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Vertical profiles of optical and microphysical particle properties above the northern Indian Ocean during CARDEX 2012
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
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Number of Authors: 92016 (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)).

Place, publisher, year, edition, pages
2016. Vol. 16, no 2, p. 1045-1064
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-128568DOI: 10.5194/acp-16-1045-2016ISI: 000371284000035OAI: oai:DiVA.org:su-128568DiVA, id: diva2:938546
Available from: 2016-06-17 Created: 2016-03-30 Last updated: 2019-08-28Bibliographically approved
In thesis
1. Multiple perspectives on absorbing aerosols over the northern Indian Ocean and Asia
Open this publication in new window or tab >>Multiple perspectives on absorbing aerosols over the northern Indian Ocean and Asia
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
aerosols, black carbon, air pollution, South Asia
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-172375 (URN)978-91-7797-791-9 (ISBN)978-91-7797-792-6 (ISBN)
Public defence
2019-10-11, William-Olssonsalen, Geovetenskaps hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
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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

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