International Regulation of Urban Smog Since the 1940s - Air Pollution and Global Warming: History, Science, and Solutions - page 187

Table 8.4. Ranking of most polluted U.S. cities in terms of ozone and year-round particulate matter
pollution, 2006-2008

Ozone Particulate matter pollution

1. Los Angeles-Long Beach-Riverside, CA 1. Phoenix-Mesa-Scottsdale, AZ

2. Bakersfield, CA 2. Bakersfield, CA

3. Visalia-Porterville, CA 3. Los Angeles-Long Beach-Riverside, CA

4. Fresno-Madera, CA 3. Visalia-Porterville, CA

5. Sacramento-Arden-Arcade-Yuba City, CA-NV 5. Pittsburgh-New Castle, PA

6. Hanford-Corcoran, CA 6. Fresno-Madera, CA

7. Houston-Baytown-Huntsville, TX 7. Birmingham-Hoover-Cullman, AL

8. San Diego-Carlsbad-San Marcos, CA 8. Hanford-Corcoran, CA

9. San Luis Obispo-Paso Robles, CA 9. Cincinnati-Middletown-Wilmington, OH-KY-IN

10. Charlotte-Gastonia-Salisbury, NC-SC 10. Charleston, WV; Detroit-Warren-Flint, MI;

Weirton-Steubenville, WV-OH

Source: American Lung Association (2010).

eliminated. Stage 3 alerts, frequent in the 1950s, when
peak ozone mixing ratios reached 0.68 ppmv, have not
occurred in Los Angeles since before 1976. The num-
ber of days of PM 2.5 standard exceedances has similarly
decreased in Los Angeles, although maximum PM 2.5
levels have not decreased to the same degree as the
number of standard exceedances (Figure 8.2b). Occa-
sional spikes in the maximum PM 10 concentration are
likely due to severe Santa Ana wind dust events.
Figure 8.3 compares the trends in the maximum
hourly ozone and annual PM 2.5 among four air dis-
tricts in California. Ozone improvements are greatest
in the South Coast district (home to Los Angeles),
but improvements inother districts, including the clean

North Central Coast district, have also occurred. Max-
imum particulate matter pollution increased in the San
Joaquin Valley between 2004 and 2009, but it decreased
or stayed constant in the other districts.
Part of the reason for the ozone reductions over time
in California has been the decrease in ozone-producing
organic gases (Figure 8.4a). Concentrations of atmo-
spheric metals, primarily from industrial sources and
also from vehicles (in the case of lead primarily), have
also decreased over time (Figure 8.4b).
The measured improvement in air quality in the
United States due to air pollution regulations corre-
lates with an improvement in the economy during the
same period. For example, between 1980 and 2009,

300 0.5 120 500

Basin maximum

250 100

PM 10

0.25 250

200 80

Fed. 8-hr

150 0 60 0

PM 2.5

100 40

H.A.

-250

Fed PM 2.5

50

Stage 1

20

Stage 2

0 0 -500

1975 1985 1995 2005 1990 2000 2010

(a) Year (b) Year

Figure 8.2. (a) Days per year that the ozone mixing ratio in the Los Angeles Basin exceeded the California state
standard (State), the U.S. National Ambient Air Quality Standard (NAAQS) (Fed.), the California health advisory
(H.A.) level, the Stage 1 smog alert level, and the Stage 2 smog alert level over time. Also shown is the
maximum ozone mixing ratio each year in the basin. (b) Same as (a), but for days of exceedances of the federal
PM 2.5 standard and the maximum PM 2.5 and PM 10 levels in the basin. From California Air Resources Board
(2011); South Coast Air Quality Management District (SCAQMD) (2011).