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Multiple perspectives on absorbing aerosols over the northern Indian Ocean and Asia
Stockholm University, Faculty of Science, Department of Meteorology .
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Aerosol particles in the atmosphere scatter and absorb solar radiation, and thereby affect the Earth's energy budget, but the magnitude of the overall radiative effect due to aerosol-radiation interactions is uncertain. In particular, the impact of absorbing aerosol particles, mainly black carbon (BC), organic carbon and dust, is not completely understood. A large variety of absorbing aerosols can be found in Asia and over adjacent oceans due to huge urban, biomass burning and desert areas. This thesis focuses on the investigation of atmospheric aerosols over the northern Indian Ocean and Asia from multiple perspectives. This includes surface and vertical observations of physical and chemical particle properties by in-situ and remote sensing instruments as well as an investigation of the representation of absorbing aerosols in general circulation models. One main focus is on the identification of BC-containing particles at the marine remote Maldives Climate Observatory in Hanimaadhoo (MCOH) which is frequently influenced by continental air masses containing anthropogenic aerosols. During an intensive field campaign, vertical measurements of aerosol particles were performed with a Lidar and unmanned aerial vehicles. Elevated layers of absorbing aerosol above the marine boundary layer were found frequently when air masses had their source over the northern Indian Peninsula. However, determining a complete profile of particle absorption only from Lidar measurements is linked to high uncertainties. Long-term surface measurements of particle absorption and scattering together with observations of aerosol chemical composition at MCOH are used to evaluate purely optical methods for aerosol characterization. These optical methods are found to give reasonable estimates of particle types but they can not replace detailed chemical measurements. An additional comparison study of various instruments for determination of BC mass give potentially very diverse results, highlighting the complexity of the quantification of BC-containing particles. The investigation of the representation of absorbing aerosols over Asia in general circulation models reveals firstly that the particle absorption is generally underestimated in global climate models, and secondly that the range in aerosol absorption determined from major changes of emissions, meteorology and particle optical properties can not reach the large inter-model diversity. 

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2019. , p. 48
Keywords [en]
aerosols, black carbon, air pollution, South Asia
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-172375ISBN: 978-91-7797-791-9 (print)ISBN: 978-91-7797-792-6 (electronic)OAI: oai:DiVA.org:su-172375DiVA, id: diva2:1346641
Public defence
2019-10-11, William-Olssonsalen, Geovetenskaps hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2019-09-18 Created: 2019-08-28 Last updated: 2019-09-12Bibliographically approved
List of papers
1. Vertical profiles of optical and microphysical particle properties above the northern Indian Ocean during CARDEX 2012
Open this publication in new window or tab >>Vertical profiles of optical and microphysical particle properties above the northern Indian Ocean during CARDEX 2012
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2016 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 2, p. 1045-1064Article in journal (Refereed) Published
Abstract [en]

A detailed analysis of optical and microphysical properties of aerosol particles during the dry winter monsoon season above the northern Indian Ocean is presented. The Cloud Aerosol Radiative Forcing Experiment (CARDEX), conducted from 16 February to 30 March 2012 at the Maldives Climate Observatory on Hanimaadhoo island (MCOH) in the Republic of the Maldives, used autonomous unmanned aerial vehicles (AUAV) to perform vertical in situ measurements of particle number concentration, particle number size distribution as well as particle absorption coefficients. These measurements were used together with surface-based Mini Micro Pulse Lidar (MiniMPL) observations and aerosol in situ and off-line measurements to investigate the vertical distribution of aerosol particles. Air masses were mainly advected over the Indian subcontinent and the Arabian Peninsula. The mean surface aerosol number concentration was 1717 +/- 604cm(-3) and the highest values were found in air masses from the Bay of Bengal and Indo-Gangetic Plain (2247 +/- 370cm(-3)). Investigations of the free tropospheric air showed that elevated aerosol layers with up to 3 times higher aerosol number concentrations than at the surface occurred mainly during periods with air masses originating from the Bay of Bengal and the Indo-Gangetic Plain. This feature is different compared to what was observed during the Indian Ocean Experiment (INDOEX) conducted in winter 1999, where aerosol number concentrations generally decreased with height. In contrast, lower particle absorption at the surface (sigma(abs)(520nm) = 8.5 + 4.2Wm(-1)) was found during CARDEX compared to INDOEX 1999. Layers with source region specific single-scattering albedo (SSA) values were derived by combining vertical in situ particle absorption coefficients and scattering coefficients calculated with Mie theory. These SSA layers were utilized to calculate vertical particle absorption profiles from MiniMPL profiles. SSA surface values for 550 nm for dry conditions were found to be 0 : 94 +/- 0 : 02 and 0 : 91 +/- 0 : 02 for air masses from the Arabian Sea (and Middle East countries) and India (and Bay of Bengal), respectively. Lidar-derived particle absorption coefficient profiles showed both a similar magnitude and structure as the in situ profiles measured with the AUAV. However, primarily due to insufficient accuracy in the SSA estimates, the lidar-derived absorption coefficient profiles have large uncertainties and are generally weakly correlated to vertically in situ measured particle absorption coefficients. Furthermore, the mass absorption efficiency (MAE) for the northern Indian Ocean during the dry monsoon season was calculated to determine equivalent black carbon (EBC) concentrations from particle absorption coefficient measurements. A mean MAE of 11.6 and 6.9m(2) g(-1) for 520 and 880 nm, respectively, was found, likely representing internally mixed BC containing particles. Lower MAE values for 880 and 520 nm were found for air masses originating from dust regions such as the Arabian Peninsula and western Asia (MAE(880 nm) = 5.6m(2) g(-1), MAE(520 nm) = 9.5m(2) g(-1)) or from closer source regions as southern India (MAE(880 nm) = 4.3m(2) g(-1), MAE(520 nm) = 7. 3m(2) g(-1)).

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-128568 (URN)10.5194/acp-16-1045-2016 (DOI)000371284000035 ()
Available from: 2016-06-17 Created: 2016-03-30 Last updated: 2019-08-28Bibliographically approved
2. Investigation of Two Optical Methods for Aerosol‐Type Classification Extended to a Northern Indian Ocean Site
Open this publication in new window or tab >>Investigation of Two Optical Methods for Aerosol‐Type Classification Extended to a Northern Indian Ocean Site
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2019 (English)In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 124, no 15, p. 8743-8763Article in journal (Refereed) Published
Abstract [en]

Methods for determining aerosol types in cases where chemical composition measurements are not available are useful for improved aerosol radiative forcing estimates. In this study, two aerosol characterization methods by Cazorla et al. (2013, https://doi.org/10.5194/acp-13-9337-2013; CA13) and Costabile et al. (2013, https://doi.org/10.5194/acp-13-2455-2013; CO13) using wavelength‐dependent particle absorption and scattering are used, to assess their applicability and examine their limitations. Long‐term ambient particle optical property and chemical composition (major inorganic ions and bulk carbon) measurements from the Maldives Climate Observatory Hanimaadhoo as well as concurrent air mass trajectories are utilized to test the classifications based on the determined absorption Ångström exponent, scattering Ångström exponent, and single scattering albedo. The resulting aerosol types from the CA13 method show a good qualitative agreement with the particle chemical composition and air mass origin. In general, the size differentiation using the scattering Ångström exponent works very well for both methods, while the composition identification depending mainly on the absorption Ångström exponent can result in aerosol misclassifications at Maldives Climate Observatory Hanimaadhoo. To broaden the applicability of the CA13 method, we suggest to include an underlying marine aerosol group in the classification scheme. The classification of the CO13 method is less clear, and its applicability is limited when it is extended to aerosols in this environment at ambient humidity.

National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-172377 (URN)10.1029/2018JD029685 (DOI)000482475000023 ()
Available from: 2019-08-28 Created: 2019-08-28 Last updated: 2019-09-16Bibliographically approved
3. Comparison of BC measurement methods at a marine receptor site
Open this publication in new window or tab >>Comparison of BC measurement methods at a marine receptor site
(English)Manuscript (preprint) (Other academic)
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-172378 (URN)
Available from: 2019-08-28 Created: 2019-08-28 Last updated: 2019-08-28Bibliographically approved
4. Absorbing aerosols over Asia: An inter-model and model-observation comparison study using CAM5.3-Oslo
Open this publication in new window or tab >>Absorbing aerosols over Asia: An inter-model and model-observation comparison study using CAM5.3-Oslo
(English)Manuscript (preprint) (Other academic)
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-168050 (URN)
Available from: 2019-04-28 Created: 2019-04-28 Last updated: 2019-08-28Bibliographically approved

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