The downward flux by the atmosphere is the same as the real greenhouse effect. This flux is excatly the same as the sum of four energy fluxes: LW radiation absorption 155.6 + SW radiation absotpion 75.0 + Latent heating 90.8 + 24.2 = 345.6 W/m2

ClimatExam   Scientific analyses of the climate change and global warming by Dr. Antero Ollila