STUDY OF AFRICAN DUST WITH MULTI-WAVELENGTH RAMAN LIDAR DURING THE “ SHADOW ” CAMPAIGN IN

1 West Africa and the adjacent oceanic regions are very important locations for studying 2 dust properties and their influence on weather and climate. The SHADOW (Study of SaHAran 3 Dust Over West Africa) campaign is performing a multi-scale and multi-laboratory study of 4 aerosol properties and dynamics using a set of in situ and remote sensing instruments at an 5 observation site located at IRD (Institute for Research and Development) Center, Mbour, 6 Senegal (14 0 N, 17 0 W). In this paper, we present the results of lidar measurements performed 7 during the first phase of SHADOW which occurred in March-April, 2015. The multiwavelength 8 Mie-Raman lidar acquired 3β+2α+1 measurements during this period. This set of measurements 9 has permitted particle intensive properties such as extinction and backscattering Ångström 10 exponents (BAE) for 355/532 nm wavelengths corresponding lidar ratios and depolarization ratio 11 at 532 nm to be determined. The mean values of dust lidar ratios during the observation period 12 were about 53 sr at both 532 nm and 355 nm, which agrees with the values observed during the 13 SAMUM 1 and SAMUM 2 campaigns held in Morocco and Cape Verde in 2006, 2008. The 14 mean value of particle depolarization ratio at 532 nm was 30±4.5%, however during strong dust 15 episodes this ratio increased to 35±5%, which is also in agreement with the results of the 16 SAMUM campaigns. The backscattering Ångström exponent during the dust episodes decreased 17 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-109, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 19 February 2016 c © Author(s) 2016. CC-BY 3.0 License.


INTRODUCTION
Africa is one of the major sources of atmospheric dust and numerous campaigns were organized to study the dust properties near the origin source [1,2,3].The SHADOW (Study of SaHAran Dust Over West Africa) campaign is performing a multiscale and multi-laboratory study of aerosol properties and dynamics using a set of in situ and remote sensing instrumentation (multi-wavelength Raman LIDAR, Wind-LIDAR,, nephelometer, aethalometer, sun/lunar photometer, airborne sunphotometer, optical particle counter) in the framework of the CaPPA (Chemical and Physical Processed in The Atmosphere) project (http://www.labex-cappa.fr/).The site is located at IRD (Institue for Research and Development) Center, Mbour, Senegal (14 0 N, 17 0 W).The objective of the experiment is to report the optical, chemical and physical properties of the aerosols as well as the source apportionment in a location where aerosol loading can be very large and aerosol type depends on the season.Two enhanced observing periods of 7 weeks are considered: March-April 2015 when dust originating from the Sahara/Sahel region is dominant and December 2015-January 2016 when dust and carbonaceous aerosols resulting from fire activities are in variable proportion and transported at different altitudes.Other types of aerosols can also be present such as sulfates from nearby urban areas or maritime aerosols depending on the air mass flow.The mixed state of these various chemical components results in different radiative properties of the aerosols.
We hereinafter focus our study on multiwavelength Mie-Raman lidar measurements performed during the first phase of the SHADOW campaign for the period 8 March -24 April.During this period approximately 40 day-and night-time measurement sessions were performed and numerous strong dust episodes were observed.

LIDAR SYSTEM DESCRIPTION
The results presented were obtained with LILAS multi-wavelength Mie-Raman lidar based on a tripled Nd:YAG Spectra Physics INDI laser with a 20 Hz repetition rate.The laser output power at =355 nm is about 2 W. The backscattered light is collected by a 40-cm aperture Newton telescope operated at 47 dg to horizon.The system is capable of detecting three backscattered signals at the laser wavelengths and two Raman signals, thus three backscattering and two extinction coefficients (so called 3β+2α set) can be calculated.The outputs of the detectors are recorded at 7.5 m range resolution using Licel transient recorders that incorporate both analog and photoncounting electronics.The full geometrical overlap of the laser beam and the telescope FOV is achieved at ~1500 m range, which determines the lower limit of the full set of our 3+2measurements due to the difficulty of calculating aerosol extinction in the overlap region.To improve the system operational capabilities, we detected rotational Raman scattering at 530 nm instead traditionally used vibrational nitrogen backscatter at 608 nm.[4].For each profile 4000 laser pulses were accumulated so the temporal resolution of the measurements was about 3 minutes.The measurements were performed both in day and night time.The system has also capability for depolarization measurements at 355 nm and 532 nm wavelengths.

DUST PARTICLE PROPERTIES DERIVED FROM RAMAN LIDAR OBSERVATIONS
The vertical distribution of particle intensive properties is strongly influenced by the origin of the air masses which during the SHADOW measurement period were arriving either from ocean or continental regions.In this section, we present the results of three measurements sessions on 13 and 29 March which were characterized by different types of air masses.As follows from fig. 1, on 13 March the air masses at all heights were transported mainly over the ocean, but the back trajectory for 1500 m presents a "loop" over continent, so the corresponding air masses may contain more dust compared to other heights.Fig. 2 shows the vertical profiles of 3+2 measurements together with lidar ratios LR

March
The backtrajectories from the night of 29-30 March indicating a strong dust case are shown in Fig. 3.The air masses at low altitude were transported over the continent and were strongly loaded with dust.Fig. 4 presents the height profiles of the same particle parameters as in fig. 2 but for 29 March.The extinction coefficient inside the dust layer (below 1500 m) is greater than 0.6 km -1 for both wavelengths.The backscattering coefficient at 355 nm inside the dust layer is lower than  532 which is consistent with the lidar ratio at 355 nm being larger than that at 532 nm with values as large as 65 sr.To estimate the impact of the m I enhancement at 355 nm on the values of EAE and BAE at 355/532 nm wavelengths, numerical simulations were performed.Extinction and backscattering Ångström exponents were calculated using the model of randomly oriented spheroids as described in [5] for a bimodal particle size distribution:

CONCLUSION
The lidar measurements performed in March-April 2015 during the first phase of the SHADOW campaign in Senegal has provided a significant amount of information about dust particle parameters.The use of rotational Raman scattering in the LILAS for 532 nm observations improved the α 532 measurements and, as a result, the calculation of lidar ratio and extinction Angstrom exponent were improved as well.The mean values of lidar ratios of pure dust were about 53±8 sr for both 532 nm and 355 nm wavelengths, which agrees with the values observed during SAMUM 1 (Morocco) and SAMUM 2 (Cape Verde) campaigns [3].The mean value of particle depolarization ratio at 532 nm was 30±4.5%, however during strong dust episodes this ratio increased up to 35±5%, which is also in agreement with the results of SAMUM campaigns.The backscattering Angstrom exponent at 355/532 nm wavelengths during the dust episodes decreased to ~-0.7, while the EAE values, though being negative, were higher than -0.2.Low values of BAE may be a result of enhanced dust absorption at 355 nm.

Fig. 4 .Fig. 5 .
Fig.4.Vertical profiles of (a) backscattering and extinction coefficients and (b) lidar ratios, depolarization ratio, backscattering and extinction Angstrom exponents measured on 29 March 2015 for period 22:00-23:30 UTC.Three size distributions used in computations are shown as insert in fig.5.The ratio N c /N f in all cases was 0.01, and the real part of CRI was 1.55 for all 355 , LR 532 , depolarization ratio  532 , and Angstrom exponents 355/532 are close to zero up to 2750 m, but above that height both EAE and BAE start to increase indicating the presence of smaller particles.The lidar ratios LR 355 and LR 532 are approximately 53±8 sr inside the dust layer.Above 2750 m uncertainties are larger but the mean values of LR do not seem to change above the 2750 m height.