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Reducing uncertainties associated with filter-based optical measurements of soot aerosol particles with chemical information.
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
(English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324Article in journal (Refereed) Submitted
Abstract [en]

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.

National Category
Meteorology and Atmospheric Sciences
Identifiers
URN: urn:nbn:se:su:diva-30836OAI: oai:DiVA.org:su-30836DiVA: diva2:274349
Available from: 2009-10-28 Created: 2009-10-28 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Characterization of soot in air and rain over southern Asia
Open this publication in new window or tab >>Characterization of soot in air and rain over southern Asia
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Filter-based optical measurements of light absorbing particulate matter at awavelength of about 550 nm, here referred to as soot, in air and rainwaterhave been performed during the period from 1st June 2005 to 31th May 2009at Godavari in Nepal, Sinhagad in India and Hanimaadhoo in The Maldives.A method for determination of water-insoluble light absorbing matter inrainwater has been developed. Analysis of environmental samples has beensuccessfully performed with the described method on samples collected atHanimaadhoo and Godavari. At Hanimaadhoo the average soot concentrationin rainwater was 48 mgl-1 and at Godavari 86 mgl-1.In order to reduce systematic errors at optical determination of soot due tothe light scattering of non-absorbing particles co-deposited on the filter, suchas inorganic salts and mineral dust, an additional sensor recording backscatteredlight was used. Two alternative protocols of corrections (optical andchemical) were applied to the samples. Simultaneous measurements of sootand inorganic ions in aerosol and precipitation at Hanimaadhoo during theperiod May 2005 to February 2007 made it possible to calculate the washoutratio (WR) of these components as a measure of how efficiently they are scavengedby precipitation. During the monsoon season the WR for soot was similarto that of sulphate and other fine mode aerosol components, indicating thatsoot containing particles in these situations were efficient as cloud condensationnuclei. During the polluted winter days, on the other hand, the WR forsoot was 3 times smaller than that of sulphate, showing that the soot containingparticles had retained a hydrophobic character even after a travel time ofseveral days.The Indian monsoon circulation with its two annual phases in combinationwith the location of the main combustion source areas dominated the observedpatterns of soot at the observatories in India and Maldives. Godavari in Nepalwas however mainly influenced by combustion sources all year around concealingpossible variability related to the monsoon circulation. At Hanimaadhoo,peak values in the soot concentration occurred during the winter season(December to April) when air was transported from the polluted Indian subcontinentout over the Indian Ocean. At least a factor of ten lower values wererecorded in air that had spent more than 10-days over the Indian Ocean duringthe monsoon season (July to September).

 

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology , Stockholm University, 2009. 42 p.
Keyword
soot, black carbon, atmospheric concentration, light absorption, wet deposition, scavenging, wash-out ratio, South-Asia.
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:su:diva-30861 (URN)978-91-7155-969-2 (ISBN)
Public defence
2009-11-27, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defence, the following papers were unpublished and had a status as follows: Paper 1: Submitted. Paper 2: Manuscript. Paper 3: Submitted: Paper 4: Submitted.Available from: 2009-11-05 Created: 2009-10-28 Last updated: 2009-10-29Bibliographically approved

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