Light Absorbing Material ("Soot") in Rainwater and in aerosol particles in the Maldives

• Simultaneous measurements of soot (absorbing material at 528 nm) and inorganic ions in aerosol and precipitation at the Maldives Climate Observatory Hanimaadhoo during the period May 2005 to February 2007 have made it possible to calculate the washout ratio (WR) of these components as a measure of how efficiently they are scavenged by precipitation. Based on air trajectories the data have been separated into days with polluted air arriving from the Indian subcontinent in a north-easterly sector during winter and clean monsoon days with southerly flow from the Indian Ocean. The average soot concentration was a factor of ten higher in the former situations. Despite considerable scatter for individual days a systematic pattern emerged when the WR for the different components were compared with each other. During the monsoon season the WR for soot was similar to that of sulphate and other fine mode aerosol components, indicating that soot containing particles in these situations were efficient as cloud condensation nuclei. The origin of the light absorbing material during the monsoon season is unclear. We speculate that light absorbing material from the tropical ocean surface could contribute to the concentration of "soot" during the monsoon season. During the polluted winter days, on the other hand, the WR for soot was 3 times smaller than that of sulphate. This indicates that, even after a travel time of several days, the soot containing particles from India have retained much of their hydrophobic property and that the soot must be mainly externally mixed. The low WR and the infrequent rain during this season probably contribute to extending the atmospheric lifetime of soot well beyond several days. Surprisingly high concentrations of non sea salt calcium were measured during the monsoon season, substantially higher than during the winter season. The origin of these high values might be long-range transport from the Australian or African continents. Another possibility might be exopolymer gels derived from the ocean surface micro-layer.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Meteorologi och atmosfärforskning (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Meteorology and Atmospheric Sciences (hsv//eng)

Nyckelord

Atmosphere and hydrosphere sciences

Atmosfärs- och hydrosfärsvetenskap

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