Black Carbon Radiative Effects on the Cryosphere: Snow-Albedo Reduction and Transport through a Melting Snowpack

Black carbon (BC) affects global and regional climate by heating the atmosphere, altering clouds, and darkening snow and ice surfaces. Modeling studies attribute increased air temperature and decline in snow and ice extent in the Northern Hemisphere partly to BC deposition to the surface and indicate that this may be the second leading cause of observed ice retreat. Observational validation of the modeled BC impact on snow has yet to be published. Field studies dating back to the early 1980s measured atmospheric BC concentrations in snow and ice in the arctic, but the BC effect on snow albedo and melting has been difficult to observe directly because the albedo reduction is small and often masked by other natural variables.

This study evaluates both the initial impact of BC on snow albedo, as well as associated feedbacks due to snow age and BC scavenging during snow melting. As snow ages, snow grains increase in size. Radiation transfer theory predicts that larger grains enhance the efficacy of BC in reducing snow albedo, thereby increasing the “aging” rate of the snow. The second potential feedback, associated with BC transport through a melting snowpack, occurs if BC is scavenged from the melt water by the ice grains thus increasing the BC concentration in the remaining snow. A model of the evolution of the vertical distribution of BC in melting snow was developed and agrees with measurements of BC transport in both laboratory-made and natural snow. These measurements provide the first observations of a BC concentration enhancement in melting snow and can be used to improve the snowpack scavenging parameter of BC used in models.