Stratspheric Chemistry-Ozone

Stratspheric Chemistry-Ozone Chapter 3

Introductions Stratosphere is the region of atmosphere that extends from 15 to 50 km above earth surface. The temperature in stratosphere ranges from ─60 0C at 15 km to ─2 0C at 50 km. Sun emits radiation over a broad region of the electromagnetic radiation spectrum corresponding to λ range of 1000 μm to less than 250 nm. The maximum intensity of light occurs at 550 nm (yellow region), this radiation is near the optimum for photosynthesis and production of biomass. longer wavelengths of light give heat and the normal light. The short wavelengths make only a small portion of total solar flux. The short wavelengths are very dangerous and most of them are filtered in the atmosphere. Principal constituents of gases are N2 and O2 High energy of radiation striking gases (oxygen in particular)lead to synthesis and decomposition of ozone. Stratosphere is called the ozone layer. Concentration of ozone in stratosphere ozone layer is relatively small (1-8ppmv) Ozone screens (effective filter ) out the harmful ultraviolet radiation (200-315 nm) what would reach the surface of the earth

UV flux categories: • UV radiations can be categorized into three types: • UV─A (λ: 315─400 nm): This type of UV makes up 7% of the total solar flux. UV─A radiations are not so harmful to living species. • UV─B (λ: 280─315 nm): This type of UV makes up 1.5% of the total solar flux. UV─B radiations are harmful to plant and living species especially after long exposure. • UV─C (λ < 280 nm): This type of UV makes up 0.5% of the total solar flux. UV─C radiations can destroy all aspect of life. • In fact UV-B and UV-C must be filtered from sun radiation before reaching earth surface. In fact, ozone (O3) is the chemical species that is able to absorb UV-B and UV-C radiations.

Presence of ozone (O3) in stratosphere: Ozone found naturally in the area between 15 – 30 km and this region, which fall in the stratosphere region, is called ozone-layer. the concentration of ozone can be reported as ppmv or as ozone number density (molecule/cm3).

Presence of ozone (O3) in stratosphere: Note that at lower altitudes, the concentration of ozone (ozone number density) is relatively high and this because of the photochemical smog processes. Ozone is a useful substance only at higher altitude (good ozone) but at lower altitudes will be dangerous on human (bad ozone). Even though ozone has a small concentration in stratosphere (maximum concentration 8 ppmv or 8 .1 × 10-4 kpa or 2×1012 molecules O3/cm3atmosphere), it prevents the harmful UV radiation to reach earth’s surface. As can be seen below, ozone is effectively absorbing radiations of wavelengths from 200–315 nm and oxygen is effectively absorbing radiations of wavelengths from 125–175 nm. UV-C (λ < 280 nm) is effectively absorbed by oxygen and ozone molecules. UV-B (λ: 280-315 nm) radiations are partially absorbed by ozone. The maximum absorption capacity of ozone occurs at λ = 250 nm.

UV-B radiation dangers to human. UV-B radiation could cause cell damage within the mid-layer of the active skin. UV-A radiation is less danger but due to long exposure it could causes cell damage and aging process. Sunscreens and sunblocks are pharmaceuticals that used to reduce the penetration of the dangerous forms of radiation . Sunscreens are made of benzyl salicylate or cinnamate that absorb strongly in the 250-325 nm range. Sun protection factor (SPF=15 corresponding to absorb 93 %of the UV-B radiation (100/(100-93)=15) Ineffective in screening out UV-A radiation Sunblocks contains solid particles (zinc oxide, or titanium oxide) that physically reflect the radiation out of the skin. It is effective in reducing the penetration of all types of radiation

Ozone Measurements It is not like water or soil samples. It is possible to grab sample of gas but in the case of ozone . It is difficult since it reacts with material of the container and get destroyed. The measurements are done in situ. The instruments are based on measuring the absorption of the light in the range of 200-315 nm and the maximum absorbance is about 255 nm . Gound-based system based on laser light and a large telescope. Such as 1-Stratospheric Ozone Lidar Trailer Experiments (STROZ-LITE) : It measures the ozone, temperature and aerosol concentration They determine the concentration of ozone based on two wavelengths (308 nm and 351 nm) and it is known as Differential absorption lidar (DIAL) method.

Ozone Measurements 2-Dobson Ozone spectrometer. Dobson invents a spectrophotometer to measure the ozone concentration in atmosphere. Dobson spectrophotometer measures the intensity of solar radiation at Four wavelengths where two of which are absorbed by ozone molecules and two of which are not One Dobson unit (DU) is defined to be 0.01 mm thickness of pure ozone layer at STP conditions. For example, if the concentration of ozone above Amman city is 300 DU, then this means that if all ozone over Amman city is collected and compressed down it will give a layer of 3 mm thick at STP conditions. 3-TOMS: Satellite-based measurements

Stratospheric Ozone concentrations The world average value is 300 DU and may reach to 250 DU at tropics and up to 450 DU at north and south areas. Daily fluctuation is 20-30 DU. Seasonal changes are large: high in winter and spring and low in summer and fall In the last decades, it is reported that the concentration of ozone has been decreased to 150 DU at Antarctic and Arctic areas. This indicates that there is depletion in ozone concentration in the atmosphere and this depletion in ozone layer will danger the life on earth. It is believed that the depletion in ozone concentration is back to the human activities on earth. Ozone hole; this definition is used to describe the thinning of the region of the ozone layer.

Oxygen-Only Chemistry – Formation and Turnover of O3 (Natural Processes) : Chapman reaction sequences The synthesis and decomposition of ozone in atmosphere can be described by four reactions known as Chapman’s cycle. The reactions involved in the cycle are: Synthesis reactions of O3: ΔH0 (kJ/mol) 1. O2 + hv (λ < 240 nm) → O + O (slow) 498.4 2. O + O2 + M → O3 + M (fast) ─106.5 (M: O2 or N2 and these species are very important to prevent dissociation of O3) Decomposition reactions of O3: 3. O3 + hv (λ ≈ 230-320 nm) → O2* + O* (fast) 386.5 4. O + O3 → O2 + O2 (very slow) ─391.9

Synthesis reaction Reaction 1 is endothermic reaction because is describes the dissociation of oxygen molecules to oxygen atoms by solar radiation. The heat of reaction can be calculated from the heat of formation of reactants: ΔH0reaction = 2 ΔH0f (O) ─ ΔH0f(O2) = 2(249.2) ─ 0 = 498.4 kJ/mol The wavelength of light associated with this amount of energy is calculated from: In fact, the radiations having wavelength grater than 240 nm are not effective to initiate this reaction. Therefore, radiations grater than 240 nm will penetrate further through the stratosphere into troposphere. Reaction 1 is slow because it is bond-breaking reaction.

Synthesis reaction This reaction (2) is very important because it describes the synthesis of ozone. This reaction is exothermic and does not need solar energy to occur. Reaction (2) explains the increase in temperature in stratosphere region (exothermic reaction). ΔH0 of the reaction can be calculated from the relation: ΔH0reaction = ΔH0f (O3) ─ ΔH0f (O) ─ ΔH0f(O2) = 142.7 ─ 249.2 ─ 0 = ─ 106.5 kJ/mol Reaction 2 is a fast reaction.

Decomposition reaction Reaction 3 is the most important reaction occurs in atmosphere. If this reaction does not occur then the radiations from 230 – 320 nm will penetrate atmosphere and reach earth surface. Simply, ozone molecules (that prepared in reaction 2) absorb radiation from (230-320 nm) and protect earth from these harmful radiations. ΔH0 of the reaction can be calculated from the relation: ΔH0reaction = ΔH0f (O) + E (O) + ΔH0f (O2) + E (O2) ─ ΔH0f(O3) = 249.2 + 190 + 0 + 90 ─ 142.7 = 387 kJ/mol. Where E(O) and E(O2) are energies (in kJ/mol) that are needed for excitation of O and O2 respectively. The wavelength of light associated with this amount of energy is calculated from: Radiations of longer wavelengths penetrate further to reach earth surface. Reaction 3 is a fast reaction.

Decomposition reaction Because reaction 4 is highly exothermic, so it is also responsible for rising the temperature of stratosphere along with reaction 2. Reaction 4 is responsible for removing 20% of the produced oxygen atoms in stratosphere. Reaction 4 can be considered as a termination step for production of oxygen atoms. This will reduce ozone production because oxygen atoms are very important for ozone production as shown in reaction 2. The heat of reaction can be calculated from the heat of formation of reactants: ΔH0reaction = 2 ΔH0f (O2) ─ [ΔH0f(O) + ΔH0f(O3)] = 2(0) ─ [249.2 + 142.7] = ─ 391.9 kJ/mol The activation energy of reaction 4 is high (18 kJ/mol) so this reaction is slow. Reaction 4 considered as one of the natural destruction cycles of ozone.

The net reaction of ozone cycle: The net reaction of ozone cycle can be obtained by adding the propagation steps in the cycle (reactions 2 and 3): O2 + O + M → O3 + M first propagation step. O3 + hn → O2 + O + heat second propagation step. ------------------------------------------ hn → heat the net reaction of ozone cycle in stratosphere. Overall, stratospheric ozone chemistry has a net effect of converting (harmful) ultraviolet radiation to heat.

Ozone formation-destruction cycle.

The Ozone Layer: Naturally, the maximum ozone concentration is present at an altitude approximately 23 km above earth surface. Above that altitude, the destruction of ozone is high due to the presence of high energy radiations and the ratio (O/O2) is high. At lower altitudes (< 23 km), no enough energy to produce oxygen atoms (O) and finally O3 (reaction 2). Based on that, O3 present at moderate altitudes in stratosphere (about 23 km above earth surface).

Catalytic decomposition of ozone: Research on ozone layer proved that many substances can destroy O3 to O and O2. Some of these substances produced naturally and some produced by human. These substances have a long residence time in troposphere and after their leakage to stratosphere can destroy ozone. The most important chemical species that can destroy ozone are presented below

Catalytic decomposition of ozone: General mechanisms of ozone destruction: There are two mechanisms for ozone destruction by the above substances: Mechanism-I: X• + O3 → XO• + O2 (ozone has loose oxygen atom) XO• + O→ X• + O2 (oxygen atoms present in stratosphere in high concentrations) ---------------------------------------- Overall O3 + O → 2O2 Where X• is any reacting species in previous table . Note that the overall reaction is the same as equation 4 in Chapman’s cycle. Mechanism-II: X• + O3 → XO• + O2 X•′ + O3 → X′O• + O2 XO• + X′O• → X• + X•′ + O2 ---------------------------------------- Overall 2O3 → 3O2 Where X• and X•′ are any pair of reacting species in previous table .

Catalytic decomposition of ozone: HOx ─ catalytic cycle. HOx species can be produced from water that present in atmosphere as following: O* + H2O → 2•OH Or as: H2O + hv → •H + •OH The ozone decomposition cycle by (•H or •OH or HOO•) can be presented according to mechanism-I as following: For •OH: •OH + O3 → HOO• + O2 HOO• + O→ •OH + O2 --------------------------------- Overall O3 + O → 2O2 For •H: •H + O3 → •OH + O2 •OH + O→ •H + O2 --------------------------------- Overall O3 + O → 2O2 For HOO•: . . – . .+ . . HOO•+ O3 → •OH + 2O2 (O3 can be imagined as :O •• O: :O) •OH + O→ HOO• ˙˙ ˙˙ --------------------------------- Overall O3 + O → 2O2

Catalytic decomposition of ozone: • NOx ─ catalytic cycle. • NO (nitric oxide) can be produced from composition of fuel in engines . Also some nitrogenous fertilizers can produce NO. NO is a stable compound in troposphere and can leak into stratosphere and destroy ozone as following: • •NO + O3 → •NO2 + O2 • •NO2 + O→ •NO + O2 • --------------------------------- • Overall O3 + O → 2O2 • NO can be produced in upper atmosphere regions (in thermosphere) as following: • N2 + hv (λ < 126 nm) → 2N* (N* is an excited nitrogen atom) • N* + O2 → •NO + O

Catalytic decomposition of ozone: ClOx ─ catalytic cycle ClOx destruction cycles contribute to 10% of total ozone destruction. There are many sources for chlorine radicals: 1) CH3Cl: This compound produced biogenically from oceans, burning of vegetation, and volcanic emissions. In atmosphere CH3Cl can be decomposed by light as: CH3Cl + hv → •CH3 + •Cl 2) Hydrochloric acid (HCl): This substance can be released into atmosphere from volcano and sea-spray. HCl can be easily decompose in atmosphere to •H and •Cl radicals. Ozone destruction cycle by •Cl: •Cl + O3 → ClO• + O2 ClO• + O→ •Cl + O2 --------------------------------- Overall O3 + O → 2O2

Anthropogenic sources of chlorine: (CFCs) compounds. CFCs (chlorinated fluorocarbon) compounds were invented in 1930s to replace normal hydrocarbon compounds. CFCs used in industry due to their excellent chemical properties like: low viscosity, low surface tension, low boiling point, and high stability. CFCs used in refrigerants, solvents for cleaning and in polymers. CFCs compounds are well-mixed in troposphere and because of their inertness they circulate in troposphere until they leak into stratosphere. At stratosphere, where high radiations of lower wavelengths are present, the destruction of CFCs occurred as following: CFCl3 + hv (UV-B or C) → •CFCl2 + •Cl (•Cl. radicals are effectively destroy ozone) The life times of CFCs compounds are very long in troposphere. Note that the life time is highly increased by increasing the number of fluorine atoms in the compound. The high stability of CFCs compounds containing more fluorine atoms in troposphere is due to the stronger C-F bond. C-F bond (∆H0 C-F = 484 kJ/mol) needs more energy to dissociate compare to C-Cl (∆H0 C-Cl = 338 kJ/mol).

Null and Holding cycles • Null cycles: In these cycles an inter-conversion between X and XO is occurred without affecting odd oxygen (O) removal. • Null Cycle for NO: Normal cycle of NO: • •NO + O3→ •NO2 + O2 •NO + O3 → •NO2 + O2 • •NO2+ hv→ •NO + O •NO2 + O→ •NO + O2 • --------------------------------- --------------------------------------- • Overall O3 + hv→ O2 + O Overall O3 + O → 2O2

Holding cycles: Holding cycles are considered as reservoirs for NOx, HOx and ClOx species. For example the reservoir for •NO2 is N2O5: •NO2 + •NO3 → N2O5 •NO2 and •OH radicals can be added to give HNO3: •NO2 + •OH → HNO3 (HNO3 is a reservoir for •NO2 and •OH radicals) •Cl can react with CH4 to give the following holding cycle: •Cl + CH4 → HCl + •CH3 (HCl is a reservoir for •Cl radical) In fact 50% of NOx is stratosphere stored as HNO3. 70% of Clx species in stratosphere are stored as HCl. In fact the reservoirs (N2O5, HNO3, and HCl) are dissociated to free the radicals once sun-rise. Because of that, the radicals that destroy ozone have an infinite life-time. In addition to the mentioned reservoirs, some new reservoirs have been identified for ClOx and NOx radicals. The following reactions produced these reservoirs: ClO• + HOO• → HOCl + O2 (hypochlorous acid) HOO• + •NO2 + M → HO2NO2 + M (M: N2 or O2) (pernitric acid) ClO• + •NO2+ M → ClONO2 + M (M: N2 or O2) (chlorine nitrate)

8. Antarctic and Arctic “ozone hole” formation: Because of its importance for absorption of harmful radiations (200–300 nm), ozone concentration had been monitored world-wide since the mid 1950s. Recently, scientists had been noted a noticeable decline in ozone concentration at the Antarctic and Arctic areas. The reduction in ozone concentration in Antarctic area was observed in October time of each year. As shown below, the variations in ozone concentration during the year 2001; note the large reduction in ozone concentration in October time.

The average of ozone concentration in October during the period 1956-1966 had been 314 DU, while from 1983-1993 the same average drooped to 182 DU. The reduction in ozone concentration in these areas can be attributed to a complex combination of climatic factors and the accumulation of reservoirs species. Mechanism of ozone – hole formation above Antarctic area: • During the long dark winter in the Antarctic area (South Pole), and due to the intense cold (T = 193 – 187 K) and rotation of earth a giant vortex will form as shown in the next Figure . Inside this vortex a polar stratospheric clouds (PSCs) form as a result of low temperature. These PSC can be categorized into two types: • PSCs of diameter 1 μm formed at 193 K and consist of HNO3 and water (ratio 1:3). • PSCs of diameter 10 μm formed at 187 K and consist of pure water. • Besides the PSCs there are many accumulated gases like: HCl, N2O5 and ClONO2. These gases known as reservoirs as discussed earlier. On the surface of PSCs, the following heterogeneous reactions take place which release more chlorine molecules and hypochlorous acid:

HCl + ClONO2 ──────> Cl2 + HNO3 On PSCs H2O + ClONO2 ──────> HOCl + HNO3 PSCs have accelerated the production of Cl2 and HOCl in Antarctic area. Once the onset of sunlight in October, the solar radiation provides energy for photo-degradation of reservoirs molecules: Cl2 + hv → 2•Cl HOCl + hv → •Cl + •OH The chlorine and other radicals are then start to destroy ozone molecules by the standard mechanisms (I or II). The degradation of ozone occurs rapidly and the concentrations of ozone reduced to half of their winter value. Ozone can be destroyed in that area without the presence of atomic oxygen (O) as following: •Cl + O3 → ClO• + O2 •Cl + O3 → ClO• + O2 ClO• + ClO• → ClOOCl ClOOCl + hv → O2+2•Cl --------------------------------- Overall: 2O3 + hv → 3O2 The destruction reactions of ozone persist until the air temperature rises. The vortex breaks up and the PSCs disappear when the temperature rises again at the late of spring. At the late of spring, the chlorine radicals tied-up again by formation of HCl and ClONO2, and the level of ozone begin to recover to the winter levels .

The Antarctic ozone hole.

Ozone – hole above Arctic Area (North Pole): Similar observations were reported for ozone concentration above Arctic area. As shown below, the variations in ozone concentrations in 1992 and 1995. As shown below, the concentration of ozone (ozone number density) had been largely declined in 1995 in comparison with 1992. The reduction in ozone concentration in 1995 is due to the high concentrations of pollutants which released by different industries.

9. Kinetic calculations of zone destruction cycles As mentioned earlier, ozone can be destroyed according to following cycle: Reaction1 X• + O3 → XO• + O2 Rate1 = K1 [•X][O3] Reaction2 XO• + O→ X• + O2 Rate2 = K2 [•XO][O] ---------------------------------------- Net reaction O3 + O → 2O2 Rate-net = Knet [O3][O] To calculate the overall rate (rate-net) of zone destruction, rate1 and rate2 should be calculated. To calculate rate 1 and rate 2, the values of K1, K2, [X], [O3], [XO], and [O] should be known. The overall-rate value (rate-net) of ozone destruction is equal to slowest rate of the two reactions (reaction1 and reaction2). In fact, kinetic calculations of zone destruction by (•H, •OH, •NO, and •Cl) can be easily carried out.

Table of Kinetic data for various radical species involved in the catalytic decomposition of zone. (Data were calculated at 235 K and 30 km above earth surface) Notes: k is the rate constant of the reaction. A and Ea are Arrhenius parameters: , Ozone concentration at 30 km is 2.0×1012 molecule cm-3. Note that the order of all reactions in the Table is second-order as concluded form the unit of the rate constant (K).

Example: Identify the rate-determining step in the catalytic cycle involving hydrogen (•H) and hydroxyl radicals (•OH), and determine the overall rate of ozone destruction as a consequence of this cycle. (Note that the calculation applies to reactions occurring at 30 km only.) Solution Catalytic cycle involving hydroxyl and hydrogen radicals for ozone destruction is: Reaction1 •H + O3 →•OH + O2 k1 = 1.9 ×10-11 molecule-1 cm3 s-1 Reaction2 •OH + O→ •H + O2 k2 = 2.3 × 10-11 molecule-1 cm3 s-1 ------------------------------------- Net reaction O3 + O → 2O2 Rate = ?? Concentration of species from the data in previous Table. [O3] = 2.0×1012 molecule cm-3 [•H] = 2.0×105 molecule cm-3 [•OH] = 1.0×106 molecule cm-3 [O] = 1.0×109 molecule cm-3 Rate1 = k1[•H][O3]

= (1.9×10-11 molecule-1 cm3 s-1)×(2.0×105 molecule cm-3)×( 2.0×1012 molecule cm-3) = 7.6×106 molecule cm-3 s-1 Rate2 = k2[•OH][O] = (2.3×10-11 molecule-1 cm3 s-1)×(1.0×106 molecule cm-3)×(1.0×109 molecule cm-3) = 2.3×104 molecule cm-3s-1 The second step (reaction 2) is the rate-determining step because it has the slower rate. The overall rate of ozone destruction is equal to Rate2 = 2.3×104 molecule cm-3s-1

The Antarctic ozone hole.

Stratspheric Chemistry-Ozone