Aggwndw aggregates and/or windows emissions from an I/O API file to a new grid. The program does not change grid projections or interpolate data; the grid boundaries of the input and output grids must be aligned and the output grid must be entirely contained within the input grid. Aggwndw automatically checks that the input and output emissions do not differ by more than 0.01% and also produces a report file containing summed input and output data.