Variation of the excitation temperature and opacity for the l_0,1 − 0_0,0 line at 72 GHz (left), and the 2_0,2 − 1_0,1 line at 145 GHz (right) with the electron fraction, kinetic temperature, and H₂ ortho-to-para ratio. The difference between the excitation temperature and the CMB is plotted. Different symbols show the values of the H₂ ortho-to-para ratio between 3 × 10^-5 and 3, their color and size indicate the kinetic temperature between 10 K and 60 K. The column density of p-H₂CO is 7.3 × 10¹¹ cm^-2, the H₂ density is set to 10³ cm^-3, and the line FWHM is 0.5 km s^-1. The 1_0,1 − 0_0,0 line at 72 GHz has not been observed in this work.