Non-sea Salt Sulfate Research Articles

Chemical characteristics of aerosols at coastal station in Hong Kong. I. Seasonal variation of major ions, halogens and mineral dusts between 1995 and 1996

Aerosols samples (total suspended particulate “TSP” and PM 10 particulate) were collected at coastal monitoring station in Hong Kong between 1995 and 1996. They were analyzed to investigate the seasonal cycle among major ions (Na +, NH 4 +–N, K +, Mg 2+, Ca 2+, Cl −, NO 3 −, and SO 4 2−), halogen elements (Br, I) and mineral dusts. The concentration of major ions showed a summer minimum and a winter maximum in a coastal region of Hong Kong. Halogen (Br, I) and dust concentrations exhibited a very similar variation. From the molar equivalence of Na + and Cl −, it was found that the Cl − in TSP or PM 10 was deviated from sea-salt (NaCl) component. Bromine (Br) in aerosols evidently originated from marine source but iodine (I) in aerosols may have been generated from natural and anthropogenic sources including possible biomass emission. The ratios of nonsea-salt sulfate (nss-SO 4 2−) to nitrate (NO 3 −) and trace elements (As, Sb, Se, Pb, V and Zn) were used to explain the different pollution emissions.

Small-Scale Horizontal Variations in Ionic Concentrations of Bulk Deposition from Hong Kong

The concentrations of major anions andcations in bulk deposition were found to varyspatially by factors of up to 65, for two samplings at17 sampling sites separated by 45 km or less in theTerritory of Hong Kong. Under predominantly maritimeairstreams, these variations resulted from localemissions, as well as from lower seasaltconcentrations at the more northerly sites. The weakacidity of the bulk deposition resulted fromneutralization of hydrometeors by basic particulatematter, leading to an unusual anticorrelation of theconcentrations of non-seasalt sulphate and hydrogenions. Greater anthropogenic activity at inland andindustrial sites was generally marked by highernitrate concentrations, with the ratio of nitrate ionconcentrations from the two samplings being stronglycorrelated with that of calcium ion. Chloride lossincreased with increasing concentrations ofnon-seasalt nitrate and sulphate. Most of the variancein the bulk deposition data for 8 major anions andcations could be explained by 2 factors: seasalt andanthropogenic, in common with the 1997 EnvironmentalProtection Department data for these species in totalsuspended particulate matter. Results for 14 solubletrace metal species in the bulk deposition at thesites are also reported.

O3 oxidation of SO2 in sea‐salt aerosol water: Size distribution of non‐sea‐salt sulfate during the First Aerosol Characterization Experiment (ACE 1)

Non sea‐salt sulfate (NSS) of 2.2–2.3 nmol m−3 total magnitude in aerosols observed during the First Aerosol Characterization Experiment (ACE‐1) at Cape Grim, Tasmania, was trimodally distributed with ∼1 nmol NSS m−3 in >0.7 μm ambient diameter (diam) coarse sea‐salt mode aerosols; despite this low NSS concentration, [H2SO4(g)] was so low that <1% of this NSS could have been due to H2SO4 scavenging. Ammonium was not associated with these coarse sea‐salt aerosols, suggesting that cloud processing was a minor contributor to the NSS found in >0.7 μm diam aerosols. The mechanism of O3 oxidation of SO2 in sea‐salt aerosol water (SSAW) is assessed for its capability to explain this coarse aerosol NSS. Limitation of this mechanism's NSS contribution is largely due to SSAW's buffering capacity since its reaction rate is reduced by 2 orders of magnitude at pH 6 versus the pH≥8 of unreacted SSAW. However, the buffering capacity of sea‐salt aerosols may have been significantly enhanced over that of bulk seawater alkalinity. This appears to be due to carbonate resulting from small fragments of biogenic CaCO3 in the ocean's surface microlayer. Given the observed nonsoil calcium excess over that in bulk seawater, the estimated actual buffering capacity of sea‐salt aerosols observed during ACE 1 was 50%, or more, enhanced over that due to bulk seawater alkalinity. Considering this enhanced buffering capacity, O3 oxidation of SO2 in SSAW can produce sufficient NSS to explain 70–90% of the ∼1 nmol m−3 found in >0.7 μm diam coarse sea‐salt aerosols with cloud processing and further oxidation of SO2 in SSAW (i.e., pH<6) by other sea‐salt conversion mechanisms contributing the remainder. The amount of NSS produced by sea‐salt conversion mechanisms during the ACE 1 remote Southern Ocean experiment vied with homogeneous and cloud processing in their contribution to the total observed NSS of 2.2–2.3 nmol m−3.

Heterogeneous sulfate production in the remote marine environment: Cloud processing and sea‐salt particle contributions

Heterogeneous sulfate production in the remote marine boundary layer is studied by combining measurements collected during the first Southern Hemisphere marine Aerosol Characterization Experiment (ACE 1) with two numerical models: one simulating heterogeneous sulfate production in sea‐salt aerosol water (SSAW) and the other simulating cloud processing. The models calculate oxidation of SO2 by O3 and H2O2 via aqueous‐phase reactions both in SSAW and in cloud droplets. Model calculations indicate that as much as 50–75% of the observed non‐sea‐salt (NSS) sulfate in particles in the ambient diameter size range of 0.9–16 μm during the ACE 1 measurement period is due to production in SSAW, with cloud processing producing the remainder. The initial alkalinity of the wind‐generated sea‐salt aerosols can strongly influence sulfate production. Changing the effective alkalinity of SSAW from a value typical of seawater to a value based on the measurements taken at Cape Grim results in a better match to the observed NSS sulfate size distribution with up to 75% of the predicted supermicrometer NSS sulfate being due to heterogeneous conversion in SSAW. The average rates of removal of sulfate due to dry and wet deposition for the 0.4–16 μm size range are 38 and 14 ng m−3 d−1 respectively.

Effects of ship emissions on sulphur cycling and radiative climate forcing over the ocean

The atmosphere overlying the ocean is very sensitive—physically, chemically and climatically—to air pollution. Given that clouds over the ocean are of great climatic significance, and that sulphate aerosols seem to be an important control on marine cloud formation1, anthropogenic inputs of sulphate to the marine atmosphere could exert an important influence on climate. Recently, sulphur emissions from fossil fuel burning by international shipping have been geographically characterized2, indicating that ship sulphur emissions nearly equal the natural sulphur flux from ocean to atmosphere in many areas3. Here we use a global chemical transport model to show that these ship emissions can be a dominant contributor to atmospheric sulphur dioxide concentrations over much of the world's oceans and in several coastal regions. The ship emissions also contribute significantly to atmospheric non-seasalt sulphate concentrations over Northern Hemisphere ocean regions and parts of the Southern Pacific Ocean, and indirect radiative forcing due to ship-emitted particulate matter (sulphate plus organic material) is estimated to contribute a substantial fraction to the anthropogenic perturbation of the Earth's radiation budget. The quantification of emissions from international shipping forces a re-evaluation of our present understanding of sulphur cycling and radiative forcing over the ocean.

Characteristics of dimethylsulfide, ozone, aerosols, and cloud condensation nuclei in air masses over the northwestern Pacific Ocean

Long‐term measurements of several trace gases and aerosols were carried out from December 1994 to October 1996 at Ogasawara Hahajima Island over the northwestern Pacific Ocean. The continental impact on the concentrations of sulfur compounds, ozone (O3), and cloud condensation nuclei (CCN) was estimated on the basis of the classification of air mass into seven types by isentropic trajectory analysis. From May to October, the air mass originating from the central North Pacific Ocean is predominant and regarded as the clean marine air for the concentrations of sulfur compounds and CCN. From the results of the molar ratio of methane sulfonic acid to non‐sea‐salt sulfate (NSS) and the positive correlation between dimethylsulfide (DMS) and CCN in this air mass it can be concluded that DMS largely contributes to the production of NSS and CCN. On the other hand, continental and anthropogenic substances are preferably transported to the northwestern Pacific Ocean by the predominant continental air mass from November to March. The enhancement of concentrations by the outflow from the Asian continent are estimated by a factor of 2.8 for O3, 3.9 for SO2, 3.5 for CCN activated at 0.5% supersaturation (0.5% CCN), 4.7 for 1.0% CCN, and 5.5 for NSS. Moreover, the CCN supersaturation spectra are also affected by the continental substances resulting in factor 2 of enhancement of cloud droplet number concentration. The diurnal variations of DMS and O3 for each air mass show a pattern of daytime minimum and nighttime maximum, which are typically found in remote ocean, even though those amplitudes are different for each air mass. Consequently, it can be concluded that the influence of nitric oxides (NOx) for the daytime O3 production and nitrate (NO3) radical for the nighttime oxidation of DMS are small even in the continental air mass.

Detection of non-sea salt sulphate aerosol at a remote coastal site in South Africa: A PIXE study

An aerosol sampling site based on a single-stage streaker sampler with four-hour time resolution was established in 1993 at Brandt-se-Baai, on the north-western coast of South Africa (31.5°S 18°E, 50 m asl). The site was deployed to monitor the generation of dust from the local mining operations and in part to test the hypothesis of large-scale air mass recirculation over Southern Africa. Streaker samples were analysed by PIXE for the usual crustal derived elements and sulphur. Sulphate aerosols over southern Africa are produced in the atmosphere by oxidation of SO2, from industry, or DMS, from the ocean. Industrial related sulphate plays a key role in tracing long-range transport of anthropogenic pollutants.We present results from selected monthly samples collected during 1996. Measured particulate sulphur has been apportioned between contributions from sea spray and non-sea salt sulphate (NSS) aerosols. Marine particulate sulphur, present in the regional background, was traced by its correlation with Na and Cl, which were routinely detected. The NSS content of these air masses was enriched with respect to the expected sea salt sulphur by two orders of magnitude. Al, Si, K, Ca, Ti and Fe, the major crustal elements, were measured only episodically with the occurrence of strong south-easterly winds. These winds are associated with wind blown dust from a mine located 4 km from the site.These results are the first direct aerosol characterisation of the remote region of the western seaboard of southern Africa. Specifically the sulphur measurements at this site provide insights into relative contributions of natural DMS, industrial and sea salt contributions of particulate sulphate.

Factors affecting the levels of hydrogen peroxide in rainwater

Measurements of hydrogen peroxide (H 2O 2) and several meteorological and chemical parameters were made for 34 rain events which occurred in Miami, Florida between April, 1995 and October, 1996. The measured H 2O 2 concentrations ranged from 0.3 to 38.6 μM with an average concentration of 6.9 μM. A strong seasonal dependence for H 2O 2 concentrations was observed during this period, with highest concentrations in the summer and lower levels in the winter, which corresponds to the stronger solar radiation and higher vaporization of volatile organic compounds (VOCs) in the summer and fall, and the weaker sunlight and lower vaporization in the winter and spring. Measurements also showed a significant increase trend of H 2O 2 with increasing ambient rainwater temperature. Rains that were out from lower latitude were exposed to higher solar irradiation and contained relatively higher levels of H 2O 2 than those from the north. All these observations indicate that photochemical reactions that involved volatile organic compounds are the predominant source of H 2O 2 observed in rainwater. During several individual rainstorms, H 2O 2 concentration was found to increase as a function of time due to electrical storm activities. This finding suggests that lightning could be an important factor that determines the level of H 2O 2 during thunderstorms. Statistical data showed that the highest concentrations of H 2O 2 were observed only in rains containing low levels of nonsea-salt sulfate (NSS), nitrate and hydrogen ion. H 2O 2 concentrations in continental originated rains were much lower than marine originated ones, indicating that air pollutants in continental rains could significantly deplete the H 2O 2 concentration in atmospheric gas-phase, clouds and rainwater.

Dimethyl sulfide oxidation in the equatorial Pacific: Comparison of model simulations with field observations for DMS, SO2, H2SO4(g), MSA(g), MS and NSS

Reported here are results from an airborne photochemical/sulfur field study in the equatorial Pacific. This study was part of NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A program. The focus of this paper is on data gathered during an airborne mission (P‐3B flight 7) near the Pacific site of Christmas Island. Using a Lagrangian‐type sampling configuration, this sortie was initiated under pre‐sunrise conditions and terminated in early afternoon with both boundary layer (BL) as well as buffer layer (BuL) sampling being completed. Chemical species sampled included the gas phase sulfur species dimethyl sulfide (DMS), sulfur dioxide (SO2), methane sulfonic acid (MSA)g, and sulfuric acid (H2SO4)g. Bulk aerosol samples were collected and analyzed for methane sulfonate (MS), non‐sea‐salt sulfate (NSS), Na+,Cl−, and NH4+. Critical non‐sulfur parameters included real‐time sampling of the hydroxyl radical (OH) and particle size/number distributions. These data showed pre‐sunrise minima in the mixing ratios for OH, SO2, and H2SO4 and post‐sunrise maxima in the levels of DMS, OH, and H2SO4. Thus, unlike several previous studies involving coincidence DMS and SO2 measurements, the Christmas Island data revealed that DMS and SO2 were strongly anticorrelated. Our “best estimate” of the overall efficiency for the conversion of DMS to SO2 is 72±22%. These results clearly demonstrate that DMS was the dominant source of SO2 in the marine BL. Using as model input measured values for SO2 and OH, the level of agreement between observed and simulated BL H2SO4(g) profiles was shown to be excellent. This finding, together with supporting correlation analyses, suggests that the dominant sulfur precursor for formation of H2SO4 is SO2 rather than the more speculative sulfur species, SO3. Optimization of the fit between the calculated and observed H2SO4 values was achieved using a H2SO4 first‐order loss rate of 1.3 × 10−3 s−1. On the basis of an estimated total “wet” aerosol surface area of 75 µm2/cm3, a H2SO4 sticking coefficient of 0.6 was evaluated at a relative humidity of ≃95%, in excellent agreement with recent laboratory measurements. The Christmas Island data suggest that over half of the photochemically generated SO2 forms NSS, but that both BL NSS and MS levels are predominantly controlled by heterogeneous processes involving aerosols. In the case of MS, the precursors species most likely responsible are the unmeasured oxidation products dimethyl sulfoxide (DMSO) and methane sulfinic acid (MSIA). Gas phase production of MSA was shown to account for only 1% of the observed MS; whereas gas phase produced H2SO4 accounted for ∼20% of the NSS. These results are of particular significance in that BL‐measured values of the ratio MS/NSS have often been used to estimate the fraction of NSS derived from biogenic DMS and to infer the temperature environment where DMS oxidation occurred. If our conclusions are correct and both products are predominantly formed from complex and still poorly characterized heterogeneous processes, it would suggest that for some environmental settings a simple interpretation of this ratio might be subject to considerable error.

Nitrite Variability in Coastal North Carolina Rainwater and Its Impact on the Nitrogen Cycle in Rain

The concentration range for nitrite (NO2-) in 115 rain samples collected in Wilmington, NC, from June 1996 through February 1998 was 0.022−0.603 μM. Nitrite concentrations did not correlate with precipitation volume, suggesting a continuous supply of nitrite during rain events possibly by slow scavenging of gas-phase material such as HONO(g) or NO2(g) or in-cloud oxidation of other reduced forms of nitrogen. Nitrite levels exhibited no seasonal oscillations, which is in contrast to other rainwater parameters at this site such as pH, nitrate, non-seasalt sulfate (NSS) and ammonium. Concentrations of nitrite did not correlate with concentrations of pollutant indicators (pH, nitrate, and NSS). The concentration of nitrite in both winter El Nino rains and summer tropical rains was less than half its concentration in non-El Nino or non-tropical events, suggesting a possible terrestrial source of nitrite or nitrite precursor. Controlled laboratory studies demonstrated that hydrogen peroxide can oxidize nitrite in rainwater at environmentally relevant H+, H2O2, and NO2- concentra tions. Nitrite oxidation by H2O2 in the aqueous phase has important ramifications with respect to N cycling and acid generation within the troposphere.

Processes relating aerosol number, aerosol volume and non-seasalt sulfate concentrations in the clean and polluted marine boundary layer

Processes relating aerosol number, aerosol volume and non-seasalt sulfate concentrations in the clean and polluted marine boundary layer

Spectrophotometric determination of hydrogen peroxide in rainwater

Hydrogen peroxide in rainwater has been determined from the measurement of absorption at 432 nm after the formation of the stable oxo-peroxo-pyridine-2,6-dicarboxylatovanadate(V) complex (OPDV) in acid media. Determinations were in good agreement with those using a modified p-hydroxyphenylacetic acid (PHPA) emission method. The linear range for the OPDV method is from 0.05 to 50 ppm H 2O 2, and the limit of detection under the analytical conditions employed was 5.8 nmol H 2O 2 for a 20 cm 3 rainwater sample. The volume weighted mean H 2O 2 concentration in rainwater collected at roof-top level in central Kowloon was 15.9 μM ( N=10), and the ratio of non-sea salt sulphate and H 2O 2 concentrations was significantly correlated with pH. The OPDV absorption method was also employed for the determination of ambient gaseous H 2O 2, and liquid-nitrogen cold-trap collection gave results about 30% lower than impinger sampling, where artifact H 2O 2 formation occurred.

Filter and impactor measurements of anions and cations during the First Aerosol Characterization Experiment (ACE 1)

During the First Aerosol Characterization Experiment (ACE 1), we measured the concentrations of aerosol non‐sea‐salt sulfate (NSS), methane sulfonate (MS), ammonium, sodium, nitrate, and a variety of other anions and cations from the National Center for Atmospheric Research C‐130Q aircraft and the Cape Grim Baseline Air Monitoring Station. We used the data to test a variety of hypotheses concerning the sources and properties of aerosol in remote environments. We noted a large gradient in free tropospheric (FT) MS, with much more in the spring (southern) hemisphere, but FT NSS showed no latitudinal gradient. Most species had strong vertical concentration gradients, generally with more material near the biogenic sources of the marine boundary layer (MBL) than in the FT. During the Lagrangian experiments and in time series measurements at Cape Grim, we saw daytime increases of photochemically derived MS, with more constant or decreasing concentrations at night. NSS also increased in the daytime during the Lagrangian experiments. At Cape Grim, we were unable to account for the gravimetric mass of particles on impactor substrates using the measured ions, which implies that a substantial fraction of the aerosol at that location must be either organic or mineral matter. Our FT ratios of MS/NSS exhibited a strong latitudinal gradient, with the southernmost values similar to those found in Antarctic plateau snowfall.

名古屋近郊域降水中に含まれるイオン濃度の季節変化

A continuous sampling at every 0.5mm of precipitation during rain (average sampling time of 19 minutes) and analyses of ions involved in the rainwater were carried out from April 1, 1993 to March 31, 1994 in Nagakute, 16km east of Nagoya, Japan, to understand temporal changes in ion concentrations in precipitation in detail. The average pH, nitrate (NO3-), and non-seasalt sulfate (nss-SO42-) in the collected rainwaters were 4.76, 14.5μeq l-1 and 18.0μeq l-1, respectively. The average concentrations of ions obtained in this study were about half of those obtained during 1984 and 1986 by the Japan Environmental Agency, which was thought to be caused by either the longer rainy season in 1993 or the reduction of sulfur dioxide emission in recent years.The seasonal changes in ion concentrations show that they were significantly influenced by Asian dust storms and typhoons. Asian dust storms and typhoons were found to play important role in characterizing the precipitation chemistry in Japan.A rapid change in ion concentrations during several minutes was frequently found in individual precipitation accompanied by weather change. Concentrations of NO3-, NH4+, and SO42- decreased with an increase in wind speed, while those of Na+ and Cl- increased, showing the differences in the production and the deposition processes of ions. Higher concentrations of NO3- and nss-SO42- in rain samples with southern wind direction were frequently observed, showing a clear relation to the industrial facilities of ion sources located in the southern direction. A greater amount of ammonium ion (NH4+) was observed to be transported from the suburbs of Nagoya rather than from the downtown area. This measurement (short period sampling and continuous observation) showed characteristics typical of a precipitation chemistry, especially for the seasonal concentration change in major cations and anions and ion transport processes, observed in suburbs in the eastern Asia region.

Springtime peaks of trace metals in Antarctic snow.

Drifting snow samples were collected at Asuka Station (71 degrees 32'S, 24 degrees 08'E, 930 m above sea level) over a period from July to December 1991; 36 elements (including Na, Mg, K, Ca, Fe, Al, Li, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Se, Rb, Sr, Cd, Pb, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Th) in snow were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) by direct sample introduction. Concentrations of Cl-, NO3-, and SO(4)2- in the snow were also determined by ion chromatography. In late September to early October, there was a pronounced peak concentration of most of the elements together with non-sea salt sulfate. Enrichment factor analyses suggest that Na, Mg, Ca, K, and Sr are of marine origin and Al, Fe, Mn, Rb, Cr, Ni, Ga, V, and all the rare earth elements are of crustal origins. Volcanic eruption of Mt. Pinatubo (June 1991) and Mt. Hudson (August 1991) could be the reason for the precipitation of Pb, Cd, Cu, Zn, and Se together with non-sea salt sulfates in the austral spring at Asuka Station.

Contribution of particulate sulfate and organic carbon to cloud condensation nuclei in the marine atmosphere

To identify the source of cloud condensation nuclei(CCN) in the marine atmosphere, simultaneous measurements of aerosol chemical components and CCN concentration were made over more than a year at Hahajima, the Ogasawara Islands, located on the Northwest Pacific Ocean in the lee of Eastern Asian large pollution sources. Large loading of CCN in continental air mass and long‐distance transportation to the remote ocean were confirmed. Non‐seasalt sulfate and water soluble organic carbon were dominant in fine aerosols and their concentration revealed significant relationships with CCN concentration, suggesting that not only sulfate but also organic aerosols play a key role in CCN formation, especially in the background atmospheric condition.

Methanesulphonate and non-sea salt sulphate in aerosol, snow, and ice on the East Antarctic plateau

This investigation reports the first simultaneous measurement of methanesulphonate (MSA) and non-sea salt sulphate (NSSS) in aerosols, surface snow, and ice core samples for a continental site in Antarctica (78°S, 139°E, elevation 2849 m). Aerosol MSA concentrations ranged from 0.09–0.43 nmol m−3 STP (median 0.14 nmol m−3) and were generally lower than those observed at coastal Antarctic sites. NSSS concentrations varied from 0.66–1.32 nmol m−3 stp (median 0.88 nmol m−3), comparable to those reported for other continental Antarctic locations. Whereas the MSA:NSSS molar ratio in aerosol samples was in the range 12.7–32.5% (median 17.0%), the ratio down a snow pit and ice profile varied from 1.14–55.6% (median 3.50%), reflecting the variability to be expected over a period of a decade. The chemical composition and low MSA content suggests an origin of aerosols consistent with long range transport from mid-latitudes.

Marine sulphur emissions

The principal volatile sulphur species found in seawater are dimethyl sulphide (DMS), carbonyl sulphide (COS) and carbon disulphide (CS2. Of these, DMS is the most abundant and widespread in its distribution. The predominant oceanic source of DMS is dimethylsulphonioproprionate (DMSP), a compatible solute synthesized by phytoplankton for osmoregulation and/or cryoprotection. Not all species have the same ability to form DMSP; for example, diatoms generally produce little, whereas prymnesiophytes and some dinoflagellates make significantly larger amounts. Much of the release of DMSP and DMS to the water occurs on death or through predation of the plankton. Our recent field data strongly suggest that oxidation of DMS to dimethyl sulphoxide (DMSO) is an important process in the water column, and it is clear that considerable internal cycling in the DMSP/DMS/DMSO system occurs in the euphotic zone. A fraction of the DMS crosses the sea surface and enters the atmosphere where it is oxidized by radicals such OH and NO3to form products such as methanesulphonate (MSA), DMSO and non-sea salt sulphate (NSSS) particles. These particles are the main source of cloud condensation nuclei (CCN) over oceanic areas remote from land.

Sources of atmospheric methanesulphonate, non-sea-salt sulphate, nitrate and related species over the temperate South Pacific

Aerosol species and trace gases were collected during three intensive sampling periods representing winter (2 July to 9 August 1991), spring (29 September to 6 November 1991) and summer (15 January to 29 February 1992) at Baring Head on the southern end of New Zealand's North Island. In remote marine air, mean winter, spring and summer aerosol phase concentrations (ng m −3) were, respectively, non-sea-salt sulphate (NSSS) = 115, 139 and 187, methanesulphonate (MSA) = 1.5, 23 and 48, nitrate (NOD = 293, 70 and 84, and ammonium (NH 4 +) = 44, 39 and 59. Mean gas phase concentrations were sulphur dioxide (SO 2 −) = 39,39 and 30, nitric acid (HNO 3) = 18,30 and 27 and ammonia (NH 3) = 74,42 and 31. Although natural sources for atmospheric reactive sulphur and nitrogen predominate in this region, evidence was also found for the existence of additional, non-oceanic sources of NOn3 and NSSS, including a long-range transport source of ammonium sulphate. Diurnal fluctuations were seen with lower SO 2and NH 3 concentrations at night, while nighttime sources of NSSS and HNO3 were indicated. Molar ratios of MSA/NSSS in remote air were 1.3, 16.3 and 25.7% for winter, spring and summer samples, respectively. These could be only partially explained by known DMS oxidation mechanisms, and a low, relatively constant background concentration of non-DMS NSSS appeared to be present at all times.

Tropospheric aerosol ionic composition in the Eastern Mediterranean region

Concentrations of the major soluble ions: sulfate (SO4=), nitrate (NO3-), chloride (Cl-), methanesulfonate (MSA), sodium (Na+), ammonium (NH4+)), potassium (K+), calcium (Ca++) and magnesium (Mg++), have been measured in 49 aerosol samples collected over a year period at a coastal site of the island of Crete in the Eastern Mediterranean. Significant correlations have been observed between nss-SO4= (nonsea salt sulfate) and NH4+ and Cl- and NO3-. The NH4+/SO4= equivalent ratio was 0.32. Comparison between total measured concentrations of anions and cations shows a clear deficit of cations. Correlation observed between the cation deficit and the nss-SO4= concentrations suggests that H+ is probably the missing cation. Taking into consideration MSA/nss-SO4= ratios, the contribution of biogenic sulfur in the Eastern Mediterranean was evaluated between 0.6 and 28.3%. The variations in the ion concentrations are discussed conjointly with meteorological data and 5-day back trajectories of air masses.