64 SCIENCE HIGHLIGHTS: Dust

Dust deposition rates derived from optical satellite observations Luca Lelli, W. von Hoyningen-Huene, M. Vountas, M. Jäger and J.P. Burrows

Satellite retrievals of aerosol optical thickness over the Atlantic Ocean have been used to estimate the rates of dust deposition. The results reveal higher dust deposition in summer than in winter and give insights into the location of desertic dust source areas. Desert dust (or dust aerosol) outflows off production, and higher particle flux can dust transport covers the entire Atlantic the West African coast impact offshore increase export fluxes of biomass from the Ocean and is therefore an invaluable com- biogeochemical processes and can be surface waters to the sea floor and hence plement to in-situ measurements. regarded as natural hazards to human increase accumulation rates in the under- activities and ecosystems. For example, lying sediments. Standard remote sensing techniques of dust storms affect aviation operations dust are based on the physical principle of reducing flight visibility, causing engine Dust is collected on land only at few extraterrestial sunlight travelling through mechanical damages and flight path observation sites, like at Cape Blanc the Earth’s atmosphere and being attenu- reassessments. On the other hand, dust (Mauritania) or Barbados (Fischer ated by suspended dust particles. Nadir- input adds nutrients to the surface of and Karakas 2009; Prospero 1999). looking spectral satellite radiometers (e.g. the seas which stimulate phytoplankton Conversely, remote satellite monitoring of SeaWiFS, MERIS, MISR, MODIS) measure

Figure 1: Monthly averages of Aerosol Optical Thickness (AOT; dimensionless) retrieved from MISR satellite data for the determination of dust sedimentation rate. Maps of AOT for January and July 2001 on a global scale (A) and zoomed in to the example region around Cape Blanc (B), where sediment traps are deployed across West Africa and the Atlantic Ocean. The time series in (C) portrays the AOT signal extracted for the adjacent boxes M1 and M2. The oscillations of both curves indicate that dust activity is more frequent in summer than in winter. Dust deposition is reflected by the difference between the red and black curves during the respective months.

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Figure 2: Time series of AOT changes between boxes M1 and M2 (see Fig.1), derived from satellite retrievals from SeaWiFS (black curve) and MISR (blue curve) instruments. The curves can be considered as approximations for the rate of dust deposition at the site of Cape Blanc. the amount of sunlight reflected back mass load. The algorithm for this is based changes, derived from SeaWiFS and MISR to space and allow us to determine the on assumptions on the dust grain size and measurements, exhibit differences that degree of sunlight attenuation. The latter dust particle density in the air. can be attributed to distinct instrumental is described by Aerosol Optical Thickness characteristics. (AOT), which is defined as the extinction Mass loss (or sedimentation rate) which coefficient of light integrated over a verti- can be directly compared to ground These preliminary results are a first step cal column through the atmosphere. AOT measurements can be derived from lateral towards a systematic application of this can be calculated from data of reflected dust transport in two ways: (i) using a method. However, the approach needs light with algorithms like the Bremen transport model with a sedimentation significant improvements in terms of the AErosol Retrieval (BAER; von Hoyningen- module or (ii) estimating the mass loss consideration of dust transport trajec- Huene et al. 2003, 2011) and monitored from the change in the aerosol columnar tories, the injection height of dust, its both in space and time. However, such mass along the trajectory. Here, the latter mixing with surrounding air and optical algorithms are challenged not only by approach is followed. parameters used for determining the local missing a-priori information on the spe- dust type. cific optical properties of dust (Dinter et The sedimentation rate is inferred from al. 2006), but also by local reflectivity of the mass difference along the main AFFILIATIONS the ground. Radiant surfaces (such as the aerosol transport path. If one takes the Institute of Environmental Physics and Remote Sensing bright Sahara) may reduce the contrast temporally averaged AOT of two adja- (IUP), University of Bremen, Germany between suspended dust and the ground cent regional boxes (see boxes M1 and CONTACT beneath, hence degrading the overall M2 in Fig. 1) along the main trajectory, a Luca Lelli: [email protected] algorithmic performance and accuracy. sedimentation rate can be estimated from the difference between the red and black REFERENCES Figure 1 shows seasonal variations in sat- AOT curves (Fig. 1C). Our new method Fischer G, Karakas G (2009) Biogeosciences 6: 85-102 ellite-derived AOT in dust-laden regions allowed us to produce estimates of dust Prospero JM (1999) J Geophys Res 13: 15917-15927 worldwide. Off West Africa, for example, deposition from monthly averages of AOT Von Hoyningen-Huene W et al. (2003) J Geophys Res 108, AOT is about three times as high in sum- on a regional grid. doi: 10.1029/2001JD002018 mer than in winter. Von Hoyningen-Huene W et al. (2011) Atmos Meas Tech Testing the method 4: 151-171 From optical thickness Preliminary qualitative comparisons have to dust deposition been made for Cape Blanc on the coast Dinter T et al. (2009) Tellus 61: 229-238 Comparisons of satellite-derived AOT and of Mauretania, using deposition data by data from sediment traps over West Africa Fischer and Karakas (2009). First, AOT and the Northern Atlantic do not show a values were calculated with the BAER direct relationship between atmospheric algorithm using SeaWiFS data. Then, dust loading and dust accumulation. two grid boxes over the site and off- However, the spatio-temporal change of shore Cape Blanc were selected. The two dust AOT can be used as an indicator for neighboring boxes are located between the fallout of aerosol along the transport 20°N and 22°N (see M1 and M2, Fig. 1). path. The method described in the fol- We assume that both boxes are located lowing, has been developed to determine along the main westward oriented dust dust deposition from satellite data. transport path. Even though this assump- tion is only of limited validity and the time The deposition rate of aerosol particles series of the difference between AOT in is reflected by the decrease of AOT along both boxes shows some negative values the transport path rather than AOT itself. (Fig. 2), the analysis of the time series Driven by this idea, a method has been reveals a potentially promising correlation developed to determine the relative (r > 0.6) between the AOT derived from change of AOT along an estimated trans- remote sensing and in-situ sedimenta- port path. This requires that the aerosol tion data from Cape Blanc (unpublished, loading is determined from AOT for the not shown). Figure 2 also shows that the region of interest, i.e. the AOT needs to method is sensitive to the satellite radiom- be transferred quantitatively into aerosol eters used. The pictured values of AOT

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