Air Quality

Each model run will automatically report emissions estimates, using rates from EPA's MOVES model. Totals are available for multiple pollutant species across a modeled day, and include running emissions (from intra- and inter-zonal trips) as well as an estimate of emissions from starting engine processes, which emit pollutants at different rates than running engines. The calculation process is consistent with past methods to estimate regional emissions for the Regional Transportation Plan (RTP) update in 2018 and GHG impacts of the 4-Part Strategy.

Emission Summaries

Two sets of emissions are calculated, which have been used for different applications:

• Regional Emissions
• Emissions by Vehicle Type

Regional Emissions are consistent with the Regional Transportation Plan analyses (e.g., FEIS addendum to the RTP). These estimates do not separate emission rates by vehicle type.

Emissions by Vehicle Type use distinct rates by light, medium, and heavy vehicles to compute totals that result in slightly different results than those calculated with aggregate rates across all vehicle types. The distinction was initially made to test policies specific to vehicle type for GHG analyses (e.g., 5% emissions reduction in medium and heavy trucks), which was not possible with regional rates.

Output Files

After a run is complete, total emissions summaries and intermediate calculation files are available in outputs\emissions. This directory contains two sub-directories that hold emissions summaries from regional and vehicle-specific rates. Two files are also available that report inter- and intra-zonal vehicle miles traveled (VMT) by rate category. Rates are applied to these VMT totals, which are grouped by unique speed bins (0-15 mph, 15-30, mph, etc.), road types (arterial, local), hour, county, and vehicle type, except for regional rates that do not differentiate by vehicle type. These intermediate VMT files are useful for tracking summaries and validating that VMT totals are calculated as expected. These files are used by both types of emissions calculated. Further results are specific to each calculation method and stored in separate folders as follows:

• by_vehicle_type
  • emissions_by_veh_type_summary.csv
  • interzonal_emissions_by_veh_type.csv
  • intrazonal_emissions_by_veh_type.csv
  • start_emissions_by_veh_type.csv
• regional
  • regional_emissions_summary.csv
  • regional_emissions_summary_detailed.csv
  • regional_interzonal_emissions.csv
  • regional_intrazonal_emissions.csv
  • regional_start_emissions.csv

The highest level results are emissions_by_veh_type_summary.csv and regional_emissions_summary.csv, which report total daily (tons) of emissions from starts, intrazonal, and interzonal travel, for the following set of pollutants. (Note that only a subset of pollutants [those which are GHG related] are tabulated for results by vehicle type, as indicated by an asterisk.)

• Total Gaseous HCs*
• CO
• NOx
• Methane*
• N20*
• Non-methane HCs
• VOCs
• Atmospheric CO2*
• Total Energy*
• CO2 Equivalent*
• PM10 Exhaust
• PM10 Brakewear
• PM10 Tirewear
• PM25 Exhaust
• Elemental Carbon
• Sulfate Particulate
• PM25 Brakewear
• PM25 Tirewear
• Composite NonECPM
• H20 Aerosol
• PM10 Total
• PM25 Total

Scripts and Inputs

The script scripts\summarize\standard\emissions.py is automatically run immediately after network_summary.py. The network summary script generates some of the necessary inputs (such as a file of link-level outputs with final assigned traffic volumes).

Assumptions regarding starts (based on vehicle population estimates) are embedded in code and comments in emissions.py , as well as in the associated file standard_summary_configuration.py, which contains constants and assumptions, such as time-of-day lookup values, and vehicle distributions by county.

Inputs

Running and starting emissions rates are generated externally by MOVES, but are stored within the Soundcast repository at scripts\summarize\inputs\network_summary in running_emission_rates.csv and start_emission_rates.csv, respectively. Rates for all model years are combined into a single input file. The model year variable, as defined in input_configuration.py will automatically select the appropriate rates by year from these files. Running emission rates for 2014, 2025, and 2040 are taken directly from the "EmissionRates_Regional_YYY" files stores at 'X:\Trans\AIRQUAL\T2040 2018 Update\EmissionCalcs\Running Emissions; start emissions rates from the same years are similar used from "Starts_YYYY" files at X:\Trans\AIRQUAL\T2040 2018 Update\EmissionCalcs\Start Emissions. Rates for 2050 were generated with MOVES in September 2018, consistent with past techniques, and archived at Y:\Air Quality\Vision2050\2050\output`.

Comparison to Past Emissions

Note that 2040 running and start emissions for Snohomish County were updated in September 2018 to reflect more accurate rates that applied Low Emission Vehicle (LEV) assumptions to Puget Sound counties as recommended by the WA Dept of Ecology. These rates will thus provide slightly different totals than those provided for the Regional Transportation Plan (difference of less than 1%). Additionally, separate time-of-day lookup dictionaries were used for intra- and inter-zonal trips when calculations were performed manually in the past. These outputs include such variations, but are consistent moving forward.