A hybrid method for reconstructing the historical evolution of aerosol optical depth from sunshine duration measurements
Abstract. A novel method has been developed to estimate aerosol optical
depth (AOD) from sunshine duration (SD) measurements under cloud-free
conditions. It is a physically based method serving for the reconstruction
of the historical evolution of AOD during the last century. In addition to
sunshine duration data, it requires daily water vapor and ozone products as
inputs taken from the ECMWF 20th century reanalysis ERA-20C, available
at the global scale over the period 1900–2010. Surface synoptic cloud
observations are used to identify cloud-free days. For 16 sites over
Europe, the accuracy of the estimated daily AOD, and its seasonal
variability, is similar to or better than those from two earlier methods when compared to AErosol RObotic NETwork measurements. In addition, it also
improves the detection of the signal from massive aerosol events such as
important volcanic eruptions (e.g., Arenal and Fernandina Island in 1968,
El Chichon in 1982 and Pinatubo in 1992). Finally, the reconstructed AOD
time series are in good agreement with the dimming/brightening phenomenon
and also provide preliminary evidence of the early-brightening phenomenon.