Perchlorate in stratospheric aerosol particles.

Perchlorate is a toxic, regulated contaminant in drinking water. According to previous isotopic studies, much of the perchlorate deposited to the Earth's surface is formed in the atmosphere, with Cl suggesting a large contribution from the stratosphere. Here, we present measurements of perchlorate in stratospheric aerosol particles and confirm that the stratosphere is an important source of perchlorate, whereas we did not observe production in the troposphere. Mass mixing ratios of aerosol perchlorate in the stratosphere were 1 to 10 parts per trillion by mass (pptm), with the highest concentrations observed in summer and in the Southern Hemisphere. Almost all of the perchlorate is in biomass burning and nitrogen-rich particles, despite those types contributing only a few percent of the aerosol particles. Such particles are less acidic than the majority of sulfuric acid particles. If the formation of perchlorate is sensitive to acidity, then the injection of some materials for solar radiation modification might significantly increase the global production of perchlorate.