Synergy between CALIOP and MODIS instruments for aerosol monitoring: application to the Po Valley

Abstract

Abstract. We propose here a synergy between Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations/Cloud-Aerosol LIdar with Orthogonal Polarization (CALIPSO/CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua and Terra in order to retrieve aerosol optical properties over the Po Valley from June 2006 to February 2009. Such an approach gives simultaneously access to the aerosol extinction vertical profile and to the equivalent backscatter-to-extinction ratio at 532 nm (BER, inverse of the lidar ratio). The choice of the Po valley has been driven by the great occurrences of pollutant events leading to a mean MODIS-derived aerosol optical thickness of 0.27(±0.17) at 550 nm over a large area of ~120 000 km2. In such area, a significant number of CALIOP level-1 vertical profiles can be averaged (~200 individual laser shots) leading to a signal-to-noise ratio greater than 10 in the planetary boundary layer (PBL) sufficient to perform a homemade inversion of the mean lidar profiles. The mean BER (together with the associated variabilities) over the Po Valley retrieved from the coupling between CALIOP/MODIS-Aqua and CALIOP/MODIS-Terra are ~0.014(±0.003) sr−1 and ~0.013(±0.004) sr−1, respectively. The total uncertainty on BER retrieval has been assessed to be ~0.003 sr−1 using a Monte Carlo approach. These mean BER values retrieved have been compared with those given by the level-2 operational products of CALIOP ~0.016(±0.003) sr−1. The values we assessed appear close to what is expected above urban area. A seasonal cycle has been observed with higher BER values in spring, summer and fall, which can be associated to dust event occurring during this period. In most of cases, the mean aerosol extinction coefficient in the PBL diverges significantly between the level-2 operational products and the result of our own inversion procedure. Indeed, mean differences of 0.10 km−1 (~50%) and 0.13 km−1 (~60%) have been calculated using MODIS-Aqua/CALIOP and MODIS-Terra/CALIOP synergies, respectively. Such differences may be due to the identification of the aerosol model by the operational algorithm and thus to the choice of the BER.