The atmosphere has a mass of about 5.15×1018 kg,[9] three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi) or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.
^Cox, Arthur N., ed. (2000), Allen's Astrophysical Quantities (Fourth ed.), AIP Press, pp. 258–259, ISBN0-387-98746-0, which rounds N2 and O2 to four significant digits without affecting the total because 0.004% was removed from N2 and added to O2. It includes 20 constituents.
^Haynes, H. M., ed. (2016–2017), CRC Handbook of Chemistry and Physics (97th ed.), CRC Press, p. 14-3, ISBN978-1-4987-5428-6, which cites Allen's Astrophysical Quantities but includes only ten of its largest constituents.
^Two recent reliable sources cited here have total atmospheric compositions, including trace molecules, that exceed 100%. They are Allen's Astrophysical Quantities[2] (2000, 100.001241343%) and CRC Handbook of Chemistry and Physics[3] (2016–2017, 100.004667%), which cites Allen's Astrophysical Quantities. Both are used as references in this article. Both exceed 100% because their CO2 values were increased to 345 ppmv, without changing their other constituents to compensate. This is made worse by the April 2019 CO2 value, which is 413.32 ppmv.[4] Although minor, the January 2019 value for CH4 is 1866.1 ppbv (parts per billion).[5] Two older reliable sources have dry atmospheric compositions, including trace molecules, that total less than 100%: U.S. Standard Atmosphere, 1976[6] (99.9997147%); and Astrophysical Quantities[7] (1976, 99.9999357%).
^Lide, David R. Handbook of Chemistry and Physics. Boca Raton, FL: CRC, 1996: 14–17