Methodology

The ECHO-AIR model works by a series of two modules. First, the model estimates annual average change in PM2.5 and, optionally, DPM concentrations as part of the Concentration Module. Second, the excess mortality and health impact excess incidence resulting from the concentration changes are calculated in the Health Module.

Concentration Module Methodology

The InMAP Source Receptor Matrix (ISRM) links emissions sources to changes in receptor concentrations. There is a matrix layer that converts each of the five precursor species into PM2.5: primary PM2.5, ammonia (NH3), oxides of nitrogen (NOx), oxides of sulfur (SOx), and volatile organic compounds (VOC). An additional and optional matrix layer converts emissions of diesel particulate matter (DPM) into concentrations of DPM. By default, the tool uses the California ISRM. For each of these species in the California ISRM, the ISRM matrix dimensions are: 3 elevations by 21,164 sources by 21,164 receptors. The three elevations of release height within the ISRM are:

  • Less than 51.8 meters
  • Between 51.8 and 95 meters
  • Greater than 95 meters.

The model is capable of reading in a different ISRM, if specified by the user.

The units of each cell within the ISRM are micrograms per meter cubed per microgram per second, or concentration per emissions.

The concentration module has the following steps. Details about the code handling each step are described in the Code Details section.

  1. Preprocessing: the tool will load the emissions shapefile and perform a series of formatting checks and adjustments. Any updates will be reported through the command line. Additionally, the ISRM layers will be imported as an object. The tool will also identify how many of the ISRM layers are required for concentration calculations.

For each layer triggered in the preprocessing step:

  1. Emissions Re-Allocation: the model will re-grid emissions to the ISRM grid.
    1. The emissions shape and the ISRM shape are intersected.
    2. Emissions for the intersection object are allocated from the original emissions shape by the percent of the original emissions area that is contained within the intersection.
    3. Emissions are summed by ISRM grid cell.
    4. Note: for point source emissions, a small buffer is added to each point to allocate to ISRM grid cells.
  2. Matrix Multiplication: Once the emissions are re-gridded to the ISRM grid, they are multiplied by the ISRM grid level for the corresponding layer.

Once all layers are done:

  1. Sum all Concentrations: concentrations of PM2.5 and DPM are summed by ISRM grid cell.

Health Module Methodology

The ISRM Tool calculations health module follows US EPA BenMAP CE methodology and CARB guidance.

Currently, for PM2.5, the tool is only built out to use the Krewski et al. (2009), endpoint parameters and functions.1 The Krewski function is as follows:

ΔM = (1 - (exp(βd x ΔCi)-1) x Ii,d,g x Pi,g

where β is the endpoint parameter from Krewski et al. (2009), d is the disease endpoint, C is the concentration of PM2.5, i is the grid cell, I is the baseline incidence, g is the group, and P is the population estimate. The tool takes the following steps to estimate these concentrations.

For DPM, excess cancer risk is calculated following OEHHA (2015) methodology:

Unit doses (breathing rate (BR) / body weight (BW) × exposure frequency (EF) × conversion factor(CF)) and age-specific factors (age sensitivity factor (ASF - OEHHA 2015 Table 8.3), fraction of time at home (FAH - OEHHA 2015 Table 8.4), duration adjustment (DA)) are applied across relevant age bins for either a 30- or 70-year averaging period, then multiplied by the cancer sensitivity factor (CSF = 1.1), summed by age group, and multiplied by ambient DPM concentrations (Cdpm) to yield risk per million people per ISRM cell.

Σage groups (EF x [BR/BW] x CF x ASF x FAH x DA x CSF) x CDPM

Additionally, the Chronic Hazard Incidence is calculated using OEHHA’s chronic exposure level:

HQ = CDPM / 5

  1. Preprocessing: the tool will merge the population and incidence data based on geographic intersections using the health_data.py object type.

  2. Estimation by Endpoint: the tool will then calculate excess mortality or incidence by endpoint:

    1. The population-incidence data are spatially merged with the exposure concentrations estimated in the Concentration Module.
    2. For each row of the intersection, the excess mortality or incidence is estimated based on the function of choice (currently, only Krewski for PM2.5).
    3. Excess mortality or incidence is summed across age ranges by ISRM grid cell and racial/ethnic group.

Once all endpoints are done:

  1. Export and Visualize: excess mortality and incidence is exported as a shapefile and as a plot.

Other Features

ECHO-AIR has a command called check-setup that allows the user to make sure that all of the code and data files are properly saved and named in order to make sure that the program will run.

References

  1. Krewski et al., (2009) Extended Follow-Up and Spatial Analysis of the American Cancer Society Study Linking Particulate Air Pollution and Mortality. HEI Research Report 140. ↩