What boundary conditions should be specified for simulating plume dispersion of a large cooling tower under influence of side wind. Is it appropriate to specify symmetry boundary at the top?

Maybe an environmental pressure boundary is more appropriate. The plume would reflect from a symmetry boundary.

sym. boundary is used when you are considering symmetry across a plane. BTW interesting topic.

Mr. Richard, I presume environmental pressure boundary is same as the pressure boundary with environment "ON". If this boundary is specified then the viscous drag that should be acting on the boundary due to sideways motion of air above it, will be absent. In literature, I have come across papers where symmetry boundary has been specified, though I agree with you that specifying symmetry boundary will necessarily cause the plume to bend in downwind direction.