Diesel engines have been identified as contributing to more than half of the transport sectors black carbon (BC) emissions in the US. This large contribution to atmospheric BC concentrations has raised concern about source specific emission rates, including off-highway engines. The European Union has recently implemented more stringent particulate regulations in the form of particle number via the Particle Measurement Programme (PMP) methodology. The PMP method counts the non-volatile fraction of particulate matter (PM) above 23 nm and below 2.5 μm via a condensation particle counter. This study evaluates a surrogate black carbon method which uses the PMP particle count method with a correlation factor to the BC fraction. The transient capable Magee Scientific Aethalometer (AE-33) 880 nm wavelength channel was used to determine the BC fraction. Total cycle PMP particle count displayed linear correlation with the cycle averaged AE-33 BC measurement (R2 = 0.96), yielding a correlation factor of 2.2×1012 ± 1.8×1011 #/mg, falling within the range of previously published BC-PN correlation factors. However under closer investigation, certain engine operating modes display poor correlation between the two methods, indicating possible changes in particle optical properties including particle size and chemical composition. As these changes are a function of engine operation, a single correlation factor is not appropriate for measuring transient BC events. The absorption angstrom exponent (AAE) method was used to determine the degree of change in the aerosols optical response. Overall the AAE of the tested off-highway engine was found to be significantly lower than other studies, the stationary cycle displaying an AAE of 1.14 ± 0.01 and the transient cycle displaying an AAE of 1.15 ± 0.03.