Aerosol optical depth measurements during the Aerosols99 experiment

The Aerosols99 cruise took place during the period from January 14 to Febru ary 8, 1999, on the R/V Ronald Brown. The cruise track was almost a straigh t line in the southeast direction from Norfolk, Virginia, to Cape Town, Sou th Africa, and afforded the opportunity to sample several different aerosol regimes over the North and South Atlantic. Handheld sunphotometers, a shad owband radiometer (FRSR), and a LIDAR were used to measure the aerosol opti cal depth (AOD) during the cruise. The AOD and angstrom exponent a (spectra l dependence of the AOD) varied strongly between regimes. Maritime regions typically had AOD (500 nm) of approximately 0.10 +/- 0.03, with a around 0. 3 +/- 0.3. An African dust event was encountered in which the AOD (500 nm) averaged 0.29 +/- 0.05 with an alpha of 0.36 +/- 0.13. At the Intertropical Convergence Zone (ITCZ), no measurements were obtained because of cloudine ss; however, after the ITCZ we encountered a biomass burning aerosol with h igh average AOD (500 nm) of 0.36 +/- 0.13, and a high alpha (0.88 +/- 0.30) . Farther south the aerosol went back to the low levels of a typical marine aerosol.