period)15. Because of this, the average weighted design value is thought to be more

representative of the baseline emissions and meteorology period than other methodologies such

as choosing the highest single design value period.

Additionally, the average design value will be more stable (less year to year variability)

than any single design value period. An analysis of recent ambient design value data at 471

ozone monitors, over the period 1993-2004, shows that the median standard deviation of design

values was 3.3 ppb whereas the standard deviation of the 5 year weighted average design values

was 2.4 ppb (Timin, 2005a). Also, moving from the period ending in 2003 to the period ending

in 2004, the median change in the ozone design values was 4.0 ppb. The median change in the 5

year weighted average ozone design values was only 0.8 ppb (there was not a long enough data

record to do the same analyses for PM2.5) . These analyses show that the average design values

are clearly more stable and will therefore provide a “best estimate” baseline year design value

(DVBI) for use in future year model projections.

The recommended averaging technique assumes that at least five complete years of

ambient data is available at each monitor. In some cases there will less than five years of

available data (especially at relatively new monitoring sites). In this case we recommend that

data from the monitor is used if there is at least three consecutive years of data. If there are three

years of data then the baseline design value will be based on a single design value. If there are

four years of data then the baseline design value will be based on an average of two design value

periods. If a site has less than three years of data, then the site should not ordinarily be used in

the attainment test.

Calculating site-specific

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castellano: DISPER CUSTIC DESCAR RADIA italiano:

castellano: DIS CUS DES RAD english: DIS CUS DES RAD

português: DIS CUS DES RAD italiano: DIS CUS DES RAD