Emissions are given by the manufacturer for NOx, CO, and HC including formaldehyde in gm/HP-Hr . Typical is 2 gms NOx/HP-hr, so a 1000 HP unit is 2 * 1000 * 24 *365//454 2000 = 19.3 ton/year.
EPA also publishes standard factors for older units separated by engine designs, 2 cycle or 4 cycle, ect...