Studying the vertical aerosol extinction coefficient by comparing in situ airborne data and elastic backscatter lidar

摘要

Vertical profiles of aerosol particle optical properties were explored in acase study near the San Pietro Capofiume (SPC) ground station during thePEGASOS Po Valley campaign in the summer of 2012. A Zeppelin NT airship wasemployed to investigate the effect of the dynamics of the planetary boundarylayer at altitudes between ∼  50 and 800 m above ground. Determinedproperties included the aerosol particle size distribution, the hygroscopicgrowth factor, the effective index of refraction and the light absorptioncoefficient. The first three parameters were used to retrieve the lightscattering coefficient. Simultaneously, direct measurements of both thescattering and absorption coefficient were carried out at the SPC groundstation. Additionally, a single wavelength polarization diversity elasticlidar system provided estimates of aerosol extinction coefficients using theKlett method to accomplish the inversion of the signal, for a verticallyresolved comparison between in situ and remote-sensing results. Note,however, that the comparison was for the most part done in the altitude rangewhere the overlap function is incomplete and accordingly uncertainties arelarger. First, the airborne results at low altitudes were validated with theground measurements. Agreement within approximately ±25 and ±20 %was found for the dry scattering and absorption coefficient, respectively.The single scattering albedo, ranged between 0.83 and 0.95, indicating theimportance of the absorbing particles in the Po Valley region. A clearlayering of the atmosphere was observed during the beginning of the flight(until ∼  10:00 LT – local time) before the mixing layer (ML) was fullydeveloped. Highest extinction coefficients were found at low altitudes, inthe new ML, while values in the residual layer, which could be probed at thebeginning of the flight at elevated altitudes, were lower. At the end of theflight (after ∼  12:00 LT) the ML was fully developed, resulting inconstant extinction coefficients at all altitudes measured on the ZeppelinNT. Lidar estimates captured these dynamic features well and good agreementwas found for the extinction coefficients compared to the in situ results,using fixed lidar ratios (LR) between 30 and 70 sr for the altitudes probedwith the Zeppelin. These LR are consistent with values for continentalaerosol particles that can be expected in this region.