Ship‐based aerosol optical depth measurements in the Atlantic Ocean: Comparison with satellite retrievals and GOCART model

Abstract

Aerosol optical depth measurements were made in October–December 2004 onboard the R/V Akademik Sergey Vavilov. The cruise area included an Atlantic transect from North Sea to Cape Town and then a crossing in the South Atlantic to Argentina. In the open oceanic areas not influenced by continental sources aerosol optical depth values were close to background oceanic conditions (τa ∼ 0.06–0.08). Spectral dependence, especially in the high latitude Southern Atlantic, can be considered as quasi‐neutral (Angstrom parameter α was less than 0.4). Back‐trajectory analysis allowed statistical division of the aerosol optical parameters and showed similar properties for the North Atlantic polar marine, South Atlantic subtropical marine and South Atlantic polar marine air. Ship‐borne aerosol optical depth comparisons to GOCART model and satellite retrievals revealed systematic biases. Satellite retrieved optical depths are generally higher by 0.02–0.07 (depending on the sensor), especially in low τa conditions. GOCART model simulated optical depths correlate well with the ship measurements and, despite overall bias and a notable disparity with the observations in a number of cases, about 30% agree within ±0.01.

Flow climatology for physicochemical properties of dichotomous aerosol over the western North Atlantic Ocean at Bermuda

“…9b). AOT values are systematically lower than AOD by about 0.02 to 0.06, consistent with previously reported comparisons (Smirnov et al, 2006). These differences result from several factors including the fact that satellite AOD measures backscattered radiance whereas AOT is primarily measuring forward scattered radiance.…”

The optical and physical properties of atmospheric aerosols over the Indian Antarctic stations during southern hemispheric summer of the International Polar Year 2007–2008

“…An examination of the surface wind over the oceanic regions adjacent to the island stations (Amsterdam and Crozet) by using QuickSCAT wind data showed variable winds varying from a minimum of 3 m s −1 to maximum of 16 m s −1 with an average of ≥10 m s −1 . Similar results depicting the surface wind speed dependence on spectral AOD has been reported over same latitude region by Smirnov et al (2006) and Vinoj et al (2007). The dominance of fine and accumulation particles at Maitri might be due to 1. the enhanced station activities there, 2. pre existing accumulation mode dominant background aerosols in the coastal Antarctic atmosphere as suggested by Herber et al (1993) and Hara et al (2005); and 3. those of biogenic species (like methane sulphonate) and non sea salt sulphate (products of dimethyl sulphide) which have been reported during Antarctic summer (Weller et al, 2008).…”

Ship-borne aerosol profiling with lidar over the Atlantic Ocean: from pure marine conditions to complex dust–smoke mixtures

“…Mean AOT at 500 nm of 0.09 ± 0.01 on 29 November (0.09 ± 0.02 on 30 November) and an Ångström exponent of 0.08 ± 0.02 (0.23 ± 0.08) clearly indicate marine conditions for remote oceanic areas, not influenced by continental aerosol sources. In these regions, the AOT at 500 nm is typically below 0.1 and the Ångström exponent less than 0.4 (Smirnov et al, 2006).…”