Gaseous Criteria Pollutants

This page last reviewed December 10, 2010

The California Air Resources Board has monitored the gaseous criteria pollutants carbon monoxide, nitrogen dioxide, ozone, and sulfur dioxide since its inception in 1968.  Monitoring is performed to demonstrate attainment or non-attainment of national and state ambient air quality standards. 

Carbon Monoxide - A colorless,odorless gas, carbon monoxide is a byproduct of incomplete combustion and is emitted directly into the atmosphere, primarily from motor vehicle exhaust. Carbon monoxide concentrations typically peak nearest a source, such as roadways, and decrease rapidly as distance from the source increases. Carbon monoxide is readily absorbed into the body from the lungs. It decreases the capacity of the blood to transport oxygen, leading to health risks for unborn children and people suffering from heart and lung disease.  The symptoms of excessive exposure-- headaches, fatigue, slow reflexes, and dizziness--also occur in healthy people. 

Nitrogen Dioxide - A reddish-brown gas with an irritating odor, nitrogen dioxide is emitted from motor vehicles, industrial facilities, and power plants.  Nitrogen dioxide and nitric oxide are products of all types of combustion.  Nitric oxide reacts with hydrocarbons in the presence of sunlight to form nitrogen dioxide. In the summer months NO2 is a major component of photochemical smog. At ambient concentration levels nitrogen dioxide is an irritating gas that may constrict the airways of asthmatics and increase the susceptibility to infection in the general population. 

Ozone - The most widespread air quality problem in the state, ozone is a colorless gas with a pungent, irritating odor. Ozone is not emitted directly into the atmosphere but is primarily formed through the reaction of hydrocarbons and nitrogen oxides in the presence of sunlight.  Large spatial and temporal separation can exist between the sources of hydrocarbons and nitrogen and the formation of ozone. Ozone's health effects can include reduced lung function; aggravated existing respiratory illness; and irritated eyes, nose, and throat.  Chronic exposure can cause permanent damage to the alveoli of the lungs. 

Sulfur Dioxide - A colorless gas with a strong, suffocating odor, sulfur dioxide is primarily a combustion product of coal, fuel oil, and diesel fuel.  Only small quantities of SO2 come from gasoline fueled motor vehicle exhaust.  Sulfur Dioxide is emitted directly into the atmosphere and can remain suspended for days allowing for wide distribution of the pollutant.  Sulfur dioxide can trigger constriction of the airways, causing particular difficulties for asthmatics.  Children can experience increased respiratory tract infections and healthy people may experience sore throats, coughing, and breathing difficulties.  Long-term exposure has been associated with increased risk of mortality from respiratory or cardiovascular disease.


Monitoring Activities summarizes the network and the monitoring methods for ozone, nitrogen dioxide, carbon monoxide, sulfur dioxide, and hydrogen sulfide.  It includes sampling schedules, number of sites operating in the State, methodology used by the ARB, U.S. EPA reference method, and data availability.

Quality Assurance

The Quality Assurance Section conducts performance and system audits to ensure the accuracy of the gaseous criteria pollutant data generated by ARB and local air quality agencies.  The performance audit is a through-the-probe assessment that tests the integrity of the entire sampling system.  The system audit is a complete assessment of the monitoring program.  Information is available on the following topics: 

Publications and Downloadable Files

Through the Probe Performance Audits of Continuous Ambient Air Analyzers, Michael V. Warren. Technical paper presented at the U.S. EPA/AWMA Symposium on Measurement of Toxic and Related Air Pollutants, Research Triangle Park, North Carolina, April 29 - May 1, 1997. 

Abstract:  The CARB's Quality Assurance Section (QAS) uses a self-sufficient audit van to conduct through-the-probe (TTP) audits. The audit gas concentrations are generated in the van using a gas calibrator to dilute multi-blend gases with zero air. The audit gas is then delivered to the station through a presentation line. The gas calibrator is capable of generating ozone for ozone audits and gas phase titration of nitrogen oxide. The TTP audit technique and the use of multi-blend gas cylinders allow for simultaneous audits of multiple analyzers. 

Comprehensive Quality Assurance Site Survey, Fredrick L. Burriell.  Technical paper presented at the Air and Waste Management Association's 91st annual Meeting and Exhibition, San Diego, California, June 14-18, 1998. 

Abstract:  The California Air Resources Board (CARB) developed the Comprehensive Quality Assurance Site Survey (Survey) as a means of evaluating the performance of ambient air monitoring stations within California. Each station is assessed, using the Survey, to determine the accuracy and representativeness of data being generated. CARB has incorporated into the Survey siting criteria set forth by the United States Environmental Protection Agency (U.S. EPA) in 40 CFR 58. Each year, a thorough evaluation is made at each station for such criteria as sampler model, purpose, objective, residence time, scale, station temperature, obstacles, traffic, local sources, and dominant influence. While most ambient air monitoring stations carefully adhere to regulations during the initial site setup, as reflected by their site reports, changes occur over time that are overlooked by the station operators. Some changes that occur include scaling problems, source problems, obstacles, and temperature requirements.