Abstract
Abstract. Heterogeneous reactions are important to atmospheric chemistry and are therefore an area of intense research. In multiphase systems such as aerosols and clouds, chemical reactions are usually strongly coupled to a complex sequence of mass transport processes and results are often not easy to interpret. Here we present a systematic classification scheme for gas uptake by aerosol or cloud particles which distinguishes two major regimes: a reaction-diffusion regime and a mass transfer regime. Each of these regimes includes four distinct limiting cases, characterised by a dominant reaction location (surface or bulk) and a single rate-limiting process: chemical reaction, bulk diffusion, gas-phase diffusion or mass accommodation. The conceptual framework enables efficient comparison of different studies and reaction systems, going beyond the scope of previous classification schemes by explicitly resolving interfacial transport processes and surface reactions limited by mass transfer from the gas phase. The use of kinetic multi-layer models instead of resistor model approaches increases the flexibility and enables a broader treatment of the subject, including cases which do not fit into the strict limiting cases typical of most resistor model formulations. The relative importance of different kinetic parameters such as diffusion, reaction rate and accommodation coefficients in this system is evaluated by a quantitative global sensitivity analysis. We outline the characteristic features of each limiting case and discuss the potential relevance of different regimes and limiting cases for various reaction systems. In particular, the classification scheme is applied to three different datasets for the benchmark system of oleic acid reacting with ozone in order to demonstrate utility and highlight potential issues. In light of these results, future directions of research needed to elucidate the multiphase chemical kinetics in this and other reaction systems are discussed.
Highlights
The conceptual framework enables efficient comparison of different studies and reaction systems, going beyond the Tropospheric aerosols are composed of organic and inorscope of previous classification schemes by explicitly resolving interfacial transport processes and surface reactions limgclaensicansdubfrsotamncceosndoeringsiaOntaiotcinnegoaffgrnoamsS-pdhciariseeecntspecemceiisessio(Knaonfakpiadrotiuited by mass transfer from the gas phase
The most striking differences between these cases and previous schemes is the under-representation of the mass transfer regime: Worsnop et al offer a mass transfer-limited case, this only applies to a bulk reaction and is not necessarily a case limited by a single process
It represents a range of cases, all of which fall within our definition of the Bmt regime
Summary
The conceptual framework enables efficient comparison of different studies and reaction systems, going beyond the Tropospheric aerosols are composed of organic and inorscope of previous classification schemes by explicitly resolving interfacial transport processes and surface reactions limgclaensicansdubfrsotamncceosndoeringsiaOntaiotcinnegoaffgrnoamsS-pdhciariseeecntspecemceiisessio(Knaonfakpiadrotiuited by mass transfer from the gas phase. Most previous studies of the gas uptake into aerosol particles have used “resistor” models which account for physical and chemical processes for a single or at most a few physical domains within the aerosol particle by analogy to electrical circuits (an overview of resistor models in the canonical system of oleic acid–ozone heterogeneous reaction is given in Zahardis and Petrucci, 2007). Such models allow analytical expressions to be derived for uptake of trace gases or loss of condensed phase material in simplified, limiting cases. This taxonomy will be useful as a common ground for discussion of heterogeneous chemical processes and as a tool for analysis
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