Criteria Air Pollutants

Learn about key air pollutants and their emissions inventory for the San Francisco Bay Area. 

Criteria Air Pollutant Definitions

To assess and regulate the air quality of a region, the U.S. Environmental Protection Agency has set national ambient air quality standards for six criteria air pollutants. These pollutants include:

  • Particulate matter
  • Carbon monoxide
  • Nitrogen dioxide
  • Sulfur dioxide
  • Ground-level ozone
  • Lead

To learn more about these and other pollutants in the Bay Area you can use the Pollutant Glossary.

Regional Emissions Inventory

An overview of the San Francisco Bay Area’s emissions inventory of criteria air pollutants and/or their precursors from years 1990 to 2040 is provided in the Bay Area Emissions Inventory: Summary Report for Criteria Air Pollutants.

The pollutants included in the emissions inventory are particulate matter with diameters 10 micrometers and smaller (PM10) and 2.5 micrometers and smaller (PM2.5), total organic gases (TOG), reactive organic gases (ROG), nitrogen oxides (NOx), carbon monoxide (CO), and sulfur dioxide (SO2). Ground-level Ozone (O3) is a secondary criteria air pollutant and formed when nitrogen oxides (NOx) react with a class of pollutants called reactive organic gases (ROG - see "Organic Compounds" in Pollutant Glossary) in the presence of sunlight. Since O3 is not directly emitted to the atmosphere, emissions of ROG, which is a precursor to O3 formation, are included in the inventory. Estimates of emissions for lead (Pb) are covered in the toxic air contaminants reporting process.

For this emissions inventory, a base year of 2015 is typically used as a reference point for estimating emissions for historical (backcasting) and future (forecasting) years. Wherever available, the most recently available data, including actual emissions, are included. 

Inventory Updates (Version: January 2024)

A few key inventory updates and improvements in the current version of the emissions inventory are listed here:

  • Historical data gaps and missing values have been backfilled using the Air District’s internal data.
  • Emissions estimation methodology for several important area source categories (e.g., composting, road dust, and industrial/commercial natural gas combustion) have been improved.
  • Methane (CH4) emissions have been consistently included in the accounting of total organic gases (TOGs).
  • Updated growth profiles that are based on the most recent population, household, and employment numbers are used to forecast future year emissions.
  • An improved backcasting approach has been applied to generate historical emission estimates, prior to the year 2000, for certain categories derived from California Air Resources Board datasets.
  • New source categories (e.g., ozone depleting substance substitutes and structures coatings) have been added and several categories have been re-scoped (e.g., domestic natural gas combustion), split (e.g., industrial wastewater treatment), or combined (e.g., composting).

Inventory Reports, Data and Documents

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Assessment, Inventory, and Modeling Division

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Spare the Air Status

Last Updated: 8/21/2023