Reactive uptake of ozone by proxies for organic aerosols: Surface versus bulk processes

Uptake measurements of ozone were conducted with two types of proxies for a tmospheric organic aerosols: organic liquids and self-assembled organic mon olayers. Alkanes and terminal alkenes were used. The monolayer surface was characterized, prior to and after reaction, using IR spectroscopy. Uptake e xperiments were conducted using a flow tube reactor coupled to a chemical i onization mass spectrometer. The reactive uptake coefficient, gamma, is sho wn to be due to reaction with the double bond. For the monolayers, gamma is composed solely of a surface reactive component and is smaller by at least an order of magnitude than values obtained for a liquid of the same chain length. Uptake by the liquids is higher due to solubility and reaction ill the bulk. The phase of the atmospheric organic aerosol will determine the a ppropriate use of a bulk or surface uptake probability in atmospheric model s. Since the aerosol surface is processed and sites are consumed, gamma is time variant. We define a parameter gamma' as the surface uptake probabilit y per reactive site and determine its value as 9 x 10(-19) cm(2) molecule(- 1). This enables the modeling of surface reactions as surface site concentr ations diminish following interaction with the gaseous species.