Spatial variability of the aerosol optical thickness over Southern Ocean and coastal Antarctica: Comparison with MODIS and MERRA-2 aerosol products

Abstract

Aerosols scatter or absorb incoming solar radiation and alter the earth's radiation budget. The Southern Ocean provides an excellent environment to study the natural/background aerosols and teleconnection with other regions of the world through long-range transport. Measurements of Aerosol Optical Thickness (AOT) (in the wavelength range 380–1020 nm) have been carried out during a 10th Southern Ocean expedition, between 20o-67oS, during December 2017–February 2018, onboard chartered ice-class vessel SA Agulhas. AOT (500 nm) in the range ~ 0.04–0.17 and an Angstrom parameter (α) (turbidity coefficient (β)) in the range ~0.03–1.29 (0.02–0.05) were observed and values decrease with the increase in latitude towards the Antarctic coast, indicative of a clean environment. AOT versus α scatter plot reveals broadly two classes, fine mode and coarse mode but exhibits different optical properties in the Southern Ocean and Antarctic coast. The HYSPLIT air mass back trajectory analysis shows that for the measurement location between 30o- 60oS, the origin of air masses is from the South Atlantic Ocean, whereas measurement locations between 60o-70oS are influenced by sources from Antarctica with no long-range transport of aerosols from other significant landmasses. The AOT data south of 30o is sparse and ship-based data are the only source to validate satellite and modelled AOT. Comparison of sunphotometer AOT with collection 6.1 MODIS (TERRA) AOT reveals a Mean Bias Error (MBE) in the range −0.01 to −0.1 and Root Mean Square Error (RMSE) in the range 0.02– to 0.1. Similarly, comparison with MERRA-2 reanalysis AOT shows the MBE (RMSE) in the range 0 to −0.11 (0.02–0.11). Both the MODIS (TERRA) and MERRA-2 reanalysis are found to underestimate AOT and show maximum errors (in terms of MBE and RMSE) in the 50oS-60oS latitude band.