J.L. Colin, R. Losno, N. Le Bris, M. Tisserant, I. Vassali, S. Madec, S. Cholbi, F. Vimeux, G. Bergametti.
In collaboration with :
Dr. T. Jickells and Dr. L. Spokes
Dr. G. Jennings
Dr. Michael Schulz and Dr. A. Rebers
Dr. W. Maenhaut and Dr F. Francois
Dr. R.V. Grieken and Dr. J. Injuk
Within EUROTRAC programme, the main objectives of our group were devoted to better understand the homogenous and heterogeneous processes which control the atmospheric aqueous phase chemistry and the evolution of particulate aerosol (mineral and carbonaceous) in cloud droplets, before their deposition. These objectives were conducted mainly in marine environment to study atmospheric systems rather homogenous and characterised by long residence times.
The main role of the cloud system is to cover aerosol particles with a layer of water in which aqueous chemical reactions start with acidic species, reducing or oxidising agents, radicals and photons (figure 1). Dissolution processes release trace metals from aerosol surface which may be complexed with inorganic ions and organic species driving thus metal speciation in cloud droplets. This speciation is critical to more accurately describe their catalytic effect on aqueous phase chemistry and after several cloud cycles their bioavailability in wet deposit and their potential impact on marine environment.
The main goals achieved in this topic were based on field experiments in marine environments, laboratory experiments and chemical modelling.
A first experiment was conducted in Corsica Island over the western Mediterranean basin. Daily aerosol samples were collected continuously over one year whilst rain samples were sampled only during short campaigns. Corsica campaign was not directly developed within EUROTRAC programme, but it is mentioned here because the results obtained on these rainwater were of first interest all along further discussion.
A second experiment occurred at Mace Head, on the west coast of Ireland, in November 1990 during a collaborative field experiment with UEA, UCG and the French group (LISA, CFR) with major aims to collect trace metals in rainwater and carbonaceous species in both aerosols and rains.
A third experiment was conducted again at Mace Head in April 1991 with several teams of the ASE group (LISA, UEA, UCG, IAAC, UNS UIA). This campaign was carried out for a general intercalibration of ultra clean procedures for aerosol and rain sampling and analysis. This intercalibration has been discussed in several workshops held in Paris (October 1992) and Arcachon (October 1993).
A wide range of atmospheric conditions are thus covered with these 2 coastal sites : At Mace Head, Atlantic rain waters are often associated with clean air masses, fairly representative of remote oceans with pH values around 5. By contrast the site in Corsica is representative of contrasting events with strong differences between polluted (pH as low as 3) and non-polluted or acid neutralised air masses with pH values over 7 for events associated with Saharan dust inputs.
We have developed a kinetic reactor to measure dissolution mechanisms in a reaction cell where fine fraction of soils are dispersed in a large amount of aqueous solution. A very small amount of well identified solid powder (clays, soil minerals, loess or Saharan dust) reacts with a controlled aqueous solution, in an open loop where new and clean water solution is continuously injected into the cell. The rate of trace metal dissolution is measured under various pH and oxidant conditions and over short time scales (around the minute). This may give a better view about weathering and transformations of aerosols into a cloud or a rain drop. Models developed can benefit immediately from these laboratory studies.
The strongest changes of the aerosol surface occur during cloud processing. We develop modelization into cloud droplets, involving aerosol. The aerosol contribute to the cloud chemistry by exchanging compounds with the aqueous phase, and especially trace metals. These metals act both as catalyst in the reactivity scheme of oxidising species (OH, HO2,...), and are modified by these species. For example, Iron(III) is changed into Iron(II) and becomes much more soluble by such processes. Modelling allow us to assess the behaviour of trace metals before their deposition to the oceans.
Sampling and analysing procedures for pH, major ions and trace metals measurements have been extensively improved during the EUROTRAC program. Rainwater were manually collected in single-use polyethylene collectors on a wet-only, event-by-event basis. Funnels and bottles were pre-cleaned using ultraclean techniques in a clean room, dried under laminar flow hoods and sealed in polyethylene bags until rain collection. At the end of the rain, the sample is filtered through a 0.4µm pore size Nuclepore membrane and two aliquot (60 ml each) are kept. The first for major ions analyses is preserved with chloroform and stored in a freezer. The second for dissolved trace metals is acidified with ultrapure Prolabo Normatom® nitric acid. In a final form we have improved this collector with an on line filtration and a direct separation of iron species with a Sep-Pak® cartridge.
The intercalibration of Mace Head in April 1991 was focused on ultraclean procedures for aerosol and rain sampling, analytical procedures for trace metals in aerosol and for major ions and trace metals in rainwater. Our contribution has been focused on major ions and reveals large variations between the results obtained by the different laboratories for a same rain event. The main variation is widely due to sampling methods influenced by non rain deposit (marine brine and sea salt) which is very dependent on the geometry of the sampler and the wind velocity. Moreover we have shown that at Mace Head and probably in all remote marine areas, the major ion content could be explained by sea salt input and then evaluated by a simple conductance measurement (figure 2) (Losno et al., 1996).
The soluble-insoluble partition of trace metals are driven by thermodynamic equilibrium for Al(III), involving mainly hydroxide (OH-) and sulphate (SO42-), and adsorption/desorption mechanisms for Pb, Cu, Zn.
This element shows the same behaviour in rainwater than in river water. At low pH (<5), Al solubility can be predicted from an equilibrium with an insoluble aluminium hydroxide sulphate salt :
Al4(SO4)3(OH)6¯ + 6H+ = 4 Al3+ + 6H2O + 3 SO42- pK= 14.4
At pH>5, Al solubility is controlled by aluminium-hydroxy compounds (figure 3). A special behaviour is observed for very low concentrated sample collected at Mace Head.
The solubility of Zn, Pb and Cu are controlled by adsorption/desorption
processes in which rainwater particles provide surface-active
sorption sites. pH is a critical parameter as illustrated by Zen
and Pb behaviour on figure 4. The pH of rainwater ranges between
4 and 7. The soluble zinc ranges between 15 and 99% of the total
zinc, the higher percentage coming for the lower pH. Results suggest
that in marine precipitation with pH<5, more than 80% of the
total Pb, Cu and Zn concentrations are delivered to the surface
oceans in the dissolved form (Lim et al., 1994).
Measurements were made at Porspoder (Brittany) and Mace Head (Ireland), after a laboratory methodological development. Among various anions, organic may have a strong chelating power, that can change the behaviour of trace metals in the rain water. The method developed here consist in adding precise amounts of a selected metal (lead was chosen) to the sample, and in measuring the remaining free metal. The difference of slope between added and free metal is considered to be related to available ligands (see legend of figure 5). If ligands were found at Porspoder (figure 5), none were found at Mace Head.
To evaluate the potentially biologically labile fraction of iron to marine phytoplankton upon wet deposition, attention has been paid to speciation of Fe(II)/Fe(III) in marine precipitation. Both kinetic and thermodynamic approaches can be used to describe the factors which control the speciation of iron in marine precipitation.
To control this iron cycle, we have developed an analytical method to measure directly Fe(II) and Fe(III) species by ionic chromatography coupled with a photometric detection. To be able to detect both species at the nanomol/l level, a cationic resin was used as a precolumn to preconcentrate rain samples so as to get satisfactory detection limits (1.8 nmol/l for iron(II) and 2.1 nmol/l for iron(III)). But this method tested with freshly collected marine rain samples showed that the concentration levels of iron(II) and iron(III) could not be determined because of quasi-missing recorded signals on chromatograms. Indeed, major cations such as Na+, K+, Ca2+ or Mg2+ were supposed to cause large interference with metallic ions in the mechanism of preconcentration as their concentrations in marine precipitation are much higher than iron concentrations (about ten to hundred µmol/l versus a few nmol/l).
Further studies were carried out with synthetic solutions in the laboratory to assess the hypothesis and as a conclusion that the precolumn was either of too low capacity or the competition between major and trace metallic ions for their retention on the sites of the resin induced partial preconcentration. Another point was the delay between the sampling of the rainwater and the analyse latter in the laboratory giving time for the oxidation of Fe(II) to Fe(III) in the sample.
To avoid this artefact we have set up a new analytical protocol allowing a separation of both species on an open column fitted directly to the funnel. This open column (Sep-Pak®, Waters) is filled with Ferrozine as a stationary phase, which is a specific chelating agent for Fe(II). After the rain collection, Fe(II) is eluted with a water/methanol eluant and then analysed by Graphite Furnace Atomic Absorption Spectroscopy.
These approach will permit to explain the equilibrium reach and observed between soluble iron and its hydroxide in various rainwater (figure 6) which are not collected taking account the reactivity of iron II.
Dissolution rates of aerosol particles are measured in an open flow chemical reactor. As the apparatus is simplistic, the main goal of this research is to insure real kinetic conditions far from dissolution equilibrium to explain the dissolution processes. As the solubility of various species are very poor, we must control operating conditions where analytical concentrations are in the range ppt (10-12) to ppb (10-9). These concentrations are consistent with those obtained in marine cloud and rain waters.
Soil (20 mg) is introduced into the dissolution area at the beginning of an experiment, then an ultra clean solution at a fixed pH and Redox property is flowed through the cell. The emerging filtered solution is sampled and analysed in trace metals. The volume of the reaction chamber is 34 ml, and the flow rate 18 ml.mn-1.The average residence time in the stirred cell is about 2 mn.
We already measured the dissolution rate of d-block metals as a function of time in a aeolian soil collected on the Cappo Verde Islands. The results show that the dissolution rate is pH dependent, but with different behaviour than equilibrium with hydroxy or oxo-hydroxy salts. As an example, figure 8 show how the copper dissolves at various pH values.
From those curves, the reaction rate can be plotted at selected positions in the reaction, versus pH (figure 9):
In marine environment, trace metals are implicated in their biogeochemical fate and also as catalysts in oxidising power of the atmosphere. Both are strongly linked. Next figures (10 and 11) show that light can produce FeII, the most soluble specie of iron, from FeIII with a very poor solubility. Alternate step of dark and light are simulated here: 0-5 s light; 5-10 s dark, 10-15 s light, 15-20 s dark. Ordinates are mol/l, abscises are s.
As indicated in figure 11, the response time of the system is
very enhanced by the trace metals catalyst, and a steady state
is reached after 1 or two seconds. In those conditions, the reactive
species concentrations could be well approached by their photostationary
values expressed as:
Reactions and values are given in ANNEXES. R4 is the main HO2 source, with R16 for several %. Sinks are R10, R13, R18, and R34. R18 and R34 are the main contribution. For OH, R1, R10, R19, R31 and R4 are the main. R31 and R34 build a catalytic cycle for CuI/CuII. The rate of R31 and R34 are the same and also far up others. The response of this system is very fast, around 10-10 mol.l1.s-1.
The general aim of this project conducted until 1993 was to study carbonaceous particles behaviour in aerosol and rainwater.
Black carbon (soot) aerosols were measured with optical equipment (aethalometer) during the first Mace Head field campaign in November 90. At Mace Head, there is a very striking sectorisation of atmospheric concentrations for particulate data. From the clean sector (West) to the dirty sector (East), concentrations vary in a range which can overlay 2 orders of magnitude. The main value of atmospheric soot carbon in western wind conditions (0.015 µg/m3) is very low, only five times the measured value in arctic area.
The two main components of the carbonaceous aerosols (Ct) are the organic fraction (Co) and the black carbon or soot fraction (Cb). As a tracer of the origin of the carbonaceous aerosols, the Cb/Ct ratio which at Mace Head is on the average of the order of 22%, attests an industrial origin for the atmospheric particulate material. Accordingly with other results obtained at other sites, a comparison between Cb/Ct ratios in aerosols and in rains points out a systematic relative increase of black carbon in rains which could show a partial disappearance of the organic fraction during its incorporation in hydrometeors (Table 1). It may be hypothesised that this dissolution of the hydrophilic organic particulate material is accompanied by that of other inorganic species such as sulphates which are likely to be absorbed at the surface of the atmospheric particles. This process could be a key step in the probable CCN role of the carbonaceous aerosols a major part of which is attached to submicron particles. Our future research will focus on that field and will rely primarily on experimental leaching of aerosols.
Scavenging ratio estimates (S = Crain (µgC.l-1)/
Cair (µgC.m-3)) reinforce the hypothesis
that the aerosol acquires a better hydrophilic character during
its atmospheric transport (S increases, Table 1). Also, S values
found for black carbon are of the same order as values obtained
for soluble species such as sulphur species. Consequently, due
to both its chemical inertia and its hydrophilic coating, black
carbon could trace the physical processes of the incorporation
of atmospheric particulate pollutants.
|Mace Head (n=18)|
|Ile de France (n=51)|
|Tropical Africa (n=21) (off fires)|
The idea of an internally mixed aerosol may be corroborated with the surprising correlation often found between excess sulphur and soot-carbon in environments where combustion inputs are dominant. Gases committed during the combustion processes are likely to adsorb onto the carbonaceous nuclei and undergo oxidative conversions leading to a solid coating of the particle. SO2 is then transformed into SO42-. In the particles, the mean C/S ratio value is 1 and may reach 2 in old aerosol phases (Cachier et al., 1993). This feature could be used in further works to trace with soot-carbon pollution inputs to rain waters. Due to this coating of hydrosoluble substances, the combustion carbonaceous particles have a hydrophilic behaviour, and their capability to be incorporated in hydrometeors is enhanced (Ducret, 1993). This may explain the important sectorisation of the black carbon scavenging ratios observed at the Mace Head site (Table 1).
The soot-carbon aerosol display optical properties which vary
with the size and the coating of the particles. As an example,
accordingly with theoretical considerations, we found that a thin
coating of sulphates would enhance the light absorption capability
of the particles whereas in old particles, a thick surface layer
would on the contrary lower the absorption. This had to be taken
into account in remote sensing retrieval of aerosol loads or in
models of the radiative impact of anthropogenic particulate. This
important result was gained primarily with the optical sensor
of an Aethalometer acquired during the first year of this EUROTRAC
Within this EUROTRAC project , we have documented the impact of mineral and carbonaceous aerosol on marine atmospheric environment. This is of considerable interest since for example some trace metals (e.g. Fe, Mn and Zn) can act to enhance marine primary activity while other metals (e.g. Cu, Hg) can affect dramatically the initial step of the marine life. Several goals were reached during this programme and are shown in this report. Numerous results are still in the interpretation process and will provide further publication.
Furthermore this EUROTRAC project involving 5 EU research groups experienced on field, laboratory and modelling studies in marine atmospheric environment was of major benefit for international collaboration. Considerable expertise in this area has been thus gained by these groups which have regularly met in annual ASE workshops since 1989.
Losno R., J.L. Colin, N. Le Bris, G. Bergametti, T. Jickells, and B. Lim. Aluminium solubility in rainwater and molten snow, J. of Atmos. Chem., 17, 29-43, 1993
Liousse C., H. Cachier, and S.G. Jennings. Optical and thermal analysis of black carbon aerosol content in different environments: variations of the specific attenuation coefficient, sigma s Atmos Environ., 27A, 1203-1211, 1993.
Liousse C., F. Dulac, C. Deveaux and H. Cachier. Aging of savanna biomass burning aerosols : consequences on their oiptical properties, J. of Atmos. Chem., sous presse.
Lim B., T.D. Jickells, J.L. Colin, and R. Losno. Solubilities of Al, Pb, Cu and Zn in rain sampled in the marine environment over the North Atlantic Ocean and Mediterranean Sea, Global Biogeochemical Cycles, 8, 349-362, 1994.
François F., W. Maenhaut, J.L. Colin, R. Losno, M. Schulz, T. Haster, L. Spokes and T. Jickells. Intercomparison of elemental concentrations in total and size-fractionated aerosol samples collected during the Mace Head experiment, April 1991, Atmos. Environ., 27, 837-849, 1995.
Schulz M., W. Dannecker, T. Church, J.L. Colin, T. Haster, M. Leermakers, R. Losno, C. Meulemann, L. Spokes. Intercomparison of rain sampling at Mace Head/Ireland, Annales Geophysicae, Suppl. 11 to Vol 10, pC223, 1992.
Losno R., J.L. Colin, , L. Spokes, T. Jickells, M. Schulz,
A. Rebers and M. Leermakers. Intercomparison of sampling and
analysing procedures for major ions and pH measurements in marine
rainwaters, in preparation, 1996.
Le Bris N., Contribution to the study of trace metals dissolved/particulate partition in wet precipitations (in french) Ph.D. Université Paris 7 (May 1993)
Ducret, J., Incorporation of carbonaceous particles into wet precipitations (in french) Ph.D. Université Paris 7 (Oct 1993)
Liousse C., Optical properties and remote sensing of tropical combustion aerosol (in french) Ph.D. Université Paris 7 (June 1993)
Reaction used and limit conditions for the modelization.
Compounds and initial concentrations:
O3: 4.500E-10 H2O2: 5.000E-05 HO2: 0.000E+00
O2-: 0.000E+00 OH: 1.000E-14 H+: 1.000E-05
hv: 1.000E+00 O2: 3.000E-04 FeII: 0.000E+00
FeIII: 5.000E-08 MnII: 3.000E-09 MnIII: 0.000E+00
CuI: 0.000E+00 CuII: 1.000E-09
H+: 1.000E-05 hv: 1.000E+00 O3: 4.500E-10
(01) H2O2 + hv --> OH + OH k=5.700E-07
(02) HO2 + OH --> O2 k=7.000E+09
(03) O2- + OH --> O2 k=1.100E+10
(04) H2O2 + OH --> HO2 k=2.700E+07
(05) O3 + OH --> HO2 + O2- k=2.000E+09
(06) HO2 + HO2 --> H2O2 + O2 k=8.600E+05
(07) HO2 + O2- --> H2O2 + O2 k=1.000E+08
(08) H2O2 + HO2 --> OH + O2 k=5.000E-01
(09) H2O2 + O2- --> OH + O2 k=1.300E-01
(10) O3 + O2- --> OH + O2 + O2 k=1.500E+09
(11) OH + MnII --> MnIII k=3.400E+07
(12) HO2 + MnII --> MnIII + H2O2 k=6.000E+06
(13) O2- + MnII --> MnIII + H2O2 k=1.100E+08
(14) HO2 + MnIII --> MnII + O2 + H+ k=2.000E+04
(15) O2- + MnIII --> MnII + O2 k=1.500E+08
(16) H2O2 + MnIII --> MnII + HO2 + H+ k=3.200E+04
(17) HO2 + FeIII --> FeII + H+ + O2 k=2.000E+04
(18) O2- + FeIII --> FeII + O2 k=1.500E+08
(19) hv + FeIII --> FeII + OH k=5.900E-04
(20) HO2 + FeII --> FeIII + H2O2 k=1.200E+06
(21) O2- + FeII --> FeIII + H2O2 k=1.000E+07
(22) OH + FeII --> FeIII k=3.000E+08
(23) O3 + FeII --> FeIII + OH + O2 k=1.700E+05
(24) FeII + MnIII --> FeIII + MnII k=2.100E+04
(25) H2O2 + FeII --> FeIII + OH k=6.010E+01
(26) O2 + FeII --> FeIII + O2- k=7.900E-04
(27) FeIII + CuI --> FeII + CuII k=1.000E+07
(28) OH + CuI --> CuII k=3.000E+08
(29) HO2 + CuI --> CuII + H2O2 k=1.500E+09
(30) O2- + CuI --> CuII + H2O2 k=1.000E+10
(31) H2O2 + CuI --> CuII + OH k=4.000E+05
(32) MnIII + CuI --> CuII + MnII k=2.100E+04
(33) HO2 + CuII --> CuI + O2 + H+ k=1.000E+08
(34) O2- + CuII --> CuI + O2
HO2 / O2- pKa= 4.68