Comparison of two Calibration Methods for Atmospheric HOx Measurements

It is now well established that HOX (= OH + HO2 ) radicals measurements are needed to provide robust modeling of atmospheric chemistry. During the last two decades, a few instruments based on fundamentally different techniques have been built around the world (DOAS, FAGE, CIMS). Some inter- comparisons have been done and the results give confidence into HOX measurements. Among these different techniques, the latter two require a calibration system to determine the instrument's response to HOX radicals. One method, widely used in the field to generate a standard of these radicals, relies on the UV-photolysis of water-vapor inside high-purity air flow (H2 O --> OH + H, H + O2 + M --> HO2 + M). The use of a single calibration method is a main concern and in order to avoid a systematic error from the calibration step, complementary accurate calibration methods are desirable. A promising method involves the ozonolysis of alkenes as a source of OH radicals. This method relies on the steady-state OH concentration produced from ozone-alkene reactions in a flow-tube reactor. We have recently built two different calibration systems based on the methods quoted above and these systems have been used to calibrate our Laser-Induced Fluorescence instrument. The agreement between both techniques will be discussed in detail and we will present our conclusions about the advantages and drawbacks of both methods.