A virtual analyzer is provided to estimate either an attribute of a reactant applied during performance of, or an amount of a reactant exhausted by, a process having multiple process parameters (MPPs) that is performed to control an amount of a pollutant emitted into the air. The virtual analyzer includes an interface which receives signals corresponding to attributes of the MPPs. If the process is a wet flue gas desulfurization (WFGD) process, the signals include a signal corresponding to a measured pH level of the applied reactant. If the process is a selective catalytic reduction (SCR) process, the signals include a signal corresponding to a measured amount of the reactant exhausted by the process. The analyzer also includes either a neural network process model or a non-neural network process model. Whichever type of model is utilize, if the process is a WFGD process, the model represents a relationship between the pH level of the applied reactant and the attributes of the MPPs other than the measure pH level of the applied reactant. On the other hand, if the process is the SCR process, the model represents a relationship between the amount of the reactant exhausted by the process and the attributes of the MPPs other than the measured amount of the reactant exhausted by the process. The analyzer also includes a processor. If the process is the WFGD process, the processor has the logic to estimate a pH level of the applied reactant based on the attributes of the MPPs, other than the measured pH level of the applied reactant, that correspond to the received signals and on the one model. On the other hand, if the process is the SCR process, the processor has the logic to estimate an amount of the reactant exhausted by the process based on the attributes of the MPPs, other than the measured amount of the reactant exhausted by the process, that correspond to the received signals and on the one model.