Five years of Synoptic-scale dust transport events in the southern Himalayas

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1 Five years of Synoptic-scale dust transport events in the southern Himalayas R. Duchi a P. Cristofanelli a, A. Marinoni a, L. Bourcier a, D.Putero a, F. Calzolari a, T.C. Landi a and P. Bonasoni a [a] CNR- ISAC, Bologna, Italy Rocco Duchi, R.Duchi@isac.cnr.it CNR Institute for Atmospheric Sciences and Climate, 40129, Bologna, Italy 1 7th International Workshop on Sand/Duststorms and Associated Dustfall, FRASCATI, 2 nd 4 th December 2013

2 Introduction The effective radiative forcing due to aerosolradiation Few in-situ, interactions continuous that dust takes observations rapid adjustments at high elevations into account in the Himalayas. (ERFari) is assessed to be 0.45 ( 0.95 to +0.05) W m 2. Mineral dust contribute to the ERFari for 0.1 ( In addition 0.3 to +0.1) to the W m effect 2,both on of radiation, anthropogenic aerosol and at high altitudes natural can origin. significantly change the snow and ice melting rate. 2 7th International Workshop on Sand/Duststorms and Associated Dustfall, FRASCATI, 2 nd 4 th December 2013

3 Khumbu Valley (Nepal), Measurement site at 5079 Nepal m a.s.l. Climate Observatory Pyramid (NCO-P) not far from the Italian Pyramid Observatory One of the 29 Global Stations of the GAW-WMO Programme Monsoon season: Relative Humidity never below 70% Southerly winds even during night time Winter season: Relative Humidity never above 70% Northerly winds during night time NCO-P ISAC-CNR Lat. N 27 57' Lon. E 86 48' Taken from Altitude: 5079 m asl from (adapted

4 Methodology The number concentration and size distribution of particles with diameters between 0.25 and 10 mm in 23 size bins were measured by an optical particle counter (OPC 190 GRIMM). A Multi-Angle Absorption Photometer (MAAP 5012), measures the transmission and the backscattering of a light beam (Petzold and Schönlinner, 2004) incident on a fibre filter where aerosol particles are deposited by sampling flow, providing particle light absorption coefficient σ abs at 670 nm. Aerosol total and back scattering coefficients at three wavelengths (450, 550 and 700 nm) are derived by an integrating nephelometer (model TSI 3563). The data provided by the MAAP and the nephelometer were used to estimate the SSA at 670 nm. The synoptic-scale origin and path of air masses reaching NCO-P were investigated thanks to the 5-day back-trajectory ensembles calculated with the Lagrangian Analysis Tool LAGRANTO (Wernli and Davies, 1997), basing on the 6-hourly operational analyses produced by the ECMWF.

5 Identification of Dust Application of the Kolmogrov-Zurbenko filter to the OPC coarse fraction particle number concentration (N 1-10 ) dataset over 5-y of measurements (March February 2011). Identification of 275 Dust Transport Days (DTDs), 22.2 % of the entire data set.

6 Variation of aerosol properties Increase Increase of particle in the the number mm 10 mm size Highest increase concentration range of N 1-10 and PM 1-10 concentrations in greater every size on range average during DTDs for with the respect Premonsoon and Post- Monsoon to background conditions and monsoon Winter season. season.

7 Mineral dust sources During In general Monsoon Indian there is also a not negligible Source domain identification: (especially 5 days LAGRANTO Back TRJ passing contribution the Thar over desert) a of domain IPA, North is Africa and is below mean seasonal associated height the predominant with the high N of aerosol predominant injection 1- (Liu et al., 2008). 10 source e PMthorught 1-10 concentrations during the at Winter NCO-P year. IN is the predominant source during all season except Winter

8 Conclusions Synoptic scale transport of mineral dust affects NCO-P for more than 20% of the time (275 days during 5 years). Large variation of aerosol properties: N 1-10 increases more than 6 times, while PM 1-10 is a characterized by a tenfold increase. The higher SSA observed at NCO-P during DTDs indicates the influence of synoptic-scale transport of mineral dust on aerosol optical properties over South Himalayas. On a yearly basis Thar desert is the main source of mineral aerosol. Seasonal differences on contribution from other sources (IPA during Monsoon, NA during Winter). 8 7th International Workshop on Sand/Duststorms and Associated Dustfall, FRASCATI, 2 nd 4 th December 2013

9 Acknowledgements This work, carried out in the framework of the UNEP ABC (Atmospheric Brown Clouds), has been funded by the Ev-K2-CNR SHARE (Stations at High Altitude for Research on the Environment) and MIUR/CNR-DTA NextData projects, in collaboration with the NAST - Nepal Academy of Science and Technology. LAGRANTO back-trajectories were provided by Michael Sprenger (ETHZ). The MODIS analyses used in this work were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. The authors also acknowledge the CALIPSO, MODIS and OMI mission scientists and associated NASA and KNMI personnel for the data production used in this research. This work has been currently submitted to the Aeolian Research journal with the title Synoptic-scale dust transport events in the southern Himalayas. 9 7th International Workshop on Sand/Duststorms and Associated Dustfall, FRASCATI, 2 nd 4 th December 2013