9.7 OH AND HO2 CONCENTRATIONS, PRODUCTION AND LOSS RATES AT THE LA PORTE SITE DURING TEXAQS 2000

The hydroxyl and hydroperoxy radicals (OH and HO2, collectively referred to as HOx) are extremely reactive species, causing most of the photochemical processing of trace gases in the atmosphere and influencing both ozone production and aerosol formation. OH and HO2 were observed during the TexAQS campaign in Houston in August/September 2000 with the Penn State GTHOS instrument (Ground-based Tropospheric Hydrogen Oxides Sensor) by laser induced fluorescence of OH. A detailed description of the technique is given in Stevens (1994). HO2 was quantitatively converted to OH by titration with NO. The 1σ precision for OH was about 2.5×105 molec/cm3 for 10-min averages or approximately 0.01 ppt. For HO2 the 1σ precision was approximately 0.1 ppt for 1-min averages. The 2-sigma-accuracy was ±40%, due to uncertainties of the absolute calibration. In addition total OH loss rates in ambient air were measured using the TOHLM expe-riment (Total OH Loss rate Measurement). OH produced by photolysis of water vapour is injected at different positions into ambient air pumped through a flow tube. The remaining OH at the end of the flow tube is measured by LIF. The slope of the OH signals plotted versus reaction time then gives the reaction rate of OH. A detailed description of the instrument can be found in Kovacs (2001). The total uncertainty for the measurements is estimated to ±25%