AirWare
air quality assessment & management
Reference and User Manual
AirWare   On-line Reference Manual

  Release Level 5.6
  Release Date 2009 09
  Revision Level 1.0
Last modified on:   Monday, 20-Jul-09 14:03 CEST

PSI air quality Index

In addition to the basic set of air pollutants, AirWare can also compute and display air quality indices such as PSI.

PSI requires values from several pollutants and for different aggregation periods that are generated by the
photochemical model CAMx, both a daily forecats and as hourly nowcasts with the corresponding moving averages over the most recent 24 hours, including the current hour.

  1. All CAMx result matrices show averages over a time period. The period can be 1 hour, 8 hours or 24 hours. All matrices are assigned to their ending time. For example, the 8-hour-average-matrix of 10:00 - 18:00 is assigned to 18:00. The 24-hour-average-matrix of September 29th, 7:00 - September 30th, 7:00 is assigned to September 30th, 7:00.

  2. For calculation of PSI matrices the following matrices are generated for each timestep:
    • CO: maximum of the 8-hour-averages of the last 24 hours (in reference to the timestep) is taken for each grid cell.
    • O3: maximum of the 1-hour-averages of the last 24 hours (in reference to the timestep) is taken for each grid cell.
    • NO2: maximum of the 1-hour-averages of the last 24 hours (in reference to the timestep) is taken for each grid cell.
    • SO2: 24-hour-average-matrix (ending at the timestep) is taken.
    • PM10: 24-hour-average-matrix (ending at the timestep) is taken

  3. The concentration values are transformed into PSI values using the following formula:

      psip = ( psi(hi) - psi(lo) ) * (concp - bp(lo) ) / ( bp(hi) - bp(low) ) + psi(low)

    where:

      psip   calculated psi index for the pollutant
      concp   the concentration of the pollutant
      bp(lo)   lower breakpoint of the interval of pollutant concentration
      bp(hi)   higher breakpoint of the interval of pollutant concentration
      psi(lo)   PSI values corresponding to bp(lo)
      psi(hi)   PSI values corresponding to bp(hi)

    bp(lo), bp(hi), psi(lo) and psi(hi) are taken from the PSI color definition.

    If the concentration is above the upper limit of the table, PSI is not defined. In this case the maximum PSI value + 1 is assigned ( psip=301 )

  4. PSI matrix is generated by taking the maximum of the psip value of the 5 pollutants for each grid cell.


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