10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Well Mixed, Ambient Conditions and Long Observation Times: Aging Experiments in a CSTR
FRANZ FRIEBEL, Amewu Mensah, ETH Zürich
Abstract Number: 1467 Working Group: Oxidation Flow Reactor: Development, Characterization, and Application to Aerosols
Abstract Aerosol particles which are emitted into the atmosphere are exposed to different reactants and therefore change their properties. The average atmospheric lifetime ranges from several hours to more than a week. Experimental approaches which investigate the occurring changes in aerosol particle properties should ideally cover these time scales. Generally, there are two approaches to mimic these time frames. One, the concentration of reactants, e.g. Ozone or OH-radicals can be increased to trigger faster reaction rates e.g. Potential Aerosol Mass (PAM) Oxidation Flow Chambers. Two, larger aerosol tanks can be constructed to extend the observation time e.g. AIDA-Chamber at KIT. Both approaches imply their specific challenges. Treating aerosols with high concentrations of oxidants bears the risk that atmospheric processes are not represented well. An example is the heterogenous oxidation of soot particles with ozone, where the reaction mechanism follows a Langmuir-Hinshelwood kinetic where an elevated ozone concentration does not lead to a faster reaction. Extending the observation time by extending the tank volume is often technically and financially challenging.
Here we present an approach that makes use of the continuously stirred tank reactor (CSTR), a concept well known in chemical engineering but rarely applied in atmospheric science. This approach decouples long observation times from high chamber volumes thereby allowing low reactant concentrations and low aerosol flows.
Generally, aerosol tanks get filled once and provide a flow of aged aerosol until the reservoir is exploited (batch mode). In contrary to that, the CSTR aerosol tank is continuously filled with fresh aerosol and provides a constant sample flow over an unlimited time scale after the tank reached steady state conditions. The sample flow does not consist of a uniform aerosol sample but of a mix of aerosol samples at different aging stages. Nevertheless, statistical challenges in subsequent data analysis can be disentangled by incorporation of the well-defined aerosol age distribution.
From Summer 2016 till winter 2017/18 we conducted several lab campaigns at ETH Zurich where we successfully applied the CSTR aerosol tank concept. Size selected soot particles were exposed to different ozone concentrations (0-200 ppb), at a temperature range of 5°C to 35°C and at different levels of humidity (0-80%) in a 3 m3 stainless steel tank. The setup generated a sample flow of 25 lpm and allowed several instruments to measure the identical and undiluted sample flow for up to 16 h.
An increase of ozone gas phase concentration by a factor of 10 results in a reduction of CCN activation time of only 1.7. At the contrary, a reaction temperature increase of 10 K results in a decrease of activation time by a factor of 1.5 - 2.0. Here, we focus on the underlying principles of CSTR-data analysis and present the advantages of this experimental approach allowing for new insights into atmospheric ageing processes.