  Solvability of Cauchy's problem in $\mathbb{R}^2$ for subcritical
quasi-geostrophic equation is discussed here in two phase spaces;
$L^p(\mathbb{R}^2)$ with $p> \frac{2}{2\alpha-1}$ and $H^s(\mathbb{R}^2)$ with
$s>1$. A solution to that equation in critical case is obtained next as a limit
of the $H^s$-solutions to subcritical equations when the exponent $\alpha$ of
$(-\Delta)^\alpha$ tends to $\frac{1}{2}^+$. Such idea seems to be new in the
literature. Existence of the global attractor in subcritical case is discussed
in the paper. In section 7 we also discuss solvability of the critical problem
with Dirichlet boundary condition in bounded domain $\Omega \subset
\mathbb{R}^2$, when $\| \theta_0 \|_{L^\infty(\Omega)}$ is small.
