Red Rain Research Articles

Iron Speciation in Precipitation in the North-Eastern Mediterranean and Its Relationship with Sahara Dust

Precipitation samples collected at Erdemli, Turkey, during February 1996–June 1997 were analysed to determine iron content and speciation. The purpose of the measurements was to examine the atmospheric abundance of iron and to quantify its solubility in the region. Spectrophotometric analyses of Fe(II) and reducible Fe(III) in precipitation samples, along with measurements of pH, conductivity, filterable iron (Fefilt), particulate aluminium (Alpar) and particulateiron (Fepar) were performed to determine iron solubility, which principally affects its bioavailability. Backward trajectories corresponding to the sampling dates were analysed to determine the sources of atmospheric constituents arriving at the site. Among these, the mineral dust transported from the Great Sahara to the region is considered to be a rich source of iron. The concentration of Fe(II) varied from below detection limit (0.02 μM) up to 0.42 μM,while the maximum concentration of total reactive Fe (referred as Fe(II) + reducible Fe(III) = Fereac) was found to be 1.0 μM in precipitation. A strong correlation was found between particulate Fe and Al fractions, both of crustal origin. No correlation was observed between the soluble and insoluble fractions of iron. The soluble iron fraction, Fe(II) concentration varied independently from the concentrations of reducible Fe(III), Fefilt, Fepar, and from the pH of the precipitation. The Fefilt fraction (size < 0.45 μm), measuredby Atomic Absorption Spectrophotometer, and frequently interpreted to be the soluble iron fraction in the literature, was found to be significantly higher than the corresponding Fereac fraction inprecipitation samples, most likely due to the colloidal iron content of the Fefilt fraction passing through the 0.45 μm pore size filter. The volume weighted mean Fefilt concentration of the precipitation samples collected during the episodic `red rain' events was found to be relatively higher. The geometric mean ratios of soluble Fe(II) and of Fereac to Total Fe (Fefilt + Fepar), werefound to be 1.6% and 2.1%, respectively, while the mean ratioof Fefilt to Total Fe was 9.6%. The flux of bioavailable iron (Fereac)fraction in most atmospheric wet deposition events was found to be sufficient for supporting the maximum primary production rates that are typical for the Eastern Mediterranean Sea.

Increasing frequency of Saharan rains over northeastern Spain and its ecological consequences

Desert areas in North Africa are important sources of mineral dust to the atmosphere, which upon deposition can influence oceanic and terrestrial biogeochemical cycles and affect forest productivity. Here, an increase of the frequency of rainfall loaded with such dust (red rain) in the last decades is documented for a site in northeastern Spain. The relevance of such a trend for the neutralization of acid rain and the delivery of nutrients in significant amounts to Mediterranean ecosystems is described.

The chemical composition of dust transported in red rains—its contribution to the biogeochemical cycle of a holm oak forest in Catalonia (Spain)

The chemistry of North African dust reaching NE Spain with red rains is here described to (1) characterize the red dust elemental composition, (2) analyze the relative contribution of dissolved and particulate forms to the total element inputs for the period 1983–1994, and (3) study the role of the particulate and dissolved inputs in red rains to the forest nutrient cycle. Five dust samples, obtained from the filtration of five red rain events, have been considered as representative of the dust reaching NE Spain as they include dust from the main source regions in North Africa. Enrichment factors were lower than four for all elements and samples, indicating the crustal character of the red dusts and their scarce mixing with anthropogenic pollutants. Back trajectory analysis of the red rain air masses computed at various isentropic surfaces showed northward fluxes at all altitudes in four out of five events. The remaining event, which presented a lower layer from European origin contacting upper layers of North African provenance, had higher trace metal concentrations in the dust (still lower than reported values for the circum-Mediterranean area) and higher S and N concentrations in dissolved form. The occurrence of red rains introduced high interannual variability in the input fluxes for the major elements. Phosphorus inputs occurred mostly in particulate form linked to red dust deposition. Red dust particulate inputs were also important for K + and Mg 2+. For Ca 2+, dissolved inputs in red rains equalled and sometimes overuled particulate inputs in red dust, due to the calcite dissolution. Sodium and S inputs in red rains were mostly in dissolved form. The amounts of base cations delivered by red rains are important contributors to the holm oak forest needs at Montseny, by providing 27% of K +, 45% of Ca 2+ and 84% of Mg 2+ fluxes needed for the above ground biomass annual increment. For Ca 2+ this is specially relevant because of the calcium-poor lithology of the site.

Mineralogical composition of African dust delivered by red rains over northeastern Spain

The African dust deposition, mineralogy, source regions, and influence on rain chemistry were analyzed for a site in the Montseny mountains (Catalonia, northeastern Spain) for the period August 1, 1983, through August 1, 1994. Dust deposition was highly variable: two events (collected on November 9–11, 1984, and March 22–25, 1991) accounted for 62% of the total dust input in the 38 red rain events recorded in the period. The average annual dust deposition was 5.3 g m−2 (SE 2.6). Three source regions in the African continent were identified with back trajectory analysis: (1) Western Sahara, (2) Moroccan Atlas, and (3) Central Algeria. Events from the Moroccan Atlas predominated in terms of number of events, amount of rainfall, and dust deposition. The meteorological situations during red rains were identified: Western Sahara events occurred principally with a depression at high latitudes (50°N) forming a trough in the North Atlantic, Moroccan Atlas events occurred with a depression in front of Portugal, and events from central Algeria were associated with a depression over Spain or North Africa. By order of abundance (median of n=13) the minerals identified in the dust by X ray diffraction were as follows: illite &gt; quartz &gt; smectite &gt; palygorskite &gt; kaolinite &gt; calcite &gt; dolomite &gt; feldspars. Differences in mineralogy between source regions were significant for smectite, kaolinite, quartz, and dolomite. Smectite and kaolinite content was highest in the Algerian events, while quartz and dolomite content was lowest. Events from the Moroccan Atlas had the lowest smectite and kaolinite. The red rain soluble chemistry was clearly influenced by the dissolution of calcite (high alkalinity, calcium concentration and basic pH) and by marine components (high Na+, Cl− arid Mg+2 concentrations). Because of the very alkaline character of red rains they had a strong weight on the annual mean pH of rainwater. This was reflected by the significant relationship (r=0.71, p&lt;0.01) between the percentage of annual precipitation as red rain and annual mean precipitation pH. Red rains provided 25–45% of the annual dissolved input of the major ions in precipitation and 34% of the calcium required for holm oak (Quercus ilex L.) forest growth at the study site.

Clocks, Clouds and Red Rain

Clocks, Clouds and Red Rain

Fluctuations of ionic composition of precipitations collected in corsica related to changes in the origins of incoming aerosols

Tremendous variations of relative abundances of major ions in rain-water have been observed in a pristine location of South Corsica (Bavella Pass). From selected data connected to air masses' back trajectories reaching the sampling zone, they can be explained by antagonistic influences of well differentiated aerosols mainly originating from the continental environment of the Mediterranean Basin, the impact of the marine source being in all cases significant. Episodes of acid rain are observed when incoming air masses pass over various remote European industrial regions (lowest pH values close to 4) and can especially be attributed to pollution by sulphuric acid which contributes to a large extent to the ionic balance. On the contrary, situations with air masses arriving from arid regions of North Africa fit with the highest pH values (6 to 7) and correspond to red rain episodes with high content in solid particulate matter including up to 30% of calcite. Due to the partial dissolution of this component, they are responsible for very high enrichments in dissolved calcium and bicarbonate, which may account for the major part of the ionic concentrations of rain-water. In situations with mixing of Saharan and European air masses, the intermediate pH values observed can be a consequence of a neutralizing effect of the incoming desert aerosol on the atmospheric acidity. It appears then that Saharan dust may counterbalance the effects of acidic rains in Mediterranean regions. Such interaction in the atmosphere and also at ground and sea level after atmospheric deposition may have implications for marine and terrestrial ecosystems of these areas.

Influence of Saharan dust on the rain acidity and atmospheric input to the Mediterranean

It has long been recognized that Saharan dust may be transported a long way from its sources, particularly over the ocean1, and especially over the tropical North Atlantic, as far as the Caribbean2–7, and over various areas of the Mediterranean8–10. The eolian transport of desertic aerosols is a major contribution to oceanic sedimentation11–13 and may have important climatic implications14. This phenomenon is responsible for episodes of so-called ‘red rain’ or ‘red snow’ which have been identified in various areas of the European continent15–17. Numerous red rains and red dust falls have been observed in Corsica. From 1984 data on the chemistry of precipitation collected in South Corsica, we show here that due to its CaCO3 content, Saharan dust significantly increases the pH of rain water. This may counteract the effects of acidic rain. An assessment of African dust deposition indicates that, compared with the sediments discharged by the Rhone river, atmospheric input to the northwestern Mediterranean contributes significantly to sedimentation in that region.

Opal phytoliths in some Victorian soils and "red rain" residues

Opaline bodies in certain Victorian soils have been regarded in the past as sponge spicules. It is now shown that the majority of these opaline bodies are the same as those developed in certain plants, hence they are recorded as opal phytoliths. This corrected identification of the opaline bodies is of fundamental significance in affecting certain aspects of the interpretation of the history of such components of soils. It is no longer necessary to consider these bodies of opal as sponge spicules carried into soils by wind transportation or other means. They are released from plants containing them and shed into the soils on the decomposition, or destruction during grass and forest fires, of phytolith-bearing vegetation, or sometimes via the dung of graminiferous animals. A few smaller phytoliths do become wind-borne, as evidenced by their occurrence in "red rain" and "red snow" residues. It is also shown that distilled water should always be used in treating soils for examination of their phytolith populations, because a small quantity of the smaller phytoliths (and diatoms) occurs in tap-water.

Australian Origin of Red Rain in New Zealand

THE sample of red rain dust collected at Ben Lomond, near Queenstown in southern New Zealand, by Dr. Kidson, which the Editor has kindly sent me for inspection, agrees in its microscopic characters with falls previously described. It contains some coarse quartz grains, doubtless of local origin, but the bulk is a fine clay and silt such as is common on the dry lake beds of the interior of Australia; it contains some diatoms and shreds of algae, which look like those that grow in the pools in such localities.

Australian Origin of Red Rain in New Zealand

WE have recently had two striking examples of the manner in which the influence of a continent may extend, in ways which might easily be overlooked, to enormous distances round about it. It is well known that in dry years large quantities of smoke from bush fires in Australia are carried by the south-east monsoon over the East Indian Archipelago. On Oct. 25 and 26, 1929, however, dense haze, which can have had none other than an Australian origin, was seen at the Island of Niue in lat. 19° S., long. 170° W., 2300 miles distant from Australia. The meteorological observer, Mr. J. P. McMahon-Box, reports that “A strange smoky haze enveloped the whole Island on the 25th and 26th. It came up about midnight on the 24th to 25th, apparently from the south-west, in which direction it was densest. Visibility was very poor indeed.… It disappeared during the night of the 26th to 27th.… We experienced a couple of thunderstorms during the period of the haze.”

Red Rain at Bordigbera, Italy

‘RED rain’ fell at Bordighera on the morning of October 31, coating the pavements and vegetation with a dense red-brown deposit which turned to salmon colour when dry. The shower, which terminated about noon (English summer time), was accompanied by a current of hot moist air from the east. The previous day was wet and overcast, and the evening after the shower there was a thunderstorm with long flashes of lightning from east to west at an estimated altitude never touching below 2500 feet. Samples of the deposit (doubtless sand from the Sahara) are being preserved for microscopical examination.

A Note on the Red Rain in Iliad 16.459

A Note on the Red Rain in Iliad 16.459

“Red Rain” and the Dust Storm of February 22

IN a letter under the above heading which you did me the honour to print in your issue of May 21, vol. lxviii. P. 53, I gave the results of the chemical examination of a sample of dust collected from the roof of Bayham Abbey, Lamberhurst, after the dust storm of February 22, and sent to me by the kindness of Lord Camden, and I stated that it would be interesting to compare its characters with those of the dust, presumably of African origin, which was observed to fall in the district of Taormina by Sir Arthur Rucker, and was the subject of an interesting communication to NATURE by Prof. Judd in 1901 (vol. lxiii. p. 514).

“Red Rain” and the Dust Storm of February 22

“Red Rain” and the Dust Storm of February 22

Analysis of the “Red Rain” of February 22

SOME of your readers will probably be interested to know something of the nature of the muddy rain which fell here on Sunday, February 22. A sample of the downfall, caught in an open field between 10 a.m. and 12 noon, was brought to me. To examine, and particulars of the partial analysis of the suspended matter which the water contained are subjoined. The large percentage of organic matter seems to me to be the most remarkable point in the analysis, and I regret not having had time to make a separate investigation of this. A rapid examination of the physical properties of the sediment, or mud, which I made, seemed to indicate that the organic matter was condensed hydrocarbon gases, or condensed volcanic vapours (such as one might expect to be evolved unburnt in a very large volcanic outburst). The sediment seems to be terrestrial, as the large amount of organic matter, coupled with the small amount of iron found, prohibits the theory of a meteoric origin.

Remarkable Meteorological Phenomena in Australia

ON Wednesday, November 13, 1902, we experienced here in Australia some most extraordinary meteorological phenomena. For the previous five or six days, exceedingly hot, dry weather had prevailed, owing to winds blowing from the Australian interior, where a Huge anticyclone was resting, in a coastward direction, the winds taking in Queensland and New South Wales a westerly, and in Victoria a northerly, direction. The hot weather culminated in terrific dust-storms in Queensland, New South Wales, Victoria and South Australia, and during these storms “fireballs” were seen hovering in the air. On the sea, “red rain” was experienced by several passing vessels.

Fall of Coloured Dust on February 22–23

I HAVE received this morning from two of the observers of the Royal Meteorological Society samples of red or muddy rain which fell on Sunday night or Monday morning.

Blood-Rain

IN view of the recent letters in NATURE regarding the fall of red rain in Italy, the following extract from Roger of Wend-over's Chronicles of the year 1223 may possibly be of interest:— “In the same year it rained blood-coloured earth at Rome for three days, to the great wonder of numbers of people (vol ii. p. 444 of Bohn's edition of Wendover's “Flowers of History.”) It is rather curious that so miserably superstitious a gobemouche as Wendover should have described the phenomenon so accurately instead of calling it a rain of blood.

Odd Showers: or, Explanations of the Rain of Insects, Fishes, and Lizards; Soot, Sand, and Ashes; Red Rain and Snow; Meteoric Stones; and other Bodies

Odd Showers: or, Explanations of the Rain of Insects, Fishes, and Lizards; Soot, Sand, and Ashes; Red Rain and Snow; Meteoric Stones; and other Bodies

Red rain supposed to arise from butterflies

Red rain supposed to arise from butterflies