Lung-Wen Antony Chen (Environmental and Occupational Health) and colleagues have published an article, "Apportionment of Vehicle Fleet Emissions by Linear Regression, Positive Matrix Factorization, and Emission Modeling," on the journal Atmosphere.

Real-world emission factors for different vehicle types and their contributions to roadside air pollution are needed for air quality management. This study applies four methods to derive emission factors for different vehicle types from tunnel measurements in Hong Kong: (1) simple linear regressions (SLR); (2) multiple linear regressions (MLR); (3) positive matrix factorization (PMF); and (4) EMission FACtors for Hong Kong (EMFAC-HK). Separable vehicle types include those fueled by liquefied petroleum gas (LPG), gasoline, and diesel. It concludes that diesel vehicles make the largest contribution to CO₂, NO_x, and PM_2.5, while gasoline vehicles make the largest contribution to CO. The article also discusses the accuracy and uncertainty of these different methods and identifies several knowledge gaps in the assessment of traffic-related impact on urban air quality.