2.1.1. Sampling by Complete Cryogenic Trapping
In this method ambient atmospheric water vapor is cryogenically trapped at -78 °C from a stream of ambient air, without any isotopic fractionation during sampling, so that isotopic composition of liquified ambient water vapor represents the true isotopic composition of ambient vapor. A stream of ambient air is diverted into a glass condenser immersed in a liquid Nitrogen + Alcohol slush maintained at -78 °C. The liquid Nitrogen + Alcohol slush temperature is carefully chosen to be slightly greater than -78 °C so that the ambient CO2 does not condense along with the water vapor. The air-flow rate is also maintained at around 450 cc/ minute to ensure that there is no isotopic fractionation due to preferential loss of lighter isotopic molecular species of water vapor. Detailed sampling procedure and sampling devices for complete cryogenic trapping is discussed by R. D. Deshpande et al. (2013).
With an average Rh of about 65 % and temperature of 27 °C, it typically took approximately 4-5 hours to collect about 2ml of the liquefied vapor by complete cryogenic trapping. Depending on the specific humidity of ambient air, sampling duration for each sampling day was estimated to ensure that minimum of 2 ml of liquified vapor could be sampled. After an adequate duration of trapping of moisture in the condenser, the condenser was taken outside the Dewar flask for it to attain room temperature. This liquid was then transferred into 15 ml sample bottles. Thus, the true ambient vapor was trapped and sampled using this method and for the purpose of this study we shall from now on refer to this sample as ‘vapor’ representing true isotopic composition of vapor.