Figure 1: Overestimation is largely due to the differences in default emission factors from USEPA AP-42 Standards and the CARB Certificates. Larger horsepower diesel engines such as fracture fluid pumps are typically Tier 2 or higher. As of 2008, Tier 4 engines have been phased in which require PM and NOx to be reduced by 90%. AP-42 assumes Tier zero while the CARB certificates take into account emission reductions imposed by the USEPA and categorized by Tier rating.