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Emissions Inventory

Emissions Inventory

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

Stationary sources include fixed facilities such as: 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 emission testing, from the Environmental Protection Agency’s AP-42 Compilation of Emission Factors, and other evaluations.

Examples of area sources are:  residential water heating, consumer products, unpaved roads, and crop tilling.  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 energy efficient home appliances, the use of low emitting paints, and solvents and other water borne products. 
Mobile sources are what we use to transport ourselves and do commerce like:  ships, planes, trains and automobiles. Mobile sources are the largest category in the San Luis Obispo County inventory – the biggest piece of the emission pie.  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.  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.  Both the CARB and Cal Poly websites have information about the pollen and VOC emissions from various types of plants and trees.  Emissions from natural sources are estimated by CARB and the District. 

Emission Inventory Trends and Projections 2000-2035

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 2000’s due to the implementation of District control measures and the effect of cleaner automobiles.

2012 Emission Inventory Summary


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.

Biogenic sources consist of emissions from naturally existing flora and fauna, emit a significant amount of organic gases.  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.  Source can be from evaporation or formed as a product of combustion.


Nitrogen dioxide (NO2) is one of a group of highly reactive gasses known as oxides of nitrogen (NOx).  Other oxides of nitrogen include nitrous oxide, ammonia, 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. 

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.  Shipping emissions are primarily from the type of fuel burned.

Ozone Precursors

Both ROG and NOx react in the presence of solar radiation react to form photochemical ozone “smog”.  The total emissions from above combine to increase ambient ozone formation. 

As stated above, natural biogenic and ocean going vessel sources of VOCs are often removed from planning inventories.  Natural sources can be excluded because they are beyond the scope of regulation; while the impact of ocean going vessels can be highly variable in terms of affecting inland air quality.  Both sources are shown above however to show the big picture of ozone producing compounds in our county.


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.


Sulfur oxides, primarily consisting of SO2, can pose significant health risks to the respiratory system by forming sulfuric acid and other compounds.  The main source of SOx emissions is the burning of sulfur containing fuels. 


Fuels obtained outside the State of California contain higher percentages of sulfur.


Particulate matter is comprised of various small particles.  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 or blood and cause health problems.   PM includes: mineral dust, acid droplets, organic chemicals, and metals.  The PM data does not include a specific estimate of emissions from the Oceano Dunes State Vehicle Recreation Area; emission factors from that source are planned for development but are not currently available.  Of note, CARB has recently updated their methodology for calculating fugitive road dust emissions which significantly lowered the PM emissions for this and all previous inventories.  Previously posted inventories will need to be adjusted when comparing PM trends. 



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