Editing Smog (section)

====={{anchor|LA}}Los Angeles and the San Joaquin Valley=====
Because of their locations in low basins surrounded by mountains, [[Los Angeles]] and the [[San Joaquin Valley]] are notorious for their smog. Heavy automobile traffic, combined with the additional effects of the [[San Francisco Bay]] and Los Angeles/[[Long Beach, California|Long Beach]] port complexes, frequently contribute to further air pollution.

Los Angeles, in particular, is strongly predisposed to the accumulation of smog, because of the peculiarities of its geography and weather patterns. Los Angeles is situated in a flat basin with the ocean on one side and mountain ranges on three sides. A nearby cold ocean current depresses surface air temperatures in the area, resulting in an [[Inversion (meteorology)|inversion layer]]: a phenomenon where air temperature increases, instead of decreasing, with altitude, suppressing [[thermals]] and restricting vertical convection. All taken together, this results in a relatively thin, enclosed layer of air above the city that cannot easily escape out of the basin and tends to accumulate pollution.

Los Angeles was one of the best-known cities suffering from transportation smog for much of the 20th century, so much so that it was sometimes said that ''Los Angeles'' was a synonym for ''smog.''<ref>{{cite book|title=The Economics and Politics of the Slowdown in Regulatory Reform|author=Roger G. Noll|year=1999}}</ref> In 1970, when the Clean Air Act was passed, Los Angeles was the most polluted basin in the country, and California was unable to create a State Implementation Plan that would enable it to meet the new air quality standards.<ref>"Early Implementation of the Clean Air Act of 1970 in California." EPA Alumni Association. [http://www.epaalumni.org/history/video/interview.cfm?id=38 Video], [https://www.epaalumni.org/userdata/pdf/2B9E3C6816EC9466.pdf#page=1 Transcript] (see p6). 12 July 2016.</ref> However, ensuing strict regulations by state and federal government agencies overseeing this problem (such as the [[California Air Resources Board]] and the [[United States Environmental Protection Agency]]), including tight restrictions on allowed emissions levels for all new cars sold in California and mandatory regular emission tests of older vehicles, resulted in significant improvements in air quality.<ref>{{Cite web|last=US EPA|first=OAR|date=2016-05-05|title=Vehicle Emissions California Waivers and Authorizations|url=https://www.epa.gov/state-and-local-transportation/vehicle-emissions-california-waivers-and-authorizations|access-date=2020-11-26|website=US EPA|language=en}}</ref> For example, air concentrations of volatile organic compounds declined by a factor of 50 between 1962 and 2012.<ref>{{Cite web|url=https://csl.noaa.gov/news/2012/119_0809.html|title=NOAA CSL: 2012 News & Events: 50-year decline in some Los Angeles vehicle-related pollutants|first=NOAA Chemical Sciences|last=Laboratory (CSL)|website=csl.noaa.gov}}</ref> Concentrations of air pollutants such as nitrous oxides and ozone declined by 70% to 80% over the same period of time.<ref>{{cite web|url=http://hbsciu.com/2015/02/15/is-clean-air-worth-the-cost-a-case-study-for-developing-megacities-2/|title=Is clean air worth the cost? A case study for developing megacities|work=HBSciU |date=15 February 2015}}</ref>