Atmospheric Measurement Techniques Discussions www.atmos-meas-tech-discuss.net 1867-8610 3 4 2010 10.5194/amtd-3-3725-2010 http://www.atmos-meas-tech-discuss.net/3/3725/2010/ http://www.atmos-meas-tech-discuss.net/3/3725/2010/amtd-3-3725-2010.html http://www.atmos-meas-tech-discuss.net/3/3725/2010/amtd-3-3725-2010.pdf 3725 3745 2010-08-24 Laboratory evaluation of the effect of nitric acid uptake on frost point hygrometer performance T. Thornberry troy.thornberry@noaa.gov T. Gierczak R. S. Gao H. Vömel L. A. Watts J. B. Burkholder D. W. Fahey NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA Meteorologisches Observatorium Lindenberg, German Weather Service, Lindenberg, Germany Department of Chemistry, University of Warsaw, Warsaw, Poland Chilled mirror hygrometers (CMH) are widely used to measure water vapour in the troposphere and lower stratosphere from balloon-borne sondes. Systematic discrepancies among in situ water vapour instruments have been observed at low water vapour mixing ratios (<5 ppm) in the upper troposphere and lower stratosphere (UT/LS). Understanding the source of the measurement discrepancies is important for a more accurate and reliable determination of water vapour abundance in this region. We have conducted a laboratory study to investigate the potential interference of gas-phase nitric acid (HNO₃) with the measurement of frost point temperature, and consequently the water vapour mixing ratio, determined by CMH under conditions representative of operation in the UT/LS. No detectable interference in the measured frost point temperature was found for HNO₃ mixing ratios of up to 4 ppb for exposure times up to 150 min. HNO₃ was observed to co-condense on the mirror frost, with the adsorbed mass increasing linearly with time at constant exposure levels. Over the duration of a typical balloon sonde ascent (90–120 min), the maximum accumulated HNO₃ amounts were comparable to monolayer coverage of the geometric mirror surface area, which corresponds to only a small fraction of the actual frost layer surface area. 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