Chemical characterization of ambient aerosol collected during the southwest monsoon and intermonsoon seasons over the Arabian Sea: Anions and cations

Ambient aerosol samples were collected over the northern Indian Ocean during two 1 month-long research cruises (German R/V Meteor) that took place during the intermonsoon (May) and SW monsoon (July/August) of 1995. A high volume and two small volume collectors were used to collect samples, which were subsequently analyzed for ferrous iron, 32 elements, and anions and cations. The present paper focuses on the bulk aerosol material, the ions, while utilizing some of the trace metal data that were presented in more detail in our previous paper [Siefert et al., 1999]. Data are analyzed and interpreted with the aid of principal component and multiple linear regression analyses. Intermonsoon samples were strongly influenced by continental material, both of crustal and anthropogenic origin. The crustal component (24.5±13% of the total suspended particulate mass (TSP), 6.0±4.4 μg m^(−3)) contained 3.2% gypsum (CaSO_4). While more than half of the TSP (21.2±9.6 μg m^(−3)) during the SW monsoon was sea-salt-derived due to the strong winds prevailing during this season, only 1.7±1.1% (0.7±0.4 μg m^(−3)) was found to be of crustal origin. Sulfate (SO_4^(2−)) sources were determined and quantified with linear regression analyses utilizing specific tracers for the independent variables. Lead (Pb) was found to be a more reliable surrogate for anthropogenic SO_4^(2−) compared to nitrate (NO_3^−) during the relatively polluted intermonsoon. Soluble calcium (Ca^(2+)) served as the tracer for gypsum, and methane sulfonate (MSA) served as the tracer for biogenically derived SO_4^(2−) during both seasons. On the basis of this analysis, 75% of the non-sea-salt sulfate (NSS-SO_4^(2−)) (0.8±0.2 μg m^(−3), representing ∼2.4% of TSP) was found to be of biogenic origin during the SW monsoon with the remaining 25% of anthropogenic origin. During the intermonsoon, NSS-SO_4^(2−) accounted for 2.1±1.2 μg m^(−3) (∼9.2% of TSP) and had a composition that was 65% anthropogenic, 21% biogenic, and 14% gypsum-derived. Linear regression analyses revealed that the bio-SO_4^(2−)/MSA weight ratios appear to be consistent with the temperature dependence proposed by Hynes et al. [1986]. In this case the yield of SO_4^(2−) increased relative to MSA with an increase in temperature. Three samples during the SW monsoon, near the coast of Oman, showed lower temperatures, due to coastal upwelling, than the rest of the samples; at 24°C the bio-SO_4^(2−)/MSA weight ratio was 6.8±0.5. The remainder of the SW monsoon samples were collected at an average temperature of 27.2°C, for which the bio-SO_4^(2−)/MSA weight ratio was 13.5±4.4. At an average temperature of 28.9°C during the intermonsoon, sampling gave a ratio of 17.7±4.8. These observations indicate a temperature dependence factor between 24° and 29°C of 2.2 (i.e., a 2.2 increase in the ratio of bio-SO_4^(2−)/MSA with every degree temperature increase). Cl− deficits determined during both seasons appear to indicate that different mechanisms may govern the observed depletion of Cl− in each season

Chemical composition of aerosols collected over the tropical North Atlantic Ocean

Ambient aerosol samples were collected over the tropical northern Atlantic Ocean during the month of April 1996 onboard the R/V Seward Johnson. Dichotomous high-volume collector samples were analyzed for ferrous iron immediately after collection, while trace metals, anions, and cations were determined upon return to the laboratory. Data are analyzed with the aid of enrichment factor, principal component, and weighted multiple linear regression analyses. Average mineral aerosol concentrations amounted to 19.3±16.4 μg m^(−3) whereby the chemical characteristics and air mass back trajectories indicated the dust to be of a typical shale composition and Saharan origin. Calcite accounted for 3.0 and 7.9% of the mineral aerosol during the first and second halves of the cruise, respectively. Total iron concentrations (averaging 0.84±0.61 μg m^(−3)) are crustally derived, of which 0.51±0.56% is readily released as Fe(II). Eighty-six percent of this Fe(II) is present in the fine (<3 μm diameter) aerosol fraction and correlates with NSS-SO_4^(2−) and oxalate. Approximately 23% of the measured NSS-SO_4^(2−) in both size fractions appears to be biogenically derived, and the rest is of anthropogenic nature. Biogenic SO_4^(2−) /methanesulfonic acid (MSA) ratios could not be easily extracted by employing a multiple linear regression analysis analogous to that of Johansen et al. [1999], possibly due to the varying characteristics of the aerosol chemistry and air temperature during the cruise. Because of the presence of anthropogenic SO_4^(2−), the non-sea-salt (NSS)- SO_4^(2−)/MSA ratio, 37.4±6.4, is elevated over what would be expected if the NSS - SO_4^(2−) were purely biogenic. Cl^− depletion is seen in all samples and averages 18.3±9.1%. The release of Cl from the aerosol phase appears to occur through acid displacement reactions with primarily HNO_3 in the coarse and H_2SO_4 in the fine fraction

Chemical characterization of ambient aerosol collected during the southwest monsoon and intermonsoon seasons over the Arabian Sea: Labile-Fe(II) and other trace metals

Atmospheric deposition of iron (Fe) to certain regions of the oceans is an important nutrient source of Fe to the biota, and the ability of the biota to uptake Fe is dependent on the speciation of the Fe. Therefore understanding the speciation of Fe in the atmosphere is critical to understanding the role of Fe as a nutrient source in surface ocean waters. Labile ferrous iron (Fe(II)) concentrations as well as total concentrations for Fe and other important trace metals, cations, and anions were determined over the Arabian Sea for two nonconsecutive months during 1995. Ambient aerosol samples were collected during the Indian Ocean intermonsoon and southwest monsoon seasons over the Arabian Sea. Sampling took place aboard the German research vessel Meteor in the months of May (leg M32/3; intermonsoon) and July/August (leg M32/5; southwest monsoon). Both cruise tracks followed the 65th east meridian, traveling for 30 days each (from north to south during leg M32/3 and from south to north during leg M32/5). A high-volume dichotomous virtual impactor with an aerodynamic cutoff size of 3 μm was used to collect the fine and coarse aerosol fractions for metal analysis. A low volume collector was used to collect aerosol samples for anion and cation analysis. The analysis for labile-Fe(II) was done immediately after sample collection to minimize any possible Fe redox reactions which might occur during sample storage. The analytical procedure involved filter extraction in a formate/formic acid buffered solution at pH 4.2 followed by colorimetric quantification of soluble Fe(II). Metals, anions, and cations were analyzed after the cruise. Total atmospheric aqueous-labile-Fe(II) concentrations during the intermonsoon were between 4.75 and 80%) was present in the fine fraction (<3.0 μm). During the southwest monsoon, atmospheric aqueous-labile-Fe(II) concentrations were consistently below the detection limit (<0.34 to <0.089 ng m^(−3), depending on the volume of air sampled). Air mass back trajectories (5 day, three dimensional) showed that air masses sampled during the southwest monsoon had advected over the open Indian Ocean, while air masses sampled during the intermonsoon had advected over northeast Africa, the Saudi Arabian peninsula, and southern Asia. These calculations were consistent with the results of the statistical analysis performed on the data set which showed that the variance due to crustal species during the intermonsoon samples was greater than the variance due to crustal species during the southwest monsoon. The factor scores for the crustal components were also greater when the back trajectories had advected over the nearby continental masses. Principal component analysis was also performed with the intermonsoon samples where aqueous labile Fe(II) was above the detection limit. Aqueous labile Fe(II) did not correlate well with other species indicating possible atmospheric processing of the iron during advection

Determination of photochemically available iron in ambient aerosols

Experiments to determine the concentration of photochemically available Fe in ambient aerosol samples were carried out using a novel photochemical extraction procedure. Ambient aerosol samples, which were collected on Teflon filters, were suspended in an aqueous solution within a photochemical reactor and irradiated. Under these conditions, which were favorable to the photochemical weathering of aerosol particles, the relative amount of Fe(II)_(aq) to Fe_(total) was shown to increase. The extent and rate of Fe(II)_(aq) photoproduction was used to characterize the Fe in aerosol samples collected from Whiteface Mountain, New York, Pasadena, California, San Nicholas Island, California, and Yosemite National Park, California. Photochemically available Fe concentrations found ranged from <4 ng m^(−3) (0.07 nmole m^(−3)) to 308 ng m^(−3) (5.52 nmole m^(−3)), Fe_(total) concentrations ranged from 10 ng m^(−3) (0.18 nmole m^(−3)) to 3400 ng m^(−3) (61 nmole m^(−3)), and the percentage of photochemically available Fe to Fe_(total) ranged from 2.8% to 100%. Aerosol samples were also collected during biomass burning events in southern California; these samples showed insignificant changes in the photochemically available Fe (compared to nonbiomass burning samples) in conjunction with large increases of Fe_(total). Calculations based on these experiments also provide further evidence that redox reactions of Fe in cloudwater could be an important in situ source of oxidants (OH, HO_2/O_2^−). The estimated oxidant production rate in cloudwater based on these experiments is between 0 and 60 nM s^(−1), with an average value of 16 nM s^(−1). This estimated in situ oxidant production rate due to Fe chemistry is approximately equal to previous estimates of the oxidant flux to cloudwater from the gas phase

An extensive bloom of the N₂-fixing cyanobacterium Trichodesmium erythraeum in the central Arabian Sea

We encountered an extensive surface bloom of the N, fixing cyanobactenum Trichodesrniurn erythraeum in the central basin of the Arabian Sea during the spring inter-monsooon of 1995. The bloom, which occurred dunng a period of calm winds and relatively high atmospheric iron content, was metabollcally active. Carbon fixation by the bloom represented about one-quarter of water column primary productivity while input by N₂ flxation could account for a major fraction of the estimated 'new' N demand of primary production. Isotopic measurements of the N in surface suspended material confirmed a direct contribution of N₂ fixation to the organic nltrogen pools of the upper water column. Retrospective analysis of NOAA-12 AVHRR imagery indicated that blooms covered up to 2 X 10⁶ km², or 20% of the Arabian Sea surface, during the period from 22 to 27 May 1995. In addition to their biogeochemical impact, surface blooms of this extent may have secondary effects on sea surface albedo and light penetration as well as heat and gas exchange across the air-sea interface. A preliminary extrapolation based on our observed, non-bloom rates of N₂ fixation from our limited sampling in the spring intermonsoon including a conservative estimate of the input by blooms, suggest N₂ fixation may account for an input of about 1 Tg N yr⁻¹. This is substantial, but relatively minor compared to current estimates of the removal of N through denitrification in the basin. However, N₂ fixation may also occur in the central basin through the mild winter monsoon, be considerably greater during the fall intermonsoon than we observed during the spring intermonsoon, and may also occur at higher levels in the chronically oligotrophic southern basin. Ongoing satellite observations will help to determine more accurately the distribution and density of Trichodesmium in this and other tropical oceanic basins, as well as resolving the actual frequency and duration of bloom occurrence

Identification of Hammerhead Ribozymes in All Domains of Life Reveals Novel Structural Variations

Hammerhead ribozymes are small self-cleaving RNAs that promote strand scission by internal phosphoester transfer. Comparative sequence analysis was used to identify numerous additional representatives of this ribozyme class than were previously known, including the first representatives in fungi and archaea. Moreover, we have uncovered the first natural examples of “type II” hammerheads, and our findings reveal that this permuted form occurs in bacteria as frequently as type I and III architectures. We also identified a commonly occurring pseudoknot that forms a tertiary interaction critical for high-speed ribozyme activity. Genomic contexts of many hammerhead ribozymes indicate that they perform biological functions different from their known role in generating unit-length RNA transcripts of multimeric viroid and satellite virus genomes. In rare instances, nucleotide variation occurs at positions within the catalytic core that are otherwise strictly conserved, suggesting that core mutations are occasionally tolerated or preferred

The role of coastal zones in global biogeochemical cycles

The unique and dynamic coastal ocean is a significant source and sink of a multitude of atmospheric species of importance to global biogeochemical cycles and climate. The transition zone between land and ocean, including the atmosphere as a medium for the exchange of matter and energy, is characterized by a strong physical-biogeochemical coupling, resulting in an inherently complex system. Important biogeochemical exchanges occurring in the coastal zone involve water, nutrients (e.g., nitrogen, phosphorous, iron, and silica),salts (e.g., chlorine,bromine,and iodine), carbon (e.g.,dissolved organic carbon (DOC),dissolved inorganic carbon (DIC), particulate organic carbon (POC), and carbon dioxide (CO2), reactive organic trace gases (e.g., nitrogenous, halogenated, and sulfurous hydrocarbons), and inorganic trace gases (e.g., nitrous oxide, N2O). Coastal zones are of particular importance to humans, as they are characterized by high per area productivity and are responsible for the majority of the world's fish catch. In addition, coastal ecosystems play an important role in the global carbon cycle as large fluxes of carbon and carbon-related tracers move between the land, ocean, and atmosphere in these regions. Most of the world's population lives near coastal zones, and anthropogenic changes and related climate change in these regions can pose serious consequences not only for fisheries but also for global biogeochemical cycles

Luxury uptake of aerosol iron by Trichodesmium in the western tropical North Atlantic

Dust transported from North Africa carries micronutrient iron (Fe) to the western tropical North Atlantic (WTNA) which may significantly influence the metabolism of the N2-fixing cyanobacteria, Trichodesmium. For the first time, we conducted shipboard incubation experiments using freshly collected aerosol, seawater, and Trichodesmium colonies. Trichodesmium assimilated significant amount of aerosol Fe up to 14 times higher than the control. The uptake amount increased proportionally to the P: Fe ratio that Trichodesmium initially contained and to the aerosol Fe added and leached to the incubation solution. Trichodesmium assimilated more aerosol Fe than needed for its maximum growth (0.14 d-1) demonstrating a high capacity of luxury uptake of Fe from the dust. Copyright 2011 by the American Geophysical Union.This work is sponsored by China Shanghai Pujiang Program (09PJ1401200), China National Natural Science Foundation (41005075), and Program for New Century Excellent Talents in University (NCET-09-0308).Peer Reviewe

Photoreduction of Iron Oxyhydroxides and the Photooxidation of Halogenated Acetic Acids

The photolytic reduction of ferrihydrite (am-Fe_2O_3*3H_2O), lepidocrocite (γ-FeOOH), goethite (a-FeOOH), hematite (α-Fe_2O_3), maghemite (γ-Fe_2O_3) and iron-containing aerosol particles (Fe_(aerosol)) in the presence of a series of halogenated acetic acids has been investigated. The fastest rates of photoreduction of Fe(lll) to Fe(ll) were achieved with ferrihydrite as an electron acceptor and fluoroacetic acid as an electron donor. The relative rates of photooxidation of the monohalogenated acetic acids with ferrihydrite in order of decreasing reactivity were as follows: FCH_2CO_2H > CICH_2CO_2H > BrCH_2CO_2H > ICH_2CO_2H; for multiple substituents the relative order of reactivity was as follows: FCH_2CO_2H > F_2CHCO_2H > F_3CCO_2H. With respect to the iron oxide electron acceptors, the relative order of reactivity toward monohaloacetate oxidation was am-Fe_2O_3-3H_2O > γ-Fe_2O_3 > γ-FeOOH ≥ α-Fe_2O_3 ≥ Fe_(aerosol) > α-FeOOH. Strong kinetic isotope effects observed for the photooxidation of CICD_2CO_2H suggest that the oxidation of the mono- and disubstituted haloacetic acids proceeds via hydrogen-atom abstraction by surface-bound hydroxyl radicals to produce haloacetate radicals, which in turn yield the corresponding halide and glycolic acid. Fully halogenated haloacetic acids appear to be oxidized via a photo-Kolbe mechanism to yield the corresponding halo acids and CO_2

Iron photochemistry of aqueous suspensions of ambient aerosol with added organic acids

Experiments to simulate cloudwater conditions were carried out by using ambient aerosol samples suspended in an aqueous solution. Electron donors known to exist in atmospheric cloudwater (oxalate, formate, or acetate) were then added to the simulated cloudwater, and the solution irradiated with ultraviolet light while important species were measured (i.e., H_2O_2, Fe_(total), Fe(II)_(aq), and pH). A total of four different ambient aerosol samples were used in the simulated cloudwater experiments; they were collected from (1) Whiteface Mountain, NY, (2) Pasadena, CA, and (1) Sequoia National Park, CA. Hydrogen peroxide (H_2O_2) photoproduction was observed in the simulated cloudwater experiments with added oxalate. Fe(II)_(aq) photoproduction was observed in the simulated cloudwater experiments with and without added acetate or added formate using ambient aerosol collected simultaneously with the ambient aerosol used in the added oxalate experiments. The production of Fe(II)_(aq) showed that Fe from the ambient aerosol was available for photochemical redox reactions. In all cases, the production rates for Fe(II)_(aq) and H_2O_2 in the light were greater than production rates in nonirradiated control experiments. The simulated cloudwater experiments (with four different aerosol samples) showed similar behavior to previous experiments carried out with synthetic Fe-oxyhydroxy polymorphs in the presence of oxalate, formate, or acetate. The Fe present in the ambient aerosol appears to be a critical component for the production of H_2O_2 in the simulated cloudwater experiments