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Emissions Inventory

Emissions Inventory

The San Luis Obispo Air Pollution Control District maintains an air pollution emissions inventory for the county with contributions from the California Air Resources Board (CARB). The air pollutants tracked by this inventory are known as ‘criteria pollutants’. Criteria pollutants include Total Organic Gases (TOG) and its subset, Volatile Organic Gases (VOC), carbon monoxide (CO), oxides of nitrogen (NOx), oxides of sulfur (SOx) and particulate matter (PM). Sources of air pollution are grouped into major categories of stationary, mobile, area-wide and natural sources.

Stationary sources include power plants, waste water treatment plants, auto body shops, and landfills. Most stationary sources are required to obtain a Permit to Operate from the District, and these facilities submit annual activity reports that are used to estimate their emissions. Emission estimation methods come from actual stack exhaust testing, from the Environmental Protection Agency’s AP-42 Compilation of Emission Factors, and other various sources.

Examples of area sources are residential water heating and the use of paints, solvents and other consumer products. Area sources may be spread out throughout the county, and each of those point sources individually may not appear to be important, but the collective emissions from many of these categories are very significant. This illustrates how the individual choices that we all make are very important to our air quality. We would encourage consumers to select water borne products, and to use energy efficiently.

Mobile sources are things like ships, planes, trains and automobiles. Most mobile source data is estimated by CARB for the entire state, but the District does have the responsibility of estimating some mobile source categories like aircraft. Mobile sources are the largest category in the San Luis Obispo County inventory – the biggest piece of the emission pie. Transportation choices that we all regularly make have a direct impact on the air quality in our county. We can all make choices to walk, ride a bike, carpool or take a bus rather than driving alone.

In addition to the man-made air pollution, there are also significant quantities of pollutants from natural sources. Natural sources include biological and geological sources such as wildfires, windblown dust, gas seeps and the biogenic emissions of VOCs from plants and trees. Emissions from natural sources are estimated by CARB. Both the CARB and Cal Poly websites have information about the pollen and VOC emissions from various types of plants and trees.

Emission Inventory Trends and Projections 1990-2020

Past and current emission inventory totals are taken from the District’s database. CARB has an extensive database of growth and control factors that are used to estimate the future emissions. These long term trends show the dramatic decreases in ozone precursor emissions that occurred in the 1990’s due to the implementation of District control measures and the effect of cleaner automobiles. Future projections illustrate the need for additional control strategies, especially for particulate matter.

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2009 Emission Inventory Summary

Summary Table (PDF) Excel File


Total Organic Gases are compounds of carbon and hydrogen. These compounds include all of the reactive organic gases (ROG) in addition to low reactivity organic compounds like methane and acetone. Common sources of organic gases include solvents, pesticides, the burning of fuels and organic wastes. In stronger concentrations organic compounds can be dangerous to health, causing eye, nose and throat irritation as well as liver, kidney and central nervous system damage. Organic gases can also react with oxides of nitrogen to form ozone.

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Natural (biogenic) sources such as plants and trees also emit a significant amount of organic gases. These natural emission sources are normally not included in a planning inventory because they are beyond the scope of regulatory programs. But they have been included here to show the big picture of all emission sources in the county.


Reactive Organic Gas (ROG) or Volatile Organic Compounds (VOC): A subset of TOG, these reactive or volatile compounds contribute to the formation of ground level photochemical smog.


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Carbon monoxide (CO) is a colorless, odorless gas. It results from the incomplete combustion of carbon-containing fuels such as gasoline or wood, and is emitted by a wide variety of combustion sources.


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Nitrogen dioxide (NO2) is one of a group of highly reactive gasses known as oxides of nitrogen (NOx). Other oxides of nitrogen include nitrous acid and nitric acid. NO2 forms quickly from emissions from cars, trucks and buses, power plants, and off-road equipment. In addition to contributing to the formation of ground-level ozone and fine particle pollution, NO2 is linked with a number of adverse effects on the respiratory system.

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The emissions from ocean-going vessels are often not considered in a planning inventory because their impact upon inland air quality can be highly variable.

Ozone Precursors

These are the chemical compounds ROG and NOx that react in the presence of solar radiation react to form photochemical ozone “smog”.


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Primarily consisting of SO2, sulfur oxides can pose significant health risks to the respiratory system. The main source of SOx emissions is the burning of sulfur containing fuels. Emissions from ships are sometimes not included in planning inventories because variables such as the distance from shore, the weather and wind patterns make the onshore impact very difficult to predict. Ocean going ships burn high sulfur fuels that result in large quantities of sulfur oxide emissions.

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Particulate matter is comprised of various small particles including acids, organic chemicals, metals and dust. Of primary concern are particles that are 10 micrometers in diameter or smaller (PM10) and particles that are 2.5 micrometers in diameter or smaller PM (2.5). Particles that fall within that range can enter the lungs and cause health problems. The PM data does not include a specific estimate of emissions from the Oceano Dunes State Vehicle Recreation Area because emission factors from that source are not currently available.

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