Optimal environmental regulation based on more than just emissions

I develop a principal-agent model of environmental regulation in which the regulator can acquire two costly signals of the firm's abatement effort. Acquisition of the second signal is conditioned on the observed value of the first, emissions signal. The optimal contract takes the form of an emissions standard when only the emissions signal is acquired, and a set of contingent emissions standards when both signals are acquired; the standards are coupled with uniform, maximal penalties for noncompliance. Acquisition of the second signal may be optimal when intermediate values of the first signal are observed but not when extreme values are observed.