Given the computational burden of running full chemical transport models, it is highly desirable to have alternative ways to obtain fast and accurate approximations to at least some of these model outputs. We propose two methods that closely approximate the ammonia wet deposition of the Community Multiscale Air Quality (CMAQ) model, a regional-scale three-dimensional chemical transport model. The first method uses a greatly simplified version of CMAQ, called here “Tracer,” which requires one fortieth the processing time of CMAQ. The second method uses an extension of the Tracer model called “Multitracer.” Both methods make use of a CMAQ run under a reference emission scenario and are shown to provide good approximations to CMAQ outputs under different emission scenarios. The first fast approximation method proposed here requires a Tracer run under the new emission scenario while the second proposed approximation only requires a matrix multiplication between a precomputed matrix that approximates the transport in the model, obtained from a single Multitracer run, and the new emission. It will be shown that this method is not a simple source-receptor approximation of the model. As an important application of the second predictor, we propose an inverse modeling method for ammonia that makes it possible to adjust emissions by a different factor for each of the 100 subregions of the spatial domain. Testing with pseudodata yields a good match between the inverse modeled emissions and the actual emissions. Estimates of emissions using actual observations in the eastern United States show a reasonable adjustment field.