10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Temperature Matters More than Concentration: CCN-activation of Soot after Exposure to Ozone under Atmospheric Conditions
FRANZ FRIEBEL, Amewu Mensah, ETH Zürich
Abstract Number: 1472 Working Group: Carbonaceous Aerosol
Abstract Freshly emitted soot particles are known to be poor cloud condensation nuclei (CCN), but from atmospheric measurements it can be deduced that a significant fraction of soot particles act as CCNs. One process which might contributes to this discrepancy is the heterogeneous oxidation of soot particles. Soot particles have an average atmospheric lifetime of one week. During this time, they are exposed to different aging processes, like oxidation with ozone, which might change how the soot particles interact with water.
The investigation of those processes is an experimentally challenging task, due to the long time span which should be covered. Many studies were conducted in which soot was treated with oxidants at concentrations exceeding the atmospheric average by a factor of 1000. This approach reduces the needed observation time, but in case of ozone it does not consider the non-linear reaction kinetic with soot.
In Winter 2017/18 we conducted a lab campaign at ETH Zurich where we investigated the reaction of ozone with soot particles, under atmospheric conditions. With the CSTR-approach we could observe 100nm size selected soot particle up to 16 h. The particles were exposed to Ozone concentration between 22 and 220 ppb at a temperature range of 5 to 35°C. We measured the CCN-activity at different super saturations between 0.3% to 1.4 %.
It was found, that an increase of the ozone concentration at 25°C, by a factor of 10 (from 22 to 220ppb) lead to a reduction in the activation time by factor 1.8 (from 7h to 3h:50min at 1.0 % super saturation). A temperature increase from 5°C to 35°C at a constant Ozone concentration of 200 ppb lead to an reduction of the activation time by factor 5. Our results indicate, that for an assessment of the cloud formation potential of soot particles the ambient temperature should be considered as much as the ozone concentration.
According to the literature, the reaction kinetics can be best described by the Langmuir-Hinshelwood-mechanism, where the reaction is initiated by the adsorption of an Ozone-monolayer. The speed of the reaction is approximately proportional to concentration of ozone adsorbed on the soot surface, but not to the gas phase concentration. [1] This agrees well with our observations. After exposing soot particle several minutes to different Ozone concentrations, the particle diameter increased by 3% and the particle mass by 20%, but remained almost constant during further exposure. Additionally the ozone concentration in the gas phase had little influence on the activation time.
Based on our temperature experiments we calculated an activation energy of 40 kJ/mol for the time limiting step causing CCN-activation. This value is in accordance with previous findings and indicates that the limiting reaction step for the CCN-activation is a conversion of physisorbed Ozone into organic Ozonides. [2]
[1] S. Kamm et.al, The heterogeneous reaction of ozone with soot aerosol, Atmos. Environ. 33 (1999) 4651-4661 [2] T. Berkemeier et.al, Ozone uptake on glassy, semi-solid and liquid organic matter and the role of reactive oxygen intermediates in atmospheric aerosol chemistry. Phys.Chem.Chem.Phys., 2016, 18, 12662