The cosmogenic radioisotopes 38Cl(t1/2=37.3 min.) and 39Cl (t1/2=55.5 min.) are formed in the stratosphere in detectable quantities, and participate in stratospheric chlorine cycles. The predominant expected chemical forms for these for these radioisotopes are C10 and/or ClONO2 throughout most of the stratosphere. The rates for formation of HCl are sufficiently slow that most 38Cl and 39Cl atoms undergo radioactive decay before ever reacting to form HCl.

The measurement of the fraction of 38Cl and 39Cl in the forms ClO and ClONO2, with a concurrent NO2 measurement, can be used to measure in situ the stratospheric formation rate for ClONO2. Such experiments could be conducted at altitudes accessible to aircraft experiments.