The Roles of Photooxidation and Biodegradation in Long-term Weathering of Crude and Heavy Fuel Oils ROGER C. PRINCE *, ROBERT M. GARRETT , RICHARD E. BARE , MATTHEW J. GROSSMAN , TODD TOWNSENDà, JOSEPH M. SUFLITAà, KENNETH LEE§, EDWARD H. OWENS  , GARY A. SERGYàà, JOAN F. BRADDOCK§§, JON E. LINDSTROM   & RICHARD R. LESSARDààà  ExxonMobil Research and Engineering Co., Annandale, NJ 08801, USA àInstitute for Energy and the Environment and the Department of Botany and Microbiology, University of Oklahoma, Norman, OK 73019, USA §Department of Fisheries and Oceans, Dartmouth, Nova Scotia, Canada B2Y 4T3   Polaris Applied Sciences, Inc., Bainbridge Island, WA 98110, USA ààEnvironment Canada, #200, 4999––98th Ave. Edmonton, AB, Canada T6B 2X3 §§Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775, USA    Shannon & Wilson, Inc., Fairbanks, AK 99709, USA àààExxonMobil Research and Engineering Co., Fairfax, VA 22037, USA Although spilled oil is subject to a range of natural processes, only combustion, photooxidation and bio- degradation destroy hydrocarbons and remove them from the biosphere. We present laboratory data that demonstrate the molecular preferences of these processes, and then examine some oil residues collected from previously documented releases to confirm the important roles that these processes play in removing spilled oil from both marine and terrestrial environments. Ó 2003 Elsevier Science Ltd. All rights reserved. Introduction Crude and heavy fuel oils that escape into the en- vironment, either from natural seeps or from acci- dental spills, become subject to a variety of physical, chemical and biological phenomena. Small molecules evaporate (Fingas, 1999; Sharma et al., 2002), and are either degraded photochemically (Poisson et al., 2000; Hurley et al., 2001), or are washed from the atmo- sphere in rain and then biodegraded (Arzayus et al., 2001). Under particularly aggressive aeration in water, as happened in the spill from the OSSA II pipeline into the flood-stage R  ıo Desaguadero on the Bolivian Al- tiplano in January 2000, this evaporation can extend into molecules with >30 carbon atoms (Douglas et al., 2002; Prince et al., 2002), but evaporation is more usually limited to molecules with less than about 15 carbon atoms (Payne et al., 1991; Fingas, 1999). Ter- restrial spills may soak into the ground, as happened in the Nipisi, Rainbow and Old Peace River pipeline spills in the Lesser Slave Lake area of Northern Al- berta spill (Blenkinsopp et al., 1996). Some molecules, particularly aromatic hydrocarbons and small polar molecules such as naphthenic acids, dissolve if suffi- cient water is present (Lafargue & Le Thiez, 1996; Burns et al., 2000), and again these are eventually biodegraded (Herman et al., 1994). Spills at sea or on lakes and rivers often disperse into the water column, Spill Science & Technology Bulletin, Vol. 8, No. 2, pp. 145–156, 2003 Ó 2003 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1353-2561/03 $ - see front matter doi:10.1016/S1353-2561(03)00017-3 145 *Corresponding author. E-mail address: roger.c.prince@exxonmobil.com (R.C. Prince).