Saharan dust absorption and refractive index from aircraft-based observations during SAMUM 2006

Abstract

During the Saharan Mineral Dust Experiment (SAMUM) conducted in summer 2006 in southeast Morocco, the complex refractive index of desert dust was determined from airborne measurements of particle size distributions and aerosol absorption coefficients at three different wavelengths in the blue (467 nm), green (530 nm) and red (660 nm) spectral regions. The vertical structure of the dust layers was analysed by an airborne high spectral resolution lidar (HSRL). The origin of the investigated dust layers was estimated from trajectory analyses, combined with Meteosat 2nd Generation (MSG) scenes and wind field data analyses. The real part n of the dust refractive index was found almost constant with values between 1.55 and 1.56, independent of the wavelength. The values of the imaginary part k varied between the blue and red spectral regions by a factor of three to ten depending on the dust source region. Absolute values of k ranged from 3.1 × 10−3 to 5.2 × 10−3 at 450 nm and from 0.3 × 10−3 to 2.5 × 10−3 at 700 nm. Groupings of k values could be attributed to different source regions.

Optical properties of Saharan dust aerosol and contribution from the coarse mode as measured during the Fennec 2011 aircraft campaign

“…The range of values shown in Fig. 9 are very similar to those estimated in other recent studies varying between 0.0001 to 0.0046 (McConnell et al, 2010;Petzold et al, 2009;Petzold et al, 2011;Formenti et al, 2011a), supporting the view that the imaginary part of the refractive index is relatively small at wavelengths of 550 nm.…”

“…due to the Mie scattering curve, it is possible for one value of scattering to correspond to more than one particle diameter). We present results assuming values of 1.53 for n, and 0.001 and 0.003 for k. Size distributions were also corrected using a range of other refractive indices (n = 1.42, 1.58; k = 0.006) reflective of those in the literature (Wagner, 2012;Otto et al, 2009;McConnell et al, 2010;Johnson and Osborne, 2011;Petzold et al, 2009;Kim et al, 2011). However, results presented in Sect.…”

“…An iterative process is used to derive the imaginary part of the refractive index at 550 nm, k 550 , using the technique described by McConnell et al (2010), and similar to that used by Petzold et al (2009). Figure 4 depicts the iteration process used here to derive k 550 .…”

Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign

“…In contrast, reported values for imaginary index, n i , vary by as much as an order of magnitude (Schladitz et al, 2009). For example, DABEX observations yielded a Saharan dust n i of 0.0004i , whereas values of approximately 0.004i were derived from SAMUM experiments Petzold et al, 2009;Schladitz et al, 2009). The variation of the n i values are attributed to its dependence on the dust iron oxide content, which naturally is a function of source region.…”

Vertical and horizontal variation of aerosol number size distribution in the boreal environment

“…The system provides aerosol optical depth, volume backscatter coefficient, cross section, and depolarization profiles with high temporal and vertical resolution (Eloranta 2005). These quantities can be used for example in identifying the dominant aerosol type for different layers using a combination of lidar ratio and depolarization (Li et al 2009, Petzold et al 2009). 15…”