The reconstruction of a 73-year time series of AOD at 500 nm by using artificial neural networks

A 73-year time series of the daily AOD at 500 nm has been reconstructed at IZO. For this purpose, we have combined AOD estimates from artificial neural networks (ANNs) from 1941 to 2001, and AOD measurements from a Precision Filter Radiometer (PFR) between 2003 and 2013. The analysis is limited to summer months (July–August–September), when the largest aerosol load is observed at IZO (Saharan mineral dust particles).

The ANN AOD time series has been comprehensively validated against coincident AOD measurements performed with a solar spectrometer Mark-I (1984–2009) and AERONET CIMEL photometers (2004–2009) at IZO, obtaining a rather good agreement on a daily basis: Pearson coefficient, R, of 0.97 between AERONET and ANN AOD, and 0.93 between Mark-I and ANN AOD estimates.

In addition, we have analysed the long-term consistency between ANN AOD time series and long-term meteorological records identifying Saharan mineral dust events at IZO (synoptical observations and local wind records). Both analyses provide consistent results, with Pearson coefficient (R) > 0.85 (Figure 1).struction of a 73-year time series of AOD at 500 nm at IZO has been achieved by using artificial neural networks (ANNs) from 1941 to 2001 and AOD measurements directly obtained with a Precision Filter Radiometer (PFR) between 2003 and 2013.The ANN method has proved to be a very useful tool for the reconstruction of daily AOD values at 500 nm from meteorological input data, such as the horizontal visibility, fraction of clear sky, and relative humidity, recorded at IZO. This methodology could be extrapolated to other sites, especially those affected by high dust loads.

Figure 1.- (a) Time series of the number of days grouped into ANN AOD intervals (AOD≥0.05; AOD≥0.10; AOD≥0.20) on the left axis, while on the right axis, the bars indicate the number of days with SYNOP data reporting dust in suspension (05–06 SYNOP codes) for the period 1941–2009. The 5-year running mean are shown in black. (b) Scatterplot of number of days with ANN AOD≥0.20 and number of days with 05–06 SYNOP codes. The least-square fit parameters are shown in the legend. (c) Time series of the ANN AOD monthly medians (blue line) and monthly percentage of time the wind blows from the second quadrant (E–S; 90°-180°) (black line) at IZO in July in the period 1941–2009. (d) Percentage of time (y axis) the wind blows from in each one of the four quadrants vs. the ANN AOD monthly medians (x axis). R indicates the Pearson coefficient. Reprinted from García et al. (2016).