Measurements and interpretation of oxygen isotopes in stratospheric carbon dioxide

Abstract

Carbon dioxide (CO2) is an important natural and anthropogenic greenhouse gas in Earth's atmosphere. Its atmospheric mole fraction has increased from about 280 ppm (parts per million) in the pre-industrial atmosphere to more than 400 ppm at present. Investigation of the stable isotopic composition of atmospheric CO2 is important for studying carbon cycle and provides a tracer for sources and sinks of this important greenhouse gas in the atmosphere. Measurements of the isotopic composition (δ17O and δ18O ) of stratospheric CO2 can be used to study the photochemical processing and transport of CO2 in the stratosphere. However due to the cost and logistical effort of sampling large amounts of air are required for traditional measurement systems. This study presents new and cost-effective experimental methods to measure isotope effects in CO2 from small stratospheric air samples and further insight into evaluation of the anomalous signatures from the measured quantities. First, we have established an online measurement system for analysis of all single substituted isotopologues (12C16O17O, 12C16O18O, 13C16O16O) in nanomolar quantities of CO2 extracted from stratospheric air samples. Second, in order to collect and to store the vertical profile of air collected with an AirCore (an alternative lightweight, and therefore relatively cheap sampling system) we have successfully developed a Stratospheric Air Sub-sampler (SAS). The SAS device consists of stainless steel tubing that is separated by valves to divide the tubing into numerous sub-samples. SAS can be easily transported and the air sub-sampled from the AirCore after recovery and after an online analysis of carbon gases, can be further analysed with slower instrumentation. However the analytical system must be capable of analysing very small air samples (~25 ml) at ambient temperature and pressure. We have successfully developed the coupling of the SAS to an analytical system that determine the 17O excess of CO2 in stratospheric air samples. The performance of both analytical measurement techniques developed in this study is demonstrated for a set of stratospheric and upper tropospheric air samples collected over Kiruna, Sweden in 2011 and over Sodankylä, Finland in 2014.