利用14C进行大气化石源CO_2示踪是目前节能减排领域的热点研究,为了更加便利、环保地对大气CO_2进行长期、积时采样,在原有排磷酸溶液集气法的基础上,西安加速器质谱中心研究组设计和建立了新的分子筛采样方法和装置。进一步的实验表明,分子筛解吸附的最佳条件为550℃下热解20 min,该条件下分子筛解吸附效率达99%以上,且重复性良好。该方法所收集的碳量随采样天数呈线性增长,碳吸附速率约为1.04 1.30 mg C · d~(-1),可以满足高精度AMS-14C测定及高分辨率大气14C研究的要求。在相同实验条件下,利用分子筛方法和排磷酸溶液集气法同时采集大气CO_2气体,结果表明,两者存在较好的一致性,说明分子筛法可提供可靠的数据。此外,分子筛法易于操作和运输,并具有便利环保的特性,值得推广使用。目前该方法已应用于大气CO_2的收集,数据表明从冬季供暖期到夏季,西安市大气化石源CO_2浓度以0.22 muL · L-1 · d~(-1)的速率逐渐降低。化石源CO_2来源分析结果进一步表明,西安市化石源CO_2主要源自于煤燃料的燃烧,而冬季采暖需求导致的煤燃烧量的增加可能是导致冬季化石源CO_2浓度较高的主要原因。
英文摘要:
Background, aim, and scope Large quantities of CO_2 has emitted due to anthropogenic combustion of fossil fuel since industrial revolution, and the concentration of atmospheric CO_2 has increased to levels unprecedented in at least the last 800000 years which is the largest contribution to the global warming. Monitoring atmospheric CO_2 sources is increasingly important in the interests of establishing controllable strategies and implementing greenhouse gas regulations. CO_2 emitted from the combustion of fossil fuels contains no ~(14)C because the half-life of ~(14)C (5730 years) is much less than the age of fossil fuels. The difference between the ~(14)C compositions of CO_2 derived from fossil fuels (CO_(2ff)) and from modern CO_2 sources is huge and this contrast makes ~(14)C a unique and sensitive tracer for quantifying CO_(2ff). The existing sampling methods all have their own shortcomings. For example, the samples collected by fl asks or gas sample bags are only representative of each point in time, and the pump-based alkaline solution method and molecular sieve method require external energy supply which could not be deployed in remote sampling sites, and absorption rate of the passive sampling method using molecular sieve was too low (0.014 0.065 mg C · d~(-1)) to make high-resolution atmospheric ~(14)C research. So there is an interest in developing an active absorption system using a molecular sieve with the pressure gradient produced by the water fl owing out of a glass bottle drop by drop. Materials and methods The active absorption system is composed of two parts: (1) a molecular sieve device and (2) pressure gradient apparatus. The molecular sieve cartridge (MSC) was made in a quartz tube, in the middle of cartridge (22 mm O.D., 60 mm length) fi lled with 6.5 g of 13* molecular sieve secured with quartz wool. The pressure gradient apparatus consists of a 6 L custom-made glass bottle with a 9 mm O.D., 60 mm length glass tube welded to the bottle, and a water flow regulator connected to the glass tube. The flow rate can be controlled by the regulator to meet a targeted sampling duration (hour, day or week). In addition, the dripping water can be collected in a separate glass bottle for re-use. A 60 mm O.D. * 250 mm length muffletube furnace was designed to heat MSC for activation and desorption. Before sampling, the MSC should be activated completely by heating at 550℃ while purging with high-purity N2. When sampling finished, the molecular sieve device was inserted into the tube furnace and connected to the vacuum system with a metal joint to desorb and purify CO_2 by cryogenic trapping. Results The results of molecular sieve absorption and desorption showed that the recovery rate was over 99% and the parameters also produced good reproducibility at the 2sigma level, when the MSC was heated at 550℃ for 20 min. So 550℃, 20 min was the optimal condition and was adopted for routine sampling. The comparative experiments were conducted by collecting atmospheric samples from the same locality at the same time using the new active absorption system and the phosphoric acid solution displacement method. And the results demonstrated that there was good overall reproducibility between the two methods at the 2sigma uncertainty level and both methods were viable for sampling. Besides, we used the active absorption system to collect air samples at different duration (one day, two days, three days and five days) at the flow rate of 5 mL · min~(-1), and the results showed that the amount of carbon increased linearly with the increasing sampling duration (R~2= 0.9784).