Observations of water vapor in the UT/LS of unprecedented accuracy with non-equilibrium corrected frost point hygrometers

We present a new retrieval protocol for chilled mirror hygrometer measurements under rapidly changing humidity conditions that enables balloon-borne frost point measurements in the upper troposphere/lower stratosphere of unprecedented accuracy. Chilled mirror hygrometers measure the frost point (or dew point) of air by quantifying the degree of saturation of the air with respect to the condensed phases of water (ice or liquid water). To this end, they attempt to determine the thermodynamic equilibrium of the condensate with the vapor phase by measuring the mirror reflectance, which changes with the thickness of the condensate. In the rapidly changing environment along the balloon trajectory, however, the adjustment of the mirror temperature to the new equilibrium point leads to frequent, damped overshoots or non-equilibrium errors. For the Cryogenic Frost Point Hygrometer (CFH), a balloon-borne chilled mirror instrument of reference quality, we (i) identify points in time along the balloon trajectory when the mirror is in true equilibrium with the gas phase, which we term ‘Golden Points’, and (ii) correct the measurements for non-equilibrium conditions between these Golden Points. For (i), we identify the points where the mirror reflectance assumes an extreme value, i.e. a maximum or a minimum. At these extreme points, the CFH mirror temperature represents the frost point with an accuracy better than 0.2 K (resulting from the uncertainties of the mirror temperature sensor and the precise timing of the Golden Points along the sounding profile). These accurately determined frost points can be used to detect and correct offsets, biases and time-lag errors in other humidity sensors flown together with CFH on the same balloon payload, such as the thin-film capacitive hygrometer of the Vaisala RS41 radiosonde. In the middle stratosphere (~ 28 km), a frost point uncertainty of 0.2 K corresponds to < 4 % uncertainty (2-σ) in H₂O mixing ratio which includes the 0.3 hPa uncertainty of the RS41 radiosonde GPS-based pressure measurement. At lower altitudes, the uncertainty is even less. For (ii), we compute the time-derivative of the mirror reflectance, which is proportional to the non-equilibrium error. The proportionality factor is related to a property of the mirror condensate, which we term ‘morphological sensitivity’, and allows correction of the CFH non-equilibrium data. The sensitivity constant is determined using an a-priori reference, such as the RS41 radiosonde humidity measurements after they have been time-lag and bias-corrected by means of (i), or the Golden Points interpolation in situations where Golden Points occur frequently enough (< 50 m) and the non-equilibrium error between Golden Points is large enough (> 0.5 K). This procedure paves the way for H₂O mixing ratio and relative humidity observations of unprecedented accuracy (< 4 % at 250 m vertical resolution) in the UT/LS. We showcase this novel measurement strategy and design philosophy on chilled mirror hygrometers with low global warming potential coolant, DIA-CFH (i.e., CFH using a mixture of dry ice and alcohol as coolant) and PCFH (thermoelectric coolant), flown in 2023-2024 over the central European alpine region as part of the Swiss H₂O Hub project.