AbstractThe molecular compositions of polar organic compounds (POCs) in particles emitted from various vessels and excavators were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and possible molecular structures of POCs were proposed. POCs were extracted with purified water and sorted by elemental composition into three groups: CHO, CHON, and S-containing compounds (CHONS and CHOS). The results show the following. (i) CHO (accounting for 49% of total POCs in terms of peak response) was the most abundant group for all tested off-road engines, followed by CHON (33 %) and CHOS (35 %) for diesel and HFO (heavy-fuel-oil)-fueled off-road engines. (ii) The abundance and structure of the CHON group in water extracts were different in terms of engine type and load. The relative peak response of CHON was the highest for excavator emissions in working mode, compared to the idling and moving modes. Furthermore, dinitrophenol and methyl dinitrophenol were potentially the most abundant emission species for high-rated speed excavators, while nitronaphthol and methyl nitronaphthol were more important for low-rated speed vessels. (iii) The composition and structure of the S-containing compounds were directly influenced by fuel oil characteristics (sulfur content and aromatic ring), with more condensed aromatic rings in the S-containing compounds proposed in HFO-fueled vessel emissions. More abundant aliphatic chains were inferred in diesel equipment emissions. Overall, higher fractions of condensed hydrocarbons and aromatic rings in POCs emitted from vessels using HFO cause strong optical absorption capacity. Different structures in POCs could provide a direction for qualitative and quantitative analysis of organic compounds as tracers to distinguish these emissions from diesel or HFO-fueled off-road engines.