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Radiative transfer in hydrogen lines in supernova remnant (SNR) shock wavesis studied taking into account the population of the hydrogen atom 2s-state.Measurements of Balmer line emission, especially of H~$\alpha$, are oftenrelied upon to derive physical conditions in the SNR shock. On the other hand,Lyman series photons, especially Ly~$\beta$, are mostly absorbed by upstreamhydrogen atoms. As a result, atoms are excited to the 3p state, and then emitH~$\alpha$ by the spontaneous transition from 3p to 2s. Thus, the nature ofH~$\alpha$ depends on how many Ly~$\beta$ photons are converted to H~$\alpha$photons. Moreover, the Balmer lines can be scattered by the 2s-state hydrogenatoms, which are excited not only by collisional excitation but also by theLyman-Balmer conversion. It is shown for example that the H~$\alpha$ photonsare scattered if the shock propagates into an H~$_{\rm I}$ cloud with a densityof $\sim30~{\rm cm^{-3}}$ and a size of $\sim 1$~pc. We find that the lineprofile of H~$\alpha$ becomes asymmetric resulting from the difference betweenline centre frequencies among the transitions from 3s to 2p, from 3p to 2s andfrom 3d to 2p. We also find that the broad-to-narrow ratio of H~$\alpha$, whichis often used to estimate the ion-electron temperature equilibrium, varies atmost $\simeq 10$ per cent depending on the ionization degree of the upstreammedium because of incomplete conversion of Lyman lines to Balmer lines.

Radiative transfer of hydrogen lines from supernova remnant shock waves: contributions of 2s-state hydrogen atoms