1.atomic absorption spectroscopy is used for elemental analysis (for atoms); UV spectroscopy is used for compound (molecule) analysis.
2.AA spectrum is characterized by sharp lines,while UV is by broad peaks.
3.AA often is used to detect trace amount of metal ions or atoms at ppb level,while UV's detection limit normally is above ppm level.
4.AA spectrometer is more complicated than UV machine.
5.too many to list them all here.