Heterogenous Chemistry of HO2 Radicals in the Troposphere

Heterogeneous chemistry in the stratosphere is of great importance, for example the heterogeneous reactions on the polar stratospheric clouds and its impact on the ozone depletion. In the troposphere the heterogeneous reactions on liquid cloud droplets lead to acid precipitation. The gase phase photochemistry of free radicals is generally considered to be too fast to be relevant. However, the relatively low overall reactivity of HOx radicals results in a typical atmospheric lifetime of several minutes, and therefore it can collide many times with atmospheric aerosols. When comparing results from field measurements with results from gas phase kinetics models, which are based solely on conventional gas phase chemical reaction schemes, one finds an overestimation of the OH and HO2 radical concentration by ~20-100%. From calculations and laboratory experiments there exists the strong indication for the existence of a heterogeneous HOx loss mechanism on aerosol particles which are typical of polluted environment. There is a clear need for improved characterization of the physico-chemical interaction between HOx radicals and tropospheric non-cloud aerosols in the laboratory, before its effect on tropospheric models can be assessed more quantitatively. In the proposed project I want to measure the rates of reaction of HO2 radicals with atmospherically relevant solid surfaces of salts and minerals representative of major inorganic components in tropospheric aerosols in dependence on temperature and relative humidity. For the kinetic studies I will employ a flow tube apparatus, the surrogates for the tropospheric surfaces will be deposited as a film on the walls of a cylindrical tube, the gas phase radicals will be monitored with chemical ionization mass spectrometry (CIMS). The results will contribute to the development of improved heterogeneous kinetic models of tropospheric HO2 chemistry.