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How to Calculate Air Quality?

Published in Air Quality Monitoring 4 mins read

Air quality is calculated by measuring the concentrations of various pollutants in the air and then comparing these measurements to established air quality standards or guidelines. This comparison results in an air quality index (AQI) value that represents the level of air pollution.

Here's a breakdown of the process:

1. Pollutant Measurement

  • Monitoring Stations: Air quality is typically monitored using specialized equipment located at fixed monitoring stations. These stations continuously measure the concentration of key pollutants in the air.
  • Pollutants Tracked: Common pollutants monitored include:
    • Particulate matter (PM2.5 and PM10)
    • Ozone (O3)
    • Nitrogen dioxide (NO2)
    • Sulfur dioxide (SO2)
    • Carbon monoxide (CO)
  • Measurement Duration: The average concentration of a pollutant is often measured over a specific period, such as one hour or 24 hours, depending on the pollutant and the regulatory requirements.

2. Comparison to Air Quality Standards

  • Air Quality Standards: Each pollutant has a specific air quality standard or guideline established by environmental agencies (e.g., the Environmental Protection Agency (EPA) in the US). These standards represent the maximum acceptable concentration of the pollutant in the air to protect public health.
  • Air Quality Index (AQI): The AQI is a scale used to represent air quality levels. Each pollutant's measured concentration is converted into an AQI value using pre-defined breakpoints. These breakpoints are defined by regulatory agencies and relate pollutant concentrations to specific health effects.

3. Calculating the AQI

The process of calculating the AQI involves the following steps for each pollutant:

  1. Determine the Pollutant Concentration: Obtain the average concentration of the pollutant for the specified averaging time (e.g., 1-hour or 24-hour average).

  2. Find the Breakpoints: Identify the AQI breakpoints that correspond to the measured concentration. Breakpoints are concentration ranges associated with specific AQI values and health risk categories.

  3. Apply the AQI Equation: Use a mathematical formula to convert the pollutant concentration into an AQI value. The formula generally used is a linear interpolation between the breakpoints:

    I = [(I_high - I_low) / (C_high - C_low)] * (C - C_low) + I_low

    Where:

    • I = The AQI value.
    • C = The pollutant concentration.
    • C_low = The concentration breakpoint that is less than or equal to C.
    • C_high = The concentration breakpoint that is greater than or equal to C.
    • I_low = The AQI value corresponding to C_low.
    • I_high = The AQI value corresponding to C_high.

  4. Determine the Overall AQI: The overall AQI for a location is the highest AQI value calculated for any of the individual pollutants. This value represents the worst air quality condition at that location.

4. Reporting and Interpretation

  • AQI Categories: The AQI scale is divided into categories, each associated with a color code and a health risk level. Examples include:

    AQI Value Air Quality Category Color Code Health Implications
    0-50 Good Green Air quality is satisfactory, and air pollution poses little or no risk.
    51-100 Moderate Yellow Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.
    101-150 Unhealthy for Sensitive Groups Orange Members of sensitive groups may experience health effects. The general public is not likely to be affected.
    151-200 Unhealthy Red Everyone may begin to experience health effects; members of sensitive groups may experience more serious effects.
    201-300 Very Unhealthy Purple Health alert: Everyone may experience more serious health effects.
    301-500 Hazardous Maroon Health warnings of emergency conditions. The entire population is more likely to be affected.

  • Public Reporting: AQI values are widely reported to the public through various channels, including websites, mobile apps, and news media, allowing individuals to take precautions and protect their health.

In summary, calculating air quality involves measuring pollutant concentrations, comparing them to air quality standards, calculating AQI values, and then communicating these values to the public in a clear and understandable way.

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