Characterization of particulate matter emitted from transit buses fueled with B20 in idle modes

• Heavy metals were emitted in much higher concentration in hot idle conditions.
• Calcium, iron, and sulfur were the most-repeatable elements in all cases.
• Biodiesel blends decrease the amount of PM as well as elemental and organic carbon.
• Four main sources of PM are fuel, oil and lubricants, engine parts, and ambient air.

Characterization of particulate matter (PM) of public transit buses in idle modes has been conducted to identify PM specification and to determine the effect of biodiesel properties on the emissions. Two different sets of transit buses that were running on B20 (20% biodiesel and 80% ultra-low sulfur diesel or ULSD in volume percent) were selected. For both hot and cold idle tests, a substantial reduction in total particulate matter (TPM) was observed compared to the TPM from ULSD. The maximum PM concentrations under hot and cold idling conditions were 2.77 and 5.59 μg/m3, respectively, while ULSD showed 247.49 and 218.27 μg/m3 under the same conditions. In elemental analyses, fifteen elements were observed in the PM emissions obtained from the buses (77–85 wt% of total emissions). The major elements were calcium (Ca), sodium (Na), and iron (Fe) in PM samples. Positive matrix factorization (PMF) method was used for source apportionment analysis, and as a result, four factors were identified as the possible sources. Organic carbon to elemental carbon ratio (OC/EC) analyses showed that more OC was emitted in cold idling (>80%) than hot idling (>65%). Furthermore, the OC/EC ratio was found to be greater for the new buses with post treatment facilities (9.57–13.37) than for the old buses without them (1.85–4.55), which indicates the shift of the ratio between organic carbon and elemental carbon in the emissions as an engine ages.