How to calculate Raw PM2.5 ALT cf=3.4 from SD card files

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Hello everyone,

In the SD card csv files of my PurpleAir Flex, I have access to the following measurements: pm1_0_cf_1, pm2_5_cf_1, pm10_0_cf_1, pm1_0_atm, pm2_5_atm, pm10_0_atm, pm2.5_aqi_cf_1, pm2.5_aqi_atm, p_0_3_um, p_0_5_um, p_1_0_um, p_2_5_um, p_5_0_um, p_10_0_um as further explained here : SD Card File Headers

Would you know how to calculate the “Raw PM2.5 ALT cf=3.4” from these values ?

Thank you

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Hello,

You’ll need to use the particle counts (p_0_3_um, p_0_5_um, p_1_0_um, p_2_5_um, p_5_0_um, p_10_0_um) as a basis to find the ALT values.
 

The formula is as follows: PM2.5 ALT = 3.4(0.00030418*N1 + 0.0018512*N2 + 0.02069706*N3) .

 
To do this, you’ll need to calculate the N1, N2, and N3 values, which are:

  • N1 = 0.3um - 0.5um
  • N2 = 0.5um - 1.0um
  • N3 = 1.0um - 2.5um
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You can compute Raw PM2.5 ALT (cf=3.4) from the SD card by using the particle counts, not the cf_1/atm mass fields.

  1. First get the size-bin counts from the CSV:
    p_0_3_um, p_0_5_um, p_1_0_um, p_2_5_um (they’re cumulative counts ≥ that size, per dL).

  2. Turn those into counts within the three PM2.5 sub-ranges:
    N1 = p_0_3_um − p_0_5_um
    N2 = p_0_5_um − p_1_0_um
    N3 = p_1_0_um − p_2_5_um

  3. Plug into the ALT formula (units come out in µg/m³):
    PM2.5_ALT = 3.4 × (0.00030418·N1 + 0.0018512·N2 + 0.02069706·N3)

That’s it, no humidity or other corrections; cf=3.4 is the updated “ALT” calibration. PurpleAir documented the exact coefficients and the N1/N2/N3 definition here, and the SD headers here for the field meanings.

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I just checked and it works! Thanks to both of you.