EmissionFactors

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Emission Factors

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EMFAC2011-SG, version 1.0 [September 19, 2011 - updated January 2013]

Background

EMFAC is used to calculate current and future inventories of motor vehicle emissions at the state, county, air district, air basin, or air basin within county level. EMFAC contains default vehicle activity data, and the option of modifying that data, so it can be used to estimate a motor vehicle emission inventory in tons/day for a specific day, month, or season, and as a function of ambient temperature, relative humidity, vehicle population, mileage accrual, miles of travel and speeds. Thus the model can be used to make decisions about air pollution policies and programs at the local or state level. Inventories based on EMFAC are also used to meet the federal Clean Air Act’s (CAA) state implementation plan (SIP) and transportation conformity requirements.

Mobile source emissions include on-road motor vehicles such as automobiles, trucks and buses, as well as off-road sources such as construction equipment; boats, ships, trains and aircraft; and small non-road engines including lawn and garden equipment. Estimates of on-road motor vehicle emissions include consideration of the number of vehicles and the fleet mix (vehicle type, model year, and accumulated mileage); miles traveled; ambient temperatures; vehicle speeds; and vehicle emission factors, as developed from Smog Check data, Caltrans vehicle counts, and California Air Resources Board (ARB) testing programs.

Emission factors are produced for summer and winter operations to reflect the type of fuel in use, such as wintertime oxygenated fuel and summer time fuel which has lower volatility (lower Reid Vapor Pressure) than winter. In addition, emission factors are generated for annual average conditions (for CO₂ emission inventories). Emission reductions resulting from California's Inspection and Maintenance ("Smog Check") program are incorporated.

ARB produced the on-road motor vehicle emission estimates using EMFAC 2011-SG (ARB’s latest version of the EMFAC model), together with updated travel activity for the region. The vehicle miles traveled for 2000 are based on prior year odometer readings recorded from vehicles registered in the Bay Area during their biennial Smog Check. ARB used factors supplied by the MTC for speed distribution on Bay Area roadways and to project travel growth between 2000 and 2035.

Methodology

Total daily regional emissions are generated by applying emission factors to VMT estimates. The steps used in the emissions estimation procedure are discussed below. The methodology used for the emissions calculation in this study is consistent with the methodology used in conformity analyses conducted in the San Francisco Bay Area.

Per ARB guidance, the VMT forecasting procedure involved the use of MTC's validated network-based travel demand forecast model in order to apply adjustment factors to the future year travel demand model VMT estimate, based on the equation below:

Forecasted VMT_(future(i)) = Actual VMT₍₂₀₀₀₎ * Travel Demand Model VMT_(future(i)) / Travel Demand Model VMT₍₂₀₀₀₎,

where I = 2020 or 2035.

Using the equation above, for each specified forecasted year, the forecasted VMT was calculated for each functional class and county combination in the San Francisco Bay Area air basin.

The adjustment factors were computed by dividing the actual 2000 VMT by the 2000 travel demand model VMT.

A series of computer programs/scripts have been developed to process the data needed to generate Bay Area regional emission inventories according to the specifications set forth by the ARB. The following detailed procedure is used instead to more adequately capture temporal variation in emissions:

a. From MTC’s travel demand model, each link's travel by individual time period daily travel (ADT) estimate is divided into volumes by hour of day and direction. Emissions estimates can then be calculated on an hour by hour basis. Diurnal factors are used to allocate travel to five time periods covering the entire day, specifically five time period-specific assignments are performed: (a) early AM, 3 am to 6 am; (b) AM peak period, 6 am to 10 am; (c) midday, 10 am to 3 pm; (d) PM peak period, 3 pm to 7 pm; and, (e) evening, 7 pm to 3 am. Importantly, the diurnal factors segmenting vehicular travel into these five time periods are adjusted per previous results of the work travel departure time choice model.

b. An average speed specific to the facility class, area type and time-of-day was selected and used to select the appropriate exhaust ROG, CO, NO_x, PM₁₀, PM_2.5 and CO₂ factors;

c. The three factors listed below are multiplied to yield emissions for the link for one hour and direction for a given pollutant:

• The VMT for one hour and direction from Step 1 (48 combinations);
• The percent of the link VMT attributable to vehicle type v for which emissions are calculated. The percent of link VMT by vehicle type is generated for four categories (urban freeways, urban arterials) and
• The pollutant emission rate (be it ROG, CO, NO_x, PM₁₀, PM_2.5 and CO₂) for vehicle type v, the speed calculated from Step 2. There are three scripts/steps that produce the activity data inputted into EMFAC2011-SG.

Step (1) / Script (1) CreateSpeedBinsBetweenZones.job;

This step and script extracts link level VMT and speeds for the 13 ARB speed cohorts.

This generic script forecast year run applies for all 5 timeperiods.

Step (2) / Script (2) CreateSpeedBinsWithinZones.job;

This step and script - a) extracts intrazonal level VMT and speeds; and b) extracts total daily trips for the 13 ARB speed cohorts

This generic script forecast year run also applies for all 5 timeperiods.

Step (3) / Script (3) SumSpeedBins1.awk

This step sums EMFAC output for two csv files 1) travel between zones and 2) travel within zones by county and the 13 ARB speed cohorts. The output of this script is two files: 1) a file with the sum of the two inputs; 2) the share of each speed bin relative to county totals; and 3) the total VMT values collapsed to county and hourly bins.

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EMFAC2007, version 2.3 [Nov. 2006]

Background

The EMFAC model (short for EMission FACtor) is a computer model that can estimate emission rates for motor vehicles for calendar years from 1970 to 2040 operating in California. Pollutant emissions for hydrocarbons, carbon monoxide, nitrogen oxides, particulate matter, lead, sulfur oxides, and carbon dioxide are output from the model. Emissions are calculated for passenger cars, eight different classes of trucks, motorcycles, urban diesel and school busses and motor homes.

The EMFAC2007 model is operated with a user-friendly graphical user interface (GUI) which facilitates data input and allows the development of alternative emissions scenarios through a What If Scenarios (WIS) generator. The WIS interface can be used to incorporate updated vehicle data, adjust ambient conditions or make changes to potential emission control programs in a specific area.

EMFAC 2007 includes data for temperature, relative humidity, and characteristics for gasoline fuel sold that vary by geographic area, calendar year, and month and season. The annual average represents an average of all the monthly inventories. As a result, EMFAC will be run to estimate information for both the annual and 24-hour PM2.5 standards.

EMFAC is used to calculate current and future inventories of motor vehicle emissions at the state, county, air district, air basin, or air basin within county level. EMFAC contains default vehicle activity data, and the option of modifying that data, so it can be used to estimate a motor vehicle emission inventory in tons/day for a specific day, month, or season, and as a function of ambient temperature, relative humidity, vehicle population, mileage accrual, miles of travel and speeds. Thus the model can be used to make decisions about air pollution policies and programs at the local or state level. Inventories based on EMFAC are also used to meet the federal CAA's state implementation plan (SIP) and transportation conformity requirements.

Mobile source emissions include on-road motor vehicles such as automobiles, trucks and buses, as well as off-road sources such as construction equipment; boats, ships, trains and aircraft; and small non-road engines including lawn and garden equipment. Estimates of on-road motor vehicle emissions include consideration of the number of vehicles and the fleet mix (vehicle type, model year, and accumulated mileage); miles traveled; ambient temperatures; vehicle speeds; and vehicle emission factors, as developed from Smog Check data, Caltrans vehicle counts, and ARB testing programs.

On-road motor vehicle emissions estimates are based on ARB’s latest available emission factor model (EMFAC 2007, Version 2.3, November 2006). Bay Area emission factors incorporate projected growth in vehicle miles of travel (VMT) for the Bay Area developed by the Metropolitan Transportation Commission (MTC) from its travel demand model for the Transportation Air Quality Conformity Analysis of the Regional Transportation Plan 2035 (2009 RTP). MTC’s travel demand model utilizes regional demographic forecasts from ABAG’s socio-economic and population projections, in this case, Projections 2009.

EMFAC 2007 consists of two major parts: EMFAC and BURDEN. EMFAC calculates emission rates for a variety of vehicle types (passenger cars, trucks, etc.) by fuel usage, control technology and mode of operation (e.g., hot start, cold start). It also accounts for vehicle age, and operating conditions such as speed and temperature. Recent improvements to EMFAC include adjustments to account for real world driving patterns (e.g., rapid acceleration), use of NOx defeat devices in trucks, use of air conditioning in cars, and fuel line leaks from cars and trucks.

Emission factors are produced for summer and winter operations to reflect the type of fuel in use, such as wintertime oxygenated fuel and summer time fuel which has lower volatility (lower Reid Vapor Pressure) than winter. Emission reductions resulting from California's Inspection and Maintenance ("Smog Check") program are incorporated.

The other model component, BURDEN, uses emission factors from EMFAC and a large database of vehicle activity for each county to calculate total daily emissions. The activity is in the form of number of in-use vehicles, number of vehicle engine starts and vehicle miles traveled (VMT) for each vehicle type. The vehicle trips and VMT data for these calculations were estimated by ARB for 2000. Vehicle population is derived from Department of Motor Vehicle (DMV) data and number of engine starts is based on the population data and ARB surveys. Vehicle speed data and vehicle trip/VMT growth projections were obtained from MTC.

ARB produced the on-road motor vehicle emission estimates using EMFAC 2007, together with updated travel activity for the region. The vehicle miles traveled for 2000 are based on prior year odometer readings recorded from vehicles registered in the Bay Area during their biennial Smog Check. ARB used factors supplied by the MTC for speed distribution on Bay Area roadways and to project travel growth between 2000 and 2006.These region-specific inputs have been incorporated into the coding of the model itself, resulting in a unique version of the EMFAC2007 model for this Bay Area SIP.

A planning inventory is a seasonal inventory representing emissions when a pollutant's concentrations are at their highest levels. For example, the emission inventory for the ozone season represents emissions occurring during the summer when ozone levels are highest. The emission inventory for the particulates season represents emissions occurring during the winter when particulate levels are highest. The seasonal inventories (summer and winter) are prepared based on published ARB and EPA guidelines, as described below.

The summer season is considered May through October and the winter season is considered November through April. Data on normal operating schedules (hours per day, days per week and weeks per year) are collected as part of routine point source inventory procedures. For area sources, representative profiles showing monthly, weekly, and daily variation in emissions are prepared for each source category. These profiles are then used to obtain average seasonal operating day emissions.

For on-road motor vehicles, the EMFAC 2007 was used to develop planning inventories. The emission estimates for these inventories are based on ambient temperature profiles representing the ten days having the highest pollution levels. Summer temperature profiles are used to generate the ozone precursor (ROG and NOx) emission inventory. BURDEN divides the day into six different time periods consistent with motor vehicle activity patterns, including the morning and evening commute periods. These six periods are: midnight - 6am, 6am - 9am, 9am - noon, noon - 3pm, 3pm - 6pm, and 6pm - midnight. For each period, specific temperatures, activity data and vehicle speeds are used to estimate emissions. The emissions from the six periods are then summed to get daily emissions.

EMFAC is a California Air Resources Board (ARB) model for estimating pollution from highway vehicles. EMFAC calculates emissions of hydrocarbons (HC), oxides of nitrogen (NOx), on monoxide (CO) and other pollutants from passenger cars, motorcycles, light- and heavy-duty trucks. EMFAC is based on emissions testing of tens of thousands of vehicles. The model accounts for the emission impacts of factors such as changes in vehicle emission standards, changes in vehicle populations and activity, and variation in local conditions such as temperature, humidity and fuel quality.

EMFAC results are used to calculate current and future inventories of these emissions at the national and local level. These inventories are used to make decisions about air pollution policy and programs at the local, state and national level. Inventories based on EMFAC are also used to meet the federal Clean Air Act's state implementation plan (SIP) and transportation conformity requirements, and are sometimes used to meet requirements of the National Environmental Protection Act (NEPA).

Methodology

Total daily regional emissions were generated by applying emission factors to VMT estimates. The steps used in the emissions estimation procedure are discussed below. The methodology used for the emissions calculation in this study is consistent with the methodology used in conformity analyses conducted in the San Francisco Bay Area.

Per ARB guidance, the VMT forecasting procedure involved the use of MTC's validated network-based travel demand forecast model in order to apply adjustment factors to the future year travel demand model VMT estimate, based on the equation below:

Forecasted VMT_(future(i)) = Actual VMT₍₂₀₀₀₎ * Travel Demand Model VMT_(future(i)) / Travel Demand Model VMT₍₂₀₀₀₎,

where I = 2020 or 2035.

Using the equation above, for each specified forecasted year, the forecasted VMT was calculated for each functional class and county combination in the San Francisco Bay Area air basin.

The adjustment factors were computed by dividing the actual 2000 VMT by the 2000 travel demand model VMT.

A series of computer programs/scripts have been developed to generate emission inventories according to the specifications set forth by the ARB. These programs do not follow the approach of assigning one emission factor for each vehicle type and road type at a single speed. The following detailed procedure is used instead to more adequately capture temporal variation in emissions:

a. Each link's average daily travel (ADT) estimate is divided into volumes by hour of day and direction. Emissions estimates can then be calculated on an hour by hour basis.

b. An average speed specific to the facility class, area type and time-of-day was selected and used to select the appropriate exhaust ROG, CO, and NO_x factors;

c. The three factors listed below are multiplied to yield emissions for the link for one hour and direction for a given pollutant:

^. The VMT for one hour and direction from Step 1 (48 combinations);

^. The percent of the link VMT attributable to vehicle type v for which emissions are calculated. The percent of link VMT by vehicle type is generated for four categories (urban freeways, urban arterials) and

^. The pollutant emission rate (be it ROG, CO, or NO_x) for vehicle type v, the speed calculated from Step 2.

There are five scripts/steps that produce the activity data inputted into EMFAC.

Step (1) / Script (1) CreateSpeedBinsBetweenZones.job;

This step and script extracts link level VMT and speeds for the 13 ARB speed cohorts.

This generic script forecast year run applies for all 5 timeperiods.

Step (2) / Script (2) CreateSpeedBinsWithinZones.job;

This step and script - a) extracts intrazonal level VMT and speeds; and b) extracts total daily trips for the 13 ARB speed cohorts

This generic script forecast year run also applies for all 5 timeperiods.

Step (3) / Script (3)

This step sums EMFAC output for two csv files 1) travel between zones and 2) travel within zones by county and the 13 ARB

speed cohorts.

The output of this script is two files: 1) a file with the sum of the two inputs; 2) the share of each speed bin relative to county totals;

and 3) the total vmt values collapsed to county and hourly bins.

Step (4) / Script (4)

This step multiplies total VMT (stratified by county and hour of day) by VMT share values for fuel type and vehicle class

The output of this script is a unique file for each county, by VMT, in each category of fuel type, vehicle class, and hour of the day

Step (5) / Script (5)

This step creates a factor for growing the vehicle population files, using county VMT total ratios for project year and base year, and an EMFAC-generated factor

The output of this script is a unique file for each county, by vehicle population for each vehicle and fuel type

Air Quality Emission Inventory Modeling Procedures (for the San Francisco Bay Area)

Instructions for generating emission estimates using California Air Resources Board's EMFAC software and Travel Model One activity data.

1. Start EMFAC by selecting the "EMFAC2007" item on the Start/Programs menu (Windows XP) or Start/Run (Windows 2000), or by double-clicking the "EMFAC2007" icon on the desktop.

2. In this example we will be creating a "New" file. Select "File New"

3. Then select "Add New Scenario"

4. Next select "Air Basin" Geographic Area

5. Select "Air Basin" as the Area Type and select ”San Francisco” from the choices of Air Basin in the "Drop Down Box".

6. Select "Calculation Method" as the Calculation Method and select ”By Sub-Area” from the choices in the "Drop Down Box".

7. Next input the "Calendar Year and Season or Month". In this example we will use 2015 and summer. Select the links below for the correct analysis years and seasons.

8. Click on the “Next” button to move onto the next input 2 steps.

9. Edit the "Title" (it is recommended to include a title). Click "Next".

10. Leave steps/buttons 5, 6 and 7 in the “All”, “All” and “Default” settings. Click on the “Next” button to move onto the mode and output steps.

11. Select "Burden – Area planning inventory" as the operation mode and select ”Detailed Planning Inventories (CSV)” as the BURDEN Inventory Files and Reports format. Leave all other boxes in the BURDEN Inventory Files and Reports section UNCHECKED.

12. For Output Frequency, select “Day”. For Output Particulate As…., select “PM2.5” (in this example). For Output Hydrocarbons As…., select “ROG” Click on the “Edit Program Constants” button to move onto the Tech/IM steps

13. Click on the “Next” button to move onto the CYr Basis (year) step.

14. Click on the “Next” button to move onto the Pop/Accrual steps and then click on "Population".

15. The Total Population by sub-area for the San Francisco Air Basin will start alphabetically with “Alameda (SF)” county tab. Click on the “By Vehicle and Fuel” editing mode button. 

16. Click on and highlight the data array table by vehicle class and fuel.

17. Copy and Paste in update vehicle population (by county) values by clicking the “Paste Data Only” button.

18. Once all vehicle population data is entered for all counties (with Sonoma (SF) county), click the “Done” button.

19. Click on the “Next” button to move onto the VMT/Trips steps and then click on "VMT"

20. The Total VMT by sub-area for the San Francisco Air Basin will start alphabetically with “Alameda (SF)” county tab. Click on the “By Vehicle/Fuel/Hour” editing mode button. The “Fuel Type” tab (on the right hand side) will start with “Gas” at the top.

21. Click on and highlight the data array table by vehicle class/fuel/hour (for the gas fuel type).

22. Copy and Paste in updated VMT (by county) values by clicking the “Paste Data Only” button. For each individual county, paste in updated VMT for the diesel and electric fuel types also.

23. For each individual county, paste in updated VMT for the diesel and electric fuel types also.

24. Once all VMT data is entered for all counties (with the Electric fuel type for Sonoma (SF) county), click the “Done” button.

25. Click on the “Next” button to move onto the Profiles/Speed steps and then click on "Speed Fractions"

26. The Speed Fractions by Scenario Year, Vehicle Class and sub-area for the San Francisco Air Basin will start alphabetically with “Alameda (SF)” county tab. Click on the “5 MPH” Basis button.

27. Click on and highlight the data array table by speed bin and hour (for the gas fuel type).

28. Copy and Paste in updated speed distribution (by county) values by clicking the “Paste Data Only” button.

29. Click the “Apply to Others” button and check the “Vehicle Class” in the parameters box and click “OK”.

30. Click on and highlight the “Light-duty Trucks (T1)”, “Light-duty Trucks (T2)”, “Medium-duty Trucks (T3)” and “Motorcycles” vehicle classes and click the to the right-arrow box.

31. Once the five passenger vehicle types have been moved to the “Apply To:” column, click the “OK” box.

32. Once all speed distribution data is entered for all counties (with Sonoma (SF) county), click the “Done” button.

33. On the Profiles/Speed tab window, click the “Finish” button.

34. Click the “Save As…” button and give this input file a filename and click “Save”.