Friday, 13 November 2009
Western Africa is one of the largest sources of mineral dust aerosol in the world. With uncertainty of how dust impacts on weather and climate, the trans-Atlantic Aerosol and Ocean Science Expeditions (AEROSE) are good opportunities to address this issue. An estimated three billion metric tons of mineral aerosols are injected into the troposphere annually from the Saharan desert [Prospero et al., 1996]. Plus, smoke from biomass burning sites in the savanna grasslands in sub-Saharan Africa contribute significant quantities of smaller-sized aerosols [e.g., Hobbs, 2000]. AEROSE constitute a comprehensive approach, in terms of both measurements and modeling, for gaining understanding of the impacts of long-range transport of mineral dust in the tropical Atlantic [Morris et al., 2006]. One of the main instruments used during AEROSE cruises is the handheld sunphotometer Microtops II. Spectral measurements of the direct solar radiation provide information to calculate the columnar aerosol optical depth (AOD) with the Microtops instrument. Additionally, the size distribution of aerosols was estimated from spectral AOD, typically from 440nm and 870nm. The negative slope (or first derivative) of AOD with wavelength in logarithmic scale is known as the Angstrom Coefficient (AC). Values of AC closer to 2.0 indicate fine mode particles (e.g., smoke particles and sulfates) exist, while values of AC near zero indicate the presence of coarse mode particles such as desert dust [Eck et al., 1999]. Data is then processed using NASA's Maritime Aerosol Network (MAN), from AERONET, method [Smirnov et al., 2009]. Collaboration with MAN was established for the 2009's AEROSE-V cruise in order to produce daily averages of AOD. After each transit of the cruise plots were generated (West-East, South-North, and East-West), including for the whole expedition. Regions of different aerosols were detected during the period of measurements performed with the Microtops instrument, July 15 to August 3. West to East transit, July 15 to July 20, showed a mix of dust and smoke, where at the end smoke was more pronounce with values of AC close to 1.0. For the South to North transit, July 21 to July 28, three regions of aerosols were classified. Smoke (with AC values above 1.0) for the first two days, July 25 mixed tending to dust (close to 0.2 AC) after the ITCZ, and the rest with dust getting closer to 0.0 AC. And for the East to West transit, July 29 to August 3, aerosol dust with AC values fluctuating between 0.1 and 0.2. This vital information, including other AEROSE cruises, will be implemented to new radiation models, including radiometers getting the irradiance for the entire cruise. Shortwave and Longwave forcing will be calculated in order to understand more the radiation budget in the atmosphere and its thermodynamics.
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