Dobson units are used to measure thickness of :

The correct answer is Ozone.

Step-by-step Explanation:

1. Understanding the Dobson Unit (DU):
A Dobson Unit is a unit of measurement used to quantify the total amount of a trace gas, particularly ozone (O₃), in a vertical column of the Earth's atmosphere. It is named after the British physicist and meteorologist G. M. B. Dobson, who developed the spectrophotometer used to measure atmospheric ozone from the ground.

2. Physical Definition:
One Dobson Unit (1 DU) is defined as the quantity of ozone that would form a layer of pure ozone gas exactly 0.01 millimeters (10 micrometers) thick under standard temperature and pressure (STP), which is 0 degrees Celsius and 1 atmosphere of pressure.

3. Significance and Scale:
A typical column of ozone in the Earth's atmosphere has a thickness of about 300 Dobson Units (300 DU). This means that if all the ozone distributed throughout the atmosphere were brought down to the Earth's surface at STP, it would form a layer only 3 millimeters thick. This layer, located primarily in the stratosphere, plays a vital role in shielding life on Earth by absorbing harmful ultraviolet (UV) radiation from the Sun.