Occurrence, latitudinal gradient and potential sources of perchlorate in the atmosphere across the hemispheres (31°N to 80°S).

Perchlorate (ClO) is harmful to human health, and knowledge on the levels and sources of natural ClO in different environments remains rather limited. Here, we investigate ClO in aerosol samples collected along a cross-hemisphere ship cruise between China and Antarctica and on a traverse between coastal East Antarctica and the ice sheet summit (Dome Argus). Perchlorate concentrations range from a few to a few hundred pg m. A clear latitudinal trend is found, with elevated ClO concentrations near populated areas and in the southern mid-high latitudes. Spatial patterns of atmospheric ClO over oceans near the landmasses support that terrestrial ClO is not transported efficiently over long distances. In the southern mid-latitudes, higher ClO concentrations in March than in November-December may be caused by significant stratospheric inputs in March. Perchlorate concentrations appear to be higher in the warm half than in the cold half of the year in the southern high latitudes, suggesting seasonal difference in main atmospheric sources. ClO may be formed in the reactions between chlorine free radical (Cl·) and ozone (O) in the stratosphere when Antarctic ozone hole occurs during September-October. And the stratosphere-produced ClO is moved to the boundary layer in several months and may be responsible for the high ClO concentrations in the warm half of the year. Perchlorate produced by photochemical reactions between O and Cl· in the Antarctic stratosphere is likely responsible for the higher ClO concentrations in Antarctica than in Arctic.