OH-initiated Oxidation of Carbonaceous Surfaces at Tropospheric Temperatures

YONG LIU, Andrey Ivanov, Mario Molina

University of California San Diego

     Abstract Number: 44
     Last modified: March 27, 2011

Abstract
Despite recent studies showing rapid heterogeneous loss of OH on carbonaceous surfaces at room temperature, our current understanding about chemical transformation of carbonaceous aerosols under upper tropospheric condition and its impacts on chemical balance of other species remains very limited. In this work, we investigate chemical transformation of carbonaceous materials initiated by a heterogeneous reaction with OH over a wide range of temperatures (218-300 K) using a flow reactor coupled to chemical ionization mass spectrometer (CIMS). Self-assembled monolayers and thin organic films (alkane, alkene, alkane and alkene acid, aromatic, and methane-soot films) are selected as proxies for atmospheric carbonaceous aerosols. Experimental results showed that heterogeneous loss of OH onto the carbonaceous surfaces is highly efficient with an uptake probability close to unity and nearly independent of temperatures relevant to the troposphere. On the other hand, product studies by CIMS and FTIR showed that the distribution of gas- and condensed-phase products is strongly temperature dependent likely due to different temperature dependence of RO reactions, such as C-C bond scission, HO2 elimination, and isomerization. The obtained results suggest that higher temperatures favor volatilization, whereas HO2 elimination becomes dominant at low temperatures. Additional modeling confirms that OH heterogeneous loss to carbonaceous aerosol is negligible relative to OH homogeneous loss by reactions with CH4, CO2, and NO2 near the surface. Modeling results also suggest that the heterogeneous sink for OH radicals could become more comparable with the homogeneous one, and therefore, to affect the OH lifetime under the upper tropospheric conditions.