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Primary vs. secondary pollutants (air pollution)

In the IFVI air pollution methodology, primary and secondary pollutants are captured differently based on their distinct pathways and impacts. Here’s a breakdown of each:

1. Primary Pollutants

  • Definition: These are pollutants directly emitted from sources such as transportation, industrial processes, or agriculture. Examples include PM2.5, PM10, NOx, SOx, NH3, and VOCs.
  • Capture Method:
  • Direct Measurement: Primary pollutants are measured at their emission sources, using data on quantities (mass in metric tons or kilograms) emitted by companies.
  • Value Factors: The methodology uses “value factors” from the Global Value Factors Database, applying pollutant-specific rates to estimate the direct societal impacts of these emissions.
  • Location Context: For primary pollutants, emission location types (e.g., urban, rural, or transport zones) are essential, as population density and activity levels influence exposure and health impacts.
  • Health and Environmental Impacts:
  • Primary pollutants have immediate impacts, including respiratory and cardiovascular diseases in humans and damage to local vegetation. These pollutants are measured and valued directly to estimate their direct costs on health and visibility.

2. Secondary Pollutants

  • Definition: These pollutants form in the atmosphere due to chemical reactions involving primary pollutants under specific conditions, such as sunlight. Common secondary pollutants include ozone (O3) and secondary particulate matter (e.g., sulfates, nitrates).
  • Capture Method:
  • Air Dispersion Models: Secondary pollutants are more complex to track directly, so the methodology uses models (such as ATMOS 4.0) to simulate their formation based on emissions of primary pollutants like NOx and VOCs.
  • Value Transfer Functions: For example, societal cost calculations for ozone are based on a multivariate transfer function, using data like ambient concentration levels, population density, and income to assess regional impact.
  • Health and Environmental Impacts:
  • Secondary pollutants also affect health but over larger areas and timescales. They increase risks of respiratory illness and reduce crop productivity, and visibility in densely populated or high-traffic areas.

Summary of Differences

Primary pollutants are captured directly from emission sources and assessed based on the pollutant type and emission location. Secondary pollutants are modeled due to their formation processes in the atmosphere and are captured based on the concentration and transformation of primary pollutants in specific conditions