An intercomparison of aerosol light extinction and 180° backscatter as derived using in situ instruments and Raman lidar during the INDOEX field campaign : Indian Ocean Experiment (INDOEX)

Aircraft in situ and Raman lidar profiles of aerosol light extinction (σ ep ) and 180° backscattering (β p ) are compared for 6 days during the Indian Ocean Experiment (INDOEX). The measurements of σ ep and β p were made from the National Center for Atmospheric Research C-130 aircraft using two integrating nephelometers to measure light scattering and one Radiance Research Particle Soot Absorption Photometer to measure light absorption. Particulate 180° backscattering was measured in situ using a new instrument, the 180° backscatter nephelometer. The Institute for Tropospheric Research Raman lidar was located on the island of Hulule (4.18°N, 73.53°E), and all of the in situ profiles presented are from descents into the Hulule airport. Aerosol optical depth was also measured from Hulule using a Sun photometer, and these data are included in the intercomparison. On average, the lidar-derived values of σ ep and β p are ∼30% larger than the in situ-derived values to a 95% confidence interval. Possible reasons for the overall discrepancy are (1) a low bias in the in situ measurements because of losses in the C-130 Community Aerosol Inlet; (2) underestimation of the humidification effect on light extinction in the in situ measurements; (3) overestimation of σ ep and β P in the lidar because of subvisible cloud contamination; (4) errors in data processing that could be biasing either measurement, though the lidar retrievals are especially sensitive to this type of error. Temporal and spatial variability also appear to be the source of at least some of the discrepancy in two of the six cases, none of which are well collocated.