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Chilled mirror hygrometers (CMH) are widely used to measure water vapour inthe troposphere and lower stratosphere from balloon-borne sondes. Systematicdiscrepancies among in situ water vapour instruments have been observed atlow water vapour mixing ratios (<5 ppm) in the upper troposphere andlower stratosphere (UT/LS). Understanding the source of the measurementdiscrepancies is important for a more accurate and reliable determination ofwater vapour abundance in this region. We have conducted a laboratory studyto investigate the potential interference of gas-phase nitric acid(HNO3) with the measurement of frost point temperature, andconsequently the water vapour mixing ratio, determined by CMH underconditions representative of operation in the UT/LS. No detectableinterference in the measured frost point temperature was found for HNO3mixing ratios of up to 4 ppb for exposure times up to 150 min. HNO3was observed to co-condense on the mirror frost, with the adsorbed massincreasing linearly with time at constant exposure levels. Over the durationof a typical balloon sonde ascent (90–120 min), the maximum accumulatedHNO3 amounts were comparable to monolayer coverage of the geometricmirror surface area, which corresponds to only a small fraction of theactual frost layer surface area. This small amount of co-condensed HNO3is consistent with the observed lack of HNO3 interference in the frostpoint measurement because the CMH utilizes significant reductions(>10%) in surface reflectivity by the condensate to determine H2O

Laboratory evaluation of the effect of nitric acid uptake on frost point hygrometer performance