A large contribution of methylsiloxanes to particulate matter from ship emissions

The chemical and stable carbon isotopic composition of the organic aerosol particles (OA) emitted by a shuttle passenger ship between mainland Naples and island Capri in Italy were investigated. Various methylsiloxanes and derivatives were found in particulate ship emissions for the first time, as identified in the mass spectra of a thermal desorption - proton transfer reaction - mass spectrometer (TD-PTR-MS) based on the natural abundance of silicon isotopes. Large contributions of methylsiloxanes to OA (up to 59.3%) were found under inefficient combustion conditions, and considerably lower methylsiloxane emissions were observed under cruise conditions (1.2% of OA). Furthermore, the stable carbon isotopic composition can provide a fingerprint for methylsiloxanes, as they have low delta C-13 values in the range of -44.91 parts per thousand +/- 4.29 parts per thousand. The occurrence of methylsiloxanes was therefore further supported by low delta C-13 values of particulate organic carbon (OC), ranging from -34.7 parts per thousand. to -39.4 parts per thousand, when carbon fractions of methylsiloxanes in OC were high. The delta C-13 values of OC increased up to around -26.7 parts per thousand under cruise conditions, when carbon fractions of methylsiloxanes in OC were low. Overall, the delta C-13 value of OC decreased linearly with increasing carbon fraction of methylsiloxanes in OC, and the slope is consistent with a mixture of methylsiloxanes and fuel combustion products. The methylsiloxanes in ship emissions may come from engine lubricants.