Air Quality Trends - NO₂

Air quality trends are used to assess if the air quality is improving and thereby determine if control and abatement strategies are effective.

Ozone • PM_2.5 • Carbon Monoxide • Nitrogen Dioxide • Sulfur Dioxide • Lead

EPA first set standards for nitrogen dioxide (NO₂) in 1971, setting both a primary standard (to protect health) and a secondary standard (to protect the public welfare) at 53 parts per billion (ppb), averaged annually.  The following figure shows the decrease in annual design value NO₂ levels since 1983, well below the annual National Ambient Air Quality Standard (NAAQS) of 53 ppb.

Please select for a larger view of the NO2 Plot

On January 22, 2010, EPA strengthened the NAAQS for NO₂ by setting a new 1-hour NO₂ standard at the level of 100 ppb.  It will protect against adverse health effects associated with short-term exposure to NO₂, including respiratory effects that can result in admission to a hospital. The form for the 1-hour NO₂ standard, is the 3-year average of the 98th percentile of the annual distribution of daily maximum 1-hour average concentrations.  This new NAAQS will require near-road monitoring, which will commence in January 2013.  The three years of data required for design value calculations and subsequent attainment/nonattainment determinations will not be available until after 2015.  In the interim, Governor Rell sent EPA a December 17, 2010 letter, recommending that EPA designate all of Connecticut as “unclassifiable” under CAA Section 107(d)(1).  EPA responded with a letter dated January 20, 2012, which designated Connecticut as “unclassifiable” for NO₂.  The following figure shows the newly calculated 1-hour NO₂ design values for monitoring sites since 1983.  It shows a general decrease in the 1-hour design values, well below the new 1-hour NAAQS of 100 ppb.

Please select for a larger view of the NO2 Plot

Content Last Updated February 26, 2020