Official websites use .gov
A .gov website belongs to an official government organization in the United States.
Secure .gov websites use HTTPS
A lock ( ) or https://
means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.
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Nephelometer data (conforms to IMPROVE network standards) from October 1993 to June 2002. This table is no longer updated.
data.gov
An official website of the GSA's Technology Transformation Services