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Lesson 14 - On-Road Mobile Sources

14.1 - On-road Emissions, Where Do They Come From, How are They Calculated?

The cars and trucks that are driven on roads are some of the biggest polluters in America, equal even to power plants for some pollutants. There is no doubt they release a lot of emissions where you live, no matter where that is.

Vehicles create emissions in several ways:

• Exhaust - from the burning of fuel
  
  
• Break and tire wear - releases particulate matter
  
  
• Evaporation (not including refueling) - Volatile organic compounds from fuel evaporating off the engine.
  
  

Because on-road vehicles are such an important source of emissions, EPA has developed a very detailed and somewhat complicated way of estimating emissions from them. Every type of vehicle will release somewhat different emissions depending on what kind of engine it has, what kind of fuel it burns and how it is driven (fast or slow, lots of idling at stoplights or cruising on the open road).

The EPA divides vehicles into the following types when estimating emissions:
Vehicle types - Gasoline:

• Light duty (passenger) gasoline vehicles and trucks (LDG Vs & Ts)
  
  
• Motorcycles (MC)
  
  
• Heavy duty gasoline vehicles (HDGV 2b-8b) (Modifier is based on vehicle weight)
  
  
  • Trucks
    
    
  • Busses
    
    

Vehicle types - Diesel:

• Light duty diesel vehicles (LDDV - cars)
  
  
• Light duty diesel trucks (LDDV < 8500 gvwr)
  
  
• Heavy duty diesel trucks (HDDV (8500 > 60K)
  
  
• School & public transportation busses
  
  

The type of driving each vehicle is used for has a big effect on how much emissions it produces, so EPA defines several road types:
Road types - Rural & Urban:

• Arterials
  
  
  • Interstates
    
    
  • Other major highways
    
    
  • Minor highways
    
    
• Collectors
  
  
  • On & off ramps
    
    
• Locals
  
  

To create emission factors that take into account all of these factors, EPA uses a model called MOBILE6. MOBILE6 is a very complicated "black box" model - the user enters input parameters into the "black box" it performs some complex operations and calculations, then gives the user an "answer" of what emission factors to use for each vehicle type/road type combination.

We won't go too far into how MOBILE6 works here, but some of the parameters it takes into account when creating emission factors are:

• Climate (Seasonal temp, humidity, elevation)
  
  
• Type of road (breaking & accelerating estimates)
  
  
• Registration records (how old are the cars)
  
  
• Fuel type (sulfur content, vapor pressure, MTBE)
  
  
• And so on...
  
  

Once you get the emission factors out of MOBILE6, you need some activity data to multiply them by. The activity data needed for on-road mobile sources is total vehicle miles traveled (VMT) per road type per year. Emission estimates for paved and non-paved road dust also rely on VMT. The next section will deal with how to calculate VMT.

End of Lesson 14.1

Lesson 14

Lesson 14.2