Laboratory evaluation of the effect of nitric acid uptake on frost point...

The core information for this publication's citation.: 
Thornberry, T., T. Gierczak, R. Gao, H. Vömel, L. Watts, J. Burkholder, and D. Fahey (2011), Laboratory evaluation of the effect of nitric acid uptake on frost point hygrometer performance, Atmos. Meas. Tech., 4, 289-296, doi:10.5194/amt-4-289-2011.

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. This small amount of co-condensed HNO3 is consistent with the observed lack of HNO3 interference in the frost point measurement because the CMH utilizes significant reductions (>10%) in surface reflectivity by the condensate to determine H2 O.

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Research Program: 
Radiation Science Program (RSP)
Upper Atmosphere Research Program (UARP)