An Outstanding Saharan Dust event at Mt. Cimone (2165 m asl) in March 2004

Erika Bratticha,b, Laura Tosittib, Angelo Riccioc

a Dept. of Biological, Geological and Environmental Sciences, Alma Mater Studiorum University of Bologna, Piazza di Porta San Donato 1 , 40126 Bologna (Italy)

erika.brattich@unibo.it b Dept. of Chemistry “Giacomo Ciamician”, Environmental Chemistry and Radioactivity Lab , Alma Mater Studiorum University of Bologna, Via Selmi 2, 40126 Bologna (Italy)

c Dept. of Applied Sciences, University of Napoli “Parthenope”, Centro Direzionale – Isola C4, 80143 Napoli (Italy)

INTRODUCTION The WMO-GAW station “O. Vittori” is a research structure managed by the Italian Institute of Atmospheric Sciences and Climate (ISAC) of the National Research Council (CNR) and is located at the mountain top of Mt. Cimone (44.18°N, 10.7°E, 2165 m asl), the most elevated peak of the Northern Apennines. The importance of environmental radionuclides in the study of atmosphere and climate dynamics has been often emphasized in the course of the last decades. The radiotracer method provides a powerful tool for the basic characterization of transfer and transformation mechanisms occurring both at local and large scale. For this reason several radionuclides, namely 7Be, 210Pb, 222Rn and others are included among the key atmospheric components that are routinely monitored within the WMO-GAW network.

Natural radionuclides 7Be and 210Pb have been continuously measured by the Chemistry Department of the University of Bologna through γ-spectrometry analysis on the PM10 fraction sampled at Mt. Cimone since 1998. The climatological record of sampled data comprehends also classical meteorological parameters (such as atmospheric pressure, temperature, relative humidity, tropopause height, wind speed and wind direction), particle number densities, and chemical composition data, such as O3, CO2, CO, and black carbon. 7Be and 210Pb, having completely different sources (7Be is a cosmogenic radionuclide originating from spallation reactions, whereas 210Pb derives from the 222Rn gas emanating from soil and deriving from 238U decaying and has therefore a crustal origin), can be used as atmospheric tracers of vertical and horizontal transport processes. The data series acquired at Mt. Cimone are being analyzed in the PhD thesis of Brattich by means of multivariate statistical methods and receptor models in order to acquire a deep understanding of the impact of vertical and horizontal transport processes on atmospheric composition, and in particular on particulate matter and ozone, two of the main secondary atmospheric pollutants of greatest environmental and chemical relevance.

Figure 1. Photo and map of Mt. Cimone

(map courtesy of Planiglobe, kk&w -

digital cartography)

MT. CIMONE WMO-GAW STATION, Recently upgraded to «GLOBAL STATION» ranking

ITALIAN AIR FORCE

Meteorology (since 1941)

CO2 (since 1979), total ozone,

UV radiation

ISAC-CNR

Superficial ozone, CO, NOx,

physical “aerosol”, bioaerosol, radiation

UNIVERSITY OF BOLOGNA

Department of Chemistry

Particulate matter (PM10),

7Be, 210Pb, radon (since 1998)

UNIVERSITY OF URBINO

CFCs, HCFCs (since 1999)

METHODS 7Be, 210Pb and aerosol mass load in the form of PM10 are measured at Mt. Cimone station since the early 90’s but measurements became regular only since 1998. Aerosol sampling is carried out with a time resolution of about 48 hours by means of a Thermo – Environmental PM10 high-volume sampler and using a rectangular glass fiber filter. 7Be and 210Pb are determined by nondestructive high-resolution γ-spectrometry of aerosol samples.

MEASUREMENT SITE Mt. Cimone (44°12’ N, 10°42’ E) is the highest peak of the Northern Apennines (2165 m asl): • 360° free horizon (i.e. no orographic obstacles) • far enough from cities and industrialized areas • above the planetary boundary layer during most

of the year

• representative of the SOUTH-EUROPEAN FREE TROPOSPHERE • suitable location to investigate the influence of regional and

long-range transport of polluted air masses on the background free troposphere

• evaluation and study of the impact of mineral dust transport on the properties of atmospheric aerosol

• most representative WMO-GAW station in Italy (since October 2010 global station)

RESULTS - Synoptics This event originated from the Bodele depression in northern Chad, a remarkable source of dust (Koren et al., 2006), and the analysis of aerosol optical depth (Figure 6) revealed that dustiness conditions occurred along the entire ITCZ. Figure 6 shows the aerosol optical depth at the beginning of the second dust outbreak (on 13th March, 2004) and the average over the period 10-15 March, 2004. On 5th March 2004 images from the visible channel of the SeaWIFS satellite (Figure 7) show a huge, dense, meridionally oriented dust plume off the northwest African coast from west of Madeira to Cape Verde, sustained by hazy and prolonged Harmattan conditions. This plume spread laterally, moved westward and formed an arc more than 5000 km long from Guinea to the northern tip of Morocco. The plume traversed the Atlantic Ocean and impacted onto the Caribbean region (Knippertz & Fink, 2006).

a

RESULTS - Time series Dust outbreaks are very common throughout the year, with a peak frequency in spring (March, April, May) towards the Atlantic Ocean, or in late spring/summer (May, June, July) towards the Mediterranean Sea (even if winter and especially autumn events, though less frequent, are usually very intense) (Prospero et al., 2002; Barkan et al., 2005). Every year strong winds blowing over the Sahara desert lift hundreds of millions of tons of dust high into the sky over North Africa.

In 2004, from 13 to 15 March, as reported by Beine et al. (2005), a severe PM episode was observed at Mt. Cimone. Figure 4, showing the 2004 time series of PM10, 210Pb, number densities of fine and coarse particles, highlights a clear increase of all these parameters during the Saharan Dust episode. A contemporary decrease of ozone was also observed. O3 decreases are linked to a double effect: reduced sources of pollution in Northern Africa together with O3-destroying reactions on the surface of mineral particles probably through catalytic mechanisms due to their chemical-physical structure (Usher et al., 2003; Bonasoni et al., 2004). In particular, PM concentration exceeded 80 μg/SCM, a value seven times higher than the mean level during the preceding and subsequent days, and the maximum PM10 concentration recorded at Mt. Cimone in more than 12 years observations. This episode has been ascribed to a long lasting Saharan dust outbreak, starting at the beginning of March, and first impacting the Atlantic Ocean and then the Mediterranean area.

b

Figure 6. Aerosol optical depth at 0.55 μm, daily average on 13th March 2004 (left), and time

averaged over the period 10-15 March 2004 (right). Every image is the average over data

from the MODIS Terra and Aqua satellites (MOD08_D3.051 and MYD08_D3.051 collections).

Deep Blue retrievals are included into the average.

Figure 3. Back-trajectories

(96-hours backward) calculated

by Hysplit-4 model (http://ready.arl.noaa.gov/HYSPLIT.php)

ending at Mt. Cimone on 15th

March 2004, 12 UTC and for

three arrival heights: 1400, 2200

and 3000 m asl.

d)

b) a)

e) d)

RELEVANT INCREASE OF PM10

80 μg/m3

+540% with respect to monthly mean

INCREASE OF 210Pb

+73% with respect to monthly mean

INCREASE OF NUMBER DENSITY OF

FINE PARTICLES

+54% with respect to monthly mean

RELEVANT INCREASE OF NUMBER

DENSITY OF COARSE PARTICLES

+360% with respect to monthly mean

Figure 4. Time series of: a) PM10 (μg/m3), b) 210Pb (mBq/m3), c) O3 (ppb), d) number density of fine particles (N/cm3) and e) number density of coarse particles (N/cm3) acquired at Mt. Cimone in 2004.

PM10

sampling

since 1998

7Be and 210Pb

from γ-

spectrometry

Very long time series of

particulate matter and

atmospheric radiotracers

Figure 7. The SEAWIFS image for 15th March 2004 shows a major dust outbreak from Western Africa

across the Atlantic. The massive storm formed a huge arc of thick dust reaching Cape Verde Islands

and the shores of Western Europe; during the following days, the dust plume continued to spread

southwards and westwards.

Figure 8. Geopotential height at 700 mbar for 14th March 2004,

12 UTC. Data from NCEP/DOE AMIP-II Reanalysis project.

Figure 5. Dust loading

resulting from the

dust regional model

DREAM for the day

15th March 2004,

00 UTC

(http://www.bsc.es/projects

/earthscience/DREAM/)

References Barkan J., Alpert P., Kutiel H., Kishcha P., 2005. Synoptics of dust transportation days from Africa toward Italy and central Europe. Journal of Geophysical Research 110, 7208–7221, D07208, doi:10.1029/2004JD005222. Beine H.J., Amoroso A., Esposito G., Sparapani R., Ianniello A., Georgiadis T., Nardino M., Bonasoni P., Cristofanelli P., Dominé F., 2005. Deposition of atmospheric nitrous acid on alkaline snow surfaces. Geophysical Research Letters 32, L10808,

doi:10.1029/2005GL022589. Bonasoni P., Cristofanelli P., Calzolari F., Bonafé U., Evangelisti F., Stohl A., Zauli Sajani S., van Dingenen R., Colombo T., Balkanski Y., 2004. Aerosol-ozone correlations during dust transport episodes. Atmospheric Chemistry and Physics 4, 1201–1215. Knippertz P., and Fink A.H., 2006. Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak. Quarterly Journal of the Royal Meteorological Society 132, 1153–1177. Koren I., Kaufman Y.J., Washington R., Todd M.C., Rudich Y., Martins J.V., Rosenfeld D., 2006. The Bodele depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest. Environmental Research Letters 1(1),

doi:10.1088/1748-9326/1/1/014005 Prospero J.M., Ginoux P., Torres O., Nicholson S.E., and Gill T.E., 2002. Environmental characterization of global sources of atmospheric soil dust identified with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product.

Review of Geophysics 40, 2-1:2-31. Tositti L., Brattich E., Cinelli G., Previti A., Mostacci D., 2012. Comparison of radioactivity data measured in PM10 aerosol samples at two elevated stations in northern Italy during the Fukushima event. Journal of Environmental Radioactivity 114, 105-112. Tositti L., Riccio A., Sandrini S., Brattich E., Baldacci D., Parmeggiani S., Cristofanelli P., Bonasoni P., 2013. Short-term climatology of PM10 at a high altitude background station in southern Europe. Atmospheric Environment 65, 145-152. Usher C.R., Michel A.E., Grassian V.H., 2003. Reactions on mineral dust. Chemical Reviews 103, 4883-4939.

Figure 2.

a) High volume PM10 sampler at Mt. Cimone

b) Hyper-Pure Germanium detectors at the Environmental Chemistry and Radioactivity

Laboratory (to the left, planar type detector, to the right coassial p-type detector)

c) DECREASE OF O3

-9% with respect to monthly mean

Acknowledgements ISAC-CNR is gratefully acknowledged for providing aerosol size distribution and ozone data and infrastructural access at the WMO-GAW Global Station Italian Climate Observatory "O. Vittori" at Mt. Cimone. The Italian Climate Observatory "O. Vittori" is supported by MIUR and DTA-CNR throughout the Project of National Interest NextData.

Mt. Cimone along the main axis of the dust plume PM10 RECORD CONCENTRATION = 80 μg/m3