On extension of Green's operator on bounded smooth domains

We prove a regularity result for Green's functions that are associated to elliptic second order divergence-type linear PDO's with coefficients in C^{1,\alpha}(\bar{\Omega}). Here \alpha\in (0,1) and \Omega\subset \R^n is a bounded C^{2,\alpha} domain in dimension n\ge 3. The regularity result gives boundary estimates for derivatives up to order (2+\alpha) and, by using these estimates, we extend the associated Green's operator to a globally defined singular integral which of Calder\'on--Zygmund type.