanism ClO + ClO + M ClOOCl + M ClOOCl + hv Cl + ClOO ClOO + M Cl + O 2 + M 2[ Cl + O 3 ClO + O 2 ] NET: 2O 3 + hv 3O 2 ClO-BrO mechanism Cl + O 3 ClO + O 2 Br + O 3 BrO + O 2 BrO + ClO Br + Cl + O 2 * NET: 2O 3 3O 2 21
Contributions of ClO dimer and ClO-BrO cycles 22
Chlorine ozone hole chemistry (excluding Bromine) 23
Observed Ozone loss in the northern polar region 24
February 1989 ER-2 measurement campaign. Flight track from Norway to Greenland and back. Highly elevated ClO in PSC processed air concentration levels comparable to Antarctica. (Shorter lifetime and coverage of PSCs explains much lower total overall ozone loss.) 25
Recap 1. PSC formation esp. Type II with large particles that remove gaseous HNO 3. 2. Reactions on PSCs that remove NO x and liberate active ClO X. 3. Cl 2, HOCl, and ClNO 2 photolyze rapidly when the sun returns. 4. NO X buffers the release of active ClO X by forming ClNO 3, which in turn reacts with PSCs to form active ClO x and HNO 3. 5. When air is denitrified, active ClO x builds up and large scale O 3 depletion occurs via ClO + ClO, ClO + BrO, etc. 26
Schematic time series of chlorine and temperature in polar ozone depletion period. 27
Final comments Chemical models now include both gas phase and heterogeneous processes and do a reasonable job of describing polar ozone loss. ClO dimer mechanism accounts for up to 60-80% of the polar ozone loss ClO-BrO mechanism is the next biggest contributor There are outstanding problems (recall Stratospheric warming and 2002). Heterogeneous chemistry in the global stratosphere is an area of active research. Natural stratospheric aerosols Aircraft emissions Volcanoes 28
Q & A on Stratospheric Ozone 20 Questions and Answers from the 2006 WMO report on Ozone Depletion: ftp://ftp.nilu.no/pub/nilu/geir/assessment-2006/10%20q&aschapter.pdf Required reading this may show up on the midterm! 29
From the 1998 Q&A set. 30
Ozone Trend Analysis Measurements of the total ozone column from ground stations and from satellites have been analyzed for trends in ozone. Some of the confounding factors include: The annual cycle Quasi-biennial oscillation (QBO) El Nino Southern Oscillation (ENSO) The 11 year solar cycle Volcanic eruptions Indications are, that on a global scale, ozone had decreased about 3% from 1980-2000. Little change (slight increase?) since then. 31
Ground-Based Ozone Trend 32
Trend reversal? 33
Satellite Ozone Trend notice time range 34
Trend by Latitude and Season 35
Trends from Three Sets of Measurements 36
2006 Assessment Report 37
More sophisticated analysis from the 2006 Assessment EESC Effective Equivalent Stratospheric Chlorine 38
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Ozone Assessments Scientific Assessment of Ozone Depletion World Meteorological Organization (WMO) 2010:http://www.esrl.noaa.gov/csd/assessments/ozone/2010/ 2006: http://www.wmo.int/pages/prog/arep/gaw/ozone_2006/ozone_a sst_report.html 2002: http://www.wmo.int/pages/prog/arep/gaw/ozone_2002/ozone_2 002.html 1998: http://esrl.noaa.gov/csd/assessments/ 1994: 1991: 1989: Atmospheric Ozone: 1985 40
From the Ozone Assessments TOTAL COLUMN OZONE 2010: Average total ozone values in 2006-2009 have remained at the same level for the past decade, about 3.5% and 2.5% below the 1964-1980 averages respectively for 90 S-90 N and 60 S-60 N. 2010: The latitude dependence of simulated total column ozone trends generally agrees with that derived from measurements, showing large negative trends at Southern Hemisphere mid and high latitudes and Northern Hemisphere midlatitudes for the period of ODS (ozone depleting substances) increase. 41
POLAR OZONE (2010 Assessment) The Antarctic ozone hole continued to appear each spring from 2006 to 2009. This is expected because decreases in stratospheric chlorine and bromine have been moderate over the last few years. Analysis shows that since 1979 the abundance of total column ozone in the Antarctic ozone hole has evolved in a manner consistent with the time evolution of ODSs. Since about 1997 the ODS amounts have been nearly constant and the depth and magnitude of the ozone hole have been controlled by variations in temperature and dynamics. The October mean column ozone within the vortex has been about 40% below 1980 values for the past fifteen years. Arctic winter and spring ozone loss has varied between 2007 and 2010, but remained in a range comparable to the values that have prevailed since the early 1990s. Chemical loss of about 80% of the losses observed in the record cold winters of 1999/2000 and 2004/2005 has occurred in recent cold winters. Recent laboratory measurements of the chlorine monoxide dimer (ClOOCl) dissociation cross section and analyses of observations from aircraft and satellites have reaffirmed the fundamental understanding that polar springtime ozone depletion is caused primarily by the ClO + ClO catalytic ozone destruction cycle, with significant contributions from the BrO + ClO cycle. Polar stratospheric clouds (PSCs) over Antarctica occur more frequently in early June and less frequently in September than expected based on the previous satellite PSC climatology. This result is obtained from measurements by a new class of satellite instruments that provide daily vortex-wide information concerning PSC composition and occurrence in both hemispheres. The previous satellite PSC climatology was developed from solar occultation instruments that have limited daily coverage. Calculations constrained to match observed temperatures and halogen levels produce Antarctic ozone losses that are close to those derived from data. Without constraints, CCMs simulate many aspects of the Antarctic ozone hole, however they do not simultaneously produce the cold temperatures, isolation from middle latitudes, deep descent, and high amounts of halogens in the polar vortex. Furthermore, most CCMs underestimate the Arctic ozone loss that 42 is derived from observations, primarily because the simulated northern winter vortices are too warm.
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Measurements plus Projections 44
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Trends and Predictions Note the large dip in the early 1990 s this led to overly high projections of ozone loss until the 1998 assessment. The extra loss is thought to be due to the effects of the Pinatubo volcano. Recovery (timing and magnitude) depends on emissions scenarios. 46
Linkage of Ozone and UV Radiation Less ozone means more UV at the Earth s surface. Analysis of this trend is also very important. Effect is not linear, but for small changes in the ozone column it can be approximated as linear. 47
Relation between ozone and UV changes Dependence of erythemal ultraviolet (UV) radiation at the Earth's surface on atmospheric ozone, measured on cloud-free days at various locations, at fixed solar zenith angles. Legend: South Pole (Booth and Madronich, 1994); Mauna Loa, Hawaii (Bodhaine et al., 1997); Lauder, New Zealand (McKenzie et al., 1998); Thessaloniki, Greece (updated from Zerefos et al., 1997); Garmisch, Germany (Mayer et al., 1997); and Toronto, Canada (updated from Fioletov et al., 1997). Solid curve shows model prediction with a power rule using RAF = 1.10. 48
Radiation Amplification Factor (RAF) Radiation Amplification Factor (RAF) is defined as the percentage increase in UV bio that would result from a 1% decrease in the column amount of atmospheric ozone. The radiation amplification factors are given in http://sedac.ciesin.org/ozone/docs/unep98/tbl1_1.html for a number of different known effects. The RAFs can generally be used only to estimate effects of small ozone changes, e.g. of a few percent, because the relationship between ozone and UV bio becomes non-linear for larger ozone changes. 49
Trends in biologically active UV from TOMS (1979-1992) - increasing. Latitude band - degrees Trend (a) - % per decade Uncertainty (b) ±2 sigma 70 S 60 S 4.5 5.5 60 S 50 S 5.5 4.5 50 S 40 S 3.5 3.5 40 S 30 S 1.5 2.5 30 S 20 S 2.0 2.5 20 S 10 S 1.5 2.5 10 S 0 1.5 3.0 0 10 N 1.0 3.5 10 N 20 N 0.0 2.5 20 N 30 N 1.0 3.0 30 N 40 N 3.0 3.0 40 N 50 N 3.5 3.0 50 N 60 N 5.0 4.0 60 N 70 N 4.0 4.0 Trends in biologically active radiation (weighted with the erythemal action spectrum of McKinlay and Diffey, 1987), derived from total ozone and cloud reflectivity measurements from the Total Ozone Mapping Spectrometer (TOMS, version 7) over 1979-1992. Adapted from Herman et al. (1996). (a) Zonally averaged trend over given latitude band, values rounded to nearest half percent. (b) As corrected by Herman et al. (1998) and includes combined instrumental error and variability of 50 UV radiances.
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Also from Ozone Assessment Ground-based UV reconstructions and satellite UV retrievals, supported in the later years by direct ground-based UV measurements, show that erythemal ("sunburning") irradiance over midlatitudes has increased since the late 1970s, in qualitative agreement with the observed decrease in column ozone. The increase in satellite-derived erythemal irradiance over midlatitudes during 1979-2008 is statistically significant, while there are no significant changes in the tropics. Satellite estimates of UV are difficult to interpret over the polar regions. In the Antarctic, large ozone losses produce a clear increase in surface UV radiation. Ground-based measurements show that the average spring erythemal irradiance for 1990-2006 is up to 85% greater than the modeled irradiance for 1963-1980, depending on site. The Antarctic spring erythemal irradiance is approximately twice that measured in the Arctic for the same season. Clear-sky UV observations from unpolluted sites in midlatitudes show that since the late 1990s, UV irradiance levels have been approximately constant, consistent with ozone column observations over this period. Surface UV levels and trends have also been significantly influenced by clouds and aerosols, in addition to stratospheric ozone. Daily measurements under all atmospheric conditions at sites in Europe and Japan show that erythemal irradiance has continued to increase in recent years due to net reductions in the effects of clouds and aerosols. In contrast, in southern midlatitudes, zonal and annual average erythemal irradiance increases due to ozone decreases since 1979 have been offset by almost a half due to net increases in the effects of clouds and aerosols. 53
Chemical Models of the Stratosphere Concepts of atmospheric models (as applied to the lower atmosphere) are presented in Chapter 25 Models needed for understanding past and present; and for predicting future outcomes. Future outcomes depend on emission scenarios. Source gas emissions for the stratosphere are better understood than those for the troposphere. 54
Models (cont.) Models include 1. Reaction Rate Coefficients 2. Photolysis Rate Coefficients (these vary with solar flux, i.e., SZA, ozone, aerosol, albedo) 3. Initial conditions for species and meteorological quantities (p, T, [species], ) 4. Met forecast model or parameterization of transport (including fluxes of source gases to the stratosphere) 5. Treatment of heterogeneous chemistry and physics Types of calculations 1. Steady-State Calculation: Fixed inputs, run until stable outputs 2. Prognostic Calculation: Estimate changes in inputs (chemical, solar, transport) and run model to predict or project possible outcomes 55
Types of Models 0-D or box model: only variable is time 1-D model: Vary time and altitude (vertical transport is estimated using eddy diffusion ) 2-D model: vary time, altitude, and latitude (transport estimated with 2x2 matrix of transport coefficients) 3-D model: becoming more common, but resolution is quite coarse in many cases, and requires greater computer resources. Usually coupled with met model. 56
Ozone Depletion Potential Start with an Ozone Depletion Model Reference Species is CFCl 3 (CFC-11 or Freon 11). All other ozone depleting species (mostly chlorine and bromine containing compounds) are compared to CFC-11. Properties of CFC-11 (CFCl 3 ): 3 Cl atoms per molecule Unreactive in troposphere, so it is (eventually) transported to the stratosphere, where its lifetime with respect to photolysis is ~45 years It is photolyzed in the stratosphere to yield Cl atoms Chlorine catalyzed ozone destruction occurs in polar regions and in midlatitudes (different mechanisms) 57
Ozone Depletion Potential (cont.) BY DEFINITION ODP OF CFC-11 = 1. The ODP is a property of the species. All other species ODP s are referenced to the ODP of CFC-11. Consider HCFC-22 HCFC-22 = CHF 2 Cl (= X in our example) Properties of HCFC-22: 1 Cl atom per molecule Reacts slowly in the troposphere (chemical lifetime ~12 years) If it penetrates into the stratosphere, it can be photolyzed to release Cl and destroy O 3. 58
Methods of Calculating ODP Better (but computationally more difficult) method 1. Find the emission rate of CFC-11 (kg/yr) that results in a 1% O 3 depletion. Call this emission rate A. 2. Find the emission rate of compound X (kg/yr) that results in a 1% O 3 depletion. Call this emission rate B. 3. ODP of X = A/B. More practical (but less rigorous) method 1. Calculate ΔO 3 per unit mass emission rate of CFC- 11. Call this ΔO 3 A. 2. Calculate ΔO 3 per unit mass emission rate of compound X. Call this ΔO 3 B. 3. ODP of X = B /A. 59
GLOBALLY AVERAGED ODP Ozone Depletion Potential ODP = ΔO ΔO z z Θ Θ t t 3 3 ( z, Θ, t) ( z, Θ, t) for X for CFC 11 cosθ cosθ z = altitude, Θ = latitude, t = time ΔO 3 = change in O 3 at steady state (per unit mass emission rate) Examples of global ODP s HCFC -22 ODP =.06-.08 HCFC-142b (CH 3 CF 2 Cl) ODP =.08-.10 CH 3 CCl 3 ODP = 0.13 CF 3 Br (H-3101) ODP = 10-12 NOTE: Local ODP s vary greatly (e.g. Polar Spring) 60
Trends of Controlled Ozone Depleting Chemicals http://www.esrl.noaa.gov/gmd/hats/graphs/graphs.html 61
EECl = Effective Equivalent Chlorine (as Cl 2 ) 62
The factor of two difference in the scale on this slide compared to the last one comes about because Cl 2 contains two Cl atoms. http://www.esrl.noaa.gov/gmd/odgi/ 63