Abstract
Abstract. Realizing the importance of aerosol characterization and addressing its spatio-temporal heterogeneities over Bay of Bengal (BoB), campaign mode observations of aerosol parameters were carried out using simultaneous cruise, aircraft and land-based measurements during the Winter Integrated Campaign for Aerosols gases and Radiation Budget (W_ICARB). Under this, airborne measurements of total and hemispheric backscatter coefficients were made over several regions of coastal India and eastern BoB using a three wavelength integrating nephelometer. The measurements include high resolution multi-level (ML) sorties for altitude profiles and bi-level (BL) sorties for spatial gradients within and above the Marine Atmospheric Boundary Layer (MABL) over BoB. The vertical profiles of the scattering coefficients are investigated in light of the information on the vertical structure of the atmospheric stability, which was derived from the collocated GPS (Global Positioning System) aided radiosonde ascents. In general, the altitude profiles revealed that the scattering coefficient remained steady in the convectively well-mixed regions and dropped off above the MABL. This decrease was quite rapid off the Indian mainland, while it was more gradual in the eastern BoB. Investigation on horizontal gradients revealed that the scattering coefficients over northern BoB are 3 to 4 times higher compared to that of central BoB within and above the MABL. A north-south gradient in scattering coefficients is observed over Port Blair in the eastern BoB, with values decreasing from south to north, which is attributed to the similar gradient in the surface wind speed, which can be replicated in the sea salt abundance. The gradients are parameterized using best-fit analytical functions.
Highlights
Marine aerosols play a key role in a number of climate processes
The ML sorties off VSK and Port Blair (PBR) were made across the longitudes from the base stations into the oceanic atmosphere over Bay of Bengal (BoB)
The scattering AOD, estimated by integrating the ambient scattering coefficient, in the altitude region 0–3000 m was ∼0.22 and 0.14 which accounted for nearly 45 % and 60 % to the columnar AOD off the coastal base Visakhapatnam (VSK, north-western BoB) and off the island base PBR respectively, suggesting most of the aerosol was confined to lower troposphere, over PBR, during winter, in contrast to that of pre-monsoon season (Satheesh et al, 2006)
Summary
Marine aerosols play a key role in a number of climate processes. In addition to the direct and indirect effects (Haywood and Boucher, 2000) on the in-coming solar radiation, marine aerosols are known to significantly affect the chemistry of atmosphere through heterogeneous and multiphase processes (von Glasow et al, 2002). The recent, exhaustive field campaign, Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) (Moorthy et al, 2008) was the first major experiment that examined the entire BoB using cruise based, as well as island based measurements supplemented with airborne measurements from the east and west coasts of India (Moorthy et al, 2008; Babu et al, 2008; Nair et al, 2008a; Ramachandran and Rajesh, 2008; Satheesh et al, 2008, 2009) These measurements provided the first experimental observation of the prevalence of elevated aerosols over the Indian landmass and coastal waters during premonsoon season, occurring in the altitude region of 2 to 4 km within which the concentration and extinction were larger than those observed close to the surface and within the MABL (Babu et al, 2008; Satheesh et al, 2009). Extensive airborne measurements were made from the island station Port Blair (PBR, 11.63◦ N; 92.71◦ E) in the BoB, in addition to the measurements from the coastal stations Visakhapatnam (VSK, 17.7◦ N; 83.3◦ E) and Chennai (CHN, 13◦ N; 80◦ E) along the east coast of India, enabling
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