Vertical profiles of optical and microphysical particle properties above the northern Indian Ocean during CARDEX 2012

Abstract

Abstract. 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 ± 604 cm−3 and the highest values were found in air masses from the Bay of Bengal and Indo-Gangetic Plain (2247 ± 370 cm−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 (σabs(520 nm) = 8.5 ± 4.2 Wm−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 6.9 m2 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) = 5.6 m2 g−1, MAE(520 nm) = 9.5 m2 g−1) or from closer source regions as southern India (MAE(880 nm) = 4.3 m2 g−1, MAE(520 nm) = 7.3 m2 g−1).

Observed correlations between aerosol and cloud properties in an Indian Ocean trade cumulus regime

“…Elevated aerosol plumes are generally seen to approach MCOH from this direction, following the upper-level wind field, consistent with the findings of Höpner et al (2016).…”

How much of the global aerosol optical depth is found in the boundary layer and free troposphere?

“…High values of AOD are observed both in the BL and FT over central (all seasons) and southern (JJA and SON) Africa, the Arabian Peninsula, India and eastern China. This distribution is expected as these regions correspond to the main continental sources of aerosol mass (Ginoux et al, 2001;Giglio et al, 2013;Janssens-Maenhout et al, 2015). Typical features such as transport of aerosols over the Bay of Bengal and the Indo-Gangetic plain (Höpner et al, 2016) or longrange transport of dust aerosols from the Sahara over the Atlantic Ocean (e.g., Generoso et al, 2008) are also observed in the CALIOP data, both in the BL and in the FT.…”

“…This distribution is expected as these regions correspond to the main continental sources of aerosol mass (Ginoux et al, 2001;Giglio et al, 2013;Janssens-Maenhout et al, 2015). Typical features such as transport of aerosols over the Bay of Bengal and the Indo-Gangetic plain (Höpner et al, 2016) or longrange transport of dust aerosols from the Sahara over the Atlantic Ocean (e.g., Generoso et al, 2008) are also observed in the CALIOP data, both in the BL and in the FT. Overall, the global daytime AOD is 0.147 in the troposphere, where 0.102 is found in the BL and 0.045 in the FT (Table 2), corresponding to 69 and 31 %, respectively, of the total AOD.…”

Assessment of aerosol optical property and radiative effect for the layer decoupling cases over the northern South China Sea during the 7‐SEAS/Dongsha Experiment

“…The differences in ω and g for the near-surface and upper layer aerosols can be considered in terms of different source regions and transport pathways at different height layers over DSI. The different ω was observed for the near-surface and upper layers over Niamey [Osborne et al, 2008]; Chung-Li, Taiwan [Wang et al, 2010]; Gwangju, South Korea [Shin et al, 2014]; and Indian Ocean [Höpner et al, 2016]. A representative day (11 April) was thus selected because of the availability of AERONET data and also because of its clear two-layer structure and heavy aerosol loading.…”

“…For example, Hsu et al [2003] showed using satellite measurements over southern China that the presence of smoke-laden clouds can reduce the shortwave radiation flux at the TOA by 100 W m À2 . Surface aerosol measurements may not show the presence of aloft aerosol layers since the vertical exchange between the marine boundary layer (MBL) and free-troposphere can be weak [e.g., Corrigan et al, 2008;Höpner et al, 2016]. Aerosol properties observed at the surface and in a column are different because of (1) different source regions from which the aerosols are transported to the receptor location at different heights [Franke et al, 2003], (2) altitudinal differences in the physical and chemical composition of the aerosols [Slater and Dibb, 2004], and (3) variations in the contributions of MBL aerosols to the total atmospheric column [Franke et al, 2003;Smirnov et al, 2000;Madhavan et al, 2008].…”