Nitrogen Oxides at Fremont School

This page last reviewed October 19, 2010


Nitrogen oxides (NOx) are defined as the sum of nitric oxide (NO) and nitrogen dioxide (NO2). NOx contributes to the formation of ozone and particulate matter pollutants, both of which are major air pollutants.

Most NOx emissions are produced when nitrogen (N2) and oxygen (O2) in the air react during the high-temperature combustion of fuels. On-road motor vehicles and other mobile sources currently contribute approximately 80% of (man-made) NOx emissions in California. Industrial sources contribute approximately 17% of anthropogenic NOx emissions. Significant natural sources of NOx emissions include lightning, biomass burning, and soil biological activities.

NOx emissions from on-road motor vehicles have declined by over 30% from 1990 to 2000 and are projected to decrease by an additional 40% between 2000 to 2010, due to stringent emission standards on motor vehicles and the introduction of a cleaner burning process of fuels. Emissions from industrial sources have also decreased, largely due to a switch from fuel oil to natural gas and the implementation of combustion controls.

No standards have been established for the total NOx concentration. Due to its role in the formation of ozone, ARB continues to work toward reducing levels of nitrogen oxides. However, there are National and California standards for NO2 which follows:

State NO2 Standards:
250 ppb* for 1 hour average
National NO2 Standards:
53 ppb for annual arithmetic mean concentration

*ppb means parts per billion.

Ambient Monitoring Results

Ambient levels of NOx are routinely monitored hourly at approximately 120 sites in the California ambient air quality measurement network. Based on the standards, daily maximum of 1-hour NOx measurements is a useful parameter to indicate the NOx levels. During year 1998 through 2001, the statewide average concentration of daily maximum of 1-hour NOx measurements was 82 ppb (parts per billion). Relative to the statewide average, the Fresno County region was 10% lower for the same time period, averaging 73 ppb. Note: The NO2 maximum 1-hour value for Fremont School was 78 ppb, which was well below the state standard.

The ambient monitoring results at Fremont School are provided here:

  • A graph comparing the monthly summaries of NOx at the community with historical statewide and regional levels
  • A table of summary statistics
  • Raw data in Excel format