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 in February and 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. 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. 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. Compared to the Indian Ocean Experiment (INDOEX) conducted in winter 1999, elevated aerosol layers with increased aerosol number concentration were observed more frequently in 2012. However, lower particle absorption at the surface (σabs(520 nm)=8.5±4.2 Wm−1) was found during CARDEX compared to INDOEX 1999. By combining vertical in-situ measured particle absorption with scattering calculated with Mie-theory, layers with single-scattering albedo (SSA) values of specific source regions were derived and utilized to calculate vertical particle absorption profiles from MiniMPL profiles. SSA surface values 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 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 profiles have large uncertainties and are generally weakly correlated to vertically in-situ measured particle absorption. 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 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 nm were found for air masses originating from dust regions such as the Arabian Peninsula and western Asia (5.6 m2 g−1) or from closer source regions as southern India (4.3m2 g−1).

Black carbon solar absorption suppresses turbulence in the atmospheric boundary layer

“…Further modeling experiments are certainly warranted to isolate the impact of aerosols from these additional factors. We note that heating of the aerosol layer by BC aerosol absorption has been measured directly and independently of meteorological influence (2), and also that the more polluted air masses have their origin over the continent, whereas the less polluted air masses originate over the ocean (29). Based on air mass history, one might expect that more polluted air masses would be warmer and drier than the less polluted air masses.…”

Investigation of Two Optical Methods for Aerosol‐Type Classification Extended to a Northern Indian Ocean Site

“…Here, backward trajectory cluster analysis was performed for the years 2005 until 2015 to create a climatology of the yearly distribution of air mass origin at MCOH, similar to Sheesley et al (). Each trajectory was calculated for an arrival height of 400 m, which is typically inside the tropical marine boundary layer (Höpner et al, ). The trajectories were separated in six different clusters based on if they passed predefined regions within 7 days before arrival at MCOH.…”

“…The instruments at MCOH, available to characterize aerosol chemical and physical properties, are described in detail by, for example, Corrigan et al (), Ramana and Ramanathan (), Engström and Leck (), Höpner et al (), and Budhavant et al (). Most of the instruments are situated inside a temperature and humidity‐controlled building, and the air is sampled 15 m above the local canopy through a stainless steel inlet tube with a laminar flow of 300 L/min.…”

“…Even though the aerosol sampling system performs roughly at ambient humidity and temperature conditions, both the Aethalometer and Nephelometer detect a 2–4 ° C higher inside temperature because of the electronics and light sources, resulting in a lower relative humidity. The humidity sensor inside the Nephelometer typically detects about 10% less than the outside humidity (Corrigan et al, ; Höpner et al, ). This drying might lead to decreasing particle sizes and hence changes in particle scattering and absorption coefficients, but with the benefit that the condition changes are similar in both instruments.…”

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

“…[] 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 −2 . 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 ., ; Höpner et al ., ]. 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 ., ], (2) altitudinal differences in the physical and chemical composition of the aerosols [ Slater and Dibb , ], and (3) variations in the contributions of MBL aerosols to the total atmospheric column [ Franke et al ., ; Smirnov et al ., ; Madhavan et al ., ].…”