Refers to the presence of ozone at the earth's surface. The term is also used in reference to the episodic observation of near total ozone depletion that occurs in certain regions of the arctic marine boundary layer in springtime. This ozone depletion (not to be confused with the stratospheric polar ozone hole) likely occurs as the result of bromine- and chlorine-catalyzed chemistry. The sources of the halogens are not well understood, but it is likely that they emanate from sea salt. See photochemical air pollution.