The uncertainty of emission estimation was calculated using error propagation method. According to Eq. (3), as the uncertainty was mainly associated with EF and VMT, the uncertainty can be propagated from the combination of variables in Eq. (4): σi, j =√σ2EFi, j× VMT j 2+ σ2VMTj× EF2i, j × VPj × αj (4)
where, σi, j is the uncertainty of the estimated emission of I carbonyl and j type of vehicle, σEF, σVMT are the standard deviation of emission factor and annual mileage