Passive Flux Samplers to measure Nitrous Oxide and Methane Emissions from Agricultural Sources, Part 2: Desorption Improvements

In the first part of this research work, it was shown that it was feasible to adsorb nitrous oxide (N2O) and methane (CH4) on to molecular sieves loaded into standard stainless-steel gas sampling tubes. Of three types of sieve examined, zeolite 5A molecular sieve gave the best performance. A thermal desorption procedure was used to recover the gas adsorbed. However, the average efficiency for desorption of N2O and CH4 was respectively 59% and 6% for zeolite 5A molecular sieve. The objective of the present study was to develop a complete desorption procedure for N2O and CH4 absorbed on zeolite 5A molecular sieve. The parameters investigated were: desorption gas flow rate, desorption duration and temperature. The complete procedure includes a two-stage desorption using in first stage a flow rate of 21 ml min−1 and 40 min purge period. A flow rate of 64 ml min−1 and 15 min purge period was used for the second stage. In this study, five temperatures were investigated (50, 100, 150, 200 and 250 °C). The N2O desorption efficiency was not significantly influenced by the temperature. A temperature of 100 °C was recommended for a desorption efficiency of N2O ranged from 75% to 94%, with an average of 88%. For CH4, the second stage desorbs almost the same amount as does the first stage and the desorption temperature has significant effect. Together, the two stages allowed recovery of up to 51% of the total amount of CH4 absorbed.