On the cohomology of the classifying spaces of $SO(n)$-gauge groups over $S^2$

Let $\mathcal{G}_\alpha (X,G)$ be the $G$-gauge group over a space $X$ corresponding to a map $\alpha : X \to B\mathcal{G}_1$. We compute the integral cohomology of $B\mathcal{G}_1 (S^2, SO(n))$ for $n = 3, 4$. We also show that the homology of $B\mathcal{G}_1 (S^2, SO(n))$ is torsion free if and only if $n \leqslant 4$. As an application, we classify the homotopy types of $SO(n)$-gauge groups over a Riemann surface for $n \leqslant 4$.