Kinetic model framework for aerosol and cloud surface chemistry and gas-particle interactions – Part 1: General equations, parameters, and terminology

Abstract

Abstract. Aerosols and clouds play central roles in atmospheric chemistry and physics, climate, air pollution, and public health. The mechanistic understanding and predictability of aerosol and cloud properties, interactions, transformations, and effects are, however, still very limited. This is due not only to the limited availability of measurement data, but also to the limited applicability and compatibility of model formalisms used for the analysis, interpretation, and description of heterogeneous and multiphase processes. To support the investigation and elucidation of atmospheric aerosol and cloud surface chemistry and gasparticle interactions, we present a comprehensive kinetic model framework with consistent and unambiguous terminology and universally applicable rate equations and parameters. It enables a detailed description of mass transport and chemical reactions at the gas-particle interface, and it allows linking aerosol and cloud surface processes with gas phase and particle bulk processes in systems with multiple chemical components and competing physicochemical processes.The key elements and essential aspects of the presented framework are: a simple and descriptive double-layer surface model (sorption layer and quasi-static layer); straightforward flux-based mass balance and rate equations; clear separation of mass transport and chemical reactions; well-defined and consistent rate parameters (uptake and accommodation coefficients, reaction and transport rate coefficients); clear distinction between gas phase, gas-surface, and surfacebulk transport (gas phase diffusion, surface and bulk accommodation); clear distinction between gas-surface, surface layer, and surface-bulk reactions (Langmuir-Hinshelwood and Eley-Rideal mechanisms); mechanistic description of The outlined double-layer surface concept and formalisms represent a minimum of model complexity required for a consistent description of the non-linear concentration and time dependences observed in experimental studies of atmospheric multiphase processes (competitive co-adsorption and surface saturation effects, etc.). Exemplary practical applications and model calculations illustrating the relevance of the above aspects are presented in a companion paper (Ammann and Pöschl, 2007).We expect that the presented model framework will serve as a useful tool and basis for experimental and theoretical studies investigating and describing atmospheric aerosol and cloud surface chemistry and gas-particle interactions. It shall help to end the "Babylonian confusion" that seems to inhibit scientific progress in the understanding of heterogeneous chemical reactions and other multiphase processes in aerosols and clouds. In particular, it shall support the planning and design of laboratory experiments for the elucidation and determination of fundamental kinetic parameters; the establishment, evaluation, and quality assurance of comprehensive and self-consistent collections of rate parameters; and the development of detailed master mechanisms for process mo...

Organic Constituents on the Surfaces of Aerosol Particles from Southern Finland, Amazonia, and California Studied by Vibrational Sum Frequency Generation

“…13,14 Similar to the case of the early research on stratospheric ozone depletion, 15 the lack of molecular information regarding these key processes has led to the situation where mismatches between atmospheric field measurements and computer simulations are often attributed to unknown chemistry. As a result, the processes that drive much of the chemistry in the climate system are the focus of intense, often multi-investigator chemical research programs.…”

Radial Diffusion and Penetration of Gas Molecules and Aerosol Particles through Laminar Flow Reactors, Denuders, and Sampling Tubes

“…where [X] gs is the concentration of X in the near-surface gas phase, within about one mean free path, λ X , of the surface 46 (see Figure 1), and ω X is the thermal molecular velocity of species X. In case of significant uptake, [X] gs is usually smaller than the average concentration of X in the gas phase away from the surface, [X] g (if it is assumed that X is homogeneously distributed).…”

“…Following the PRA approach for spherical particles, 46 the net flow of gas-phase diffusion through a virtual tube envelope for an absorbing cylinder can be expressed as…”

“…As outlined by Poschl et al, 46 C g,X can also be converted into a gas-phase diffusion conductance term Γ g,X for application in traditional resistor model formulations of gas uptake: 75,76 …”

“…40−42 For spherical surfaces and application to aerosol particles, Fuchs 43 and Fuchs and Sutugin 44 provided analytical solutions to the problem. Following up on their approaches, Poschl, Rudich, and Ammann (PRA) 45,46 derived and introduced a gas-phase diffusion correction factor that provides a straightforward description and robust quantification of gas-diffusion effects on gas−particle interactions in aerosols, clouds, and related kinetic models. 45−52 The most commonly applied methods for prediction of penetration and gas-phase diffusion in flow-tube reactors and denuders are the numerical method by Brown 53 and interpolation method by Cooney−Kim−Davis (CKD).…”