I have a time series of the concentration of a gas (in parts per million, or ppm) in the atmosphere with time over several years. The concentration with time approximates a sinusoid with frequency 2*pi/365 days, but it also has a 6-monthly term (first harmonic) of frequency 4*pi/365 days. I've run an fft of this time series, which comes out showing peaks at 1-year and 6-month frequencies. My only issue is that I'm not sure what the units on the y axis ("power") are. Can anyone help with this?