Reducing uncertainties associated with filter-based optical measurements of soot aerosol particles with chemical information.

• Of the many identified and potential effects ofatmospheric aerosol particles on climate, those of soot particlesare the most uncertain, in that analytical techniques concerningsoot are far from satisfactory. One concern whenapplying filter-based optical measurements of soot is thatthey suffer from systematic errors due to the light scatteringof non-absorbing particles co-deposited on the filter, suchas inorganic salts and mineral dust. In addition to an opticalcorrection of the non-absorbing material this study providesa protocol for correction of light scattering based onthe chemical quantification of the material, which is a novelty.A newly designed Particle Soot Absorption Photometerwas constructed to measure light transmission on particleaccumulating filters, which includes an additional sensorrecording backscattered light. The choice of polycarbonatemembrane filters avoided high chemical blank values and reducederrors associated with length of the light path throughthe filter.Two protocols of corrections were applied to aerosol samplescollected at the Maldives Climate Observatory Hanimaadhooduring episodes with either continentally influencedair from the Indian/Arabian subcontinents (wintermonsoon) or pristine air from the Southern Indian Ocean(summer monsoon). The two ways of correction (optical andchemical) lowered the particle light absorption of soot by 63 to 61 %, respectively, for data from the Arabian Sea sourcedgroup, resulting in median soot absorption coefficients of 4.2 and 3.5 Mm-1. Corresponding values for the South IndianOcean data were 69 and 97 % (0.38 and 0.02 Mm-1). A comparison with other studies in the area indicated anoverestimation of their soot levels, by up to two orders ofmagnitude. This raises the necessity for chemical correctionprotocols on optical filter-based determinations of soot, before even the sign on the radiative forcing based on their effectscan be assessed.

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

Publikations- och innehållstyp

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