Here is an easy way to calculate the ALT cf=3 PM2.5 value from the particle numbers given in the first four size categories in the “Secondary” part of the download using the widget in the Map page and selecting the “No” conversion. Recall that these categories are the numbers (per deciliter but that won’t matter) of all the particles greater than the given category. Therefore the first category 0.3-0.5 um is all particles greater than 0.3 um To get the number of particles in that category alone, you must subtract the number of particles >0.5 um (given in the next larger category of 0.5-1 um) from the number greater than 0.3 um. This is the number of particles in the smallest size category 0.3-0.5 um. By repeating this for the next two size categories you get the number of particles in each of the first three categories. There are also some steps putting in the density and calculating the masses of the particles in each size category, but all that is contained in the three coefficients shown below. Suppose N1 is the number of particles/dl in the smallest size category (0.3-0.5 um), and N2 and N3 are the number in the next two categories up to 1-2.5 um. Now the total mass will be given by the following equation:
PM2.5 = 3 (0.00030418*N1 + 0.0018512 *N2 + 0.02069706 * N3)
The factor of 3 is the calibration factor.
For those persons using the API site, you should get exactly the same values using the PM2.5 alt variable.