On the age of stratospheric air and inorganic chlorine and bromine release

Abstract

We estimate the average transport time from the tropical tropopause to various regions of the northern hemisphere lower stratosphere (stratospheric age) using simultaneous mixing ratio measurements of CFC-115 and CO2 measured by the Whole Air Sampler (WAS) during Airborne Arctic Stratospheric Expedition II (AASE II). Our inferred ages are consistent with those presented in previous studies. We discuss sources of uncertainties that affect age estimates in general, as well as specific uncertainties arising from inferring ages using CO2 and CFC-115 abundances. We infer inorganic chlorine (Cly) and bromine (Bry) at various lower stratospheric locations using the WAS organic chlorine and bromine measurements in combination with modeled tropospheric halocarbon trends and with our estimated ages. Inferred Cly and Bry abundances generally increase with increasing latitude and altitude. For our analyzed locations inside the polar vortex, we estimate a maximum Cly abundance of about 2.7 parts per billion by volume (ppbv) and a maximum Bry abundance of about 13.7 parts per trillion by volume (pptv). The locations of these maxima correspond to an average N2O mixing ratio of about 100 ppbv, and to a fractional dissociation of organic chlorine and bromine of 0.85 and 0.90, respectively. Finally, we discuss the expected future limitations of using CFC-115 to estimate stratospheric age due to the production limitations prescribed by the amendments and adjustments to the Montreal Protocol.