Measurements of aerosol properties from aircraft, satellite and ground‐based remote sensing: a case‐study from the Dust and Biomass‐burning Experiment (DABEX)

Abstract

This paper presents aircraft measurements of aerosol optical properties and radiative effects from the Dust and Biomass-burning Experiment (DABEX) over West Africa. On 19 January 2006 cloud-free skies and high aerosol loading provided ideal conditions for an intercomparison of aircraft, satellite and ground-based remote sensing instruments. Aerosol size distributions, optical properties, aerosol optical depth (AOD) and downwelling solar radiation were measured by the UK FAAM aircraft in the region of Niamey, Niger. The aircraft in situ measurements showed a mixture of dust and biomass-burning aerosols and indicated an AOD of 0.79 (at 550 nm) that compared well against AODs from the Banizoumbou Aerosol Robotic Network (AERONET) site (0.74) and a Microtops sunphotometer (0.72). AERONET size distributions showed a good degree of similarity with the aircraft in situ measurements. AERONET single-scattering albedos were also in fairly close agreement with the aircraft, having values of 0.85 and 0.87, respectively (at 550 nm). Measurements of downwelling solar radiation from the aircraft compared well with measurements from the Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) at Niamey. Radiative transfer modelling suggested a 130-160 W m −2 instantaneous reduction of downwelling solar radiation by the aerosol column (15-18% of the total flux). Measurements of downwelling solar radiation compared reasonably well against radiative transfer modelling based on the aircraft in situ data. Satellite retrievals of AOD from MISR and MODIS Deep Blue were within 0.05 of the ground-based sunphotometers measurements although there were discrepancies in optical properties retrieved by MISR, as compared to AERONET and the aircraft.

Motivation, rationale and key results from the GERBILS Saharan dust measurement campaign

“…Milton et al (2008) show that around 40% of this bias is due to dust in January, but greater than 90% of this bias is due to dust by mid March. Johnson et al (2009) used aircraft instrumentation to measure instantaneous reductions in near-surface SW radiation owing to biomass burning and mineral dust aerosols of around 150 W m −2 in the vicinity of Niamey, Niger.…”

On realistic size equivalence and shape of spheroidal Saharan mineral dust particles applied in solar and thermal radiative transfer calculations

“…While Schmid et al (2000) performed cut-off corrections, McConnell et al (2010) refer the optical properties, based on size-limited particle information, to the observed accumulation mode of the entire dust ensemble. The same should be done, e.g., for imaginary parts of the complex refractive index or single scattering albedo in the visible spectral range derived from limited particle data (Haywood et al, 2003;Osborne et al, 2008;Johnson et al, 2008b;Raut and Chazette, 2008;Johnson et al, 2009;McFarlane et al, 2009), which then may not be representative for the entire dust population. For demonstration how these particle losses affect the dust optical properties and radiative effects we perform integrations of size distributions stepwise up to various maximum particle diameters D e,max (Sects.…”

Facile Method for Determining the Aspect Ratios of Mineral Dust Aerosol by Electron Microscopy

“…Current uncertainties in the radiative impacts of mineral dust aerosol include size distributions, particle shape, and composition-resolved emission concentrations (Johnson et al 2009(Johnson et al , 2012Kok 2011). Both satellite and ground-based lidar measurements use algorithms that include particle shape to accurately derive aerosol mass loadings.…”