Hmmmm. here it is:
http://lungaction.org/reports/SOTA04_methodology.html-snip-
The grades for ozone and short-term particle pollution (24-hour PM2.5) were based on a weighted average for each county calculated using the Air Quality Index as noted above. The number of orange days experienced by each county was assigned a factor of 1; red days were assigned a factor of 1.5 and purple days were assigned a factor of 2. By multiplying the total number of days within each category by their assigned factor, a total was determined. Because the monitoring data was collected over a 3-year period, the total was divided by three to determine the weighted average. Each county’s grade was determined using the weighted average. Counties were ranked by weighted average. Metropolitan areas were ranked by the highest weighted average among the counties in the Census Bureau-defined Metropolitan Statistical Area.
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and look at this bit
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Weighted averages allow comparisons to be drawn based on severity of air pollution. For example, if one county had 9 orange days and 0 red days, it would earn a weighted average of 3.0 and a D grade. However, another county which had only 8 orange days, but it also had 2 red days, which signify days with more serious air pollution, would receive a F. That second county would have a weighted average of 3.7.
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This makes it rough on the places which have occassional bad days, you see. That's how we angelenos manage to maintain our record. And since all they're measuring is ozone and pm 2.5's, a smoky fall with major fires can really raise your score.
And a bit on county-by-county here in California, for you curious OC residents
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California has 10 to 13 of the most polluted counties on each list, usually ranking among the worst ten counties in the nation for each pollutant. San Bernardino County remains the most ozone-polluted, but Riverside County tops the lists for both short and year-round particle pollution. Riverside, San Bernardino, Los Angeles Kern, Fresno, Kings and Tulare counties are on all three lists, while Merced, Orange, Stanislaus and Sacramento counties are on two of the worst lists each. El Dorado, Nevada, Santa Clara, San Diego Placer and Ventura counties made one list each.
California has historically led the nation in the battle against air pollution. At the same time, California has some of the most heavily polluted air in the nation. Aggressive strategies long at work in the state have resulted in significant improvements in the last three decades. In fact, California has more protective standards for year-round particle exposure than the national standards.2
There were fewer days with unhealthful ozone levels in several counties over the 2000-2003 time period than in previous reports. Four California counties moved to a passing grade, three for their first passing grade ever. One of those, Shasta County, improved from an F to a B. Four counties improved into the A category, registering no unhealthful ozone days; most notably, Solano County received its first A, having just improved from an F to a D in the 2003 report. Still, almost 70 percent of Californians live in counties that scored F for ozone.
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See, we're at least working hard on it.
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