Mean age of stratospheric air derived from AirCore observations

Mean age of stratospheric air can be derived from observations of sufficiently long-lived trace gases with approximately linear trends in the troposphere. Mean age can serve as a tracer to investigate stratospheric transport and long-Term changes in the strength of the overturning Brewer-Dobson circulation of the stratosphere. For this purpose, a low-cost method is required in order to allow for regular observations up to altitudes of about 30ĝ€km. Despite the desired low costs, high precision and accuracy are required in order to determine mean age. We present balloon-borne AirCore observations from two midlatitude sites: Timmins in Ontario/Canada and Lindenberg in Germany. During the Timmins campaign, five AirCores sampled air in parallel with a large stratospheric balloon and were analysed for CO2, CH4 and partly CO. We show that there is good agreement between the different AirCores (better than 0.1ĝ€%), especially when vertical gradients are small. The measurements from Lindenberg were performed using small low-cost balloons and yielded very comparable results. We have used the observations to extend our long-Term data set of mean age observations at Northern Hemisphere midlatitudes. The time series now covers more than 40 years and shows a small, statistically non-significant positive trend of 0.15ĝ€±ĝ€0.18 yearsĝ€decadeĝ'1. This trend is slightly smaller than the previous estimate of 0.24ĝ€±ĝ€0.22 yearsĝ€decadeĝ'1 which was based on observations up to the year 2006. These observations are still in contrast to strong negative trends of mean age as derived from some model calculations.