Vertical Profiles of Airborne PM in Po Valley During Wheat Harvest Activities

C. Telloli; F. Coren; E. Marrocchino; C. Vaccaro

Abstract

Po Valley and Friuli Plain in Italy and Belgian Plain in Europe， are areas with the highest concentration of solid particulate matter in all the world （European Space Agency， 2004）. This implies that those areas does not respect the limits imposed by European Parliament in 2008.1 Aim of this study is the characterization of the particulate matter， through direct sampling in atmosphere to define physical properties and source of this particulate. A first campaign has been carried out in June-July 2009 in the Po Valley during farming activities of threshing， by means of a small aircraft （Cessna172P）， that has been used as platform for collecting measure particles. Particle concentration has been measured for five aerodynamic equivalent diameters （0.5， 1.0， 2.5， 5.0， 10.0μm） using a laser counter （LIGHTHOUSE HH3016）. The acquisition has been carried out vertically profiling the atmosphere from 150 to 2400m. SEM， as well as SEM-EDS analysis on single particles， have been carried out with the aim to obtain detailed dimensional and morphological information to define origin， toxicity and the nature of organic matter （Germani & Buseck， 1991； Grassi， Narducci， & Tognotti， 2004）.

INTRODUCTION

In the total PM concentration the natural sources contribute up to 94%， leaving the human factor less than 10% （Chabas & Lefèvr， 2000； Brewer & Belzer， 2001； Querol et al.， 2002； Ryall et al.， 2002； Bernabé& Carretero， 2003； Bogo et al.， 2003； Almeida， Pio， Freitas， Reis， & Trancoso， 2005； Salvador， Artí？ano， Querol， Alastuey， & Costoya， 2007）. Scientific studies have shown a positive correlation between increased natural particles with increase in anthropogenic particles， but there is no information on the causes of this correlation. Most data relate to exposure in urban and industrial areas， while contributions related to agricultural practices are rarely investigated. The relationship between natural and anthropogenic factors is strongly site depending （Scientific Committee.， 2005； Scientific Committee.， 2006）. The rise of human and industrial activities， especially in big cities， has led to the increased production of aerosols released to the atmosphere. In the lower troposphere， they cause visibility degradation， especially when these aerosols are located in the boundary layer， this is commonly experienced during the dust events （Husar et al.， 2001； Reis et al.， 2002； Vanderstraeten et al.， 2008）. In addition， highly concentrated aerosols， also known as suspended particulate matters （SPM）， give rise to health problems. Epidemiological and environmental studies （Pope 3rd et al.， 1995； Schwartz， Dockery， & Neas， 1996） have shown that exposure to fine particles， especially particulate matter smaller than 2.5 μm （PM2.5）， can result in serious respiratory disorders as cardiovascular and respiratory diseases （Abbey， Ostro， & Peterson， 1995； Pope 3rd， 2000； Pope 3rd et al.， 2002； Laden， Schwartz， Speizer， & Dockery， 2006； Hopke， 2008； Kelly， 2008）， but also to the reduction of light （Vasconcelos， Macias， & White， 1994） to the change of biodiversity and the deterioration of monuments （Sabbioni， 1995； Torfs & Van Grieken， 1997； Maravelaki-Kalaitzaki & Biscontin， 1999； Brimblecombe& Grossi， 2009）.

From several European publications between 1987 and 2009， it can be identifies four main sources of PM： locally from vehicles （Amodio， Caselli， Gennaro， & Tutino， 2009） and industrial fuel anthropogenic （Ragosta et al.， 2006） and from widely crustal and sea spray of natural origin（Viana et al.， 2008）. Many of these publications are devoted both to the description of their spatial-temporal behaviour（Chow， Watson， & Lowenthal， 1999； Viana， Querol， Alastuey， Gangoiti， & Menéndez， 2003； Rodriguez et al.， 2004） and to the analysis of their chemical composition（Marcazzan， Vaccaio， Valli， & Vecchi， 2001； Mazzei， DAlessandro， Lucarelli， & Marengo， 2006； Amodio et al.， 2008； Takahashi， Minoura， & Sakamoto， 2008； Samek， 2009）. In situ concentration of atmospheric particulate， its size distribution and its chemical characteristics depend on emission sources， local meteorology and geographical features （Ragosta et al.， 2006）. For this reason， it is important to study the behaviour of particulate in zones with specific geomorphologic or anthropogenic characteristics where only few data are available. The scientific community is focusing hazard assessment of pollutants through the establishment of procedures for sampling and analysis and characterization of sources. Such information are generally available for the first meters of the troposphere， while rare are the information on chemical， particle size and morphology in the lower troposphere （from 0 to 2500 m）.

This work， through an aircraft， aims to define the particulate before and after the threshing， to know the impact that this agricultural activity produce in the low troposphere behind the boundary layer， using a particle counter. Different studies from the literature concern he use of particle counters to assess air pollution concentration （Tuch， Brand， Wichmann， & Heyder， 1997； Weijers， Khlystov， Kos， & Erisman， 2004） and to investigate the relationship between particle number and particle mass （Harrison， Jones， & Collins， 1999； Hoek et al.， 2008）. In particular we investigated the nature of particulates in a rural area in the lower Po Valley and see if the major contribution is given either by natural or anthropogenic sources.

1. MATERIALS AND METHODS

The Po Valley is located in the northern of Italy， a flat region surrounded by the Alpine chain in the north and northwest， by the Apennine Hills in the south and by the Adriatic Sea on the eastern boundary. It is a flat plain mostly composed by terrigenous sediments transported and deposited by the river Po. There is a dominant presence of debris， clays impermeable or poorly permeable in the eastern areas proximal to the mouth. The outcropping sediments can be easily eroded an resuspended by wind action and contribute to particulate nature. Two measurements campaigns were carried out during the summer 2009 in an agricultural land in the east part of Po Valley of the Agricultural Cooperative SORGEVA near Argenta （Ferrara， Italy） （Figure 1）. The sampling area is one of the greater agricultural area of the Emilia Romagna region， located near the sea in the Comacchio Valleys （44°3640.79" N—12°0410.52 E，-1m）. The soil is a silt clay soil， made from terrigenous sediments transported and deposited by the river Po（Bianchini， Laviano， Lovo， & Vaccaro， 2001）. The rural area is far enough from the Adriatic Sea and from main and secondary roads， and therefore the contribution of particulate matter from sea spray and anthropogenic pollution from vehicular traffic and other combustion sources is negligible. The area is also far from factories or industries. The power plant Polesine Camerini in Porto Tolle 2640 Mw （RO） （44°56 N—12°19 E， 1m）， the only significant source in the area was closed during the campaign. The aerosol background monitoring stations of ARPA Emilia-Romagna2 “Ostellato”， have measured an averaged mean PM2.5 concentration of 8±2.6μg/m3 during threshing operation. The contributions of biomass burning are also zero because there was no combustion event in the nearby area （information from MODIS fire/area burnt） during the sampling.

2.1 Vertical Profiles （150 to 2400 m） to the Size and Meteorological Parameters Measured With a Particle Counter

The particle counter LIGHTHOUSE HHIAQ3016 is an analysis system capable of counting and measuring the size of particulates on the basis of Fraunhofer diffraction of a laser beam. With this instrument tool it was possible to create vertical profiles describing the concentration of different sizes of the particles； also some basic meteorological parameters， such as temperature and relative humidity are contemporary measured.

Figure 2 shows the relationship between temperature（°C： black line） and relative humidity （%： red line） with the altitude （m）. Relative humidity shows a strong lowering around 1000m which coincides with an increase in temperature， in the closeness of the boundary layer. The clouds support the condensation of humidity and the increase of the temperature， creating a barrier for the particles of particulate that remain below the boundary layer.

In the period of acquisition， adverse weather conditions have produced a delay in harvesting activities， for which it was possible to detect a report to compare air quality data collected during activities of wheat harvest. On 9 June 2009 （pre-activities of threshing） conditions on the ground， recorded by the meteorological station of the ARPA Emilia Romagna “Ostellato”， marked warm climate with temperatures ranging between 19°C and 25°C during the hottest hours， no precipitation， relative humidity 81% and a wind speed of 8km/h 2. On 14 July 2009 （activities of threshing） conditions on the ground， recorded always by the meteorological station of the ARPA Emilia Romagna “Ostellato”， marked warmer climate with temperatures ranging between 25°C and 30°C during the hottest hours， no precipitation， relative humidity 80% and a wind speed of 9km/h 2. Meteorological variables are very important during the sampling. Relative humidity had a major influence： when relative humidity was high， PM10 particles were high， whereas PM2.5 were significantly low. A likely explanation is that particles are hygroscopic（Witting et al.， 2004） and increase in size on absorbing water （Wilson & Suh， 1997）.

The second sampling （Figure 3B）， performed in the

same area， 14 July 2009 during the activities of wheat harvest， shows a percentage increase of all particle size classes with significant increases for both the ultra-fine， fine and coarse particulate. Again there is a strong reduction of the share at between 550 and 700m（inversion layer）， which seems to indicate a tendency for the accumulation of particulate matter in the lower troposphere below the inversion layer. The rapid decrease of the number of particles affects all size classes. Unlike the pre-harvest period at altitudes above 750m the increase in concentration is not observed for coarse particles， probably because the organic component released into the atmosphere by the operation of harvesting （pollen， fungi and bacteria）， consisting of cohesive walls and characterized by low bulk density， tend to congregate with other inorganic particles， favouring an increase in size and weight and consequently killing gravitational phenomena.

2.2 SEM Analysis

The choice of the manual impactor has been demonstrated very effective for collecting particulates that further have been observed by SEM， especially because the particles have been not disturbed. Chemical analysis and SEM observations showed the presence of three types of cluster， mainly microliths of calcite， quartz and subordinate silicate clay particles.

？ The chemical composition of calcite crystals can incorporate these particles into the category of predominantly carbonates with calcite， have well-formed prismatic habitus despite the modest size （Figure 4A）. These particles are derived mainly from erosion of soil and surrounding rocks and they are present at a rate of 35%.

？ For silicate particles are distinguished those formed with silica granules more or less rounded likely source of cross-border and other silicate consisting of alkali feldspar（Si， Al， Ca or Si， Al， K）， plagioclase and clay minerals （Si， Al or Si， Al， Fe） （Figs. 4B and 4C）. The low percentage of clay minerals， the main constituents of soil， are assumed only from local contributions and a low prevalence of cross-border contributions. These particles are present at a rate of 60%.

？ The organic particulate is present in very small quantities （≈ 5%）， despite the monitoring was made during collection activities of wheat.

Figures 5 show， for both samplings， the averages abundances （percentages） of the three main components of particulate matter observed by SEM. Threshing activity produces coarse particulate （ < 10μm）， which is affected by the gravity and falls to the ground. For this reason， the contribution of particulate recorded at high altitude is modest.

This preliminary study was aimed at evaluating the contributions of the agricultural practice of threshing in the troposphere， characterized by a propensity for the accumulation of particulates below the inversion layer. The dynamics of spread of particulates in the troposphere， hypothesized on the basis of data， can be confirmed by the results obtained by SEM observation of samples taken during the flight.

The agricultural practice of threshing produces the issue close to the ground to airborne particulate matter， which produces a significant increase in all size classes. This increase is pronounced in the first 500m， especially for organic particles， while at higher altitudes the decrease interested in the bigger particles， which， from the SEM study， seem to be affected by gravitational aggregation. Indeed at higher altitudes to 550m are not detected significant amounts of organic particles. At high altitudes the prevalence of particles of natural origin， with carbonate composition and quartz and only less of clayey nature， is consistent with scientific works that define a major long range contribution to particulate inorganic solid. The threshing is carried out during June and July， which is the most favourable period for the contributions to long range particulate linked to current African from south-west. This is consistent with the composition found. The very low percentage of clay particles is probably due to vegetation cover， which limits the phenomena of resuspension of clay soil， typical of the area， and therefore very low contribution of inorganic particles of local origin.

AKNOWLEDGEMENTS

The authors wish to thank M.R. Bovolenta from Centre for Electron Microscopy Ferrara University for their assistance in the sampling and analysis.

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