Temperature . The temperature will affect the evaporation rates 

 because it affects the vapor pressures of the components. The temperature 

 also affects the viscosity of the oil and the rate of migration of 

 the components. However, the range of ocean temperatures is limited, 

 and the effect of temperature is expected to be small compared to 

 the effect of differences in composition. 



Wind. Wind will affect the evaporation rate because it provides 

 fresh air that is not already partly saturated by the components 

 of the oil. In a completely stagnant situation an equilibrium 

 would be created in which all components would be present in 

 the air in direct proportion to their vapor pressures, and any 

 net additional evaporation would cease. Rapidly moving fresh 

 air will prevent any approach to saturation and will have the 

 same net effect as an increase in vapor pressure. 



Evaporative Losses 



The experimental results were in agreement with the above 

 considerations. The evaporation experiments with thin films of 

 fuel oils show that under rapidly moving fresh air the major 

 portions of the distillate fuels will evaporate. About 95% or 

 more of the distillate fuels were lost in a week under these 

 conditions, whereas only 33% of the NSFO was lost, as shown in 

 Figure 5. However, the thickness of these fuel oil films was only 0.1 mm, 

 and much lower evaporation rates would be expected for thicker films. 



In the simulated weathering of Navy distillate fuels spilled on 

 seawater, the fuel losses in one week were from about 10% to about 25%. 

 The oil thickness in these experiments was 5 mm. The thickness of a 

 typical oil spill might be about half this thickness. 



The thickness of an oil spill will vary, of course. In a 

 controlled spill of No. 2 fuel oil there was reported to be a 

 compact region of oil 2.4 ± 0.3 mm thick, surrounded by a thin region 

 only 0.002 to 0.004 mm thick [5]. The thick region contained more than 

 90% of the oil in less than 10% of the area of the visible slick. 

 According to the specifications. No. 2 fuel oil [6] is a slightly 

 lighter oil than Navy distillate fuel [3], but fuel oils A to D 

 exhibited properties very close to, or within the range, specified for 

 No. 2 fuel oil. Thus, the evaporation characteristics of a typical spill 

 of Navy distillate fuel are likely to be closer to those of a 5 -mm 

 layer, which is about twice as thick, than to a 0.1 -mm layer, which 

 is about one twenty fifth the thickness of the bulk of the reported spill. 



One reason for using an oil thickness of 5 mm in the simulated 

 weathering was to have sufficient oil residues to study their 

 properties. A second reason for using a relatively thick layer 

 of oil was to have enough material to collect the bulk of the 

 weathered oil directly without a complicated recovery system. 



