Reconstruction of historical aerosol optical depth time series over Romania during summertime

摘要

ABSTRACT Ground‐based meteorological observations are used here as proxy data, as well as aerosol reanalysis estimates for training, to reconstruct aerosol optical depth ( AOD ) time series over Romania during a multidecadal period (1961–2015). The approach uses sunshine duration fraction ( SDF ) data computed for cloudless summer days (identified from 6‐hourly cloud observations) and AOD data from the Modern‐Era Retrospective analysis for Research and Applications, version 2 ( MERRA ‐2) reanalysis. MERRA ‐2 AOD data were validated first by comparing them to high‐quality AErosol RObotic NETwork ( AERONET ) ground observations at eight stations located in Central and Eastern Europe. The comparison of MERRA ‐2 AOD data with AERONET ground observations showed that the reanalysis estimates are reasonably accurate, but tend to underestimate AOD in the region of interest. Spaceborne AOD observations from Moderate Resolution Imaging Spectroradiometer ( MODIS ) were also considered, but were found too biased and not complete enough for this task. A significant linear relationship between MERRA ‐2's AOD and SDF data at the 57 of the 86 available stations was found, allowing the use of both a linear and bivariate regression models to back‐calculate the historical AOD time series from SDF observations. This reconstruction is restricted to clear summer days here because of the limited temporal resolution of the available cloud observations. The long‐term trends of the computed AOD time series correctly delineate the expected dimming and brightening periods that have been widely reported over Europe. The proposed method is suitable over areas where very long and concomitant time series of daily sunshine and hourly cloud observations are available, but also has limitations that are discussed.