Aerosol optical depth measurements during the Aerosols99 experiment

The Aerosols99 cruise took place during the period from January 14 to February 8, 1999, on the R/V Ronald Brown. The cruise track was almost a straight line in the southeast direction from Norfolk, Virginia, to Cape Town, South Africa, and afforded the opportunity to sample several different aerosol regimes over the North and South Atlantic. Handheld sunphotometers, a shadowband radiometer (FRSR), and a LIDAR were used to measure the aerosol optical depth (AOD) during the cruise. The AOD and angstrom exponent α (spectral dependence of the AOD) varied strongly between regimes. Maritime regions typically had AOD (500 nm) of approximately 0.10 ± 0.03, with α around 0.3 ± 0.3. An African dust event was encountered in which the AOD (500 nm) averaged 0.29 ± 0.05 with an α of 0.36 ± 0.13. At the Intertropical Convergence Zone (ITCZ), no measurements were obtained because of cloudiness; however, after the ITCZ we encountered a biomass burning aerosol with high average AOD (500 nm) of 0.36 ± 0.13, and a high α (0.88 ± 0.30). Farther south the aerosol went back to the low levels of a typical marine aerosol.