<br/>PART 50 — NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS [ 40 CFR50 ]PART 50 — NATIONALPRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS [ 40 CFR 50 ]Table of Contents§ 50.1 — Definitions.[ Last FR update: 03/22/2007 ]§ 50.2 — Scope.[ Last FR update: 02/12/1998 ]§ 50.3 — Reference conditions.[ Last FR update: 11/12/2008 ]§ 50.4 — National primary ambient airquality standards for sulfur oxides (sulfur dioxide).[ Last FR update: 06/22/2010 ]§ 50.5 — National secondary ambient airquality standard for sulfur oxides (sulfur dioxide).[ Last FR update: 05/22/1996 ]§ 50.6 — National primary and secondaryambient air quality standards for PM10.[Last FR update: 10/17/2006 ]§ 50.7 — National primary and secondaryambient air quality standards for PM2.5.[Last FR update: 07/30/2004 ]§ 50.8 — National primary ambient airquality standards for carbon monoxide.[Last FR update: 09/13/1985 ]§ 50.9 — National 1-hour primary andsecondary...