A correction for the effect of drying of air samples and its significance to the interpretation of atmospheric CO2 measurements

Non-dispersive infrared gas analysers used for the measurement of atmospheric carbon dioxide concentration are influenced by water vapour in the air sample. It has thus become common practice to dry the air prior to analysis. The subsequent measurement must be corrected by multiplying the indicated concentration by (1 + 1.61r)⁻¹, where r is the water vapour mass mixing ratio of the undried air. The correction is of the order 1% near the surface, varying in both space (e.g. with latitude) and time (e.g. day-today and seasonally), and of the same order of magnitude as the measured but uncorrected horizontal, vertical and seasonal CO₂ variations. Thus serious doubt is thrown on previous comparisons of global CO₂ measurements. Ideally each CO₂ analysis of an air sample should be corrected using knowledge of the water vapour concentration of the undried air. For data obtained in the past, appropriate correction may be possible after consideration of the water vapour conditions in the atmosphere at each of the monitoring stations. This should be done by the research personnel involved with each monitoring programme, together with a consideration of the effects of carrier gas composition on the gas analyser(s) employed.