Size resolved dust emission fluxes measured in Niger during 3 dust storms of the AMMA experiment

Abstract

Abstract. During the 2006 and 2007 special observing periods of the African Monsoon Multidisciplinary Analysis campaign an original experimental system has been implemented in Banizoumbou (Niger) for measuring the size-resolved dust emission flux in natural conditions and documenting the possible influence of wind speed on its size distribution. The instrumental set-up, associated methodology, and the quality tests applied to the data set are described before the results acquired during 2 events of the Monsoon type and 1 of the convective type are analyzed in detail. In good agreement with the theory of sandblasting, it is found in all cases that saltation must take place for a vertical emission flux to be detected. During a particular erosion event, the magnitude of the vertical flux is controlled by the surface roughness, which conditions the saltation threshold, and by the wind friction velocity. The dust flux released by the high energy convective event is also found to be much richer in very fine (<2µm) particles than those of the relatively moderate Monsoon event, which shows that aerodynamic conditions definitely influence the initial size distribution of the erosion flux as previously suggested by wind tunnel experiments. However, the size distribution of the dust released by a given event is fairly constant and insensitive to even relatively important variations of u*. This is interpreted as a possible result of the rather long duration (15 min) over which wind fluctuations must be averaged for computing u*, which could make it an inadequate parameter for representing the very short response-time physical processes that are at the origin of fine dust emission at the measurement sites.

Measurements and modeling of surface–atmosphere exchange of microorganisms in Mediterranean grassland

“…The flux-gradient method was used to estimate microbial fluxes from concentrations measured with the Burkard samplers. This methodology has been widely used to measure atmospheric fluxes of different scalars such as hydrogen (Meredith et al, 2014), nitrates and nitrogen compounds (Beine et al, 2003;Griffith and Galle, 2000;Taylor et al, 1999), mercury (Edwards et al, 2005;Fritsche et al, 2008;Lindberg et al, 1995), and particulate matter (Bonifacio et al, 2013;Kjelgaard et al, 2004;Park et al, 2011;Sow et al, 2009). The method follows the Monin-Obukhov similarity theory (Monin and Obukhov, 1954) and therefore assumes that in the atmospheric surface layer the flux of a certain scalar is a function of the gradient of the scalar measured at different heights, the heights themselves (z i ), and a transport velocity that is dependent on atmospheric turbulence and stability (a more detailed description of the methodology is provided in the Supplement).…”

Physico‐chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign

“…Additionally, in summertime the Sahel is a source region of mineral dust emitted by soil erosion during the passage of mesoscale convective systems or by intense monsoon winds (Sow et al, 2009). These additional emissions occur on human-disturbed soils only (Rajot and Valentin, 2001) and therefore may be considered to be anthropogenic in origin.…”

Size Distribution and Optical Properties of Particulate Matter (PM10) and Black Carbon (BC) during Dust Storms and Local Air Pollution Events across a Loess Plateau Site

“…8(d), the peak value of dN/dlogD p for pollution events was up to 1500 # cm -3 at 0.7 µm. During dust events, the peak value was below 200 # cm -3 at around 1.0 µm (Sow et al, 2009). The variation in dN/dlogD p was very small at diameters of 0.5-2.0 µm, as indicated by Sow et al (2009).…”

“…During dust events, the peak value was below 200 # cm -3 at around 1.0 µm (Sow et al, 2009). The variation in dN/dlogD p was very small at diameters of 0.5-2.0 µm, as indicated by Sow et al (2009). Large differences in dN/dlogD p between dust and air pollution events occurred at around 1.0-2.0 µm, and dN/dlogD p was ~10 times higher for fine-mode particles during air pollution events than during dust events.…”