Brown and Black Carbon Emitted by a Marine Engine Operated on Heavy Fuel Oil and Distillate Fuels: Optical Properties, Size Distributions, and Emission Factors

We characterized the chemical composition and optical properties of particulate matter (PM) emitted by a marine diesel engine operated on heavy fuel oil (HFO), marine gas oil (MGO), and diesel fuel (DF). For all three fuels, similar to 80% of submicron PM was organic (and sulfate, for HFO at higher engine loads). Emission factors varied only slightly with engine load. Refractory black carbon (rBC) particles were not thickly coated for any fuel; rBC was therefore externally mixed from organic and sulfate PM. For MGO and DF PM, rBC particles were lognormally distributed in size (mode at d(rBC)similar to 120nm). For HFO, much larger rBC particles were present. Combining the rBC mass concentrations with in situ absorption measurements yielded an rBC mass absorption coefficient MAC(BC,780nm) of 7.8 +/- 1.8m(2)/g at 780nm for all three fuels. Using positive deviations of the absorption angstrom ngstrom exponent (AAE) from unity to define brown carbon (brC), we found that brC absorption was negligible for MGO or DF PM (AAE(370,880nm)similar to 1.0 +/- 0.1) but typically 50% of total 370-nm absorption for HFO PM. Even at 590nm, similar to 20 of the total absorption was due to brC. Using absorption at 880nm as a reference for BC absorption and normalizing to organic PM mass, we obtained a MAC(OM,370nm) of 0.4m(2)/g at typical operating conditions. Furthermore, we calculated an imaginary refractive index of (0.045 +/- 0.025)(lambda/370nm)(-3) for HFO PM at 370nm> lambda > 660nm, more than twofold greater than previous recommendations. Climate models should account for this substantial brC absorption in HFO PM.