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Progress Towards a More Environmentally Friendly NOAA Frost Point Hygrometer

D. Hurst1,2, E. Hall1,2, A. Jordan1,2, P. Cullis1,2 and K. Xiong1,2

1Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309; 303-478-5150, E-mail: Dale.Hurst@noaa.gov
2NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305

Balloon-borne frost point hygrometers (FPHs) require a powerful cooling source to be able to measure frost point temperatures as low as –95°C. The source must be compact (<1500 cm3), lightweight (<500 g), non-toxic and able to provide sufficiently strong and persistent cooling to achieve mirror temperatures below –95°C for at least 4 hours. The historically optimal FPH coolant has been HFC-23 (CHF3) because it meets all of these requirements and is relatively easy and inexpensive to purchase. Unfortunately, HFC-23 is a greenhouse gas with a very high global warming potential, so its production and consumption are now being phased out by the Kigali Amendment to the Montreal Protocol.

NOAA Global Monitoring Laboratory (GML) has been working since early 2020 to develop a different cooling source for the FPH. Progress during the pandemic has been slow, but preliminary results indicate we are on the right path to finding a viable replacement for HFC-23. Dry ice (solid carbon dioxide) meets all of the aforementioned size, weight and safety requirements and is colder than –95°C at pressures <100 hPa. However, since the sublimation point of dry ice is 5–15°C higher than the boiling point of HFC-23 over the pressure range 1000–10 hPa, dry ice provides less cooling power than HFC-23. The greatest challenge in using dry ice to cool the FPH is to ensure the solid constantly remains in good thermal contact with the FPH mirror as it sublimates. We have found that adding about 200 mL of high-purity ethanol to dry ice pellets produces a reliable “cold bath” that continuously and efficiently conducts heat away from the FPH mirror. To increase the consistency of this heat transfer, a solid copper block that that is thermally connected to the mirror is added to the bottom of the coolant reservoir.

To date, the new dry ice-cooled FPH has been launched twice from Boulder. The first flight (in February 2020) was a simple proof-of-concept test to demonstrate the capabilities of the new coolant. The second flight, in early April 2021, was performed 3 days after a standard HFC-23 FPH was launched. The stratospheric water profiles measured by the GML FPH during the two flights are presented in Figure 1. For the most part, the water vapor profiles from both soundings are very similar. Most importantly, the dry ice and ethanol bath provided enough cooling power throughout the flight to accurately measure frost point temperatures below –90°C in the stratosphere.

The ultimate tests of this new cooling source will be conducted in the tropical stratosphere, where the FPH must be able to measure frost point temperatures below –95°C. Once successful tropical flights have been performed, we will follow strict change management protocols by launching dry ice and HFC-23 instruments in succession for a period of one year at each of the three GML upper atmospheric water vapor monitoring sites (Boulder, Colorado; Hilo, Hawaii; Lauder, New Zealand).

Figure 1

Figure 1. Water vapor profiles over Boulder measured by a GML FPH cooled using (a) HFC-23 and (b) dry ice and ethanol. (c) The good agreement between the two sets of profiles demonstrates that the dry ice cooled the FPH mirror sufficiently to measure frost point temperatures as low as –90°C.