Climatologies of aerosol backscatter profiles, derived from airborne and ground-based CO2 lidar returns, indicate a high frequency of occurrence for a narrow range of low backscatter values within the troposphere—a background mode.
Jeffry Rothermel, David A. Bowdle, J. Michael Vaughan, and Madison J. Post, "Evidence of a tropospheric aerosol backscatter background mode," Appl. Opt. 28, 1040-1042 (1989)