Influence of Relative Humidity on Multiphase Chemical and Photochemical Kinetics
Abstract
Organic aerosol particles are exposed to air which can be either dry or humid. They can take up water from the air or dry out depending on their trajectory in relative humidity (RH). Water acts as a plasticizer to viscous organic matter and therefore, their atmospheric trajectory can lead to a large range of viscosity through the RH and temperature space. It has been reasonably well established that increasing viscosity leads to increasing molecular diffusion and multiphase reaction kinetic limitations. For chemical processes without light, the standard heterogeneous kinetics framework can be used to explain humidity dependent kinetics by taking into account measured or estimated diffusivity of the reactive trace gas under question. Cases where well-mixed conditions are not granted require depth resolved modelling, but still using established transport and reaction kinetics. For photochemical processes, the case is more challenging. We present an example of an indirect photochemical process, the degradation of citric acid initiated by imidazole carboxaldehyde as a photosensitizing brown carbon compound. Light absorption triggers a triplet state excitation, formation of a reduced ketyl radical which transfers an H atom or electron to an acceptor such as O2 and finally the formation of HOx species. We have quantified this indirect chemistry by observing the flux of HO2 to the gas phase. We take a step further and also attempt at understanding Fe(III) induced degradation of secondary organic aerosol with similar methods, but also including measurement of volatile products released to the gas phase. In combination, we derive optimum photochemical processing rates under a wide range in RH and present a discussion of the driving chemical kinetics and parameters we know with confidence and those which remain unconstrained.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A54A..07A
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0320 Cloud physics and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0368 Troposphere: constituent transport and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES