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. "Gas and particle" means a system consisting of both gas and suspended liquid or solid particles in air (except cloud droplets). The chmical formula for o,p-DDD is C14H10Cl4. It is a member of the group of persistent organic pollutants (POPs). The IUPAC name is 1-chloro-2-[2,2-dichloro-1-(4-chlorophenyl)ethyl]benzene