Atmospheric Chemistry Questions and Answers

Q: Stratosphere is known to protect life on earth from harmful radiation from sun is found between :

The stratosphere lies between the troposphere and the mesosphere (printed 'Toposphere & Mesosphere').The layers of the atmosphere, from the ground up, are the troposphere (to about 8-18 km), the stratosphere (to about 50 km), the mesosphere (to about 85 km) and the thermosphere. The stratosphere contains the ozone layer, which absorbs most of the sun's harmful ultraviolet radiation, which is why it is said to protect life.The mesosphere and thermosphere lie above the stratosphere, and the ionosphere and exosphere are higher still, overlapping the thermosphere and the outer edge of the atmosphere. None of these pairs encloses the stratosphere.

Q: Generally the rain water having pH ......................... is considered as acid rain.

Acid rain is rain with a pH less than 5.6.Even unpolluted rain is slightly acidic: carbon dioxide dissolves in it to form carbonic acid, and water in equilibrium with atmospheric carbon dioxide has a pH of about 5.6. Rain is called acid rain only when it is more acidic than that, which happens when sulphur dioxide and nitrogen oxides form sulphuric and nitric acids.A pH above 5.6 is less acidic than natural rain. A pH of 7 is neutral, and a pH of 10 is alkaline, so neither describes acid rain.

Q: Primary cosmic rays are composed of very energetic :

Primary cosmic rays are mostly very energetic protons.About 90 per cent of the primary cosmic ray particles arriving at the top of the atmosphere are protons (hydrogen nuclei), about 9 per cent are alpha particles (helium nuclei), and the remaining 1 per cent are heavier nuclei and electrons.Mesons such as pions and muons are secondary cosmic rays, created when the primary particles collide with nuclei of air in the upper atmosphere. Free neutrons are also secondary products and would decay in transit from distant sources. Electrons make up only a small fraction of the primary flux.

Q: Harmful UV radiation emanating from the sun is prevented from reaching the earth surface by the presence of ozone in the :

The ozone that blocks harmful UV is in the stratosphere.About 90 per cent of atmospheric ozone lies in the stratosphere, mainly between about 15 and 35 km, forming the ozone layer. Ozone absorbs ultraviolet-B and ultraviolet-C radiation, protecting life at the surface from skin cancer, eye damage and harm to plants and plankton. The absorbed energy is also why temperature rises with height in the stratosphere.The mesosphere and thermosphere above contain very little ozone. Ozone in the troposphere is small in amount and is a pollutant there, not the protective layer.

Q: Atmosphere is a mechanical mixture of several gases and principal components are Nitrogen, Oxygen and Argon. The remain gases which amount to ................. of the total mass are called minor gases.

The minor gases amount to less than 1% of the atmosphere.Dry air is about 78% nitrogen, 21% oxygen and just under 1% argon by volume; by mass the three are roughly 75.5%, 23.1% and 1.3%. Together they make up more than 99.9% of the atmosphere, so everything else, carbon dioxide, neon, helium, methane, krypton, hydrogen, ozone and the rest, is only a small fraction of one per cent.Figures of 5, 10 or 15 per cent would make the minor gases larger than argon and comparable to oxygen, which is far from the measured composition. Water vapour varies from near zero to about 4% but is treated separately from the dry-air mixture.

Q: The mean molecular weight of AIR is nearly 28.9. It remains constant upto a height of ....................... and then ..................... .

The mean molecular weight stays near 28.9 up to about 80 km, and then decreases.Below about 80 km lies the homosphere, where turbulent mixing keeps the proportions of nitrogen, oxygen and argon the same at every height, so the mean molecular weight of air does not change. Above it, in the heterosphere, mixing gives way to diffusive separation: heavier molecules settle and lighter ones dominate, and ultraviolet radiation splits molecular oxygen into atomic oxygen. Both effects lower the mean molecular weight with height.An increase is impossible for that reason, which rules out every option ending in 'increases'. The 50 km and 60 km heights fall inside the well-mixed region.

Q: Formation of O('D) atom in the \(\mathrm{O_{3}} + hv \rightarrow \mathrm{O_{2}} + \mathrm{O('D)}\) reaction is due to :

The excited oxygen atom forms by Photolysis.The reaction shown is driven by a photon (the \(hv\) term). Ultraviolet light of wavelength shorter than about 310 nm splits ozone into an oxygen molecule and an oxygen atom in the excited singlet state written O(1D). This atom is important because it reacts with water vapour to make the hydroxyl radical, the main cleaning agent of the troposphere.Collisions do the opposite: they quench O(1D) back to the ground-state atom. A termolecular reaction (O + O2 + M) is how ozone is formed, not split. Nuclear fusion has no place in atmospheric chemistry.

Q: Ozone-hole in the atmosphere is largely caused by the presence of :

The ozone hole is largely caused by Chloro-fluoro-carbon.CFCs are stable in the lower atmosphere, so they drift up to the stratosphere, where ultraviolet light breaks them and releases chlorine atoms. Each chlorine atom destroys thousands of ozone molecules in a catalytic cycle (Cl + O3 gives ClO + O2, and ClO + O regenerates Cl). Over Antarctica, polar stratospheric clouds free extra chlorine each spring, producing the seasonal hole. The Montreal Protocol of 1987 phases these chemicals out.Oxygen is the raw material from which ozone forms, not a cause of its loss. Hydrogen and radioactive waste are not the agents behind the ozone hole.

Stratosphere is known to protect life on earth from harmful radiation from sun is found between :

A.Mesosphere & Thermosphere
B.Thermosphere & Ionosphere
C.Toposphere & Mesosphere
D.Ionosphere & Exosphere

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

The stratosphere lies between the troposphere and the mesosphere (printed 'Toposphere & Mesosphere').

The layers of the atmosphere, from the ground up, are the troposphere (to about 8-18 km), the stratosphere (to about 50 km), the mesosphere (to about 85 km) and the thermosphere. The stratosphere contains the ozone layer, which absorbs most of the sun's harmful ultraviolet radiation, which is why it is said to protect life.

The mesosphere and thermosphere lie above the stratosphere, and the ionosphere and exosphere are higher still, overlapping the thermosphere and the outer edge of the atmosphere. None of these pairs encloses the stratosphere.

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Generally the rain water having pH ......................... is considered as acid rain.

A.More than 5.6
B.Less than 5.6
C.10
D.7

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

Acid rain is rain with a pH less than 5.6.

Even unpolluted rain is slightly acidic: carbon dioxide dissolves in it to form carbonic acid, and water in equilibrium with atmospheric carbon dioxide has a pH of about 5.6. Rain is called acid rain only when it is more acidic than that, which happens when sulphur dioxide and nitrogen oxides form sulphuric and nitric acids.

A pH above 5.6 is less acidic than natural rain. A pH of 7 is neutral, and a pH of 10 is alkaline, so neither describes acid rain.

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Primary cosmic rays are composed of very energetic :

A.Electron
B.Mesons
C.Protons
D.Neutron

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

Primary cosmic rays are mostly very energetic protons.

About 90 per cent of the primary cosmic ray particles arriving at the top of the atmosphere are protons (hydrogen nuclei), about 9 per cent are alpha particles (helium nuclei), and the remaining 1 per cent are heavier nuclei and electrons.

Mesons such as pions and muons are secondary cosmic rays, created when the primary particles collide with nuclei of air in the upper atmosphere. Free neutrons are also secondary products and would decay in transit from distant sources. Electrons make up only a small fraction of the primary flux.

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The temperature at Tropical Tropopause (Ht 17 km) is about ............. °C.

A.-75
B.-95
C.-50
D.-60

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

The tropical tropopause, at about 17 km, has a temperature of about -75 °C.

Temperature in the troposphere falls with height at roughly 6.5 °C per km. Over the tropics the troposphere is deepest, so the air keeps cooling up to about 16 to 18 km, and the tropopause there is the coldest part of the lower atmosphere, typically around -75 to -80 °C. Above it, in the stratosphere, temperature rises again because ozone absorbs ultraviolet radiation.

-50 and -60 °C are typical of the lower, warmer tropopause over middle and high latitudes, and -95 °C is colder than the tropical tropopause normally gets.

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Harmful UV radiation emanating from the sun is prevented from reaching the earth surface by the presence of ozone in the :

A.Mesosphere
B.Thermosphere
C.Stratosphere
D.Troposphere

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

The ozone that blocks harmful UV is in the stratosphere.

About 90 per cent of atmospheric ozone lies in the stratosphere, mainly between about 15 and 35 km, forming the ozone layer. Ozone absorbs ultraviolet-B and ultraviolet-C radiation, protecting life at the surface from skin cancer, eye damage and harm to plants and plankton. The absorbed energy is also why temperature rises with height in the stratosphere.

The mesosphere and thermosphere above contain very little ozone. Ozone in the troposphere is small in amount and is a pollutant there, not the protective layer.

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Which one of the following is the most abundant gas in the stratosphere ?

A.\(\mathrm{O_{3}}\)
B.\(\mathrm{NO_{2}}\)
C.CO
D.\(\mathrm{C_{6}H_{6}}\)

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

Among the gases offered, the most abundant in the stratosphere is ozone.

Strictly, nitrogen and oxygen make up most of the stratosphere, as they do the whole atmosphere, but neither is an option. Of the gases listed, ozone is by far the most plentiful there: the stratosphere holds about 90 per cent of all atmospheric ozone, at concentrations of several parts per million in the ozone layer, which is why it is the gas associated with that layer.

Nitrogen dioxide is present only in parts per billion. Carbon monoxide and benzene are surface pollutants found mainly in the lower troposphere.

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Atmosphere is a mechanical mixture of several gases and principal components are Nitrogen, Oxygen and Argon. The remain gases which amount to ................. of the total mass are called minor gases.

A.10%
B.15%
C.5%
D.Less than 1%

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

The minor gases amount to less than 1% of the atmosphere.

Dry air is about 78% nitrogen, 21% oxygen and just under 1% argon by volume; by mass the three are roughly 75.5%, 23.1% and 1.3%. Together they make up more than 99.9% of the atmosphere, so everything else, carbon dioxide, neon, helium, methane, krypton, hydrogen, ozone and the rest, is only a small fraction of one per cent.

Figures of 5, 10 or 15 per cent would make the minor gases larger than argon and comparable to oxygen, which is far from the measured composition. Water vapour varies from near zero to about 4% but is treated separately from the dry-air mixture.

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The mean molecular weight of AIR is nearly 28.9. It remains constant upto a height of ....................... and then ..................... .

A.80 km, increases
B.60 km, increases
C.80 km, decreases
D.50 km, increases

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The mean molecular weight stays near 28.9 up to about 80 km, and then decreases.

Below about 80 km lies the homosphere, where turbulent mixing keeps the proportions of nitrogen, oxygen and argon the same at every height, so the mean molecular weight of air does not change. Above it, in the heterosphere, mixing gives way to diffusive separation: heavier molecules settle and lighter ones dominate, and ultraviolet radiation splits molecular oxygen into atomic oxygen. Both effects lower the mean molecular weight with height.

An increase is impossible for that reason, which rules out every option ending in 'increases'. The 50 km and 60 km heights fall inside the well-mixed region.

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Formation of O('D) atom in the \(\mathrm{O_{3}} + hv \rightarrow \mathrm{O_{2}} + \mathrm{O('D)}\) reaction is due to :

A.Collision
B.Photolysis
C.Termolecular reaction
D.Nuclear fusion

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

The excited oxygen atom forms by Photolysis.

The reaction shown is driven by a photon (the \(hv\) term). Ultraviolet light of wavelength shorter than about 310 nm splits ozone into an oxygen molecule and an oxygen atom in the excited singlet state written O(¹D). This atom is important because it reacts with water vapour to make the hydroxyl radical, the main cleaning agent of the troposphere.

Collisions do the opposite: they quench O(¹D) back to the ground-state atom. A termolecular reaction (O + O₂ + M) is how ozone is formed, not split. Nuclear fusion has no place in atmospheric chemistry.

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10.

Ozone-hole in the atmosphere is largely caused by the presence of :

A.Oxygen
B.Hydrogen
C.Chloro-fluoro-carbon
D.Radioactive-waste

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

The ozone hole is largely caused by Chloro-fluoro-carbon.

CFCs are stable in the lower atmosphere, so they drift up to the stratosphere, where ultraviolet light breaks them and releases chlorine atoms. Each chlorine atom destroys thousands of ozone molecules in a catalytic cycle (Cl + O₃ gives ClO + O₂, and ClO + O regenerates Cl). Over Antarctica, polar stratospheric clouds free extra chlorine each spring, producing the seasonal hole. The Montreal Protocol of 1987 phases these chemicals out.

Oxygen is the raw material from which ozone forms, not a cause of its loss. Hydrogen and radioactive waste are not the agents behind the ozone hole.

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