Looking closely.

Measuring and analyzing air quality is our first step to reducing air pollution.

The air in our lower atmosphere is a dynamic, constantly shifting mixture of gases, liquid droplets, and small particles. It swirls and eddies around the globe like the water in the ocean, with winds and weather patterns resulting from this movement. It’s also not as light as it seems. A column of air one foot square and extending from sea level to the outer limit of the atmosphere would weigh nearly one ton. And contrary to what one might expect, the air we breathe in the lower atmosphere is not primarily composed of oxygen. Instead, it contains 78 percent nitrogen, 21 percent oxygen, and less than 1 percent gases like argon and carbon dioxide. Unfortunately, it can also contain substances that are unhealthy for us to inhale.

In the Bay Area, as in the entire state of California, a certain amount of air pollution comes from stationary industrial sources, such as refineries and power plants. But a greater percentage of harmful air emissions come from cars and trucks, construction equipment, and other mobile sources. California has more cars per household (1.8) than any other state, along with a diverse business community and a continually expanding population. All of these factors contribute to the state’s air quality challenges.

There are three major types of air pollutants that constitute a public health concern for the Bay Area: ozone, particulate matter, and toxic air contaminants.

Ozone

Ozone is the main ingredient in the pollution commonly called “smog.” Primarily a problem in the summertime, ozone is a colorless gas formed through a complex series of photochemical reactions involving sunlight and heat. It is not emitted directly into the air in significant quantities, but is formed in the presence of sunlight from chemical reactions involving other directly released precursor pollutants: reactive organic compounds and oxides of nitrogen.

Particulate Matter (PM10 and PM2.5)

Particulate matter, or PM, consists of microscopically small solid particles or liquid droplets suspended in the air. PM can be emitted directly into the air, or it can be formed from secondary reactions involving gaseous pollutants that combine in the atmosphere. Particulate pollution is primarily a problem in the winter, accumulating when cold, stagnant weather comes to the Bay Area.

PM is usually measured and monitored in two size distributions: PM10 and PM2.5. PM10 refers to particles with diameters that are less than or equal to 10 microns in size (a micron is one-millionth of a meter), or about 1/7 the diameter of a human hair. PM2.5 consists of particles with diameters that are less than or equal to 2.5 microns in size. PM2.5 is a more serious health concern than PM10, since smaller particles can travel more deeply into our lungs and cause more harmful effects.

Winter Sources of Bay Area Fine Particulates

Summer Sources of Bay Area Ozone-Forming Pollutants

Toxic Air Contaminants (TACs)

Toxic Air Contaminants, or TACs, are a category of air pollutants that in relatively small concentrations can potentially cause serious human health effects, such as cancer. The state of California has listed more than 180 TACs, which are emitted by mobile sources such as cars and trucks, large industrial plants such as refineries and power plants, and smaller facilities such as gas stations and dry cleaners. PM from diesel exhaust is listed as a TAC by the state of California.

Air Monitoring

The Air District maintains one of the most comprehensive air quality monitoring networks in the country, consisting of 31 monitoring stations distributed among the nine Bay Area counties. This network measures concentrations of pollutants for which health-based ambient air quality standards have been set by the U.S. Environmental Protection Agency, and by the California Air Resources Board, or CARB. These pollutants include ozone, particulate matter, carbon monoxide, nitrogen dioxide, and sulfur dioxide. The Air District’s network also measures concentrations of 19 toxic air contaminants and various other pollutants of concern.

The Air District’s network also includes two re-locatable air monitoring stations similar to those used by CARB under the Children’s Environmental Health Protection Program. These stations are placed in communities of interest for one to two years, in order to compare local air measurements with those obtained by the agency’s monitoring network.

Laboratory

The Air District maintains an extensive laboratory with state-of-the-art equipment for testing air quality samples collected from ambient monitors, from source tests, or during accidental releases at Bay Area facilities. The laboratory also analyzes samples submitted by the Enforcement Division to assess compliance with Air District regulations.

Forecasting

Weather patterns play a fundamental role in determining, on any given day, whether air pollution will disperse or accumulate. Air District meteorologists collect and analyze data from a network of meteorological sensors located throughout the nine Bay Area counties. This information—in combination with air monitoring measurements, computer models, and satellite feeds from weather services—is used to make daily air quality forecasts for the public.

The Air District prohibits open burning throughout the Bay Area, with the exception of a few types of fires (generally for agricultural or natural-resource management purposes) that are allowed on designated “burn” days. The Air District’s meteorological staff issues “burn” or “no-burn” notices for these types of permissible burns every day of the year.