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Iron speciation and hydrogen peroxide concentrations in New Zealand rainwater
Atmospheric Environment 35 (2001) 6041–6048
Iron speciation and hydrogen peroxide concentrations in New Zealand rainwater Robert J. Kieber*, Barrie Peake1, Joan D. Willey, Bethany Jacobs Department of Chemistry and Marine Science Program, University of North Carolina at Wilmington, Wilmington NC 28403-3297, USA Received 01 November 2000; accepted 17 March 2001
Abstract Rain was collected on the southern portion of the South Island of New Zealand during the summer of 1999 (January– March) during which time significant losses of ozone and increased UV were reported in the stratosphere over New Zealand. Iron and hydrogen peroxide concentrations were measured in rainwater because these analytes are directly influenced by photochemical processes and therefore are particularly susceptible to increasing UV levels. The absolute concentration of dissolved Fe(II) in New Zealand samples was very similar to summertime rain received in Wilmington, NC however the relative contribution of Fe(II) to total Fe was approximately twice as great in New Zealand samples. The larger percentage of reduced iron may reflect higher UV levels in New Zealand since Fe(II) is generated via photochemical reduction of particulate or dissolved Fe(III). No dissolved Fe(III) was detected in New Zealand rainwater, in contrast to the Wilmington site, where summertime Fe(III) concentrations are approximately equal to Fe(II) concentrations. Summertime hydrogen peroxide concentrations and diel variability in New Zealand were similar to other coastal and marine values in both the northern and southern hemispheres suggesting the increasing UV in New Zealand is not significantly increasing hydrogen peroxide concentrations at this location. Any excess photochemically produced hydrogen peroxide in New Zealand may be consumed through oxidation of Fe(II) which is rapidly reformed from photochemical reduction of Fe(III) by the higher UV levels in New Zealand. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen peroxide; Iron; Rainwater; New Zealand
1. Introduction There is a great deal of interest around the world in the impact of increasing levels of UV irradiation arising from decreasing stratospheric ozone, particularly over the polar regions in early summer. Such an increase has been observed in New Zealand where McKenzie et al. (1999) recently reported 12% more high energy, short wavelength UV radiation received at Lauder, southern New Zealand (451S) during the summer of 1998–1999 compared to a decade earlier. This increase was *Corresponding author. Tel.: +1-910-962-3865; fax: +1910-962-3013. E-mail address: [email protected] (R.J. Kieber). 1 Permanent address: Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
correlated with decreasing summertime ozone (O3) levels in the stratosphere above this site and is attributed to anthropogenic activities. Increasing UV radiation may alter the rates of many photochemical processes in the troposphere. These photochemical effects would be most enhanced during summer in New Zealand because midday UV radiation during winter months is significantly lower relative to summertime values (Mckenzie et al., 1999). Interconversions between Fe(III) and Fe(II) in rainwater may be particularly sensitive to changing UV radiation levels because recent studies have shown the speciation of Fe in rainwater is predominately photochemically driven (Kieber et al., 2001; Willey et al., 2000). Our earlier modeling work (Willey et al., 2000) suggests the strongly UV absorbing humic substances are one of the dominant Fe chromophores in rainwater therefore the
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 1 9 9 - 6
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chemistry of organic Fe complexes are also potentially very susceptible to increasing UV levels. The speciation of iron in the troposphere is significant because Fe is involved in a host of important redox reactions including interconversion of S(IV) to S(VI) and . . generation of reactive free radicals including OH; HO2 . @ and O2 . Any changes in iron speciation caused by increasing UV radiation will directly impact these redox reactions and hence may potentially alter the oxidizing capacity of the troposphere in this region. Changes in Fe speciation in rain may also influence the rate of marine primary productivity because speciation directly affects the bioavailability of iron as a nutrient and the residence time of iron in seawater (Kieber et al., 2001). Atmospheric hydrogen peroxide concentrations may also be altered by changing UV levels. Hydrogen peroxide (H2O2) is a chemically labile oxidant and reductant which plays a role in a variety of important redox processes occurring within the troposphere including the conversion of a number of highly reactive free radicals and trace metals. It has also received a great deal of attention in atmospheric oxidant studies because of its central role in the conversion of sulfur dioxide to sulfuric acid in cloud and rainwater (Calvert and Stockwell, 1983; Calvert et al., 1985). Concentrations of H2O2 in atmospheric samples may be directly impacted by increasing UV radiation in New Zealand because it is produced predominately via the self . reaction of the hydroperoxy radical (HO2 ) which is photochemically produced in the gas phase and subsequently scavenged into the aqueous phase (Gunz and Hoffmann, 1990; Sakugawa et al., 1990). The objective of the present study was to measure concentrations of several photosensitive analytes in New Zealand rainwater including iron speciation and hydrogen peroxide during the summer of 1999 to look for potential changes caused by increasing UV radiation. Total and dissolved Fe values were compared to earlier New Zealand data and values obtained at other coastal locations around the globe. Iron speciation values were compared to speciation results in Wilmington, North Carolina which is the only other location where detailed speciation data exists. Hydrogen peroxide values were compared to earlier New Zealand data and to other marine dominated concentrations in both the northern and southern hemispheres. The timing of our study is significant because it coincided with the measured increases in high energy UV radiation observed by McKenzie et al. (1999).
2. Experimental 2.1. Reagents and standards All bottles and containers used for standards and samples were thoroughly cleaned before use employing
previously-described trace metal-clean protocols (Bruland et al., 1979; Bruland, 1980; Tramontano et al., 1987). Poreticst polycarbonate filters used for filtration (0.4 mm) were first rinsed with deionized (DI) water, placed in 6 M Optima HCl for at least one week and thoroughly rinsed with DI water prior to use. All reagents were Fisher Scientific (Fairlawn, NJ) Reagent Grade unless otherwise noted. Ultra-pure (>99.9%) ferrozine (3-(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine) was obtained from Sigma Chemical Company (St. Louis, MO) and 99.995+% magnesium nitrate hexahydrate was obtained from Aldrich Chemical Company (Milwaukee,WI). All water for standards and reagents was obtained from the Milli-Q Ultra Plus water system. Iron standards were prepared daily in synthetic rain (Willey et al., 2000).
2.2. Sample collection and storage Rainwater samples were collected at Ross Creek near Dunedin (461C) and at Doubtful Sound (451S), on the east and west coasts respectively of the South Island of New Zealand. Ross Creek samples were collected 3 km from the city of Dunedin in a county park approximately 10 m above ground level. Doubtful Sound samples were collected at the University of Otago’s laboratory in a remote area of Fiordland National Park. These were compared with samples collected using identical methods in Wilmington, North Carolina (341N), on the southeast coast of the US. Samples were collected on an event basis using Aerochem Metrics (Bushnell, FL) ACM Model 301 Automatic Sensing Wet/Dry Precipitation Collectors equipped with high density polyethylene (HDPE) buckets. A trace metal cleaned HDPE funnel was placed in the wet deposition bucket. Tygon tubing with a fluoropolymer FEP (inert) liner connected the funnel to a trace metal clean 2 l Teflon bottle inside a closed HDPE bucket. Procedural blanks performed with MQ water in place of rainwater had undetectable Fe and H2O2 concentrations. Because of the remoteness of the Doubtful Sound location, manual rain collection systems were used instead of automatic samplers. Rain filtration and preparation of standards and samples were all conducted in a class 100 laminar flow clean bench. Iron speciation analyses were usually begun immediately upon collection or samples were stored in the dark at 41C in Teflon bottles and analyzed within 24 h. Previous work has shown Fe speciation in rain is stable for this time period (Kieber et al., 2001). Because iron speciation analyses had to be undertaken quickly and required larger volumes of sample, not as many rain samples were analyzed for iron speciation as for total iron.
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2.3. Analytical methods Concentrations of FeTotal containing both particulate and dissolved species were determined on stored, unfiltered, acidified samples. These 30 ml samples were acidified to pH 2 with Optima HCl (100 ml) in Teflon vials and stored in the dark at 41C until later analysis by flameless atomic absorption spectrophotometry. FePart was calculated as the difference between FeTotal and iron concentrations in rain that had been filtered through a precleaned 0.4 mm polycarbonate filter before acidification. These samples were analyzed using a Perkin Elmer 1100B Atomic Absorption Spectrometer equipped with an HGA-300 Perkin Elmer programmer and a Perkin Elmer AS 40 autosampler. Compressed argon was utilized as the carrier gas at a pressure of 230 psi and magnesium nitrate hexahydrate (50 ppb) was used as a matrix modifier. Samples were refrigerated (41C) until analyzed (approximately 1 month after collection) and compared to a standard curve ranging from 20 to 500 nM Fe. The detection limit for total Fe was 2 nM with a precision of 13% RSD at typical rainwater concentrations. Fe(II)(aq) and Fe(III)(aq) concentrations were determined on 30 ml aliquots of filtered rainwater (precleaned 0.4 mm polycarbonate filter) using a modification of the ferrozine colorimetric method developed by Stookey (1970) with modifications discussed in Kieber et al. (2001). Each rain sample was analyzed in triplicate for both Fe(II) and Fe(III). Analytical blanks of pH 4.5 H2SO4 with the appropriate concentrations of reagents typically had Fe concentrations of 1 nM. The detection limit was 10 nM with a corresponding precision of 4% RSD at typical rainwater concentrations. It should be noted that the difference between FeTotal and particulate iron is not simply the sum of Fe(II) and Fe(III) concentrations because this fraction also contains colloidal iron and possibly some strongly complexed Fe species which were unreactive towards ferrozine. Hydrogen peroxide was analyzed by a fluorescence decay technique involving the peroxidase-mediated oxidation of the fluorophore scopoletin by H2O2 in rain buffered to pH 7 with a phosphate buffer (Kieber and Helz, 1986; Kieber et al., 2000). Calibration curves were obtained by recording the decrease in fluorescence upon addition of hydrogen peroxide spikes to the sample. The method has an analytical precision of 2% RSD at ambient rainwater concentrations with a detection limit of 2 10@9 M (Kieber and Helz, 1986; Kieber et al., 2000). 2.4. H2O2 photochemical experiments Rainwater was added to glass stoppered 500 ml quartz, round-bottom flasks and exposed to natural midday irradiation for 6 h in a constant temperature
water bath (B281C). Each flask was sampled in triplicate initially and after irradiation, for subsequent peroxide analysis. All dark controls were covered with aluminum foil to restrict light exposure and irradiated in the same manner as light-exposed flasks.
3. Results and discussion 3.1. Concentrations of Fe species in NZ rainwater The concentration of various Fe species collected during nine rain events in New Zealand are listed in Table 1 along with comparable data determined during the summers of 1998 and 1999 in Wilmington, North Carolina (Kieber et al., 2001; Willey et al., 2000). No correlation was found between the levels of any Fe species and precipitation volume suggesting iron is not being simply washed out of the atmosphere which is consistent with our earlier work (Kieber et al., 2001). The volume-weighted total Fe concentration was 172 nM in New Zealand which is approximately half the value for Wilmington summer rain (Table 1) and is also smaller than the annual average of 276 nM reported for rain in Lewes, Delaware, another coastal site in the southeast US (Church et al., 1984). The larger total Fe concentration at Wilmington and Lewes most likely reflects greater continental influences which increase total iron concentrations via several mechanisms (Willey et al., 2000). The concentration of total Fe in New Zealand rainwater reported in Table 1 is somewhat larger than an earlier study by Halstead et al. (2000), who found a volume weighted total Fe concentration of 37 nM during a study of trace metal deposition in Paradise, New Zealand (Halstead et al., 2000). The lower concentra-
Table 1 Volume weighted average concentrations (nM) of Fe total, particulate Fe, Fe(II)(aq) and Fe(III)(aq) in rainwater collected at Wilmington, North Carolina, in the summers of 1997 and 1998 (June–August) and New Zealand summer 1999 (January– March). The % Fe(II)(aq) is relative to FeTotal. n=number of samples Site Wilmington summer
New Zealand summer
Analyte
n
VW average (nM)
n
VW average (nM)
FeTotal FePart Fe(II) Fe(III) % Fe(II)
37 37 21 21 21
330 258 40 31 12
9 4 7 7
172 107 40 nd 24
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tions reported by Halstead et al. (2000) probably reflect the influence of seasonality on total iron concentrations because the majority of their data was collected during winter and fall while all the data in Table 1 was collected during the summer. Willey et al. (2000) found total Fe concentrations in rain collected from Wilmington varied by a factor of three by season with the highest concentrations observed in summer events and lowest concentrations in fall and winter events. The lower concentrations at the Paradise location may also reflect site variability since the Paradise site is removed from the coast and in the lee of the Southern Alps and hence most likely experiences a different climate from that of either of the coastal sites used in the present study. The total iron values reported in Table 1 are also slightly higher than an earlier study on the northern end of the North Island of New Zealand by Arimoto et al. (1990) who reported low and variable concentrations of total iron ranging from non-detectable to 59 nM (Arimoto et al., 1990). The higher FeTotal concentration in New Zealand rain reported in Table 1 may result from summer dust storms in Australia whose dust plumes are likely to affect our study sites in the southern portion of the South Island to a greater extent than the northern portion of the North Island (Boone et al., 1997, Renwick, 1999). The influence of Australian dust on New Zealand iron concentrations in rainwater is supported by data form an event that occurred on 19–20 February which had the highest total iron concentrations observed during this study. Air mass trajectory calculations indicate this event traversed over the entire southeastern third of Australia before arriving in New Zealand (New Zealand Institute of Water and Atmospheric Research). The lowest Fe concentrations in Table 1, in contrast, were found in a rain event which occurred 1 March. Back trajectory analysis showed this storm originated in the southern ocean with no air mass travel over land prior to arriving in New Zealand. One of the most significant global aspects of this research is that, in addition to total Fe concentrations, we present measurements of Fe(II)(aq) and Fe(III)(aq) concentrations in rainwater in the southern hemisphere. During our New Zealand study the volume-weighted concentration of Fe(II)(aq) was 40 nM which is equivalent to Fe(II) concentrations found in Wilmington, NC, summer rain (Table 1). Our Fe(II)(aq) concentrations are also similar to the 50 nM concentration reported by Zhuang et al. (1995) for rain collected in coastal Massachusetts during 1992–1993. The rainwater concentrations of Fe(II)(aq) reported in Table 1 are much lower than those observed in fog or cloudwater in which Fe(II)(aq) often occurs in micromolar concentrations (Behra and Sigg, 1990; Pehkonen et al., 1992; Erel et al., 1993; Siefert et al., 1998). Although the absolute concentration of Fe(II) was similar to the Wilmington data the relative contribution
of Fe(II) to total iron FeTotal signal in these samples was very different. Approximately a quarter of the FeTotal in the New Zealand rain occurred as Fe(II) compared to only about 10% in Wilmington samples. The higher Fe(II) content in the New Zealand samples may be a direct result of higher UV levels in New Zealand since Fe(II) is generated via reduction of particulate Fe(III) containing iron complexes in rainwater which would cause a relative shift from particulate to dissolved Fe(II) species (Willey et al., 2000). The larger proportions of Fe(II) determined in New Zealand are most likely not the result of matrix differences since the pH of summer Wilmington rain averaged 4.3 whereas the average pH of the New Zealand summer rain was 5.5. The higher pH at the New Zealand site would tend to destabilize dissolved Fe(II) species rather than increasing its concentration (Kieber et al., 2001). No Fe(III) was detected in rainwater at the New Zealand sites during the summer, in marked contrast to the Wilmington site where approximately half the dissolved Fe occurs as Fe(III) in summer rain. Comparison of Fe(III)(aq) concentrations in New Zealand to Zhuang et al. (1995) are not possible because Fe(III) was not measured during the latter study. Several mechanisms may contribute to the relatively high ratio of Fe(II) to Fe(III) found in New Zealand rainwater. Fe(III) may be more soluble and stable at the lower pH values observed at the Wilmington site. This would be particularly important if the Wilmington site contained more readily mobilized Fe2O3 arising from anthropogenic activity compared to the New Zealand site being dominated by crustal material. Control of the redox potential by a couple such as H2O2/O2 with H2O2 acting as a reductant, would also favor Fe(II) over Fe(III). Another explanation for high levels of Fe (II) in New Zealand rain might be increased photoreduction of Fe(III) to Fe(II) in rain (Willey et al., 2000), clouds and fog waters (Behra and Sigg, 1990; Faust and Hoigne, 1990; Erel et al., 1993; Zuo, 1995). Willey et al. (2000) found the ratio of [Fe(II)(aq)]/[Fe(III)(aq)] in rainwater was highest during afternoon hours and suggested this resulted from photochemical production of Fe(II)(aq) at the expense of Fe(III)(aq) during this time of greatest light intensity. The lack of detectable Fe(III) reported in Table 1 is consistent with rapid photochemical production of Fe(II) from Fe(III). This process would be enhanced in New Zealand because of the increased UV reported by McKenzie et al. (1999) during the present study period whereby Fe(II) would be photogenerated faster than it is reoxidized back to Fe(III).
4. Fe diurnal variations Four 6 h time periods beginning with 6 a.m. (local time) were used to investigate diurnal variability of Fe
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Fig. 1. Volume-weighted concentration of total iron in New Zealand (January–March 1999) and Wilmington rain (June– September 1997) plotted as a function of time of day. Error bars represent one standard deviation. n=number of rain events included in the volume weighted average for the indicated time interval.
speciation in summer rain (Fig. 1). Each bar in Fig. 1 represents the volume weighted average concentration and standard deviation of all rain events which occurred during the indicated time interval with the number of events listed above the bar. Events were excluded from classification if they occurred during more than one time period. Total Fe was approximately twice as high during afternoon events compared to early morning or late evening in either Wilmington or New Zealand rain (Fig. 1). The higher total concentrations during the afternoon may reflect the greater intensity of regional convection including land, sea breezes during this time period since both sampling locations were near the coast. There was also a distinct diurnal variation in the concentration of Fe (II) observed in summer rain at both the New Zealand and Wilmington sites. The amplitude or difference between maximum and minimum values was approximately a factor of three in the New Zealand rain whereas the amplitude was closer to a factor of 2 in the Wilmington rain (Fig. 2). Concentrations of Fe(II) at
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Fig. 2. Volume-weighted concentration of Fe (II) (aq) in New Zealand (January–March 1999) and Wilmington rain (June– September 1997) plotted as a function of time of day.
both the New Zealand and Wilmington sites were highest in the noon to 6 p.m. time frame which corresponds to the period of greatest light intensity. After the mid-afternoon maximum concentrations declined through the evening reaching an early morning minimum. The higher Fe(II)(aq) concentrations during the period of greatest light intensity reported in Fig. 2 most likely reflects photochemically initiated dissolution of particulate Fe in the afternoons in New Zealand rain since no Fe(III)(aq) was detected in the rain at this location.
5. Hydrogen peroxide The volume weighted average concentration of H2O2 measured in New Zealand rainwater from 19 rain events during the summer of 1999 (January–March) was 6.1 mM with a volume weighted S.D. of 2 mM. This is lower than open ocean rainwater hydrogen peroxide concentrations reported elsewhere in the world which generally fall between 30 and 40 mM during summer events (Cooper et al., 1987; Kieber et al., 2000; Yuan
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and Shiller, 2000). It is also lower than the 15 mM volume weighted average warm season concentration reported by Willey et al. (1996) at the Wilmington coastal location (Willey et al., 1996). The concentration measured in New Zealand during this study are, however, very similar to a New Zealand study by Herrmann (1996) earlier in the decade who reported an average concentration of 8 mM during 6 rain events in the spring of 1994 (Herrmann, 1996). Based on these data, it appears the increasing UV light intensity in New Zealand reported by McKenzie et al. (1999) is not resulting in increasing rainwater hydrogen peroxide concentrations. This is surprising because rainwater H2O2 is photochemically produced predominately from gas phase scavenging of the hydroperoxy radical (HO2 . ) (Gunz and Hoffmann, 1990; Sakugawa et al., 1990). One reason we may not have observed increased H2O2 levels in our study is the increased levels of ultraviolet light in New Zealand may have lead to increased photochemical generation of peroxide reactive components which would act to reduce H2O2 concentrations. This would be particularly important for redox active constituents because hydrogen peroxide can act as both an oxidant and reductant in atmospheric waters. One potential peroxide sink which may have increased in New Zealand is the oxidation of Fe(II) by hydrogen peroxide which is known to occur rapidly in rainwater (Willey et al., 2000). Any excess photochemically produced hydrogen peroxide in New Zealand apparently is also consumed, possibly by oxidation by Fe(II), which is rapidly reformed from photochemical reduction of Fe(III) by the higher UV levels in New Zealand.
6. Hydrogen peroxide diel variability in rainwater The concentration of hydrogen peroxide measured in New Zealand rainwater during four 6 h time intervals during the summer of 1999 is presented in Fig. 3. The concentration of hydrogen peroxide during analogous time intervals during summer (June–August) 1997 at Wilmington, NC are also presented for comparison. Rainwater peroxide concentrations exhibited a cyclical diel pattern during the summer in New Zealand with rising concentrations during daylight reaching a maximum during the evening (6 p.m. to 12 a.m.) followed by declining concentrations in the late evening hours. A similar diel pattern was observed at the Wilmington site during the summer season (June–August) 1997 even though samples were collected several years apart in different hemispheres (Fig. 3). Gas phase scavenging of the photochemically produced hydroperoxy radical is probably not the only mechanism for hydrogen peroxide production in New Zealand rainwater. There have been suggestions that
Fig. 3. Volume-weighted concentration of hydrogen peroxide in New Zealand (January–March 1999) and Wilmington rain (June–August 1997) plotted as a function of time of day.
Fig. 4. Change in hydrogen peroxide concentration in light exposed flasks and dark controls during 6 h irradiation of 2 different rain events. The first bar in each case represents initial concentrations and the second bar represents concentrations after 6 h of irradiation. Error bars represent one S.D.
hydrogen peroxide can also be generated photochemically directly in rainwater (Kieber et al., 2001). In order to determine if this production pathway could account for some of the H2O2 observed in the New Zealand samples several photolysis experiments were undertaken. Two different rain events (OU5 and OU11) were added to glass stoppered 500 ml quartz, round-bottom flasks and exposed to natural midday irradiation (6 h, solar noon cloudless sky) in a constant temperature water bath (B281C). Each flask was sampled in triplicate initially and after irradiation for hydrogen peroxide. All dark controls were covered with aluminum foil to restrict light exposure and irradiated in the same manner as the light-exposed flasks. There was a
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significant increase in the concentration of hydrogen peroxide in the light exposed flasks for both rain events sampled (t test, po0:001). In contrast, there was no observable increase in hydrogen peroxide concentrations in dark controls (t test, po0:001). The net production of H2O2 measured for light exposed flasks relative to dark controls suggests hydrogen peroxide was photochemically produced in the aqueous phase in both these rain events (Fig. 4). These production values should be viewed as minimum estimates since exposure to sunlight may also have lead to increased photochemical generation of peroxide reactive components, which would act to reduce H2O2 concentrations.
7. Summary In summary, summertime hydrogen peroxide levels in New Zealand rain are not significantly enhanced compared to rain collected in summer at several locations in the northern hemisphere and an earlier New Zealand study. This is in spite of increased levels of UV reported in the vicinity of the New Zealand site at the same time rain was collected suggesting increased photochemical destruction mechanisms of hydrogen peroxide may be as important as photochemical production processes. In addition, diel variations of hydrogen peroxide, Fe total and Fe(II) in rain were not significantly different relative to profiles in the northern hemisphere even though total iron concentrations were lower in New Zealand rain. No Fe(III) was detected in New Zealand rain which is in contrast to rain from Wilmington, NC where concentrations of Fe(III) were approximately equal to Fe(II). These observations suggest Fe(II) is rapidly produced at the expense of Fe(III) by the UV radiation increases reported in the southern parts of New Zealand as a result of ozone depletion. Any excess photochemically produced hydrogen peroxide in New Zealand, in turn, may also be consumed by oxidation of Fe(II) which is rapidly reformed from photochemical reduction of Fe(III). The increasing UV levels in New Zealand may therefore, have significant environmental consequences because both H2O2 and iron are involved in a host of important redox reactions occurring in the troposphere. Any changes in Fe speciation caused by increasing UV levels may also influence the rate of marine productivity through changes in the bioavailability of iron and the residence time of iron in seawater (Kieber et al., 2001; Willey et al., 2000).
Acknowledgements This work was supported by NSF Grant ATM9729425 and the Chemistry Department at the
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University of Otago. The Marine and Atmospheric Chemistry Research Laboratory group at UNC-Wilmington assisted with sampling and analyses. James Renwick, New Zealand National Institute of Water and Atmospheric Research, provided the air trajectory analysis.
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Stooky, L., 1970. Ferrozine—a new spectrophotometric reagent for iron. Analytical Chemistry 42, 779–781. Tramantano, J.M., Scudlark, J.R., Chruch, T.M., 1987. A method for collection, handling and analysis of trace metals in precipitation. Environmental Science and Technology 21, 749–753. Willey, J.D., Kieber, R.J., Lancaster, R.D., 1996. Coastal rainwater hydrogen peroxide: concentration and deposition. Journal of Atmospheric Chemistry 25, 149–165. Willey, J.D., Kieber, R.J., Williams, K.H., Crozier, J.S., Skrabal, S.A., Avery, G.B., 2000. Temporal variability of iron speciation in coastal rainwater. Journal of Atmospheric Chemistry 37, 185–205. Yuan, J., Shiller, A.M., 2000. The variation of hydrogen peroxide in rainwater over the South and Central Atlantic Ocean. Atmospheric Environment. Zhuang, G., Yi, Z., Wallace, G., 1995. Iron (II) in rainwater, snow, and surface seawater from a coastal environment. Marine Chemistry 50, 41–50. Zuo, Y., 1995. Kinetics of photochemical cycling of iron coupled with organic substances in cloud and fog droplets. Geochimica et Cosmochimica Acta 59, 3123–3130.
Iron speciation and hydrogen peroxide concentrations in New Zealand rainwater Robert J. Kieber*, Barrie Peake1, Joan D. Willey, Bethany Jacobs Department of Chemistry and Marine Science Program, University of North Carolina at Wilmington, Wilmington NC 28403-3297, USA Received 01 November 2000; accepted 17 March 2001
Abstract Rain was collected on the southern portion of the South Island of New Zealand during the summer of 1999 (January– March) during which time significant losses of ozone and increased UV were reported in the stratosphere over New Zealand. Iron and hydrogen peroxide concentrations were measured in rainwater because these analytes are directly influenced by photochemical processes and therefore are particularly susceptible to increasing UV levels. The absolute concentration of dissolved Fe(II) in New Zealand samples was very similar to summertime rain received in Wilmington, NC however the relative contribution of Fe(II) to total Fe was approximately twice as great in New Zealand samples. The larger percentage of reduced iron may reflect higher UV levels in New Zealand since Fe(II) is generated via photochemical reduction of particulate or dissolved Fe(III). No dissolved Fe(III) was detected in New Zealand rainwater, in contrast to the Wilmington site, where summertime Fe(III) concentrations are approximately equal to Fe(II) concentrations. Summertime hydrogen peroxide concentrations and diel variability in New Zealand were similar to other coastal and marine values in both the northern and southern hemispheres suggesting the increasing UV in New Zealand is not significantly increasing hydrogen peroxide concentrations at this location. Any excess photochemically produced hydrogen peroxide in New Zealand may be consumed through oxidation of Fe(II) which is rapidly reformed from photochemical reduction of Fe(III) by the higher UV levels in New Zealand. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen peroxide; Iron; Rainwater; New Zealand
1. Introduction There is a great deal of interest around the world in the impact of increasing levels of UV irradiation arising from decreasing stratospheric ozone, particularly over the polar regions in early summer. Such an increase has been observed in New Zealand where McKenzie et al. (1999) recently reported 12% more high energy, short wavelength UV radiation received at Lauder, southern New Zealand (451S) during the summer of 1998–1999 compared to a decade earlier. This increase was *Corresponding author. Tel.: +1-910-962-3865; fax: +1910-962-3013. E-mail address: [email protected] (R.J. Kieber). 1 Permanent address: Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
correlated with decreasing summertime ozone (O3) levels in the stratosphere above this site and is attributed to anthropogenic activities. Increasing UV radiation may alter the rates of many photochemical processes in the troposphere. These photochemical effects would be most enhanced during summer in New Zealand because midday UV radiation during winter months is significantly lower relative to summertime values (Mckenzie et al., 1999). Interconversions between Fe(III) and Fe(II) in rainwater may be particularly sensitive to changing UV radiation levels because recent studies have shown the speciation of Fe in rainwater is predominately photochemically driven (Kieber et al., 2001; Willey et al., 2000). Our earlier modeling work (Willey et al., 2000) suggests the strongly UV absorbing humic substances are one of the dominant Fe chromophores in rainwater therefore the
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 1 9 9 - 6
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chemistry of organic Fe complexes are also potentially very susceptible to increasing UV levels. The speciation of iron in the troposphere is significant because Fe is involved in a host of important redox reactions including interconversion of S(IV) to S(VI) and . . generation of reactive free radicals including OH; HO2 . @ and O2 . Any changes in iron speciation caused by increasing UV radiation will directly impact these redox reactions and hence may potentially alter the oxidizing capacity of the troposphere in this region. Changes in Fe speciation in rain may also influence the rate of marine primary productivity because speciation directly affects the bioavailability of iron as a nutrient and the residence time of iron in seawater (Kieber et al., 2001). Atmospheric hydrogen peroxide concentrations may also be altered by changing UV levels. Hydrogen peroxide (H2O2) is a chemically labile oxidant and reductant which plays a role in a variety of important redox processes occurring within the troposphere including the conversion of a number of highly reactive free radicals and trace metals. It has also received a great deal of attention in atmospheric oxidant studies because of its central role in the conversion of sulfur dioxide to sulfuric acid in cloud and rainwater (Calvert and Stockwell, 1983; Calvert et al., 1985). Concentrations of H2O2 in atmospheric samples may be directly impacted by increasing UV radiation in New Zealand because it is produced predominately via the self . reaction of the hydroperoxy radical (HO2 ) which is photochemically produced in the gas phase and subsequently scavenged into the aqueous phase (Gunz and Hoffmann, 1990; Sakugawa et al., 1990). The objective of the present study was to measure concentrations of several photosensitive analytes in New Zealand rainwater including iron speciation and hydrogen peroxide during the summer of 1999 to look for potential changes caused by increasing UV radiation. Total and dissolved Fe values were compared to earlier New Zealand data and values obtained at other coastal locations around the globe. Iron speciation values were compared to speciation results in Wilmington, North Carolina which is the only other location where detailed speciation data exists. Hydrogen peroxide values were compared to earlier New Zealand data and to other marine dominated concentrations in both the northern and southern hemispheres. The timing of our study is significant because it coincided with the measured increases in high energy UV radiation observed by McKenzie et al. (1999).
2. Experimental 2.1. Reagents and standards All bottles and containers used for standards and samples were thoroughly cleaned before use employing
previously-described trace metal-clean protocols (Bruland et al., 1979; Bruland, 1980; Tramontano et al., 1987). Poreticst polycarbonate filters used for filtration (0.4 mm) were first rinsed with deionized (DI) water, placed in 6 M Optima HCl for at least one week and thoroughly rinsed with DI water prior to use. All reagents were Fisher Scientific (Fairlawn, NJ) Reagent Grade unless otherwise noted. Ultra-pure (>99.9%) ferrozine (3-(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine) was obtained from Sigma Chemical Company (St. Louis, MO) and 99.995+% magnesium nitrate hexahydrate was obtained from Aldrich Chemical Company (Milwaukee,WI). All water for standards and reagents was obtained from the Milli-Q Ultra Plus water system. Iron standards were prepared daily in synthetic rain (Willey et al., 2000).
2.2. Sample collection and storage Rainwater samples were collected at Ross Creek near Dunedin (461C) and at Doubtful Sound (451S), on the east and west coasts respectively of the South Island of New Zealand. Ross Creek samples were collected 3 km from the city of Dunedin in a county park approximately 10 m above ground level. Doubtful Sound samples were collected at the University of Otago’s laboratory in a remote area of Fiordland National Park. These were compared with samples collected using identical methods in Wilmington, North Carolina (341N), on the southeast coast of the US. Samples were collected on an event basis using Aerochem Metrics (Bushnell, FL) ACM Model 301 Automatic Sensing Wet/Dry Precipitation Collectors equipped with high density polyethylene (HDPE) buckets. A trace metal cleaned HDPE funnel was placed in the wet deposition bucket. Tygon tubing with a fluoropolymer FEP (inert) liner connected the funnel to a trace metal clean 2 l Teflon bottle inside a closed HDPE bucket. Procedural blanks performed with MQ water in place of rainwater had undetectable Fe and H2O2 concentrations. Because of the remoteness of the Doubtful Sound location, manual rain collection systems were used instead of automatic samplers. Rain filtration and preparation of standards and samples were all conducted in a class 100 laminar flow clean bench. Iron speciation analyses were usually begun immediately upon collection or samples were stored in the dark at 41C in Teflon bottles and analyzed within 24 h. Previous work has shown Fe speciation in rain is stable for this time period (Kieber et al., 2001). Because iron speciation analyses had to be undertaken quickly and required larger volumes of sample, not as many rain samples were analyzed for iron speciation as for total iron.
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2.3. Analytical methods Concentrations of FeTotal containing both particulate and dissolved species were determined on stored, unfiltered, acidified samples. These 30 ml samples were acidified to pH 2 with Optima HCl (100 ml) in Teflon vials and stored in the dark at 41C until later analysis by flameless atomic absorption spectrophotometry. FePart was calculated as the difference between FeTotal and iron concentrations in rain that had been filtered through a precleaned 0.4 mm polycarbonate filter before acidification. These samples were analyzed using a Perkin Elmer 1100B Atomic Absorption Spectrometer equipped with an HGA-300 Perkin Elmer programmer and a Perkin Elmer AS 40 autosampler. Compressed argon was utilized as the carrier gas at a pressure of 230 psi and magnesium nitrate hexahydrate (50 ppb) was used as a matrix modifier. Samples were refrigerated (41C) until analyzed (approximately 1 month after collection) and compared to a standard curve ranging from 20 to 500 nM Fe. The detection limit for total Fe was 2 nM with a precision of 13% RSD at typical rainwater concentrations. Fe(II)(aq) and Fe(III)(aq) concentrations were determined on 30 ml aliquots of filtered rainwater (precleaned 0.4 mm polycarbonate filter) using a modification of the ferrozine colorimetric method developed by Stookey (1970) with modifications discussed in Kieber et al. (2001). Each rain sample was analyzed in triplicate for both Fe(II) and Fe(III). Analytical blanks of pH 4.5 H2SO4 with the appropriate concentrations of reagents typically had Fe concentrations of 1 nM. The detection limit was 10 nM with a corresponding precision of 4% RSD at typical rainwater concentrations. It should be noted that the difference between FeTotal and particulate iron is not simply the sum of Fe(II) and Fe(III) concentrations because this fraction also contains colloidal iron and possibly some strongly complexed Fe species which were unreactive towards ferrozine. Hydrogen peroxide was analyzed by a fluorescence decay technique involving the peroxidase-mediated oxidation of the fluorophore scopoletin by H2O2 in rain buffered to pH 7 with a phosphate buffer (Kieber and Helz, 1986; Kieber et al., 2000). Calibration curves were obtained by recording the decrease in fluorescence upon addition of hydrogen peroxide spikes to the sample. The method has an analytical precision of 2% RSD at ambient rainwater concentrations with a detection limit of 2 10@9 M (Kieber and Helz, 1986; Kieber et al., 2000). 2.4. H2O2 photochemical experiments Rainwater was added to glass stoppered 500 ml quartz, round-bottom flasks and exposed to natural midday irradiation for 6 h in a constant temperature
water bath (B281C). Each flask was sampled in triplicate initially and after irradiation, for subsequent peroxide analysis. All dark controls were covered with aluminum foil to restrict light exposure and irradiated in the same manner as light-exposed flasks.
3. Results and discussion 3.1. Concentrations of Fe species in NZ rainwater The concentration of various Fe species collected during nine rain events in New Zealand are listed in Table 1 along with comparable data determined during the summers of 1998 and 1999 in Wilmington, North Carolina (Kieber et al., 2001; Willey et al., 2000). No correlation was found between the levels of any Fe species and precipitation volume suggesting iron is not being simply washed out of the atmosphere which is consistent with our earlier work (Kieber et al., 2001). The volume-weighted total Fe concentration was 172 nM in New Zealand which is approximately half the value for Wilmington summer rain (Table 1) and is also smaller than the annual average of 276 nM reported for rain in Lewes, Delaware, another coastal site in the southeast US (Church et al., 1984). The larger total Fe concentration at Wilmington and Lewes most likely reflects greater continental influences which increase total iron concentrations via several mechanisms (Willey et al., 2000). The concentration of total Fe in New Zealand rainwater reported in Table 1 is somewhat larger than an earlier study by Halstead et al. (2000), who found a volume weighted total Fe concentration of 37 nM during a study of trace metal deposition in Paradise, New Zealand (Halstead et al., 2000). The lower concentra-
Table 1 Volume weighted average concentrations (nM) of Fe total, particulate Fe, Fe(II)(aq) and Fe(III)(aq) in rainwater collected at Wilmington, North Carolina, in the summers of 1997 and 1998 (June–August) and New Zealand summer 1999 (January– March). The % Fe(II)(aq) is relative to FeTotal. n=number of samples Site Wilmington summer
New Zealand summer
Analyte
n
VW average (nM)
n
VW average (nM)
FeTotal FePart Fe(II) Fe(III) % Fe(II)
37 37 21 21 21
330 258 40 31 12
9 4 7 7
172 107 40 nd 24
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tions reported by Halstead et al. (2000) probably reflect the influence of seasonality on total iron concentrations because the majority of their data was collected during winter and fall while all the data in Table 1 was collected during the summer. Willey et al. (2000) found total Fe concentrations in rain collected from Wilmington varied by a factor of three by season with the highest concentrations observed in summer events and lowest concentrations in fall and winter events. The lower concentrations at the Paradise location may also reflect site variability since the Paradise site is removed from the coast and in the lee of the Southern Alps and hence most likely experiences a different climate from that of either of the coastal sites used in the present study. The total iron values reported in Table 1 are also slightly higher than an earlier study on the northern end of the North Island of New Zealand by Arimoto et al. (1990) who reported low and variable concentrations of total iron ranging from non-detectable to 59 nM (Arimoto et al., 1990). The higher FeTotal concentration in New Zealand rain reported in Table 1 may result from summer dust storms in Australia whose dust plumes are likely to affect our study sites in the southern portion of the South Island to a greater extent than the northern portion of the North Island (Boone et al., 1997, Renwick, 1999). The influence of Australian dust on New Zealand iron concentrations in rainwater is supported by data form an event that occurred on 19–20 February which had the highest total iron concentrations observed during this study. Air mass trajectory calculations indicate this event traversed over the entire southeastern third of Australia before arriving in New Zealand (New Zealand Institute of Water and Atmospheric Research). The lowest Fe concentrations in Table 1, in contrast, were found in a rain event which occurred 1 March. Back trajectory analysis showed this storm originated in the southern ocean with no air mass travel over land prior to arriving in New Zealand. One of the most significant global aspects of this research is that, in addition to total Fe concentrations, we present measurements of Fe(II)(aq) and Fe(III)(aq) concentrations in rainwater in the southern hemisphere. During our New Zealand study the volume-weighted concentration of Fe(II)(aq) was 40 nM which is equivalent to Fe(II) concentrations found in Wilmington, NC, summer rain (Table 1). Our Fe(II)(aq) concentrations are also similar to the 50 nM concentration reported by Zhuang et al. (1995) for rain collected in coastal Massachusetts during 1992–1993. The rainwater concentrations of Fe(II)(aq) reported in Table 1 are much lower than those observed in fog or cloudwater in which Fe(II)(aq) often occurs in micromolar concentrations (Behra and Sigg, 1990; Pehkonen et al., 1992; Erel et al., 1993; Siefert et al., 1998). Although the absolute concentration of Fe(II) was similar to the Wilmington data the relative contribution
of Fe(II) to total iron FeTotal signal in these samples was very different. Approximately a quarter of the FeTotal in the New Zealand rain occurred as Fe(II) compared to only about 10% in Wilmington samples. The higher Fe(II) content in the New Zealand samples may be a direct result of higher UV levels in New Zealand since Fe(II) is generated via reduction of particulate Fe(III) containing iron complexes in rainwater which would cause a relative shift from particulate to dissolved Fe(II) species (Willey et al., 2000). The larger proportions of Fe(II) determined in New Zealand are most likely not the result of matrix differences since the pH of summer Wilmington rain averaged 4.3 whereas the average pH of the New Zealand summer rain was 5.5. The higher pH at the New Zealand site would tend to destabilize dissolved Fe(II) species rather than increasing its concentration (Kieber et al., 2001). No Fe(III) was detected in rainwater at the New Zealand sites during the summer, in marked contrast to the Wilmington site where approximately half the dissolved Fe occurs as Fe(III) in summer rain. Comparison of Fe(III)(aq) concentrations in New Zealand to Zhuang et al. (1995) are not possible because Fe(III) was not measured during the latter study. Several mechanisms may contribute to the relatively high ratio of Fe(II) to Fe(III) found in New Zealand rainwater. Fe(III) may be more soluble and stable at the lower pH values observed at the Wilmington site. This would be particularly important if the Wilmington site contained more readily mobilized Fe2O3 arising from anthropogenic activity compared to the New Zealand site being dominated by crustal material. Control of the redox potential by a couple such as H2O2/O2 with H2O2 acting as a reductant, would also favor Fe(II) over Fe(III). Another explanation for high levels of Fe (II) in New Zealand rain might be increased photoreduction of Fe(III) to Fe(II) in rain (Willey et al., 2000), clouds and fog waters (Behra and Sigg, 1990; Faust and Hoigne, 1990; Erel et al., 1993; Zuo, 1995). Willey et al. (2000) found the ratio of [Fe(II)(aq)]/[Fe(III)(aq)] in rainwater was highest during afternoon hours and suggested this resulted from photochemical production of Fe(II)(aq) at the expense of Fe(III)(aq) during this time of greatest light intensity. The lack of detectable Fe(III) reported in Table 1 is consistent with rapid photochemical production of Fe(II) from Fe(III). This process would be enhanced in New Zealand because of the increased UV reported by McKenzie et al. (1999) during the present study period whereby Fe(II) would be photogenerated faster than it is reoxidized back to Fe(III).
4. Fe diurnal variations Four 6 h time periods beginning with 6 a.m. (local time) were used to investigate diurnal variability of Fe
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Fig. 1. Volume-weighted concentration of total iron in New Zealand (January–March 1999) and Wilmington rain (June– September 1997) plotted as a function of time of day. Error bars represent one standard deviation. n=number of rain events included in the volume weighted average for the indicated time interval.
speciation in summer rain (Fig. 1). Each bar in Fig. 1 represents the volume weighted average concentration and standard deviation of all rain events which occurred during the indicated time interval with the number of events listed above the bar. Events were excluded from classification if they occurred during more than one time period. Total Fe was approximately twice as high during afternoon events compared to early morning or late evening in either Wilmington or New Zealand rain (Fig. 1). The higher total concentrations during the afternoon may reflect the greater intensity of regional convection including land, sea breezes during this time period since both sampling locations were near the coast. There was also a distinct diurnal variation in the concentration of Fe (II) observed in summer rain at both the New Zealand and Wilmington sites. The amplitude or difference between maximum and minimum values was approximately a factor of three in the New Zealand rain whereas the amplitude was closer to a factor of 2 in the Wilmington rain (Fig. 2). Concentrations of Fe(II) at
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Fig. 2. Volume-weighted concentration of Fe (II) (aq) in New Zealand (January–March 1999) and Wilmington rain (June– September 1997) plotted as a function of time of day.
both the New Zealand and Wilmington sites were highest in the noon to 6 p.m. time frame which corresponds to the period of greatest light intensity. After the mid-afternoon maximum concentrations declined through the evening reaching an early morning minimum. The higher Fe(II)(aq) concentrations during the period of greatest light intensity reported in Fig. 2 most likely reflects photochemically initiated dissolution of particulate Fe in the afternoons in New Zealand rain since no Fe(III)(aq) was detected in the rain at this location.
5. Hydrogen peroxide The volume weighted average concentration of H2O2 measured in New Zealand rainwater from 19 rain events during the summer of 1999 (January–March) was 6.1 mM with a volume weighted S.D. of 2 mM. This is lower than open ocean rainwater hydrogen peroxide concentrations reported elsewhere in the world which generally fall between 30 and 40 mM during summer events (Cooper et al., 1987; Kieber et al., 2000; Yuan
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and Shiller, 2000). It is also lower than the 15 mM volume weighted average warm season concentration reported by Willey et al. (1996) at the Wilmington coastal location (Willey et al., 1996). The concentration measured in New Zealand during this study are, however, very similar to a New Zealand study by Herrmann (1996) earlier in the decade who reported an average concentration of 8 mM during 6 rain events in the spring of 1994 (Herrmann, 1996). Based on these data, it appears the increasing UV light intensity in New Zealand reported by McKenzie et al. (1999) is not resulting in increasing rainwater hydrogen peroxide concentrations. This is surprising because rainwater H2O2 is photochemically produced predominately from gas phase scavenging of the hydroperoxy radical (HO2 . ) (Gunz and Hoffmann, 1990; Sakugawa et al., 1990). One reason we may not have observed increased H2O2 levels in our study is the increased levels of ultraviolet light in New Zealand may have lead to increased photochemical generation of peroxide reactive components which would act to reduce H2O2 concentrations. This would be particularly important for redox active constituents because hydrogen peroxide can act as both an oxidant and reductant in atmospheric waters. One potential peroxide sink which may have increased in New Zealand is the oxidation of Fe(II) by hydrogen peroxide which is known to occur rapidly in rainwater (Willey et al., 2000). Any excess photochemically produced hydrogen peroxide in New Zealand apparently is also consumed, possibly by oxidation by Fe(II), which is rapidly reformed from photochemical reduction of Fe(III) by the higher UV levels in New Zealand.
6. Hydrogen peroxide diel variability in rainwater The concentration of hydrogen peroxide measured in New Zealand rainwater during four 6 h time intervals during the summer of 1999 is presented in Fig. 3. The concentration of hydrogen peroxide during analogous time intervals during summer (June–August) 1997 at Wilmington, NC are also presented for comparison. Rainwater peroxide concentrations exhibited a cyclical diel pattern during the summer in New Zealand with rising concentrations during daylight reaching a maximum during the evening (6 p.m. to 12 a.m.) followed by declining concentrations in the late evening hours. A similar diel pattern was observed at the Wilmington site during the summer season (June–August) 1997 even though samples were collected several years apart in different hemispheres (Fig. 3). Gas phase scavenging of the photochemically produced hydroperoxy radical is probably not the only mechanism for hydrogen peroxide production in New Zealand rainwater. There have been suggestions that
Fig. 3. Volume-weighted concentration of hydrogen peroxide in New Zealand (January–March 1999) and Wilmington rain (June–August 1997) plotted as a function of time of day.
Fig. 4. Change in hydrogen peroxide concentration in light exposed flasks and dark controls during 6 h irradiation of 2 different rain events. The first bar in each case represents initial concentrations and the second bar represents concentrations after 6 h of irradiation. Error bars represent one S.D.
hydrogen peroxide can also be generated photochemically directly in rainwater (Kieber et al., 2001). In order to determine if this production pathway could account for some of the H2O2 observed in the New Zealand samples several photolysis experiments were undertaken. Two different rain events (OU5 and OU11) were added to glass stoppered 500 ml quartz, round-bottom flasks and exposed to natural midday irradiation (6 h, solar noon cloudless sky) in a constant temperature water bath (B281C). Each flask was sampled in triplicate initially and after irradiation for hydrogen peroxide. All dark controls were covered with aluminum foil to restrict light exposure and irradiated in the same manner as the light-exposed flasks. There was a
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significant increase in the concentration of hydrogen peroxide in the light exposed flasks for both rain events sampled (t test, po0:001). In contrast, there was no observable increase in hydrogen peroxide concentrations in dark controls (t test, po0:001). The net production of H2O2 measured for light exposed flasks relative to dark controls suggests hydrogen peroxide was photochemically produced in the aqueous phase in both these rain events (Fig. 4). These production values should be viewed as minimum estimates since exposure to sunlight may also have lead to increased photochemical generation of peroxide reactive components, which would act to reduce H2O2 concentrations.
7. Summary In summary, summertime hydrogen peroxide levels in New Zealand rain are not significantly enhanced compared to rain collected in summer at several locations in the northern hemisphere and an earlier New Zealand study. This is in spite of increased levels of UV reported in the vicinity of the New Zealand site at the same time rain was collected suggesting increased photochemical destruction mechanisms of hydrogen peroxide may be as important as photochemical production processes. In addition, diel variations of hydrogen peroxide, Fe total and Fe(II) in rain were not significantly different relative to profiles in the northern hemisphere even though total iron concentrations were lower in New Zealand rain. No Fe(III) was detected in New Zealand rain which is in contrast to rain from Wilmington, NC where concentrations of Fe(III) were approximately equal to Fe(II). These observations suggest Fe(II) is rapidly produced at the expense of Fe(III) by the UV radiation increases reported in the southern parts of New Zealand as a result of ozone depletion. Any excess photochemically produced hydrogen peroxide in New Zealand, in turn, may also be consumed by oxidation of Fe(II) which is rapidly reformed from photochemical reduction of Fe(III). The increasing UV levels in New Zealand may therefore, have significant environmental consequences because both H2O2 and iron are involved in a host of important redox reactions occurring in the troposphere. Any changes in Fe speciation caused by increasing UV levels may also influence the rate of marine productivity through changes in the bioavailability of iron and the residence time of iron in seawater (Kieber et al., 2001; Willey et al., 2000).
Acknowledgements This work was supported by NSF Grant ATM9729425 and the Chemistry Department at the
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University of Otago. The Marine and Atmospheric Chemistry Research Laboratory group at UNC-Wilmington assisted with sampling and analyses. James Renwick, New Zealand National Institute of Water and Atmospheric Research, provided the air trajectory analysis.
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