Probabilistic Nonroad Mobile Source Emission Factors

Quantitative methods for characterizing both variability and uncertainty are applied to case studies of nitrogen oxides (NOx) and total hydrocarbon air pollutant emission factors for construction, farm and industrial engines. A database was developed and statistical methods were used to identify statistically significant categories, including gasoline engines, two-stroke diesel engines, and four-stroke diesel engines. Emissions were not found to be statistically significantly different for older versus newer nonroad diesel engines nor when comparing naturally aspirated and turbocharged engines. Interengine variability in emissions was quantified using Weibull, gamma, and lognormal distributions. Bootstrap simulation was used to characterize confidence intervals for the mean. The 95% confidence intervals for the mean emission factors were as small as −10 to +11% and as large as −48 to +49%, with an average range of −26 to +27%. Asymmetry in the confidence intervals is attributable to small sample sizes and skewness in the data. Estimates of uncertainty convey information regarding the quality of the emission factors and serve as a basis for calculation of uncertainty in emission inventories.