Ozone Concentration versus Temperature: Atmospheric Aging of Soot Particles

The oxidation of soot particles with ozone (O₃) increases the particles' ability to act as cloud condensation nuclei (CCN). To assess if this process is a relevant source for CCN in the atmosphere, the reaction rate at atmospheric conditions must be known. Here we investigate the increase in CCN activity of soot particles rich in organic carbon at O₃ concentrations ranging from 0-200 ppb and between 5 and 35 °C. We operated an ∼3 m³ aerosol chamber as a continuous-flow stirred tank reactor which allows for aging times of up to 12 h and beyond and of particle size selection prior to the aging step. We applied the activation time (t_act) concept to retrieve kinetic data. It was found that 100 nm soot particles can be CCN-active down to supersaturations of 0.3% after 12 h of exposure to 200 ppb O₃ at 35 °C. The reaction rate was found to be not directly proportional to the O₃ concentration. Instead, a Langmuir-type reaction kinetic was found to be the best fit to parametrize the reaction rates. The initial reaction step is therefore the adsorption of O₃ molecules, which could be detected by an increase in the particle diameter of up to 3.7 nm within several minutes after exposure. The increase in particle diameter agrees well with the calculated change in the O₃ surface coverage, which was obtained from CCN activation data under the assumption of a Langmuir-sorption isotherm. Further, we found that a temperature increase from 5 to 35 °C increases the reaction rate by a factor of 5 which corresponds to an activation energy of 38.5 kJ·mol^-1. Extrapolation to atmospheric conditions allows for the conclusion that the temperature is as important as the O₃ concentration for the CCN activation of soot particles within the atmospheric range.