Remote sensing of aerosol properties over oceans using the MODIS/EOS spectral radiances

Abstract

Abstract. Spectral radiances measured at the top of the atmosphere in a wide spectral range (0.55-2.13/xm) are used to monitor the aerosol optical thickness and the aerosol size distribution (integrated on the vertical column) of the ambient (undisturbed) aerosol over the oceans. Even for the moderate resolution imaging spectrometer (MODIS) wide spectral range, only three parameters that describe the aerosol loading and size distribution can be retrieved. These three parameters are not always unique. For instance, the spectral radiance of an aerosol with a bilognormal size distribution can be simulated very well with a single lognormal aerosol with an appropriate mean radius and width of distribution. Preassumptions on the general structure of the size distribution are therefore required in the inversion of MODIS data. The retrieval of the aerosol properties is performed using lookup table computations. The volume size distribution in the lookup table is described with two lognormal modes: a single mode to describe the accumulation mode particles (radius <0.5/xm) and a single coarse mode to describe dust and/or salt particles (radius >1.0/xm). Note that two accumulation modes may be present, one dominated by gas phase processes and a second dominated by cloud phase processes. The coarse mode can also be split into several partially overlapping modes describing maritime salt particles and dust. The aerosol parameters we expect to retrieve are r/, the fractional contribution of the accumulation mode to scattering; z, the spectral optical thickness; and rm, the mean particle size of the dominant mode. Additional radiative quantities such as asymmetry parameter and effective radius are derived subsequently. The impact of the surface conditions, wind speed and chlorophyll content on the retrieval is estimated, the impact of potential sources of error like the calibration of the instrument is also tested. The algorithm has been applied successfully to actual data sets provided by the Thematic Mapper on Landsat 5 and by the MODIS airborne simulator on the ER-2 and tested against ground and airborne measurements. A first estimate of the general accuracy is Az = _+0.05 +_ 0.05z (at 550 nm), Ar m = 0.3rm, At/ = _+0.25.

Fire emission heights in the climate system – Part 2: Impact on transport, black carbon concentrations and radiation

“…MODIS AOT values are calculated by retrieval algorithms based on lookup tables for different particles which depend on scattering geometries (Tanre et al, 1997). Error estimations of MODIS AOT retrievals have been investigated by comparison with ground-based AERONET (e.g., Remer et al, 2005;Bréon et al, 2011) and MAN observations (e.g., Adames et al, 2011;Smirnov et al, 2011;Schutgens et al, 2013).…”

Response to “Toward unified satellite climatology of aerosol properties. 3. MODIS versus MISR versus AERONET”

“…MISR and MODIS, both of which fly aboard the NASA Earth Observing System's Terra spacecraft, represent a significant advance over the previous generation of space-based aerosol instruments. Relatively high spatial resolution imaging, calibration accuracy and radiometric stability, along with an increased number of spectral bands for MODIS and the combination of spectral bands and multiple view angles from MISR, have led to more robust aerosol optical depth (AOD) retrievals over both water and land, with less restrictive algorithmic assumptions [1][2][3][4][5]. In addition, the MODIS algorithm derives coarse vs. fine-mode ratio over water, whereas MISR can distinguish about a dozen aerosol air mass types, under favorable retrieval conditions, based on particle size, shape, and single-scattering albedo constraints.…”

Growing up MODIS: Towards a mature aerosol climate data record

“…In high concentrations near the surface, aerosols (or particulate matter, PM) affect visibility, impact air quality, and contribute to poor health. Kaufman et al [1] recognized the importance of aerosols to climate, and along with Tanré et al [2] helped to design new instrumentation and algorithms to retrieve and quantify global aerosol properties. One instrument, known as the Moderate Imaging Resolution Spectro-radiometer (MODIS, [3]) was deployed on two satellites, Terra and Aqua, both part of NASA's Earth Observing System (EOS).…”

“…What we now call the "dark target" aerosol retrieval algorithm was designed as separate algorithms for over ocean [2] and over land [1]. In simple terms, the goal is to separate the relatively brighter "aerosol" signal from the relatively darker "surface" signal, retrieving both the amount (e.g.…”