Which of the following is measured in Dobson Units?
Stratospheric Ozone
Understanding Dobson Units and Stratospheric Ozone Measurement
Atmospheric scientists use various units to measure the concentration or total amount of gases in the atmosphere. The question asks which specific atmospheric component is measured using Dobson Units.
What are Dobson Units (DU)?
Dobson Units ($\text{DU}$) are a unit of measurement used primarily to quantify the total amount of a trace gas in a vertical column stretching from the ground to the top of the atmosphere. If you were to take all the gas in this column and compress it to standard temperature ($0^\circ\text{C}$) and standard pressure ($1 \text{ atmosphere}$), the thickness of the resulting layer would be measured in Dobson Units.
One Dobson Unit ($\text{DU}$) is defined as a layer of gas that is $0.01$ millimeters thick under standard temperature and pressure. For example, $300 \text{ DU}$ of ozone would form a layer $3 \text{ mm}$ thick if compressed to $0^\circ\text{C}$ and $1 \text{ atm}$.
What is Stratospheric Ozone?
Stratospheric ozone is ozone ($\text{O}_3$) located primarily in the stratosphere, a layer of the Earth's atmosphere between about $10$ and $50$ kilometers altitude. This region is commonly referred to as the ozone layer. The ozone layer plays a vital role in absorbing most of the Sun's harmful ultraviolet (UV) radiation, protecting life on Earth.
Measurement of Stratospheric Ozone in Dobson Units
The total column ozone, which includes about $90\%$ stratospheric ozone, is typically measured in Dobson Units. Instruments like the Dobson spectrophotometer or satellite-based sensors measure the amount of UV radiation absorbed by ozone in the atmospheric column. From this absorption, the total amount of ozone present is calculated and expressed in Dobson Units. A typical value for the total column ozone is around $300 \text{ DU}$.
Analyzing the Options
Let's look at why the other options are not typically measured in Dobson Units:
- Chloro Fluoro Carbon (CFC): CFCs are chemicals that were used in refrigerants and aerosols. Their concentration in the atmosphere is very low and is usually measured in parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt) by volume or mass, which represent ratios of molecules or mass. They are not measured for their total column thickness in Dobson Units.
- Ultraviolet Rays: Ultraviolet rays are a type of electromagnetic radiation (energy). Radiation is measured by its intensity or flux, typically in units like Watts per square meter ($\text{W/m}^2$) or through indices like the UV Index, not in units of gas thickness like Dobson Units.
- Solar Radiation: Solar radiation is the total energy received from the Sun. Like UV rays, it is radiation and is measured by its intensity or power, usually in Watts per square meter ($\text{W/m}^2$). It is not a gas measured by its thickness in Dobson Units.
Conclusion
Based on the definitions and standard scientific practice, Dobson Units are the standard unit for measuring the total amount of ozone in a column of the atmosphere, which is predominantly stratospheric ozone.
Revision Table: Atmospheric Measurements
| Substance / Phenomenon | Common Measurement Unit(s) | Description of Measurement |
|---|---|---|
| Stratospheric Ozone (Total Column Ozone) | Dobson Units (DU) | Total amount (equivalent thickness at STP) in a vertical column of air. |
| Chloro Fluoro Carbons (CFCs) | ppm, ppb, ppt (concentration) | Ratio of CFC molecules to total air molecules. |
| Ultraviolet (UV) Radiation | $\text{W/m}^2$ (intensity), UV Index | Energy flux per unit area. |
| Solar Radiation | $\text{W/m}^2$ (intensity) | Total energy flux from the Sun per unit area. |
Additional Information on Ozone and the Atmosphere
The measurement of stratospheric ozone in Dobson Units is crucial for monitoring the health of the ozone layer. Scientists use this measurement to track changes over time, such as the formation and recovery of the ozone hole over Antarctica. The decrease in ozone levels, particularly in the stratosphere, allows more harmful UV-B radiation to reach the Earth's surface, posing risks to human health (like skin cancer and cataracts) and ecosystems.
International agreements, such as the Montreal Protocol, have led to the phasing out of ozone-depleting substances like CFCs, which were causing the thinning of the ozone layer. Monitoring ozone levels in Dobson Units helps assess the effectiveness of these global efforts.
Important Questions from Miscellaneous
A stone is thrown horizontally from the top of a 20 m high building with a speed of 12 m/s. It hits the ground at a distance R from the building. Taking g = 10 m/s2 and neglecting air resistance will give :
A sphere of volume V is made of a material with lower density than water. While on Earth, it floats on water with its volume f1V (f1 < 1) submerged. On the other hand, on a spaceship accelerating with acceleration a < g (g is the acceleration due to gravity on Earth) in outer space, its submerged volume in water is f2V. Then:
A railway wagon (open at the top) of mass M1 is moving with speed v1 along a straight track. As a result of rain, after some time it gets partially filled with water so that the mass of the wagon becomes M2 and speed becomes v2. Taking the rain to be falling vertically and the water stationery inside the wagon, the relation between the two speeds v1 and v2 is :
Consider the following statements:
1. Distance between the longitudes becomes zero on North Pole and South Pole.
2. Distance between the longitudes is maximum on the Equator.
3. Number of longitudes is more than number of latitudes.
Which of the statements given above is/are correct?
One block of 2⋅0 kg mass is placed on top of another block of 3⋅0 kg mass. The coefficient of static friction between the two blocks is 0⋅2. The bottom block is pulled with a horizontal force F such that both the blocks move together without slipping. Taking acceleration due to gravity as 10 m/s2, the maximum value of the frictional force is :
