Multiple Types of Light Absorbing Carbon Aerosol in East Asian Outflow: Variatons in Morphology and Internal Structure as Characterized by Transmission Electron Microscopy

The importance of light absorbing carbon (LAC) aerosols to climate forcing is well established, but such aerosols are typically treated in climate models as uniform in optical properties. When examined by electron microscopy, however, LAC aerosols from regions with significant anthropogenic pollution show a wide variety of morphologies and internal structures. Electron energy loss spectral analysis to date on brown carbon and black carbon, albeit limited, suggests a linkage between internal structure and fundamental optical properties. Some of these LAC varieties can be easily defined as distinct “types” and other varieties show a continuum of variation within which general “types” can be defined. The data discussed here are from a research flight of the NCAR C-130 aircraft flown in April 2001 above the Yellow Sea during the ACE-Asia project. Perhaps the most common LAC type is “soot”, branched and chainlike aggregates of carbonaceous spherules. The spherule size in East Asian soot particles is 20-60 nm in many cases, but soot with large spherules (100 nm or larger) are also present. Spherule size is a “source effect” and not something altered during transport and aging. Some laboratory studies have suggested that as soot ages, the aggregates become more compact, but in these aerosols both compact and open soot particles coexist and compact soot is known to be the initial LAC product under some combustion conditions. In cases where the spherule size of the compact soot is different from that of open-structured soot, clearly the compact soot is not an aged form of the latter. Variability of ordering of the graphene sheets that make up the spherules is also a source effect. The more ordered soot particles consist of graphene sheets that curve concentrically, onion-like, around the spherule center, probably indicative of a high degree of carbonization that accompanies high temperature combustion. There is a range of ordering from highly ordered down to completely disordered where individual graphene sheets are indistinguishable at high resolution. The majority of soot from these samples is highly ordered. Another LAC type present in the sampled aerosols is individual disordered spheres, most or all probably being brown carbon. Blocky char particles are also present as are hollow carbon cenospheres, the latter a common product of inefficient combustion of heavy fuel oil. As the relative concentrations of these different types vary with altitude, the optical properties of the ensemble of LAC particles may well also vary with altitude. Clearly the assumption of a uniform LAC aerosol is incorrect.