Abstract: Atom Trap Trace Analysis (ATTA), a laser-based atom counting method, has been used to analyze atmospheric 39Ar (half-life = 269 yr), a cosmogenic isotope with an isotopic abundance of 0.8 parts-per-quadrillion (8.x10-16). In addition to the superior selectivity demonstrated in this work, both the counting rate and counting efficiency of ATTA have been improved by two orders of magnitude over prior results. Along with the previously demonstrated detection of 81Kr (229,000 yr) and 85Kr (10.8 yr) at the 10-12 level, ATTA can now be used to analyze all three long-lived noble gas radioisotopes covering a wide range of ages and applications.