Surface shortwave aerosol radiative forcing during the Atmospheric Radiation Measurement Mobile Facility deployment in Niamey, Niger

The Atmospheric Radiation Measurement (ARM) Program's Mobile Facility (AMF) was deployed to Niamey, Niger, during 2006. Niamey, which is located in sub-Saharan Africa, is affected by both dust and biomass burning emissions. Column aerosol optical properties were derived from multifilter rotating shadowband radiometer, measurements and the vertical distribution of aerosol extinction was derived from a micropulse lidar during the two observed dry seasons (January-April and October December). Mean aerosol optical depth (AOD) and single scattering albedo (SSA) at 500 nm during January-April were 0.53 +/- 0.4 and 0.94 +/- 0.05, while during October December mean AOD and SSA were 0.33 +/- 0.25 and 0.99 +/- 0.01. Aerosol extinction profiles peaked near 500 m during the January-April period and near 100 m during the October-December period. Broadband shortwave surface fluxes and heating rate profiles were calculated using retrieved aerosol properties. Comparisons for noncloudy periods indicated that the remote sensing retrievals provided a reasonable estimation of the aerosol optical properties, with mean differences between calculated and observed fluxes of less than 5 W m(-2) and RMS differences less than 25 W m(-2). Sensitivity tests showed that the observed fluxes could be matched with variations of <10% in the inputs to the radiative transfer model. The calculated 24-h averaged SW instantaneous surface aerosol radiative forcing (ARF) was -21.1 +/- 14.3 W m(-2) and was estimated to account for 80% of the total radiative forcing at the surface. The ARF was larger during January April (-28.5 +/- 13.5 W m(-2)) than October-December (-11.9 +/- 8.9 W m(-2)).