Northern Hemisphere ozone column minimum in Dobson Units 1980-2020 

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RELATED POSTS ON OZONE HOLES: [LINK] [LINK] [LINK] [LINK]  

THIS POST IS A CRITICAL REVIEW OF ALARMING REPORTS IN THE MEDIA THAT THE LARGEST OZONE HOLE EVER RECORDED HAS OPENED UP OVER THE NORTH POLE. IT IS PRESENTED IN THREE PARTS. PART-1 IS THE 2020 ARCTIC OZONE HOLE REPORT ON THE COPERNICUS WEBSITE. PART-2 IS ITS CRITICAL EVALUATION. 

ABSTRACT

OZONE HOLES ARE LOCALIZED AND EVANESCENT DECLINES IN POLAR OZONE LEVELS. THESE EVENTS HAVE NO INTERPRETATION IN TERMS OF THE CHEMICAL THEORY OF OZONE DEPLETION DESCRIBED BY ROWLAND AND MOLINA THAT FORMS THE BASIS OF THE MONTREAL PROTOCOL. NEITHER THE COPERNICUS ARCTIC POLAR OZONE HOLE OF 2020 NOR THE FARMAN ETAL ANTARCTIC POLAR OZONE HOLE OF 1985 SERVE AS EMPIRICAL EVIDENCE FOR THE CHEMICAL THEORY OF ANTHROPOGENIC OZONE DEPLETION. THE FARMAN ETAL 1985 PAPER IS DISCUSSED IN A RELATED POST [LINK] . 

PART-1: THE COPERNICUS REPORT ON THE ARCTIC OZONE HOLE [LINK] 

CAMS tracks a record-breaking Arctic ozone hole: DATE: 6th April 2020:  Ozone columns over large parts of the Arctic have reached record-breaking low values this year, and the ozone layer over the Arctic is severely depleted at altitudes of around 18 km. The last time similarly strong chemical ozone depletion was observed over the Arctic was during spring 2011, and ozone depletion in 2020 seems on course to be even stronger. The Copernicus Atmosphere Monitoring Service (CAMS*) has been closely following the rather unusual ozone hole that has formed over the Arctic this spring. Total column ozone in Dobson Units from CAMS on 29 March 2020 showing values below 250 DU over large parts of the Arctic. While we are used to ozone holes developing over the Antarctic every year during the Austral spring, the conditions needed for such strong ozone depletion are not normally found in the Northern Hemisphere.

The Antarctic ozone hole is mainly caused by human-made chemicals including chlorine and bromine that migrate into the stratosphere – a layer of the atmosphere around 10–50 kilometres above sea level. These chemicals accumulate inside the strong polar vortex that develops over the Antarctic every winter where they remain chemically inactive in the darkness. Temperatures in the vortex can fall to below -78 degrees Celsius and polar stratospheric clouds (PSCs) can form, which play an important part in chemical reactions involving the human-made chemicals that lead to ozone depletion once sunlight returns to the area. This depletion has been causing an ozone hole to form annually over the last 35 years, but the 2019 Antarctic ozone hole was actually one of the smallest we have seen during that time.

The Arctic stratosphere is usually less isolated than its Antarctic counterpart because the presence of nearby land masses and mountain ranges disturbs the weather patterns more than in the Southern Hemisphere. This explains why the polar vortex in the Northern Hemisphere is usually weaker and more perturbed than in the Southern Hemisphere, and temperatures do not fall so low. However, in 2020 the Arctic polar vortex has been exceptionally strong and long lived. Furthermore, temperatures in the Arctic stratosphere were low enough for several months at the start of 2020 to allow the formation of PSCs, resulting in large ozone losses over the Arctic. 

Minimum temperatures north of 60⁰N in the stratosphere at an altitude where the pressure measures 50 hPa 1980-2020 are shown. Data are from CAMS (2003–2020) and C3S (1980–2002). The two years shown for each decade are those with the highest column minimum and lowest column minimum, illustrating that minimum stratospheric temperatures at 50 hPa during winter and spring 2020 (black line) were below the temperature threshold for PSC formation (-78 degrees Celsius) for several months. Ozone depletion over the Arctic in 2020 has been so severe that most of the ozone in the layer between 80 and 50 hPa (an altitude of around 18 km) has been depleted. Shown below is the ozone profile chart in two panels – the left panel and the right panel.

The left panel is a comparison of ozone profiles (in millipascals) from CAMS (red) and ground based ozonesonde instruments (black) on 26 March 2020. The right panel shows mean ozone profiles from the two sources averaged over the years 2003–2019. The shaded area denotes +/- 1 standard deviation. Our forecasts suggest that temperatures have now started to increase and observations from the Microwave Limb Sounder instrument on NASA’s Aura satellite show that the stock of active chlorine is exhausted so that ozone depletion will slow down and eventually stop. Once the polar vortex breaks down, ozone-depleted air will mix with ozone-rich air from lower latitudes.

ABOUT CAMS:  CAMS monitors the ozone layer by combining information from its detailed numerical models of the atmosphere with satellite and ground-based (in situ) observations through a process called data assimilation. Currently, CAMS uses ozone satellite observations from the SBUV-2, OMI, MLS, GOME-2 and Sentinel-5P/TROPOMI instruments. CAMS is implemented by the European Centre for Medium-Range Weather Forecasts on behalf of the European Commission.

PART-2: CRITICAL EVALUATION OF THESE CLAIMS

(1) STRATOSPHERIC OZONE CHEMISTRY:  The ultraviolet spectrum in incident solar radiation comes in three frequency bands. The high energy band (200-240 nanometers in wavelength) and the medium energy band (240-300 nanometers in wavelength) are harmful to living matter and are absorbed in the ozone layer while the low energy band (300-480 nanometers in wavelength) reaches the earth’s surface and causes tanning.Ozone plays a role in the absorption of harmful UV radiation. It is both created and destroyed in the absorption process. The high-energy band UV is absorbed by oxygen molecules. The energy absorbed causes the oxygen molecule to break apart into extremely reactive oxygen atoms. A subsequent chance collision of these particles with other oxygen molecules causes the formation of ozone. The ozone thus formed then absorbs the medium-energy UV band and disintegrates back into oxygen.The UV absorption process is a cyclical one that begins and ends with oxygen. Ozone is a transient intermediate product of this process. The reason that there is any ozone accumulation at all in the stratosphere is that, of the three reactions, the second is the slowest. Sunset finds the stratosphere with an excess of single oxygen atoms still looking for a date with an oxygen molecule. Overnight, with no radiation to destroy their product, these particles build up an inventory of ozone whose destruction will begin anew at sunrise.There is therefore, a diurnal cycle in the ozone content of the stratosphere whose amplitude, incidentally, is of the same order of magnitude as reported ozone depletion that caused Montreal Protocol to be invoked. A longer but irregular cyclical pattern in stratospheric ozone coincides with the sunspot cycle. The period is approximately eleven years. It has been as long as 17 and as short as 8 years. High-energy band UV increases by 6 to 10% during periods of high sunspot activity but the medium-energy UV emission is largely unaffected. Therefore, high sunspot activity favors ozone accumulation and low sunspot activity is coincident with ozone depletion. A somewhat similar pattern exists in the case of polar ozone holes. The UV induced reactions described above occur only over the tropics where sunlight is direct. Stratospheric zone is formed ONLY over the equator and not over the poles. Stratospheric ozone is found in a layer of the stratosphere described as the ozone layer as shown in the chart below provided by NOAA [LINK] where we find that stratospheric ozone is found mostly between altitudes of 20km to 25km. Ozone concentration in the ozone layer falls gradually above 25km and sharply below 20km. All references to ozone in this context is a reference to stratospheric ozone and not to pollution and air quality issues haveing to do with lower tropospheric ozone.

(2) OZONE DISTRIBUTION: Equatorial ozone is distributed to the poles by the Brewer-Dobson Circulation (BDC), a stratospheric air circulation pattern. The shape and position of the BDC changes seasonally and also shifts over a longer time cycle. Therefore, the efficiency of the BDC in transporting ozone to the greater latitudes changes seasonally and also over longer time cycles. {Brewer, A. W. “Evidence for a world circulation provided by the measurements of helium and water vapor distribution in the stratosphere.” Quarterly Journal of the Royal Meteorological Society 75.326 (1949): 351-363}. When the distribution of ozone is not efficient, localized “ozone depletion” appears to occur in the extreme latitudes in the form of what has come to be called an ozone hole. These holes come and go in natural cyclical changes and are not the creation of chemical ozone depletion. Climate scientists say that the warming trend has weakened the Brewer Dobson circulation . This connection between climate and ozone appears to indicate that global warming can create more frequent and larger ozone holes. Butchart, N., et al. “Simulations of anthropogenic change in the strength of the Brewer–Dobson circulation.” Climate Dynamics 27.7-8 (2006): 727-741. However, the effect of global warming or of changing atmospheric composition on the Brewer Dobson Circulation remains controversial. Garcia, Rolando R., and William J. Randel. “Acceleration of the Brewer–Dobson circulation due to increases in greenhouse gases.” Journal of the Atmospheric Sciences 65.8 (2008): 2731-2739. RELATED POST ON THE BDC: [LINK] .

(3) THE ROWLAND MOLINA THEORY OF OZONE DEPLETION: The case against CFCs is that they are long lived such that once they get into the atmosphere they can hang around there for a centuries such that diffusion can eventually get them to the stratosphere. Once there, they absorb high-energy band UV and form unstable and reactive chlorine atoms. The chlorine atom particles then participate as catalytic agents to convert ozone back to oxygen. In other words they mediate the reaction between atomic oxygen particles and ozone. It is alleged that the destruction of ozone by this mechanism exposes the surface of the earth to dangerous levels of medium-band UV because there is not enough ozone in the stratosphere to absorb them. Although these reactions can be carried out in the chemistry lab, there are certain rate constraints that complicate the simplistic assumptions in the Rowland -Molina theory of anthropogenic ozone depletion.

(4) RATE CONSTRAINTS IN STRATOSPHERIC OZONE CHEMISTRY: The air up in the stratosphere is rather sparse, containing less than one percent of the molecular density of air at sea level. It is not easy for a molecular particle in random thermal motion to find another particle to react with. Photo-chemical reactions occur instantaneously while those that require a collision of two particles take much longer. This difference in the reaction rate is the reason that ozone accumulates overnight and why there is an inventory of ozone in the ozone layer. The atomic oxygen particles that react with oxygen molecules to form ozone could in theory react with an ozone molecule instead and cause its destruction or it could react with another atomic oxygen particle and form oxygen instead of ever forming any ozone. Some of the oxygen atoms do behave in this manner but these reactions proceed too slowly to be important to the chemistry of the stratosphere. The reason is that the stratospheric chemicals in question exist in minute quantities. One in a million particles is an ozone molecule or an atomic oxygen particle and one in a billion is CFC or chlorine generated from CFC. The accidental collision between chlorine atoms and ozone molecules or between chlorine atoms and oxygen atoms are rarer than those between two oxygen atoms or that between an oxygen atom and an ozone molecule. Therefore the latter collisions are more important to ozone depletion than those mediated by chlorine. Considering that more than 200,000 out of a million molecular particles in the stratosphere are oxygen molecules it is far more likely that charged oxygen atoms will collide with oxygen molecules rather than with each other or with ozone. Therefore ozone rather than oxygen is formed. Ozone formation is a rate phenomenon. Since chlorine atoms are a thousand times rarer in the stratosphere than atomic oxygen particles, it is not likely that chlorine’s mediation in short circuiting ozone generation will occur sufficiently fast to be important. Nature already contains an ozone destruction mechanism that is more efficient than the CFC mechanism but ozone forms anyway. However, the argument can be made that overnight after sunset, as charged oxygen atoms are used up the charged chlorine atoms take on a greater role in ozone destruction and also when these chemicals are distributed to the greater latitudes where sunlight is less direct and too weak to be ionizers of oxygen, the only ozone destruction chemistry left is that of charged chlorine atoms colliding with ozone. This overnight chemistry is the basis of the Rowland-Molina theory of anthropogenic ozone depletion.

(5) LATITUDINALLY WEIGHTED MEAN GLOBAL TOTAL COLUMN OZONE: The relative importance of these overnight and greater latitude reactions in making changes to latitudinally weighted mean global ozone can be checked by examining its overall long term trends as well as its trend profiles. These data are summarized in a related post [LINK] . The data do not show the ozone depletion inferred from the South Pole ozone hole found by Farman etal 1985 and described in the Montreal Protocol. The reason for that is that the Farman etal 1985 paper, the sole empirical evidence that served as the basis for the Montreal Protocol, is flawed. The problems with Farman etal 1985 are described in a related post [LINK] and summarized below.

(6) SCIENCE GONE WRONG: FARMAN ETAL 1985 & THE MONTREAL PROTOCOL: The overall structure of changes in total column ozone in time and across latitudes shows that the data from the two stations in Antarctica prior to 1995 are unique and specific to that time and place. They cannot be generalized into a global pattern of ozone depletion. The findings imply that declining levels of total column ozone in Antarctica during the months of October and November prior to 1995 do not serve as empirical evidence that can be taken as validation of the Rowland-Molina theory of chemical ozone depletion. The chemical theory implies that ozone depletion must be assessed across a greater range of latitudes and over a much longer period of time than what is found in Farman etal 1985 which serves as the only empirical basis for the ozone depletion hypothesis that led to the Montreal Protocol and the ascendance of the UN as a global environmental authority.

(7) IMPLICATIONS FOR THE INTERPRETATION OF COPERNICUS OZONE DATA FOR THE NORTH POLAR REGION AS OZONE DEPLETION: Copernicus data for total column ozone in the North Polar region show that stratospheric ozone levels measured as total column ozone at an elevation of 18km had fallen to an unusually low value of 250 Dobson Units (DU) in March 2020. They concluded that the observed low value of ozone at that altitude was an ozone hole created by chemical ozone depletion as described in the Montreal Protocol in terms of the Rowland-Molina chemical theory of ozone depletion. As for where these destructive chemicals came from, they decided that it had to have been brought there from the lower latitudes by the polar vortex air circulation due to changes to the circulation system caused by anthropogenic global warming. The implication is that the observed ozone hole over the Arctic was therefore human caused.

An ozone level of 250 DU is indeed an unusually low level for stratospheric total column ozone in the North Polar region in March as seen in the table below where 5-year moving averages for total column ozone above Barrow, Alaska at 71-North latitude, show values above 400DU for the 45-year period 1971 to 2015. However, it should be noted that the station data from Barrow are inclusive of the higher values of ozone concentration at the greater altitudes, higher than 18km, seen in the chart below the Barrow data table. The chart indicates that ozone levels below 18km are on the low end of the vertical distribution. It should also be considered that the traditional definition of an ozone hole in terms of the Montreal Protocol and since Farman 2tal 1985 refers to ozone levels less than 220 Dobson Units, meaning that 250 DU is, technically, not really an ozone hole.

(8) THE MEASUREMENT OF ROWLAND-MOLINA CHEMICAL OZONE DEPLETION AS  DESCRIBED IN THE MONTREAL PROTOCOL:  RELATED POST: [LINK] . The more serious issue in the Copernicus ozone study is that the theory of chemical ozone depletion in the Montreal Protocol refers to long term trends in latitudinally weighted mean global total column ozone over long time spans. In that context, a depletion detected in a single latitudinal zone has no interpretation particularly so when that latitudinal zone is an extreme polar latitude with very little weight in the latitudinal weighting. Also, the depletion should be found over a long time span for all calendar months. In that context, ozone holes are not evidence of ozone depletion in accordance with the chemical theory of ozone depletion described in the Montreal Protocol. Localized and brief changes in total column ozone should be understood in terms of the ozone distribution anomalies of the Brewer-Dobson circulation. The assessment of chemical ozone depletion in terms of ozone holes by Farman etal in 1985 suffers from the same methodological issue as described in a related post [LINK] .

SUMMARY

OZONE HOLES ARE LOCALIZED EVANESCENT DECLINES IN POLAR OZONE LEVELS. THESE EVENTS HAVE NO INTERPRETATION IN TERMS OF THE CHEMICAL THEORY OF OZONE DEPLETION  DESCRIBED BY ROWLAND AND MOLINA THAT FORMS THE BASIS OF THE MONTREAL PROTOCOL. NEITHER THE COPERNICUS ARCTIC POLAR OZONE HOLE OF 2020 NOR THE FARMAN ETAL ANTARCTIC POLAR OZONE HOLE OF 1985 SERVE AS EMPIRICAL EVIDENCE FOR THE CHEMICAL THEORY OF ANTHROPOGENIC OZONE DEPLETION.  THE FARMAN ETAL 1985 PAPER IS DISCUSSED IN A RELATED POST [LINK] . 

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