  By tropical Abel-Jacobi theorem, the Jacobian of a tropical curve is
isomorphic to the Picard group. A tropical curve in $\mathbb{R}^2$ corresponds
to an immersion from a tropical curve to $\mathbb{R}^2$. In this paper, we show
that any principal divisor on a tropical curve is the restriction of a
principal divisor on the ambient plane $\mathbb{R}^2$.
