Passive Flux Samplers to measure Nitrous Oxide and Methane Emissions from Agricultural Sources, Part 1: Adsorbent Selection

The present article is the first of a series reporting studies to develop cheaper techniques for measuring emissions of nitrous oxide (N2O) and methane (CH4). The objective of this first stage was to determine the best way to capture and, later, recover N2O and CH4. The three adsorbents studied were the molecular sieves zeolite 5A, Carboxen 1018 and Carboxen 1021.Zeolite 5A adsorbed N2O easily, but CH4 less easily. On average, with the procedure developed, the zeolite, the Carboxen 1018 and the Carboxen 1021 adsorbed 3·96 (±0·57), 1·31 (±0·41) and 2·48 (±1·00) ng of N2O for each mg of sieve material exposed, respectively. These amounts represent 77%, 23% and 20% of the total amount of N2O pumped through the trial sampler, respectively. For CH4, the adsorption patterns of all three molecular sieves were similar to each other, showing rapid saturation of the bed of sieve in the sampler. The adsorption capacities were, on average, 7·17 (±5·2), 4·38 (±2·3) and 6·47 (±3·3) ng of CH4 per mg of zeolite 5A, of Carboxen 1018 and of Carboxen 1021 molecular sieve exposed, respectively.The average efficiencies for desorption of the N2O using a temperature of 175 °C and a flow rate of 21·3 ml min−1 of electron capture detector (ECD) grade nitrogen were 59%, 57% and 44% for zeolite 5A, Carboxen 1018 and Carboxen 1021, respectively. The CH4 desorption efficiency was never higher than 14%, with an overall average of 7·6%.The present study has shown the feasibility of using molecular sieves as the basis of a passive flux sampler, for measuring N2O and CH4 emissions. To achieve optimal sampler design, sampling and analysis procedures, additional tests need to be made, e.g. on adsorption procedure, on adsorption capacity, on the ratio of inside to outside air velocity, and on sampler performance during field measurements.