Evaluation of the Dekati mass monitor for the measurement of exhaust particle mass emissions

The Dekati mass monitor (DMM) is an instrument which measures the mass concentration of airborne particles in real time by combining aerodynamic and mobility size particle classification. In this study, we evaluate the performance of the DMM by sampling exhaust from five engines and vehicles of different technologies in both steady-state and transient tests. DMM results are found higher than the filter-based particulate matter (PM) by 39 +/- 24% (range stands for +/- one standard deviation) for 62 diesel tests conducted in total and 3% and 14% higher, respectively, in two gasoline tests. To explore whether the difference occurs because of the different measurement principles of DMM and filter-based PM, the DMM operation is replicated over steady-state tests by combining an electrical low-pressure impactor (ELPI) and a scanning mobility particle sizer (SMPS). The correlation of ELPI and SMPS derived mass and filter-based PM is satisfactory (R-2 = 0.95) with a mean deviation of 5 +/- 15%. For the same tests, the correlation of DMM with PM was also high (R-2 = 0.95), but DMM exceeded PM by 44 +/- 23% on average. The comparison of ELPI and SMPS and DMM results reveals that the latter overestimates both the geometric mean diameter and especially the width of the particle mass-weighted size distribution. These findings demonstrate that the statistically significant difference between the DMM and the filter-based PM cannot just originate from the different measurement principles but also from the actual implementation of the combined aerodynamic-mobility measurement in the DMM. Optimizing the DMM will require changes in its design and/or the calculation algorithm to improve the resolution and width of the aerodynamic size distribution recorded.