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Aging aerosol in a well-mixed continuous-flow tank reactor: an introduction of the activation time distribution

Abstract. Two approaches are commonly used to simulate atmospheric aging processes in
the laboratory. The experiments are either performed in large aerosol
chambers (several m3) in order to achieve extended observation times or
in small chambers (< 1 m3), compensating for the short
observation times by elevated reactant concentrations. We present an
experimental approach that enables long observation times at atmospherically
relevant reactant concentrations in small chamber volumes by operating the
aerosol chamber as a continuous-flow stirred tank reactor (CSTR). We
developed a mathematical framework that allows the retrieval of data beyond
calculating mean values, such as O3 exposure or equivalent
atmospheric aging time, using the new 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 ozone oxidation. We found very good agreement
between the experimental results and the theoretical predictions. This
experimental approach and data analysis concept can be applied for the
investigation of any transition in aerosol particles properties that can be
considered a binary system. Furthermore, we show how tact can be
applied to the analysis of data originating from other reactor types such as
oxidation flow reactors (OFRs), which are widely used in atmospheric
sciences. The new tact concept significantly supports the
understanding of data acquired in OFRs, especially those from deviating
experimental results in intercomparison campaigns.
