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We present a novel spectral method to measure atmospheric carbon dioxide (CO₂) with high precision and stability without resorting to calibration tanks during long-term operation. This spectral null method improves precision by reducing spectral proportional noise associated with laser emission instabilities. We employ sealed quartz cells with known CO₂ column densities to serve as the permanent internal references in the null method, which improve the instrument's stability and accuracy. A prototype instrument – <b>AB</b>solute <b>C</b>arbon dioxide (ABC) is developed using this new approach. The instrument has a one-second precision of 0.02 ppm, which averages down to 0.007 ppm within one minute. Long-term stability of within 0.1 ppm is achieved without any calibrations for over a one-month period. These results have the potential for eliminating the need for calibration cylinders for high accuracy field measurements of carbon dioxide.