We study existence and uniqueness of weak solutions to (F) $\partial_t u+ (-\Delta)^\alpha u+h(t, u)=0 $ in $(0,\infty)\times\R^N$, with initial condition $u(0,\cdot)=\nu$ in $\R^N$, where $N\ge2$, the operator $(-\Delta)^\alpha$ is the fractional Laplacian with $\alpha\in(0,1)$, $\nu$ is a bounded Radon measure and $h:(0,\infty)\times\R\to\R$ is a continuous function satisfying a subcritical integrability condition. In particular, if $h(t,u)=t^\beta u^p$ with $\beta>-1$ and $0 < p < p^*_\beta:=1+\frac{2\alpha(1+\beta)}{N}$, we prove that there exists a unique weak solution $u_k$ to (F) with $\nu=k\delta_0$, where $\delta_0$ is the Dirac mass at the origin. We obtain that $u_k\to\infty$ in $(0,\infty)\times\R^N$ as $k\to\infty$ for $p\in(0,1]$ and the limit of $u_k$ exists as $k\to\infty$ when $1 < p < p^*_\beta$, we denote it by $u_\infty$. When $1+\frac{2\alpha(1+\beta)}{N+2\alpha}:=p^{**}_\beta< p < p^*_\beta$, $u_\infty$ is the minimal self-similar solution of $(F)_\infty$ $\partial_t u+ (-\Delta)^\alpha u+t^\beta u^p=0 $ in $(0,\infty)\times\R^N$ with the initial condition $u(0,\cdot)=0$ in $\R^N\setminus\{0\}$ and it satisfies $u_\infty(0,x)=0$ for $x\neq 0$. While if $1< p < p^{**}_\beta$, then $u_\infty\equiv U_p$, where $U_p$ is the maximal solution of the differential equation $y'+t^\beta y^p=0$ on $\R_+$.