The objective of this paper is to quantify the variability in emissions of off-road equipment using a portable emission measurement system.A total of 53 commonly used equipment for agriculture,base construction,paving construction,and material handling were selected.Time-based and fuel-based emissions were quantified by different duty and engine modes.Three duty modes (idling,moving,and working) were used.Ten engine modes were defined based on normalized engine revolutions-per-minute and manifold absolute pressure,respectively.Composite emission factors taking into account both duty modes and its corresponding time percentage during a typical duty cycle were estimated.Results showed that there existed a large off-road equipment variability in emissions.Depending on duty and engine modes,time-based NO emissions ranged from 3.1 to 237.9,29.1-1475.6,83.2-681.6,and 3.2-385.2 g/h for agriculture,base construction,paving construction and material handling equipment,respectively while for fuel-based NO emissions these ranges were 5.3-52.0,11.7-69.0,4.8-30.8,and 11.0-54.6 g/kg,respectively.Furthermore,emission factors derived from this study exhibited a much larger variability compared to those used in NONROAD by US EPA and National Guideline for Off-road Equipment of China.This implied that localized measurements of emissions are needed for improvement of accuracy of emission inventory.Furthermore,both equipment types and operations should be considered for development of emission inventory and control strategy.