The partial pressure of CO2 is equal to the total pressure molar fraction of CO2.
To give 4.3 10^-3, they are using molar fraction = 6.2 10^ - 6
Because, 695 6.2-10^-6 = 0.004309 = 4.309 10^-3 = 4.3 10^-3.
Then the new question is why the molar fraction is consideres to be the same ppm concentration. This is because ppm means parts per million, and in this case, they are considering that the parts are moles (usually it is grams, but it is not the case, given the result).
6.2 ppm = 6.2 / 1,000,000 = 6.210^-6, which is the number used above to multiply the total pressure and get the partial pressure of CO2.
If you had not told me that the result was 4.3 10^-3, I had assumed
that 6.2 was referred to grams and not moles and I had made a conversion
using the molar masses of the air and the CO2 but, again, this is not the case.
