Explain parts per million (ppm) in brief.

(N/A) When a solute is present in trace quantities,it is convenient to express concentration in parts per million $(ppm)$.
Parts per million = $\frac{\text{Number of parts of the component}}{\text{Total number of parts of all components}} \times 10^{6}$
As in the case of percentage,concentration in $ppm$ can also be expressed as mass to mass,volume to volume,or mass to volume.
For example,a litre of sea water (which weighs $1030 \ g$) contains about $6 \times 10^{-3} \ g$ of dissolved oxygen $(O_2)$. Such a small concentration is expressed as $5.8 \ g$ per $10^{6} \ g$ $(5.8 \ ppm)$ of seawater. The concentration of pollutants in water or atmosphere is often expressed in terms of $\mu g \ mL^{-1}$ or $ppm$.