Empirical correction of multifilter rotating shadowband radiometer (MFRSR) aerosol optical depths for the aerosol forward scattering and development of a long-term integrated MFRSR-Cimel dataset at Lampedusa

Abstract

Aerosol optical properties have been measured on the island of Lampedusa (35.5°N, 12.6°E) with seven-band multifilter rotating shadowband radiometers (MFRSRs) and a CE 318 Cimel sunphotometer (part of the AERONET network) since 1999. Four different MFRSRs have operated since 1999. The Cimel sunphotometer has been operational for a short period in 2000 and in 2003-2006 and 2010-present. Simultaneous determinations of the aerosol optical depth (AOD) from the two instruments were compared over a period of almost 4 years at several wavelengths between 415 and 870 nm. This is the first long-term comparison at a site strongly influenced by desert dust and marine aerosols and characterized by frequent cases of elevated AOD. The datasets show a good agreement, with MFRSR underestimating the Cimel AOD in cases with low Ångström exponent; the underestimate decreases for increasing wavelength and increases with AOD. This underestimate is attributed to the effect of aerosol forward scattering on the relatively wide field of view of the MFRSR. An empirical correction of the MFRSR data was implemented. After correction, the mean bias (MB) between MFRSR and Cimel simultaneous AOD determinations is always smaller than 0.004, and the root mean square difference is ≤0.031 at all wavelengths. The MB between MFRSR and Cimel monthly averages (for months with at least 20 days with AOD determinations) is 0.0052. Thus, by combining the MFRSR and Cimel observations, an integrated long-term series is obtained, covering the period 1999-present, with almost continuous measurements since early 2002. The long-term data show a small (nonstatistically significant) decreasing trend over the period 2002-2013, in agreement with independent observations in the Mediterranean. The integrated Lampedusa dataset will be used for aerosol climatological studies and for verification of satellite observations and model analyses.

Autonomous marine hyperspectral radiometers for determining solar irradiances and aerosol optical properties

“…Handheld Microtops measurements are generally taken on an opportunistic footing, when a dedicated operator is not available; CIMEL and POM measurements are generally taken on a 10-15 min time interval. As observed by di Sarra et al (2015), the shadowband-type technology can take readings on a sub-minute timescale, which allows almost continuous observations of AOD and the resolution of short-length and time-scale atmospheric aerosol features and variability. Although placing Spectrometer 1 and 2 on a ship, with many other sources of error such as motion and variable ship shading, may preclude accurate observation of such features, a land-based deployment should allow this opportunity.…”

“…This is typically between 5 and 10 • , whereas the POM and CIMEL instruments' FOV is ∼ 1 • . The difference between shadowband radiometer and sun-photometric retrievals of AOD has previously been observed, and subsequently empirically corrected for by di Sarra et al (2015), Table 2. Correction values applied to AOD measured using Spectrometer 2.…”

Synergistic use of Lagrangian dispersion and radiative transfer modelling with satellite and surface remote sensing measurements for the investigation of volcanic plumes: the Mount Etna eruption of 25–27 October 2013

“…By inspecting the spatial distribution of sulfur dioxide mixing ratio at 10-14 km, values of up to 60 ppt (part per trillion) are found a few tens of km from Lampedusa and up to 300 ppt less than 100 km northwards. Values greater than about 100 ppt are considered a significant perturbation from background values and are expected to perturb the secondary sulfate aerosol content (Doeringer et al, 2012).…”

On the radiative impact of aerosols on photolysis rates: comparison of simulations and observations in the Lampedusa island during the ChArMEx/ADRIMED campaign

“…MFRSR data were also used for calculating J (NO 2 ) and (Pace et al, 2006;di Sarra et al, 2015) for the periods when the AERONET data were not available, namely 6 to 16 and 27 June. It was shown in di Sarra et al (2015) that the mean bias of the MFRSR AOD relative to the AERONET measurements is always smaller than 0.004 for long-term series (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013), with a r 2 correlation coefficient always above 0.97 at all wavelengths between the AERONET and the MFRSR measurements.…”