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Atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters. Stable nitrogen isotope data can provide important information regarding the sources and processing of atmospheric fixed-nitrogen species and is thus important in controlling eutrophication. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland and also aboard the RRS Challenger in the Eastern Atlantic Ocean. Aerosol concentrations of ammonium and nitrate were determined prior to δ 15 N isotopic analysis. The isotopic enrichment factor, e , was calculated for both the species by subtracting the respective δ 15 N values of the coarse-mode fraction (>1 μm diameter) from those of the fine-mode fraction ( 4 NO 3 followed by scavenging of the HNO 3 produced appears to be significantly more important than in samples of onshore, marine dominated air. In contrast, ammonium appears to be transferred to the coarse mode during offshore flow largely via the dissolution and coagulation of aerosol ammonium, nitrate and sulphate-containing species. During onshore flow, the uptake of gaseous NH 3 , arising from the continued dissociation of NH 4 NO 3 , seems to become increasingly important.

Direct atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters and can be enhanced by heterogeneous reactions between gaseous atmospheric nitrogen species and aerosol sea salt, which increase deposition rates. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland. Major-ion aerosol concentrations were determined and temporal patterns were interpreted with the use of air-mass back trajectories. Low levels of terrestrially derived material were seen during periods of clean, onshore flow, with respective concentration ranges for nitrate and ammonium of 0.47–220 and below detection limit to 340 nmol m −3 . Corresponding levels of marine derived material during these periods were high, with sodium concentrations ranging from 39 to 1400 nmol m −3 . Highest levels of terrestrially derived material were seen during polluted, offshore flow, where the air had passed recently over strong source regions of the UK and northern Europe, with concentration ranges of nitrate and ammonium of 5.6–790 and 9.7–1000 nmol m −3 , respectively. During polluted flow ∼40–60% of the nitrate was found in the coarse mode (>1 μm diameter) and under clean marine conditions almost 100% conversion was seen. In addition, our data suggests strong evidence for dissolution/coagulation processes that also shift nitrate to the coarse mode. Furthermore, such processes are thought also to give rise to the size-shifting of aerosol ammonium, since significant coarse-mode fractions of this species (∼19–45%) were seen at both sites. A comparison of the relative importance of nitrate and ammonium in the overall dry deposition of inorganic fixed-nitrogen at each site indicates that at Weybourne the mass-weighted dry deposition velocity of the latter is around double that seen at Mace Head with its resultant contribution to the overall inorganic nitrogen dry flux exceeding that of nitrate.

Atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters. Stable nitrogen isotope data can provide important information regarding the sources and processing of atmospheric fixed-nitrogen species and is thus important in controlling eutrophication. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland and also aboard the RRS Challenger in the Eastern Atlantic Ocean. Aerosol concentrations of ammonium and nitrate were determined prior to δ15N isotopic analysis. For both species a significant difference in mean isotopic composition was seen between samples from Weybourne ( +6±6‰ for ammonium and +7±6‰ for nitrate) and Mace Head and RRS Challenger campaigns ( −9±8‰ for ammonium and −1±3‰ for nitrate). At each site a strong dependence of isotopic composition on the geographical origin of the sampled air mass was also observed. For aerosol ammonium, marine and terrestrially dominated samples were found to be isotopically distinct, perhaps reflecting the presence of oceanic sources of ammonia in addition to anthropogenic or natural terrestrial sources. Further distinctions were made within terrestrially dominated samples, possibly indicative of different types of animal husbandry regimes or other forms of anthropogenic activity. For aerosol nitrate, there was found to be generally less variation between samples at each site, although at Weybourne a significant difference was observed between the mean isotopic composition of samples originating from the northern UK ( +15±3‰ ) and that of those originating from the southern UK (+10±3‰) , suggesting that aerosol δ15N data might possibly facilitate source apportionment between NOx emissions from power stations and those from vehicle exhausts. The nitrate data also appeared to show seasonality with higher concentrations and lower δ15N values seen in the summer.

Converting measured concentrations into fluxes and using estimates of biological productivity in the coastal waters of the eastern Atlantic Ocean enables us to determine the role of the atmosphere as a source of biologically essential species, including nitrate and ammonium, to the marine biota. To understand the effects of the atmosphere as a source of nitrogen capable of promoting new production, we need to know both the seasonality of the input as well as the effects of extreme high deposition events which, while small in overall annual budget terms, maybe able to extend, or even promote, phytoplankton growth under nutrient depleted summer conditions. Aerosols and rainwater were collected at both Mace Head and at sea aboard RRS Challenger. Temporal patterns have been interpreted using airmass back trajectories which give the predicted air path prior to arrival at the sampling site. Low levels of both nitrate and ammonium are seen associated with marine westerly flow across the Atlantic and northerly air originating in the Arctic region. As expected, marine derived sodium, chloride, magnesium and seasalt sulphate are high during these periods. High concentration nitrate and ammonium events are seen associated with south-easterly flow where the airmass passes over the UK and northern Europe prior to arrival on the west coast of Ireland. In the polluted atmosphere, nitrate exists as nitric acid and as fine mode (1 μm diameter) sodium nitrate: HNO3 (g) + NaCl(s) → NaNO3(s) + HCl(g). This seasalt displacement reaction not only enhances dry nitrate deposition through more efficient gravitational settling of large particles, but also increases the efficiency of precipitational scavenging via inertial impaction. By looking at the size distribution of nitrate, we can see evidence for the seasalt displacement reaction. Under the polluted south-easterly flow, ∼40−60% of the nitrate occurs in the coarse mode fraction. Under clean marine conditions, the seasalt displacement reaction results in almost complete conversion of nitrate from the fine to the coarse aerosol mode. By converting measured wet and dry nitrate, ammonium and organic nitrogen concentrations into fluxes and comparing the data with estimates of biological productivity in the surface waters, our data suggest that ∼30% of new production in eastern Atlantic surface waters off Ireland can be supported by atmospheric inputs in May 1997 and that most of the input occurs during short lived, high-concentration, south-easterly transport events.DOI: 10.1034/j.1600-0889.2000.00062.x

Previous work has shown that the type and pH history of an aerosol governs trace metal solubility in rainwater. This study concentrates on the crustal elements Al, Fe and Mn and identifies additional processes which affect dissolution not only in the atmosphere but also on mixing into seawater. Aerosol dissolution experiments (at aerosol concentrations of about 30 mg 1−1) show manganese exhibiting high solubility at the low pH values typical of clouds (54±2.5% at pH 2, with results expressed in mole percent units) with 85% of this increase occurring within 6 hours of acidification. The percentage dissolution decreases to 50% at pH values representative of rainwater (pH 5.5) and to 26±4% at pH 8, typical of seawater. No such dramatic solution phase removal occurs at pH 8 in the presence of inorganic anions (to a final solubility of 44±2%). Thus the extent of manganese dissolution depends strongly on whether aerosols are cycled through acidic environments and on subsequent inorganic complexation once rainwater mixes into sea. Aluminium shows highest dissolution (7.1±0.6%) at low pH with 78% of this increase occurring within 6 hours of acidification. Rapid solution phase removal occurs on increasing the pH to that representative of rainwater (to 0.9±0.4% with 87% of this decrease occurring within 15 min). As a consequence of acid cycling and aluminium's amphoteric nature, solubility is enhanced at seawater pH (2.3±0.3%) over that in rain. Iron shows a strong pH-solubility relationship with highest solubility at low pH (4.7±0.2%), 70% of this value being reached within 6 hours of acidification, and decreasing rapidly to 0.17% as pH is raised to 8. Addition of inorganic anions at pH 8 to simulate mixing into seawater causes a further decrease in solubility, perhaps due to anion induced colloid destabilisation. Photochemical reduction also effects solubility under low pH conditions with Fe(II) comprising 1% of the total iron in the Saharan Aerosol used and 8.4% in an Urban material at a pH of ≈ 2. This element shows rapid solution phase removal with increasing particulate load which is tentatively rationalised in terms of a simple Kd approach.

During an intercomparison field experiment, organized at the Atlantic coast station of Mace Head, Ireland, in April 1991, aerosol samples were collected by four research groups. A variety of samplers was used, combining both high- and low-volume devices, with different types of collection substrates: Hi-Vol Whatman 41 filter holders, single Nuclepore filters and stacked filter units, as well as PIXE cascade impactors. The samples were analyzed by each participating group, using in-house analytical techniques and procedures. The intercomparison of the daily concentrations for 15 elements, measured by two or more participants, revealed a good agreement for the low-volume samplers for the majority of the elements, but also indicated some specific analytical problems, owing to the very low concentrations of the non-sea-salt elements at the sampling site. With the Hi-Vol Whatman 41 filter sampler, on the other hand, much higher results were obtained in particular for the sea-salt and crustal elements. The discrepancy was dependent upon the wind speed and was attributed to a higher collection efficiency of the Hi-Vol sampler for the very coarse particles, as compared to the low-volume devices under high wind speed conditions. The elemental mass size distribution, as derived from parallel cascade impactor samplings by two groups, showed discrepancies in the submicrometer aerosol fraction, which were tentatively attributed to differences in stage cut-off diameters and/or to bounce-off or splintering effects on the quartz impactor slides used by one of the groups. However, the atmospheric concentrations (sums over all stages) were rather similar in the parallel impactor samples and were only slightly lower than those derived from stacked filter unit samples taken in parallel.

The atmosphere has now been recognized as a major source of both trace metals and nutrients to the oceans, with wet deposition being a major contributor to the flux. Solution pH has been suggested to be the major control on metal solubility in rainwater, but for many trace metals this relationship is not a simple one. Aerosols are typically exposed to [approximately]10 condensation/evaporation cloud cycles before removal in rain and, as a result of H[sub 2]SO[sub 4] and HNO[sub 3] uptake and SO[sub 2] oxidation, cloudwater pH can be very low. Laboratory studies have been conducted, using Saharan and Urban aerosols, to assess the effect of pH on trace metal solubility. The results for the crustal elements Al and Fe in the Saharan aerosol show that metal solubility is a strong function of pH with highest solubilities seen under low pH conditions, comparable to those found in clouds. Increasing the pH to simulate neutralization of acidity by ammonia and crustal material results in almost complete removal of these elements from the solution phase, suggesting that a simple relationship between pH and solubility should exist in rainwater. For Al and Fe in the Urban aerosol there is evidence that some ofmore » the material solubilized at low pH is kept in solution at high pH, perhaps as the result of complexation by organic matter. Manganese shows high solubility after the initial acidification from both the Saharan and Urban materials with only slight removal from solution at increased pH. For this element it appears, therefore, that the pH-dependent dissolution process is not reversible. It is tentatively suggested that Fe in the Urban aerosol, under these experimental conditions, is under solubility control.« less

[1] Ammonium salts represent a major component of fine mode marine aerosol that play an important role in climate regulation, atmospheric pH control and nutrient supply to the oceans. We report here the results of analyses of aerosol samples collected in remote areas of the Atlantic Ocean. We show the isotopic abundance of aerosol ammonium varies with concentration, with isotopically light ammonium (−5 to −8‰) associated with the lowest ammonium concentrations compared to isotopically heavy ammonium (+10‰) associated with higher ammonium concentrations. We interpret the light ammonium signature as arising from marine emissions of ammonia. These results provide further evidence for the existence of a marine ammonia source and offers a method to define the relative contribution of marine and terrestrial ammonium to aerosols in remote areas and thereby investigate the role of these emissions in climate regulation.

Within the EUROTRAC programme, the main objectives of our group were devoted to a better understanding of the homogenous and heterogeneous processes which control the atmospheric aqueous phase chemistry and the evolution of paniculate 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.