H2O2 and CH2O

M. Hutterli and R. Bales

University of Arizona

In-situ observations of H2O2 and CH2O at the surface during the first year (2003) of ANTCI are critical to understanding both HOx chemistry and snow chemistry. Snow emission of CH2O and H2O2 have been found to be closely linked to the HOx and NOx chemistry, and estimated emission fluxes are large enough to influence HOx concentrations in the boundary layer. Gas phase CH2O and H2O2 measurements will be performed at the South Pole during 2003 using established methods by drawing air and pumping CH2O and H2O2 free carrier solution (water) together into coil scrubbers resulting in 100% collection efficiencies. Subsequently, the CH2O and H2O2 contents in the carrier solution are continuously analyzed by fluorescence spectrometry after adding a reagent. Both methods have been successfully employed in Greenland and Antarctica in the past years.
The CH2O detection scheme is based on reaction of CH2O with pentane-2,4-dione and NH3 followed by fluorometric detection (Hantzsch reaction). Typical LOD (3 sigma) is 30 pptv with accuracy better than 25% and a time resolution of less than 2 minutes (10% to 90% signal). There is no known interference as opposed to the similar method based on the 1-3-cyclohexanedione reagent used by other groups.
The H2O2 content in the carrier stream is continuously analyzed by fluorescence spectrometry after derivatization with 4-ethylphenol in the presence of peroxidase. The method is also sensitive to higher organic peroxides, however to a much lower degree, so that this contribution can generally be neglected at polar sites. To assure the latter and to estimate the concentrations of higher organic peroxides, mainly methylhydroperoxide, we will employ a dual channel instrument with one channel monitoring total peroxides and the other higher organic peroxides by selectively destroying H2O2 with catalase in the carrier water before analyses. Typical LOD (3 sigma) of 40 pptv are achieved for H2O2 with accuracy better than 20% and a time resolution of less than 1 minute (10% to 90% signal). The baselines of both the HCHO and H2O2 instruments are determined every hour by measuring CH2O and H2O2-free air (zero air) through the PFA intake line, which is heated to prevent condensation of water vapor. Zero air is generated by pumping ambient air through a MnO2-CuO column (Hopcalite®, Callery Chemical Company, USA). Calibrations are done twice a day by running liquid standards through the coil scrubbers while zero air is sampled. The collocation of a LIF based CH2O instrument will also allow for measurement intercomparisons for this important compound.