Abstract. Simulating atmospheric aging processes in the laboratory under atmospheric conditions typically requires large aerosol chambers (several m3) in order to achieve extended observation times. We developed an experimental approach that enables long observation times in small chamber volumes by operating the aerosol chamber in a Continuous Flow Stirred Tank Reactor (CSTR) mode. We present a mathematical framework which allows the retrieval of data beyond calculating mean values using the newly introduced metric activation time (tact). This concept was developed and successfully tested to characterize the change in cloud condensation nuclei (CCN) activity of soot particles due to heterogeneous oxidation with ozone. We show that this concept can be applied to other systems investigating non-gradual transitions. The change in CCN-activity was parameterized with tact and agreed well with theoretical predictions. Furthermore we show how tact can be applied for the analysis of data originating from other oxidation flow reactors widely used in atmospheric sciences. This concept allows to explain discrepancies found in intercomparison of different chambers.