Mass concentration means mass per unit volume and is used in the construction mass_ concentration_ of_ X_ in_ Y, where X is a material constituent of Y. A chemical species denoted by X may be described by a single term such as "nitrogen" or a phrase such as "nox_ expressed_ as_ nitrogen". "Aerosol" means the system of suspended liquid or solid particles in air (except cloud droplets) and their carrier gas, the air itself. Aerosol particles take up ambient water (a process known as hygroscopic growth) depending on the relative humidity and the composition of the particles. The chmical formula for o,p-DDT is is C14H9Cl5. It is a member of the group of persistent organic pollutants (POPs). The IUPAC name is 1-chloro-2-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene