[ { "id": "authors:6rc8e-jf844", "collection": "authors", "collection_id": "6rc8e-jf844", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150812-143043894", "type": "article", "title": "Measurements of Trace Metal (Fe, Cu, Mn, Cr) Oxidation States in Fog and Stratus Clouds", "author": [ { "family_name": "Siefert", "given_name": "Ronald L.", "clpid": "Siefert-R-L" }, { "family_name": "Johansen", "given_name": "Anne M.", "clpid": "Johansen-A-M" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" }, { "family_name": "Pehkonen", "given_name": "Simo O.", "clpid": "Pehkonen-S-O" } ], "abstract": "The oxidation state of four transition metals (Fe, Mn, Cu, and Cr) in cloudwater has been investigated during several cloud events at Whiteface Mountain (NY), one cloud event at San Pedro Hill (CA), and one fog event at Bakersfield (CA). Samples were collected and immediately analyzed for the oxidation states of four transition metals: Fe(II) [44 measurements], Cu(I) [30 measurements], Mn(IV) [27 measurements], and Cr(III) [3 measurements]. Extreme care was taken to minimize contamination and interferences when measuring these oxidation states. Particulate and dissolved concentrations of these metals were also determined. Other measurements performed\u2014relevant to the redox chemistry of these metals\u2014included pH, total elemental concentrations (Fe, Cu, Mn, Cr, Al, K, Ca, Na, and Mg), organic anions (formate, acetate, glycolate, oxalate), inorganic anions (chloride, sulfate, nitrate, sulfite), cations (sodium, calcium, magnesium, potassium), peroxides, and formaldehyde. These measurements were then used in thermodynamic-speciation models to understand the speciation of ambient fog and cloudwater. From this analysis, two different cases were found for Fe(III)soluble speciation. Fe(III) was found to exist either as Fe(OH)_2 ^+ or Fe(Oxalate)_2. However, an unidentified strong chelating ligand with Fe(III) was also suggested by the data. Cu(I) and Cu(II) were calculated to be predominantly Cu^+ and Cu^(2+) (with less than 10% as Cu(II)-oxalate complexes). A chemical kinetic model was also used to investigate the transition-metal chemistry. The model results indicate that Fe(II) should be the predominant chemical form of Fe during daylight conditions. This prediction is in agreement with the field measurements in which the highest ratios of Fe(II)/Fe total were found in samples collected during the day. The model results also indicated that Fe(III) should be the predominant form of Fe during nighttime conditions, which is also in agreement with the field measurements. In the model, Cu(II) and Mn(II) were the predominant oxidation states during daylight and nighttime conditions with Cu(I) and Mn(III) increasing during daylight conditions. Mn(III) concentrations were never high enough to influence the redox chemistry of Cr. Overall, Cr(VI) in cloudwater is predicted to be reduced to Cr(III) if free S(IV) is present.", "doi": "10.1080/10473289.1998.10463659", "issn": "1096-2247", "publisher": "Taylor & Francis", "publication": "Journal of the Air & Waste Management Association", "publication_date": "1998-02", "series_number": "2", "volume": "48", "issue": "2", "pages": "128-143" }, { "id": "authors:6tzxw-a4n32", "collection": "authors", "collection_id": "6tzxw-a4n32", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150810-083411943", "type": "article", "title": "Photoreduction of Iron Oxyhydroxides and the Photooxidation of Halogenated Acetic Acids", "author": [ { "family_name": "Pehkonen", "given_name": "Simo O.", "clpid": "Pehkonen-S-O" }, { "family_name": "Siefert", "given_name": "Ronald L.", "clpid": "Siefert-R-L" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "abstract": "The photolytic reduction of ferrihydrite (am-Fe_2O_3*3H_2O), lepidocrocite (\u03b3-FeOOH), goethite (a-FeOOH), hematite (\u03b1-Fe_2O_3), maghemite (\u03b3-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 > \u03b3-Fe_2O_3 > \u03b3-FeOOH \u2265 \u03b1-Fe_2O_3 \u2265 Fe_(aerosol) > \u03b1-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.", "doi": "10.1021/es00005a012", "issn": "0013-936X", "publisher": "American Chemical Society", "publication": "Environmental Science and Technology", "publication_date": "1995-05", "series_number": "5", "volume": "29", "issue": "5", "pages": "1215-1222" }, { "id": "authors:vfz4d-aj658", "collection": "authors", "collection_id": "vfz4d-aj658", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150810-083413666", "type": "article", "title": "Stability, Stoichiometry, and Structure of Fe(II) and Fe(III) Complexes with Di-2-pyridyl Ketone Benzoylhydrazone: Environmental Applications", "author": [ { "family_name": "Su\u00e1rez Iha", "given_name": "Maria E. V.", "clpid": "Su\u00e1rez-Iha-M-E-V" }, { "family_name": "Pehkonen", "given_name": "Simo O.", "clpid": "Pehkonen-S-O" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "abstract": "The stoichiometry, structure, and thermodynamics of Fe-(II) and Fe(III) complexes of di-2-pyridyl ketone benzoylhydrazone (DPKBH) were studied at pH 5.3 and 25.0 \u00b0C in water-ethanol solutions. The spectrophotometric method of corresponding solutions was used to obtain overall formation constants for the 1:1 (\u03b2_1 = 4.693 X 10^6 M^(-1)) and 1:2 (\u03b2_2 = 2.195 X 10^(10) M^(-2)) Fe(III)(H_20)_6^(3+)DPKBH complexes and 1:1(\u03b2_1 =1.217 X 10^5 M^(-1)) and 1:2 (\u03b2_2 = 7.814 X 10^9 M^(-2)) Fe(II)(H_2O)_6^(2+)-PKBH complexes. Conductance and pH titrations indicate that DPKBH coordinates preferentially as an anion, while FTIR analysis shows that DPKBH exists in the enol form and exhibits tridentate ligand behavior. DPKBH has been shown to be a useful analytical reagent for the simultaneous spectrophotometric determination of Fe(II) and Fe(III) in natural waters: Fe(H_20)_6^(3+) + DPKBH \u21c6 Fe(H_2O_)_3- (DPKBH)^(3+) + 3H_2O and Fe(H_2O)_6^(3+) + 2DPKBH \u21c6 Fe-(DPKBH)_2^(3+) + 6H_2O.", "doi": "10.1021/es00061a016", "issn": "0013-936X", "publisher": "American Chemical Society", "publication": "Environmental Science and Technology", "publication_date": "1994-11", "series_number": "12", "volume": "28", "issue": "12", "pages": "2080-2086" }, { "id": "authors:02sr9-5tt18", "collection": "authors", "collection_id": "02sr9-5tt18", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150821-144956798", "type": "article", "title": "The dynamic chemistry of transition metals in the troposphere", "author": [ { "family_name": "Pehkonen", "given_name": "Simo O.", "clpid": "Pehkonen-S-O" }, { "family_name": "Erel", "given_name": "Yigal", "clpid": "Erel-Y" }, { "family_name": "Siefert", "given_name": "Ronald L.", "clpid": "Siefert-R-L" }, { "family_name": "Klewicki", "given_name": "Ken", "clpid": "Klewicki-K" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" }, { "family_name": "Morgan", "given_name": "James J.", "clpid": "Morgan-J-J" } ], "abstract": "The redox chemistry of Fe, Cu, Mn, and Cr in fog and cloudwater, ambient aerosols, and surface microlayer has been investigated at coastal and inland locations in the Los Angeles basin, in Bakersfield, California, at Whiteface Mt., New York, and on Lake Erie and Lake Ontario. Samples were collected and analyzed for Fe(II), Fe(III), Fe-_(tot), Cu(I), Cu-_(tot), Mn(III+IV), Mn_(tot), Cr(III), Cr(VI), S(IV), S(VI), organic ligands (formate, acetate, oxalate). TOC (total organic carbon), pH, major cations (Na^+, Ca^(2+), NH_4^+, Mg_2^+, K^+), chloride, sulfate, nitrate, peroxides (RO_2H), and aldehydes (RCHO); the amount of sunlight was also measured. The concentrations and the ratios between the measured oxidation states and the total metal concentrations are as follows:\n\nFe(II)(\u03bcM) Fe(II)/Fe_(tot) Cu(I)(\u03bcM) Cu(I)/Cu_(tot)\n 0.1-5 0.02-0.55 0.02-0.27 0.3-0.96\nMn(IV)(\u03bcM) Mn(IV)/Mn_(tot) Cr(III)(\u03bcM) Cr(III)/Cr_(tot)\n0.026-0.21 0.25-0.97 0.002-0.051 0.09-0.74\n\nThe atmospheric redox cycle of Fe involves both dissolved and aerosol surface species and appears to be related to the presence of organic compounds which act as electron donors for the reduction of Fe(III). Fe(III) reduction is enhanced by light, but significant Fe(II) levels were observed in the dark. We suggest that reduction of Fe(III) species by organic electron donors may be an important pathway that affects the speciation of Fe in both urban and rural atmospheres. We found that 40-70% of the total chromium (-20 nM) was Cr(VI) in three cloudwater samples at Whiteface Mt. In the surface micro layer samples, a significant amount of Cr(III) was detected when the concentration of TOC was elevated. The concentrations of Cu(I) and Mn(III+IV) are below detection limit in most samples. In the few samples with measurable concentrations of Mn, Mn is in the reduced, Mn(II), state. In addition to field observations, the photolytic reduction of amorphous iron hydroxide (am-Fe(OH)_3), lepidocrocite (y-FeOOH), goethite (a-FeOOH), hematite (\u03b1-Fe_2O_3), and natural iron containing aerosol particles in the presence of formaldehyde, formate, acetate, oxalate, and butyrate has been investigated in the laboratory. Important parameters in the photoreduction experiments are pH, wavelength of the irradiating light, the nature of the electron donor, and the characteristics of the iron phase. The results show that the fastest rates of photoreduclion of Fe( III) to Fe(II) are achieved with am-Fe(OH)_3 as the electron acceptor and formate as the electron donor. Ambient iron-containing aerosol particles with oxalate as the electron donor resulted in a significant photochemical production of H_2O_2.", "issn": "0021-2164", "publisher": "National Council for Research and Development", "publication": "Israel Journal of Earth-Sciences", "publication_date": "1994", "series_number": "3-4", "volume": "43", "issue": "3-4", "pages": "279-295" }, { "id": "authors:0pnvf-2vz17", "collection": "authors", "collection_id": "0pnvf-2vz17", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150811-111819632", "type": "article", "title": "Redox chemistry of iron in fog and stratus clouds", "author": [ { "family_name": "Erel", "given_name": "Yigal", "clpid": "Erel-Y" }, { "family_name": "Pehkonen", "given_name": "Simo O.", "clpid": "Pehkonen-S-O" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "abstract": "The redox chemistry of Fe in fog and cloudwater has been investigated at coastal and inland locations in the Los Angeles basin, in Bakersfield California, and in Delaware Bay. Samples were collected and analyzed for Fe (Fe(II)), Fe(III), total(Fe), sulfur (S(IV), S(VI)), organic ligands (formate, acetate, oxalate), total organic carbon (TOC), pH, major cations (sodium, calcium, magnesium, potassium, ammonium), chloride, sulfate, nitrate, peroxides, and aldehydes (HCHO); the amount of sunlight was also measured. The ratio Fe(II)/Fe(total) varied between 0.02 and 0.55. The concentration of Fe(II) varied between 0.1 and 5 micromole, and the concentration of total Fe varied between 2 and 27 micromole. The atmospheric redox cycle of Fe involves both dissolved and aerosol surface species and appears to be related to the presence of organic compounds which act as electron donors for the reduction of Fe(III). Fe(III) reduction is enhanced by light but significant Fe(II) levels were observed in the dark. We suggest that reduction of Fe(III) species by organic electron donors may be an important pathway that affects the speciation of Fe in both urban and rural atmospheres. It is possible that reactions involving Fe and organic compounds might be an important source of carboxylic acids in the troposphere.", "doi": "10.1029/93JD01575", "issn": "0148-0227", "publisher": "American Geophysical Union", "publication": "Journal of Geophysical Research D", "publication_date": "1993-10-20", "series_number": "D10", "volume": "98", "issue": "D10", "pages": "18423-18,434" }, { "id": "authors:4k967-nhe96", "collection": "authors", "collection_id": "4k967-nhe96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150811-111819970", "type": "article", "title": "Photoreduction of iron oxyhydroxides in the presence of important atmospheric organic compounds", "author": [ { "family_name": "Pehkonen", "given_name": "Simo O.", "clpid": "Pehkonen-S-O" }, { "family_name": "Siefert", "given_name": "Ron", "clpid": "Siefert-R-L" }, { "family_name": "Erel", "given_name": "Yigal", "clpid": "Erel-Y" }, { "family_name": "Webb", "given_name": "Sam", "clpid": "Webb-S-M" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "abstract": "The photolytic reduction of amorphous iron hydroxide [am-Fe(OH)_3], lepidocrocite (\u03b3-FeOOH), goethite (\u03b1-FeOOH), hematite (\u03b1-Fe_2O_3), and natural iron-containing aerosol particles in the presence of formaldehyde, formate, acetate, oxalate, and butyrate has been investigated. Important parameters in the photoreduction experiments are the pH, wavelength of the irradiating light, nature of the electron donor, characteristics of the iron phase. The present results show that the fastest rates of photoreduction of Fe(III) to Fe(II) are achieved with am-Fe(OH)_3 as the electron acceptor and formate as the electron donor. Maximum rates of photoreduction were observed at 330 nm with a continuous decrease to 405 nm. Natural iron-containing aerosol particles show photochemical behavior similar to am-Fe(OH)_3 and \u03b3-FeOOH. These results suggest that a significant fraction of the reactive atmospheric iron in urban aerosol could be present as am-Fe-\n(OH)_3 and \u03b3-FeOOH. Ambient iron-containing aerosol particles with oxalate as the electron donor resulted in a significant photochemical production of H_2O_2.", "doi": "10.1021/es00047a010", "issn": "0013-936X", "publisher": "American Chemical Society", "publication": "Environmental Science and Technology", "publication_date": "1993-10", "series_number": "10", "volume": "27", "issue": "10", "pages": "2056-2062" }, { "id": "authors:g5h9p-8w864", "collection": "authors", "collection_id": "g5h9p-8w864", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150811-111820669", "type": "article", "title": "Simultaneous spectrophotometric measurement of iron(II) and iron(III) in atmospheric water", "author": [ { "family_name": "Pehkonen", "given_name": "Simo O.", "clpid": "Pehkonen-S-O" }, { "family_name": "Erel", "given_name": "Yigal", "clpid": "Erel-Y" }, { "family_name": "Hoffmann", "given_name": "Michael R.", "orcid": "0000-0001-6495-1946", "clpid": "Hoffmann-M-R" } ], "abstract": "A new analytical method employing di-pyridyl ketone benzoylhydrazone (DPKBH) as a colorimetric chelating agent for the simultaneous determination of iron(II) and iron (III) in cloudwater has been developed. A spectrophotometric detection limit of 4 nM for both Fe(III) and Fe(II) with a linear response from 4 nM to 0.1 \u00b5M was established for samples extracted with CHCl_3-H_2O. DPKBH chelation without CHCl_3 extraction showed a linear response from 0.1 to 30 \u00b5M. The molar extinction coefficients of the Fe(II)-bis(DPKBH) (\u0454_2) and Fe(III)-bis(DPKBH) (\u0454_1) complexes are \u0454_l and \u0454_2 = 3.6 X 10^4 L mol^(-1) cm^(-1) at 370 nm and \u0454_2 = 1.1 X 10^4 L mol^(-1) cm^(-1) at 660 nm. Analytical interference studies on the possible changes in the oxidation state of iron with S(IV), oxalate, and other potential electron donors have also been carried out. This analytical method has been used to determine iron(II) and iron(III) simultaneously in cloudwater samples collected within the Los Angeles basin airshed. The concentration of Fe(II) varied from 0.3 to 5 \u00b5M, and the concentration of Fe(III) varied from 0.6 to 1.4 \u00b5M during several stratus cloud events.", "doi": "10.1021/es00033a003", "issn": "0013-936X", "publisher": "American Chemical Society", "publication": "Environmental Science and Technology", "publication_date": "1992-09", "series_number": "9", "volume": "26", "issue": "9", "pages": "1731-1736" } ]