Hourly values of the concentration of atmospheric carbon dioxide at Mauna Loa Observatory (MLO) formed the basis for an investigation of concentration fluctuations on daily to monthly time scales. In agreement with earlier studies we found no evidence linking the fluctuations with large-scale variations in the atmospheric circulation. Neither did local cane fires produce any measurable effects. A diurnal variation that comprises daytime forest depletion and nighttime enhancement by outgassing from the summit caldera need not affect overall data quality. We conclude that MLO is an excellent site for monitoring atmospheric carbon dioxide.