THE INFLUENCE OF OZONE ON LIGHT NONMETHANE HYDROCARBONS DURING CRYOGENIC PRECONCENTRATION

A number of recent measurement series of nonmethane hydrocarbons (NMHC s) based on in situ analysis report very low alkene concentrations in the remote troposphere. It was speculated that during preconcentration or thermal desorption of the sample, atmospheric ozone may react with the reactive hydrocarbons, e.g., alkenes. Therefore the behavior of o zone in different inlet systems at different conditions was investigat ed, in order to indicate where O-3 interferences may arise. The result s for the inlet and preconcentration system used for our measurements show that up to 50% of the ambient ozone is lost during passage of a h eated stainless steel inlet line. The remaining ozone is preconcentrat ed together with the hydrocarbons. During the process of thermal desor ption the remaining ozone is lost within minutes leading to a loss of reactive hydrocarbons of the order of 2-10% which is usually less than the error of measurement. These results were confirmed when different amounts of ozone were added to samples of pressurized air with modera te and low NMHC concentrations. For ozone mixing ratios df up to 100 p pb no significant change in the concentration of light alkenes was obs erved. The results show that our system used for cryogenic preconcentr ation of NMHC with subsequent thermal desorption is suitable for quant itative measurements even of reactive light alkenes in the atmosphere without an additional ozone trap.