Satellite remote sensing of aerosols generated by the Island of Nauru

Abstract

[1] We use imagery from the Multispectral Thermal Imager (MTI) to search for aerosols generated by the Island of Nauru, an island located in the tropical western Pacific Ocean. Nauru frequently displays linear cloud trails for many kilometers downwind of the island, and this study was intended to investigate the presence of aerosols as an aid to understanding those features. The study had three components: (1) a search for specific aerosol plumes, (2) a comparison of downwind and upwind aerosol loading to look for asymmetries, and (3) application of matched filters to increase the visibility of aerosol plumes. The plume search resulted in the finding of three aerosol plumes, two of which are presented as imagery and also radiance profiles. The comparison of downwind and upwind reflectance spectra demonstrated that the radiance is slightly higher downwind of the island, and the residual spectra (downwind minus upwind) have a spectrum consistent with sea salt aerosol. Application of clutter-matched filters to MTI imagery accentuated the upper (near source) sections of an aerosol plume when utilizing a single-scatter albedo signature for coarse-grained sea salt aerosol. We combine our observations with models from the literature to describe a simple mechanism by which Nauru aerosols are created by wave breaking and wind tearing of sea spray and then entrained by island-influenced wind motions.

A pure marine aerosol model, for use in remote sensing applications

“…It is possible that the larger radii observed by AERONET here arise as a characteristic of this wave‐breaking, or the frequent cloud trails mean residual cloud contamination is more likely. Henderson et al [2006] also note that wind at Nauru is predominantly from the east; it is possible that an asymmetric aerosol field could lead to a bias in the AERONET inversion. Because of this strong surf zone source, results at Nauru may be less representative of the open ocean.…”

“… Mulcahy et al [2008] found, for stable wind conditions, a very strong relationship between bin‐average AOD and the approximate square of the wind speed. However, this was based upon measurements at a coastal site, and it is possible that breaking waves on the rocky shore would lead to the production of additional aerosol above what would be observed in open‐ocean (i.e., rock‐free) conditions, or that there are differences in wind conditions between the coast and nearby ocean [ Blanchard and Woodcock , 1980; Henderson et al , 2006]. It is uncertain whether these trends continue for high wind speeds, due to a paucity of data for ws ≈ 10 ms −1 or higher, and the few observations in these conditions have shown either increases, leveling‐off, or decreases in aerosol loading [ Blanchard and Woodcock , 1980; Exton et al , 1985; Mulcahy et al , 2008; Pant et al , 2008; Grandey et al , 2011; Kiliyanpilakkil and Meskhidze , 2011].…”

Seven years of global retrieval of cloud properties using space-borne data of GOME

“…Meas. Tech., 5, 1551-1570, 2012 www.atmos-meas-tech.net/5/1551/2012/ frequently westward downwind cloud trails (Henderson et al, 2006), which are, in turn, linked to aerosol production. It is therefore likely that, on the GOME pixel scale, the assumption of a single-layer cloud is not appropriate.…”

Validation of SeaWiFS and MODIS aerosol products with globally distributed AERONET data

“…This is further compounded by specific challenges facing the atmospheric correction in coastal waters, such as effects due to land adjacency and bottom. Even for isolated sites, local heterogeneity cannot be excluded (see Henderson et al, 2006, for an example at Nauru).…”