Environmental context. Perchlorate ion is a potentially harmful trace pollutant that can interfere with the proper absorption of iodine by the thyroid gland. Natural, atmospheric chemical sources of perchlorate have been proposed based on isotopic composition of mineral deposits, ground and surface waters. Understanding this natural source is crucial to the formulation of effective mitigation and control strategies.

Abstract. Observations of perchlorate (ClO₄^–) in a variety of environmental media imply that there is an atmospheric source related to nitrate and ozone photochemistry. Formation of perchlorate from chloride in the atmospheric condensed phase (cloud droplets, aerosol particles) requires considerable oxidation, most likely starting with the conversion to a 0 or +1 oxidation-state compound, a process that has been loosely termed ‘chlorine activation’. There are several features of mid-latitude tropospheric chlorine activation chemistry, insolubility of known precursors, nitryl chloride (ClNO₂) and molecular chlorine (Cl₂), and rapid and efficient cycling of hypochlorous acid (HOCl) to molecular chlorine, that make these pathways highly improbable candidates for the source of perchlorate. It is more probable that environments such as the chlorine-impacted stratosphere, or arid, high-salt desert regions have the necessary high concentrations of chlorine intermediates that could lead to perchlorate, although the detailed chemical steps are not completely known. Measurements of perchlorate in aerosol particles, and in polar snow, firn and ice cores, as well as further fundamental chemical kinetics studies are recommended.