We investigate the ground-state properties and the effects of temperature and magnetic field on elementary magnetic excitations in the s = 1/2 rouaite Cu₂(OH)₃NO₃ using bulk magnetization and high-field Raman spectroscopic experiments. The unique exchange topology of rouaite, consisting of weakly coupled alternating ferromagnetic and antiferromagnetic chains, gives rise to a rich array of field-induced phases, including a 2/3-plateau phase at μ₀H_2/3 ≈ 15 T, corresponding to a fully polarized ferromagnetic chain. Across μ₀H_2/3, the field dependence of low-energy phonons shows marked anomalies, indicating the presence of strong magnetoelastic coupling. In addition, as an applied field approaches μ₀H_2/3 ≈ 15 T, one-magnon excitations exhibit an order-parameter-like softening, while depleted continuum excitations become gapless. In the 2/3-plateau phase, magnetic excitations are dominated by a fractionalized spinon continuum pertaining to the decoupled antiferromagnetic chains and a spin-flip excitation arising from the polarized ferromagnetic chains. Our results demonstrate that a dimensional reduction occurs at μ₀H_2/3 ≈ 15 T, transiting from a three-dimensional magnet to a one-dimensional antiferromagnet through the magnetic quenching of the ferromagnetic chain subsystem.