Comment on acp-2022-380

Abstract

An intense and long-lasting Saharan dust outbreak over the Iberian Peninsula (IP) in springtime 2021 is studied in this work with the aim of investigating the optical and mass contribution of both the fine and coarse dust particles along their transport by crossing the IP from the south-west to the north-east. Five Iberian lidar stations (El Arenosillo/Huelva, Granada, Torrejón/Madrid and Barcelona in Spain and Évora in Portugal) were monitoring the transport and evolution of the Saharan dust particles across the Iberian Peninsula from 25 March until 7 April 2021. Particular meteorological conditions determined the aerosol scenario along the overall dust event, differing the first part of the event (25–31 March), with the strongest dust incidence on 29–31 March at the south and central stations, and 1 April at Barcelona, from the second one (1–7 April). Both the fine dust (Df) and coarse dust (Dc) components of the total particle backscatter coefficient (total dust, DD = Dc + Df) were separately derived. The dust plume was well-mixed and no significant differences were found in the vertical structure of both the Dc and Df particle backscatter coefficients. The vertical total dust mass concentration, and its Df / DD ratio, were estimated from the extinction profiles using the well-defined conversion factors for North African dust. From the beginning of the dust outbreak until 1 April, the vertical Df / DD mass ratio was nearly constant in time at each station and also almost constant in altitude with values ~ 10 %. Regarding the columnar properties, the dust optical depth at 532 nm (DOD532), the relative mass loading and the centre-of-mass height were calculated, distinguishing the corresponding fine- and coarse-dust contributions. Firstly, the dust outbreak crossed the Iberian Peninsula from South-West to North-East and the mean DOD532 was decreasing along that dust pathway, reporting values from SW to NE stations of 0.34 at El Arenosillo/Huelva, 0.28 at Granada, 0.20 at Évora, 0.28 at Torrejón/Madrid and 0.14 at Barcelona, although its Df / DD ratio kept almost constant (28–30 %). On the other hand, a similar pattern was found for the total dust mass loading and its Df / DD ratio, i.e. mostly decreasing mean mass values were reported: 586 mg m-2 (El Arenosillo/Huelva), 483 mg m-2 (Granada), 332 mg m-2 (Évora), 464 mg m-2 (Torrejón/Madrid) and 248 mg m-2 (Barcelona), being constant its Df / DD ratio (~ 10 %) along the SW-NE dust pathway In addition, the episode-mean centre-of-mass height increased with latitude in overall, showing a high variability, i.e. being greater than 0.5 km at the southern sites (El Arenosillo/Huelva, Granada, Évora) and ~1.0 km at Torrejón/Madrid and Barcelona. However, despite the relatively high intensity of the dust intrusion, the expected ageing of the dust particles was barely observed. This is on the basis that the Iberian Peninsula is relatively close to the Saharan dust sources and then, under certain dust transport conditions, any potential ageing processes in the dust particles remain unappreciated. It must be highlighted the different relative contribution of the fine dust particles to the total dust found for their optical properties (~ 30 %), associated to the radiative effect of dust, with respect to the mass features (~ 10 %), linked to air quality issues, along the overall dust event by crossing the IP. Finally, the methodology introduced, which is mainly based on the two-step POLIPHON algorithm, shows the relevance of the use of the polarized lidar measurements, mainly with elastic systems, for separating the aerosol properties for the dust fine and coarse particles as an added value to the simple one-step approach used for simple dust/non-dust separation.