The thickness of the ozone layer from the base to the top of the atmosphere is measured in:
- (a)Dobson unit
- (b)Decibel unit
- (c)Angstrom unit
- (d)Centigrade unit
Correct — A, Dobson unit.
The stem describes ozone counted over a whole vertical shaft of air, from the base of the atmosphere up to its top. That column total is exactly the quantity the Dobson unit was created to report.
One Dobson unit is 0.01 mm. It is the thickness the ozone in that column would have if it were brought to 0 °C and one atmosphere of pressure and squeezed into a pure layer. A reading near 300 DU is about 3 mm of ozone.
Carry away the trick inside the definition: a Dobson unit converts a thinly spread column of gas into an equivalent thickness of pure gas.
- (b)Decibel unit — The decibel expresses a ratio of powers on a logarithmic scale; applied to sound it gives the sound level relative to a reference pressure of 20 µPa.
It is the unit of the zone-wise limits in the Noise Pollution (Regulation and Control) Rules, 2000, which set permissible levels for residential, commercial and industrial areas and for silence zones. A column of gas has no loudness to report.
- (c)Angstrom unit — The angstrom is a length equal to one ten-billionth of a metre, that is 0.1 nanometre. It is the unit a question wants when it asks about the wavelength of light or the size of atoms and molecules.
This option tempts because column ozone genuinely is expressible as a thickness, so a units check alone will not eliminate it.
What settles the matter is the reporting convention: the standard 300 DU column written in angstroms becomes about 30 million angstroms, which is why column ozone is reported in Dobson units and not in angstroms.
- (d)Centigrade unit — The centigrade degree, the older name for the Celsius degree, measures temperature.
In atmospheric science it supplies the temperature term in the lapse rate — about 6.5 °C per km through the troposphere — and in the warming with height inside the stratosphere that ozone's absorption of ultraviolet light produces.
It grades heat, not how much of a gas stands above a point on the ground.
Ozone is spread thinly through the atmosphere rather than sitting as a solid sheet, so its amount at a place is reported as a column total: all the ozone in a vertical shaft of air above one spot on the ground.
The Dobson unit makes that total tangible. Compress the column's ozone to 0 °C and one atmosphere, then measure how thick the resulting pure layer would be. Each 0.01 mm of it counts as one Dobson unit.
Total column ozone is the number that tells you whether the ultraviolet shield above a place is intact, which is why it, and not a concentration at one altitude, is the headline measurement.
Ground stations using Dobson spectrophotometers and satellite instruments both report in the same unit, so ground records and satellite data can be compared on one scale, which is how the springtime Antarctic depletion and the post-Montreal-Protocol recovery are tracked.
- One Dobson unit equals a layer of pure ozone 0.01 mm thick at 0 °C and one atmosphere of pressure.
- A total column reading near 300 Dobson units corresponds to roughly 3 mm of ozone under those standard conditions.
- The unit is named after G. M. B. Dobson, who developed the ground-based spectrophotometer that measures column ozone from sunlight.
- The Antarctic ozone hole is conventionally identified where total column ozone falls below 220 Dobson units.
- Most atmospheric ozone lies in the stratosphere, largely between about 15 and 35 km, but a column reading counts ozone at every height.
- The decibel expresses a logarithmic power ratio such as sound level, the angstrom measures length at atomic scale, and the centigrade degree measures temperature.
All four are real units of physical science; the ozone column is the quantity reported in Dobson units.
- The phrase 'from the base to the top of the atmosphere' signals a column total, not an altitude; read as a height question it steers you toward a length unit.
- The angstrom is dimensionally capable of expressing a thickness, so eliminating it needs the reporting convention rather than a units check.
- A Dobson unit is defined only after the column is compressed to 0 °C and one atmosphere; treating 0.01 mm as an in-situ stratospheric thickness misstates it.
- Ozone is asked from several different angles — for example where the layer sits, how its amount is measured, what destroys it, the date of the international observance — and an answer taken from the wrong angle can still look plausible.
- A concentration in ppm or ppb describes ozone at one level; it does not answer a question about the whole column.
This idea surfaces as a one-line unit question — MPPSC 2024 Paper-I asked the same fact in almost the same words, keyed to the Dobson unit.
Ozone also appears on other axes: which layer of the atmosphere holds it (CDS 2021-I), and multi-statement items covering the altitude band, how ozone is produced and its three-atom structure (CDS 2020-II). There, recalling the unit alone will not settle the answer.
MPPSC_2024_PRE_PaperI_Q52024The same fact asked in almost the same words — the unit for the thickness of the ozone layer, keyed to the Dobson unit. The company it keeps is close too: here it stands against a sound unit, the same angstrom (a length unit) and a temperature unit; what differs is that MPPSC set it against ppm and ppb — concentration units — instead of decibel and centigrade.
CDS_GK_2021_I_Q552021Same subject, a different axis of it. That item asks in which layer of the atmosphere the ultraviolet-absorbing ozone is found; this one asks in what unit its amount is expressed. Knowing the ozone maximum sits in the stratosphere does not by itself supply the unit, which is why the two are worth revising as a pair.
CDS_GK_2020_II_Q362020Also on ozone, but built as a find-the-incorrect-statement item that ranges across the altitude band, the way ozone is produced, the molecule's three-atom structure and the date of the international observance for the ozone layer. This UKPSC question narrows to one measurement convention; that one rewards holding several ozone facts at once. Two things to fix in memory while revising the pair: its altitude option prints 15-55 km, a wider band than the 15-35 km quoted in this card's key facts for where ozone concentrates; and the statement keyed as incorrect there is the observance date, because the International Day for the Preservation of the Ozone Layer is 16 September, not 16th November.
- practice — not a real PYQ
One Dobson unit corresponds to a layer of pure ozone of what thickness at 0 °C and one atmosphere of pressure?
- (a)0.001 mm
- (b)0.01 mm
- (c)0.1 mm
- (d)1 mm
Answerb — One Dobson unit is defined as a 0.01 mm thick layer of pure ozone at 0 °C and one atmosphere.The other three are the same definition shifted by powers of ten. They would make a 300 DU column 0.3 mm, 30 mm and 300 mm thick respectively, against the standard figure of about 3 mm.
- practice — not a real PYQ
The Dobson unit is used to express which one of the following?
- (a)The concentration of ozone at a single altitude
- (b)The total amount of ozone in a vertical column of the atmosphere
- (c)The intensity of ultraviolet radiation reaching the ground
- (d)The rate at which chlorofluorocarbons diffuse into the stratosphere
Answerb — The Dobson unit reports the ozone in a vertical column of air, summed over every height above a point on the surface.Option (a) is a mixing ratio at one level, given in ppm or ppb. Option (c) is a radiation intensity, carried by the ultraviolet index or by watts per square metre. Option (d) is a transport rate, which an amount of ozone does not describe.
- practice — not a real PYQ
Which one of the following pairs of a unit and the quantity it measures is correctly matched?
- (a)Dobson unit — loudness of sound
- (b)Decibel — total column ozone
- (c)Angstrom — wavelength of light
- (d)Degree centigrade — atmospheric pressure
Answerc — The angstrom, one ten-billionth of a metre, is the standard unit for wavelengths of light and for atomic dimensions, so this pairing holds.Options (a) and (b) swap two units: the sound reading belongs to the decibel and column ozone to the Dobson unit. Option (d) misassigns a temperature scale, since atmospheric pressure is reported in units such as the hectopascal or the millibar.