Lidar and in situ observations of continental and Saharan aerosol: closure analysis of particles optical and physical properties

[摘要] Single wavelength polarization lidar observations collected at Mt.\ Cimone(44.2º N, 10.7º E, 1870 m a.s.l.) during the June 2000 MINATROC campaign are analyzed to derive tropospheric profiles ofaerosol extinction, depolarization, surface area and volume. Lidar retrievals for the2170-2245 m level are compared to the same variables as computed from in situ measurements of particles sizedistributions, performed at the mountain top Station (2165 m a.s.l.) by a differential mobility analyzer(DMA) and an optical particle counter (OPC). A sensitivity analysis of this closure experimentshows that mean relative differences between the backscatter coefficients obtained by the twotechniques undergo a sharp decrease when hygroscopic growth to ambient humidity is consideredfor the DMA dataset, otherwise representative of dry aerosols. Minimization of differences betweenlidar and size distribution-derived backscatter coefficients allowed to find values of the"best" refractive index, specific to each measurement. These results show the refractive index to increasefor air masses proceeding from Africa and Western Europe. Lidar depolarization was observed tominimize mainly in airmasses proceeding from Western Europe, thus indicating a spherical,i.e. liquid nature for such aerosols. Conversely, African, Mediterranean and East Europe aerosolshowed a larger depolarizing fraction, mainly due to coexisting refractory and soluble fractions. Theanalysis shows average relative differences between lidar and in-situ observations of5% for backscatter, 36% for extinction 41% for surface area and 37% for volume. These values are wellwithin the expected combined uncertainties of the lidar and in situ retrievals. Average differencesfurther decrease during the Saharan dust transport event, when a lidar signal inversion modelconsidering non-spherical scatterers is employed. The quality of the closure obtained betweenparticle counter and lidar-derived aerosol surface area and volume observations constitutes avalidation of the technique adopted to retrieve such aerosol properties on the basis of single-wavelengthlidar observations.

[学科分类] 大气科学