Dobson units are used to measure thickness of :

Dobson units (DU) are a specific unit of measurement used in atmospheric science.

One Dobson unit is defined as the thickness of a gas layer that would occupy 0.01 mm (10 µm) if it were compressed to standard temperature and pressure (STP) and spread uniformly over a 1 cm² area.

Mathematically, this can be expressed as:

$\hspace{1em}1 DU\; =\; 0.01 mm · (1 cm²)⁻¹\; \hspace{1em}(at STP)$

This definition is particularly useful for quantifying the total amount of a trace gas present in a vertical column of the atmosphere, from the surface to the top of the atmosphere.

Among the atmospheric constituents, the column amount of ozone (O₃) varies seasonally and geographically, and scientists need a convenient way to express its total vertical abundance.

Therefore, Dobson units are specifically employed to measure the **thickness of the ozone layer** in the Earth's atmosphere.

Consequently, the correct answer to the question “Dobson units are used to measure thickness of:” is **Ozone**.