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Variability of tropospheric hydroperoxides at a coastal surface site in Antarctica
Atmospheric Environment 34 (2000) 5225}5234
Variability of tropospheric hydroperoxides at a coastal surface site in Antarctica Katja Riedel, Rolf Weller*, Otto Schrems, Gert KoK nig-Langlo Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany Received 13 December 1999; received in revised form 13 March 2000; accepted 13 June 2000
Abstract The annual cycles of hydrogen peroxide (H O ) and methylhydroperoxide (MHP) have been investigated at a remote site in Antarctica in order to study seasonal variations as well as chemical processes in the troposphere. The measurements have been performed from March 1997 to January 1998 and in February 1999 at the German Antarctic research station Neumayer which is located at 70339 S, 8315 W. The obtained time series for hydrogen peroxide and methylhydroperoxide in near-surface air represents the "rst all-year measurements in Antarctica and indicates clearly the occurrence of seasonal variations. During polar night mean values of 0.054$0.046 ppbv (range(0.03}0.11 ppbv) for hydrogen peroxide and 0.089$0.052 ppbv (range(0.05}0.14 ppbv) for methylhydroperoxide were detected. At the sunlit period higher Mixing ratios were found, 0.20$0.13 ppbv (range(0.03}0.91 ppbv) for hydrogen peroxide and 0.19$0.10 ppbv (range(0.05}0.89 ppbv) for methylhydroperoxide. Occasional long-range transport of air masses from mid-latitudes caused enhanced peroxide concentrations at polar night. During the period of stratospheric ozone depletion we observed peroxide mixing ratios comparable to typical winter levels. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen peroxide; Methylhydroperoxide; Seasonal variation; Photooxidants; Ozone depletion
1. Introduction Photooxidation is the most dominant sink for many of the environmentally important natural and anthropogenic atmospheric trace gases such as CH , CO, SO , and DMS (dimethyl sul"de). Besides hydroxyl radicals (OH) and ozone (O ), hydroperoxides are the main oxidising compounds in the troposphere (see review article by: Jackson and Hewitt, 1999, and literature therein). Hydroperoxides in#uence the oxidising capacity of the atmosphere and represent a major reservoir for OH radicals. The whole complexity of the atmospheric chemistry of peroxides was described by numerous former publications (see review article by: Gunz and Ho!mann, 1990, and literature therein). In the Antarctic troposphere
* Corresponding author. Tel.: #49-471-4831-1253; fax: #49-471-4831-1425. E-mail address: [email protected] (R. Weller).
the peroxide chemistry is characterised by a limitation to some important sources and sinks on which we want to concentrate in the following. At remote polar sites hydroperoxides are mainly produced by recombination of HO and RO radicals HO #HO PH O #O , (1) RO #HO PROOH#O . (2) Peroxy radicals on their part are mostly formed by photooxidation of hydrocarbons and CO initiated by OH radical attack (Lightfoot et al., 1992). The dominant source of OH radicals is photolysis of O followed by reaction of the formed O(D) atoms with water vapour. Hydroperoxides are also involved in the odd-hydrogen (H, OH, HO ) and odd-oxygen (OH, HO , CH OO) chemistry of the troposphere. They can be used as a diagnostic tool for the extent of HO free radical chemistry V occurring in the troposphere (Kleinmann, 1994). The main sinks for hydrogen peroxide are dry and wet
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 3 2 2 - 8
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deposition, reaction with OH radicals (3) and photolysis (4) (Logan et al., 1981): H O #OHPH O#HO , (3) H O #hlP2 OH(j(350 nm). (4) In contrast to the short living radicals (OH, HO ), H O itself is rather stable in the atmosphere. At mid latitudes, the overall lifetime within the planetary boundary layer is presumed to be around q"30 h (Kleinmann, 1986). In polar regions the in#uence of the local photochemistry on the production of photooxidants is very small, especially during the polar night when temperatures and solar radiation are low or absent, respectively. Dry deposition to ice and snow surfaces is not signi"cant even for water-soluble species (Bales et al., 1987). Thus, longer lifetimes for hydrogen peroxide could be expected in polar regions (q'2}3 d). Consequently, e!ects due to long-range transport have to be included in the discussion of peroxide mixing ratios in Antarctica. Whereas in polluted air various organic hydroperoxides such as methylhydroperoxide (MHP, CH OOH), ethylhydroperoxide (EHP, CH CH OOH), hy droxyethylhydroperoxide (HEHP, CH CH(OH)OOH) and hydroperoxides of other hydrocarbons can be detected (Hewitt and Kok, 1991), at remote marine and polar regions methane is the only signi"cant organic peroxy radical precursor. Reaction of methane with hydroxyl radicals produce methylperoxy radicals: CH #OHPH O#CH , (5) CH #O #MPCH O #M. (6) Ship-based HPLC investigations indicated that no other organic hydroperoxides than methylhydroperoxide could be detected in remote marine areas (Weller et al., 2000). In comparison to hydrogen peroxide the organic hydroperoxides have lower Henry law constants, for example K "304 M atm\ and K "1.02;10 +&. &M atm\ at 298 K (Lind and Kok, 1994). Thus, wet deposition of organic hydroperoxides is less important, which is supported by higher gaseous organic peroxide/hydrogen peroxide ratios during or after rainfalls (Weller and Schrems, 1993) and an enrichment of H O in rainwater (Hellpointer and GaK b, 1989). Since hydrogen peroxide seems to be preserved for millenia (Neftel et al., 1984) in glacial ice and snow, investigations of hydroperoxides in polar regions have received special attention due to the importance of polar ice caps as inexhaustible climate archives. Atmospheric hydrogen peroxide washed out by precipitation and deposited in snow and ice may potentially provide valuable information concerning the oxidative capacity of the ancient troposphere (Thompson et al., 1993). However, there is still a vital lack of information for tropospheric photochemistry transport mechanisms of hydroperox-
ides in polar regions, about peroxide deposition, postdepositional air}snow exchange and possible chemical reactions in buried snow. Hence, one aim of our investigations was to obtain basic experimental data which are needed to understand peroxide photochemistry in the pristine polar and marine troposphere free of any anthropogenic in#uence. The crucial point of our investigations was to measure for the "rst time the seasonal variations of tropospheric H O and MHP mixing ratios in Ant arctica. Analysis of the obtained data and interpretation of the results have been performed along with parameters characterising the photochemical activity, transport and deposition processes.
2. Experimental The measurements were carried out at the German Antarctic station Neumayer (70339 S/08315 W) during the overwintering season 1997/98 and during a summer campaign which was performed from January to March 1999. The station is located on the EkstroK m Ice Shelf at a distance of 7 km from the southeast part of Atka Bay, 42 m above sea level (see Fig. 1). The instruments were installed in the Air Chemistry Observatory of the Neumayer station which is located 1500 m south of the main base. Since northerly winds are hardly ever observed there, no serious contamination problem due to the main station existed. The mean annual temperature at Neumayer is !153C. The maximum solar incidence angle is 42.83. The sun stays permanently above the
Fig. 1. Map of local area around Neumayer station on the EkstroK m Ice shelf. Continental areas are in black, ice shelf areas in grey.
K. Riedel et al. / Atmospheric Environment 34 (2000) 5225}5234
horizon from 19 November to 24 January and permanently below the horizon from 19 May to 27 July (KoK nig-Langlo et al., 1998). A #uorometric two-channel technique, described in detail by Lazrus et al. (1985), was applied for continuous quanti"cation of hydroperoxides in ambient air. An air stream of 1000 cm min\, was sucked through the sampling line, a 2.5 m long Te#on hose with an inner diameter of 4 mm. This leads to a residence time of 1.9 s for the air sample in the tube. In the instrument (Aerolaser H O analyser, Model AL2002) the incoming air #ow was led into a coil where H O was stripped from the air by a bu!er solution (pH"5.8}6.0). The stripping coil was an approximately 53 cm long spiral glass tube with an inner diameter of 2 mm. The #ow rate of the stripping solution was regulated by a peristaltic pump at 0.42 cm min\. In the "rst channel, the #uorimetric reagent (para-hydroxyphenylacetic acid, POPHA) and horseradish peroxidase was added to determine the total amount of hydroperoxides. POPHA forms a dimer when it reacts with peroxidase and organic hydroperoxides or H O . This dimer can be excited with a cadmium vapour lamp at 320 nm. The #uorescence signal at 415 nm is linearly dependent on the peroxide concentration. A distinction between hydrogen peroxide and organic hydroperoxides was achieved by selective destruction of H O with the enzyme catalase in the second parallel channel. Weller et al. (2000) showed by HPLC analysis that no other organic peroxide than methylhydroperoxide (MHP, CH OOH) could be detected in remote mar ine areas. In this paper, we assume that the signal from the catalase channel corresponds to MHP. Thus, a continuous quantitative determination of MHP mixing ratios with the Lazrus method was feasible. However, a fraction of the methylhydroperoxide is inherently destroyed by catalase, making a correction necessary. The evaluation of the raw data were carried out similarly as described in literature (Claiborn and Aneja, 1991; Ayers et al., 1996; Sta!elbach et al., 1996; Weller et al., 2000). The raw data from the peroxidase channel S and the signal of the catalase channel S were processed by the following expression: aI S !S S !a S [CH OOH]" , [H O ]" aI !a e(aI !a ) where e is the collection e$ciency for MHP and a is the residual fraction of H O remaining in the catalase channel (ratio of response in the catalase channel to that of the total hydroperoxides channel to span solutions of H O ). The destruction e$ciency of the catalase solution was determined twice a day and adjusted to be between 70 and 90%, corresponding to a "0.1}0.3. Our laborat ory experiments showed that at catalase e$ciencies between 90 and 95% the MHP decay was around 5}7%. Lower destruction e$ciencies (70}75%) lead to MHP losses of 2}3% (Tolu, 1993). The ratio between pseudo
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"rst-order reaction rate coe$cients for the reaction of catalase with a mixture of hydroperoxides and for the reaction with pure hydrogen peroxide is given by k"k /k . Based on laboratory investigations with pure G MHP we presumed a value of k"0.017. This ratio was also used for all calculations in this work. Another crucial factor is the collection e$ciency e of the stripping coil, which must be carefully determined since Henry's law coe$cients for MHP and H O show a strong temper ature dependence (Lind and Kok, 1994). For hydrogen peroxide and MHP the Henry's law constant K is given by K "e 2\ K "e 2\ . &+&. In Neumayer's Air Chemistry Observatory the temperatures varied during the whole year over the range of 18}263C (minimum and maximum temperatures of 13 and 363C were also observed). Calculations according to Lazrus et al. (1986) showed that under the given conditions, viz., 1000 cm min\ air#ow and 0.42 cm min\ stripping solution #ow, equilibrium is reached in the stripping coil. The sampling e$ciency for hydrogen peroxide was always better than 99% independent of temperature, whereas the sampling e$ciency e of methylhydroperoxide showed strong temperature dependence. It was calculated to be 0.82 at 183C and 0.75 at 263C. Uncertainty in the collection e$ciency accounted for the largest contribution of estimated uncertainty in the reported mixing ratios of MHP. Measurements of hydroperoxides in air require a periodic determination of the systems sensitivity by using a calibration source. Thus, an automatic gas calibration was performed every 12 h. Signals of zero air and of an internal hydrogen peroxide standard were compared and used to determine the sensitivity of the instrument. Zero air was generated internally by passing ambient air through a zero trap with manganese-dioxide-coated charcoal. The internal gas phase H O source consisted of a short piece of polyethylene tube in a small bottle "lled with 30% H O . The temperature of the device was stabilised to approximately 253C. The permeation source itself was calibrated every week by standard solutions of 10\ mol l\ H O , freshly prepared from a stock solu tion of 10\ mol l\ H O . The absolute permeation rate is calculated by comparing the permeation signal with the signal of the prepared liquid standards. The concentration of the 10\ M hydrogen peroxide stock solution was determined every three months by titration with KMnO . MHP calibration was performed in our home laboratory with aqueous solutions of di!erent MHP concentrations before the over-wintering period. Due to the explosive nature of MHP, calibration was not repeated during the expedition. Unfortunately no suitable permeation sources are available for MHP. Especially during winter season, H O and MHP values were sometimes extremely low and thus close to the detection
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limit of the instrument. Therefore, regular zero measurements were carried out. An external zero trap "lled with manganese-dioxide-coated charcoal was used to determine the daily zero value. The trap was mounted between the instrument and inlet tube for 30 min each day. The main air inlet was installed on top of the Air Chemistry Observatory 8 m above the ground. The inlet was protected against drifting snow with a specially designed tumbler. The sampling e$ciency of the Te#on inlet system was determined to be in the range of 0.85}0.95 by comparing measurements of indoor air (contaminated with hydroperoxides). For some minutes indoor air was directly sucked into the instrument and the results were compared with the peroxide concentrations obtained with the te#on tube. All solutions were prepared with ultra pure water (milliQ-water) and reagent grade chemicals. Under these conditions a detection limit of 50 pptv is achievable. This corresponds with 3p of the standard deviation of the noise level. The accuracy of the instrument has been calculated to be in the order of 15}20% at [H O ]) 500 pptv and 12}15% at [H O ]"500}1000 pptv. The time constant for a change from 10 to 90% signal is 30 s with a delay time of 300 s. Further supporting data were provided by the long-term measuring programme established at the Air Chemistry and Meteorological Observatory at Neumayer station. UV solar radiation measurements were carried out with an UV radiometer (selenium photocell pyranometer, Eppley, USA, 300}370 nm). Relative humidity was measured with two pernix hair hygrometers (Lambrecht 800L100) (KoK nig-Langlo and Herber, 1996). Ozone column density data were obtained by ozonesondes launched weekly at Neumayer station. Additionally data from satellite-based total ozone mapping spectrometer (TOMS) were used, available in the world wide web: http://jwocky.gsfc.nasa.gov/. Daily 5day-back trajectories have been made available by the German Weather Service (DWD).
Fig. 2. Annual variation of hydrogen peroxide (drawn black line) and methylhydroperoxide (drawn grey line): Five day running means of hydroperoxides gas-phase concentrations (in ppbv) are plotted against day of the year (1"1 January and 365"31 December). The presented time series is a result of two di!erent campaigns: March 1997 to January 1998 and February 1999. The thin line shows annual variations in UV solar radiation.
3. Results Fig. 2 presents our results of the "rst all-year-round investigations of hydroperoxides in the Antarctic troposphere. The seasonal variations of hydrogen peroxide and methylhydroperoxide gas-phase mixing ratios at Neumayer station are compared with UV solar radiation for the same time period. A clear seasonal variation is evident. During the polar night we observed mixing ratios of 0.054$0.046 ppbv (range 0.03}0.40 ppbv) for hydrogen peroxide and 0.089$0.052 ppbv (range 0.05}0.35 ppbv) for methylhydroperoxide. Most values (97% of the measured H O and 86% of the measured MHP values) are in the range from below 0.05 to 0.150 ppbv. Fig. 3 illustrates the frequency distribution of
Fig. 3. Frequency distribution of measured peroxide concentrations. Frequency (in %) is depicted versus concentration intervals of 0.05 ppbv: (a) distribution of hydrogen peroxide, and (b) methylhydroperoxide, grey columns represent winter values (1 April to 15 September, 91}258 day of the year, total number of measurements N+11000), black columns represent summer values (15 September to 1 April, total number of measurements N+7800).
K. Riedel et al. / Atmospheric Environment 34 (2000) 5225}5234
measured peroxide mixing ratios during winter (1 April to 15 September, 91}258 day of the year) and summer season (15 September to 1 April, 259}90 day of the following year). As depicted in Fig. 3 of the winter data are close to, respectively, below the detection limit of 0.05 ppbv. An increase of peroxide mixing ratios could be observed in the middle of polar night (H O "0.15 ppbv and MHP"0.11 ppbv (see Fig. 2). Due to that the winter mean values are slightly shifted to higher levels. Calculation of the winter means without these values leads to 0.048$0.046 ppbv for hydrogen peroxide and 0.084$0.055 ppbv for methylhydroperoxide. During summer, higher peroxide mixing ratios were found, i.e. 0.20$0.13 ppbv (range(0.03}0.91 ppbv) for atmospheric hydrogen peroxide and 0.19$0.10 ppbv (range(0.05}0.89 ppbv) for methylhydroperoxide.
4. Discussion 4.1. Seasonal variations Since production of hydroperoxides is closely linked with the HO chemistry, variation in the solar radiation V should entail changes in atmospheric peroxide concentrations. Fig. 2 illustrates the relation between UV solar radiation and observed peroxide mixing ratios. As expected, the decline of atmospheric peroxide concentrations coincide with the drop of solar radiation to 0.0 W/m during polar night (139}208th day of the year) since photochemical oxidation is the only relevant source of atmospheric hydroperoxides. Nevertheless surprisingly high atmospheric concentrations could be observed during polar night when the sun stays permanently below the horizon with a minimum solar incidence angle of !433 at 26 June (177th day of the year). Obviously in this case local photochemistry could not contribute to the peroxide production. The South Pole photochemical models (McConnell et al., 1997a) predict mixing ratios of around 1 pptv for hydrogen peroxide during polar night. A reasonable explanation for these signi"cant di!erences might be an underestimation of transport processes (Bales and Choi, 1996). Previous investigations of hydrogen peroxide in polar regions were carried out almost exclusively during summer season (Table 1). Generally the values for hydrogen peroxide mixing ratios are in the same order of magnitude. However, in our time series concentration levels up to 1 ppbv are only reached sporadically. In comparison with "ndings of Jacob and Klockow (1993), we observed lower H O concentrations around 0.24$0.12 ppbv in December and 0.16$0.09 ppbv in January. The only measurements performed at polar night so far were carried out by de Serves (1994) at Alert, Canada. From January to February 1992 he observed total peroxide concentrations from below detection limit (0.01 to
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0.04 ppbv. Our measurements of total hydroperoxides indicate higher values between (0.05 and 0.26 ppbv with a mean of 0.13$0.07 ppbv during winter. Several approaches from di!erent groups were made to estimate atmospheric hydrogen peroxide mixing ratios from the measured concentration values in snow pits and ice cores (van Ommen and Morgan, 1996; McConnell et al., 1997b). This method provides a suitable possibility to reconstruct atmospheric concentration signals: For Law Dome, Antarctica van Ommen and Morgan (1996) predict hydrogen peroxide mixing ratios of 0.01 ppbv at winter and between 0.12 and 0.80 ppbv at summer. Obviously the derived winter values are signi"cantly lower than our measurements. Most probably uncertainties concerning post deposition processes and air}snow transfer coe$cients are responsible for this discrepancy. In comparison to H O , there are only very few "eld measurements of organic hydroperoxides reported in literature. In polar regions, as far as we know, our investigations represent the "rst measurements at all. As pointed out in the experimental section, we attribute the catalase channel signal to methylhydroperoxide (MHP). Mean concentrations of 0.19$0.10 ppbv MHP at summer and 0.09$0.05 ppbv at winter were observed. In comparison with the measurements conducted in the continental troposphere by other groups (Heikes et al., 1987; Barth et al., 1989, Jackson and Hewitt, 1996), our investigations exhibit a higher MHP/(H O #MHP) ratio of 0.57$0.26 (range 0.1}1.0). Ship-based investigations across the Atlantic from northern mid-latitudes to southern mid-latitudes revealed ratios between 0.1 and 0.6 (mean: 0.32$0.12) (Weller et al., 2000) and between 0.17 and 0.98 (mean: 0.48$0.14) (Slemr and Tremmel, 1994). Airborne measurements over the western North Paci"c report about ratios of 0.58 (Heikes et al., 1996) while in the continental boundary over the United States much lower ratios of 0.1 have been found (Heikes et al., 1987; Barth et al., 1989). Investigations at the European continent show slightly higher MHP/(H O #MHP) ratios. Fels and Junkermann (1994) observed ratios of 0.35}0.58 at a mountain site in southern Germany and Jackson and Hewitt (1996) report on ratios of 0.05}0.37 in central Portugal. Airborne measurements over the South Atlantic, Brazil and southern Africa during TRACE-A (Lee et al., 1997, 1998) exhibit distinct di!erences according to air mass origin. MHP/(H O # MHP) ratios of 0.2 were found over continents while in marine air masses the ratio was typically twice as high. Obviously, the highest MHP/(H O #MHP) ratios were found at Neumayer station and in remote marine regions. The overall deposition rates for MHP and H O , including dry and wet deposition should be very di!erent over land, ocean, snow and ice and within the planetary boundary layer compared to the free troposphere. Especially dry deposition on the ocean surface, which is an important removal process of water soluble
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Table 1 Comparison of previous measurements of tropospheric hydroperoxides concentrations in polar regions with our recent results Measuring site
Reference
Measuring period
Atmospheric H O concentrations (ppbv)
East Greenland sea (703}803N) Alert (Canada, 82.53N, 62.33W)
Weller and Schrems (1996)
July}August 1994
de Serves (1994)
Feb.}April 1992
Summit (Greenland, 72.23N, 37.83W) Summit (Greenland, 72.23N, 37.83W) Summit (Greenland, 72.23N, 37.83W) Cape Grim (Tasmania, 413S, 1453E)
Sigg et al. (1992)
June}July 1990
Mean: 0.42$0.26 Range: (0.1}1.0 Total peroxides Dark period: (0.01}0.04 Sunlit period: 0.10}0.40 0.3}3.5
Bales et al. (1995)
May}July 1993
McConnell et al. (1997b)
April}July 1995
Ayers et al. (1996)
Feb. 1991}March 1992
Weller and Schrems (1993)
Jan. 1992
Total peroxides Summer: 0}2 (mean Dec.: 1.4) Winter: 0}1.2 (mean July: 0.16) 0.47}0.86
McConnell et al. (1997a) Fuhrer et al. (1996)
Nov.}Dec. 1994 Jan. 1996 Dec. 1993}Jan. 1994
0.20}0.25 0.90}1.00 ( 0.5
Jacob and Klockow (1993)
Dec. 1989}Jan. 1990
This work
April}Sept. 1997 Sept. 1997}March 1998 and Feb. 1999
Mean: 0.394 Range:(0.10}1.05 Mean: 0.054$0.046 Range:( 0.03}0.40 Mean: 0.197$0.130 Range:(0.03}0.90
Northwest Weddell Sea, Brans"eld Street, (61}633S, 51}593W) South Pole (Antarctica, 903S) Dronning Maud Land (Antarctica, 73}763S, 12}93W) Neumayer (Antarctica, 70.13S, 8.03W) Neumayer (Antarctica, 70.13S, 8.03W)
trace gases in the marine boundary layer, leads to a preferential depletion of H O (Weller and Schrems, 1993). Di!erent air mass histories may explain the depicted variations measured at Neumayer station. Unfortunately, literature data even for dry deposition rates of MHP and H O over di!erent surfaces are disturbingly incon sistent (Hauglustaine et al., 1994; Hough, 1991). Our data indicate that deposition of H O is favoured relative to MHP over the ocean: Air masses passing the sea before reaching Neumayer station showed a signi"cantly higher MHP/(H O #MHP) ratio. On 7 September when a MHP/(H O #MHP) ratio between 0.3 and 0.7 was found, the corresponding trajectory indicates advection of marine air masses, passing a frontal system of a depression over the southern Atlantic masses (Fig. 4: 6 (circles), 7 (squares) and 8 September (triangles)). Most probably, wet deposition occurred along the trajectory.
Atmospheric MHP concentrations (ppbv)
Mean: 1.0$0.4 Range: 0.4}2.0 0.50}1.75
Range:( 0.05}0.35 Mean: 0.089$0.052 Mean: 0.191$0.103 Range:(0.05}0.89
In contrast, continental air masses caused lower MHP/(H O #MHP) ratios during the day before and after this event. 4.2. Long-range transport ewects and tropospheric lifetimes of hydroperoxides A striking increase of peroxide mixing ratios was found in the middle of the polar night (177th day of the year, of 26 June). Local photochemistry could not contribute to the production of photooxidants, because of missing solar radiation. Thus, long-range transport of hydroperoxides from sunlit areas to Antarctica seems to be the most plausible explanation. Analysis of the 5-day-back trajectories con"rms this assumption. Fig. 5 shows trajectories from 25 (circles), 26 (squares) and 27 June (triangles). While air parcels reaching Neumayer station on 25 and
K. Riedel et al. / Atmospheric Environment 34 (2000) 5225}5234
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ing such long-range transport, atmospheric lifetimes have to be considered. Former estimations of hydrogen peroxide lifetimes vary between q"1 d in polluted atmosphere (Kleinmann, 1986), 10 d as e!ective tropospheric lifetime (Logan et al., 1981) and a `rainouta lifetime of q"50 d, calculated with a simple box model and observed meteorological conditions at Summit, Greenland (Sigg et al., 1992). All these calculations assume three main H O removing processes: Reaction with OH radicals (3), photolysis (4) and deposition. First of all, we will estimate and compare the H O lifetime over ice for polar day and night. During polar day the H O lifetime q (H O ) can be calculated by the expression q (H O )"(k #k [OH]#J )\.
Fig. 4. 5-day-back trajectories reaching Neumayer station at surface level on September the 6th (circles), 7th (squares) and 8th (triangles). All trajectories showed only marginal vertical movements within the planetary boundary layer.
The H O deposition rate over ice (k ) was esti mated with the following considerations: Deposition should occur inside the mixed layer which is then identical with the Ekman layer. This layer extends to a height of from 300 to 500 m depending on the surface roughness of the terrain (Seinfeld and Pandis, 1998). In the case of ice the surface roughness is typically small. Therefore, a mixing height of 300 m was assumed. With k\ "h v\, an adopted dry deposition velocity v for hydrogen peroxide over ice of 0.05 cm s\ (Hough, 1991), and a mixing height h"300 m, a peroxide lifetime k of 1.6;10\ s\ results. J is the photolysis frequency of H O , estimated according to Tremmel (1992) to be around 2;10\ s\, and k "1.6;10\ cm\s\ (DeMore et al., 1994), is the rate coe$cient of reaction (3) at !103C. With a mean 24 h OH concentration of 1.1;10 cm\ (Je!erson et al., 1998), q (H O ) is roughly 3 d. During polar night, we can neglect J and k [OH], but take into account the reaction H O #NO PHO #HNO (5), thus q (H O )"(k #k [NO ])\.
Fig. 5. 5-day-back trajectories reaching Neumayer station at surface level on the 25 (circles), 26 (squares) and 27 June (triangles). Origin of the air parcel arriving on 26 is a sunlit area east of South America (533S, 433W). Again, trajectories showed only marginal vertical movements within the planetary boundary layer.
27 June originated from inner Antarctica, the source region on June 26 was a marine area east of South America (533S, 433W). The corresponding local sunshine duration in this region was around 7.5 h with a maximum solar incidence angle of 13.73. Photochemical production of hydrogen peroxide and methylhydroperoxide may have occurred enhancing the atmospheric peroxide concentrations at Neumayer station "ve days later. Regard-
The steady-state NO concentration was derived con sidering the main source and loss reactions NO #O PNO #O (6) and NO #NO PN O (7), respectively, to be [NO ]"k [O ]/k + 2.7;10 cm\ (k "6;10\ cm\ s\ and k " 1.7;10\ cm\ s\ at !253C (DeMore et al., 1994), measured O mixing ratio: 32 ppbv). Unfortunately, to our knowledge there exists no data for the rate coe$cient of the reaction H O #NO , but typically hydrogen abstraction reactions by NO are 3}4 orders of magni tude lower compared to the corresponding OH reaction (DeMore et al., 1994). As a consequence, also the reaction H O #NO can be neglected, yielding q (H O )+ 7 d. Concerning the situation at 26 June, polar night predominated in the region south of 66330 S and deposition should be the dominant sink for atmospheric hydrogen peroxide, indicating that e!ective H O advection by long-range transport could occur.
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4.3. Impact of stratospheric ozone depletion Reductions in stratospheric ozone (O ) cause in creased penetration of ultraviolet-B (UV-B, 280}315 nm) radiation to the troposphere, and therefore increase the photochemical activity. Radical production in the atmosphere should react very sensitively to enhanced tropospheric UV-B actinic #uxes. Since at remote polar sites hydroperoxides are mainly produced by recombination of HO and RO radicals the in#uence of enhanced actinic #uxes on the formation of these radicals, must be regarded. Photochemistry is basically initiated by OH radicals generated by O photolysis and subsequent re action of O(D) with water vapour O #hlPO(D)#O (j(350 nm), (7) O(D)#H OP2 OH. (8) Due to higher photolysis frequencies of tropospheric O , signi"cantly higher HO mixing ratios might be V expected. Model calculations (Fuglestvedt et al., 1994) suggest that tropospheric H O concentrations should strikingly increase in parallel to the predicted increases in HO . However, it is obvious that OH production by V reaction (9) also depends on the O and the water vapour concentration which has to be considered when dealing with the in#uence of the ozone hole on local photochemistry. In the following we will focus on the observed hydroperoxide mixing ratios during the ozone hole period (271st}283rd day of the year), shortly after this event (284}300 day of the year) and during fall at comparable solar incidence angles (63rd}75th day of the year). Fig. 6a compares the peroxide mixing ratios with measured total ozone column densities from regularly launched ozonesondes and satellite-based total ozone mapping spectrometer (TOMS). Due to its position at 703S latitude Neumayer station could be inside as well as outside of the polar vortex. Thus, ozone column densities show strong #uctuations. During stratospheric ozone depletion, peroxide mixing ratios comparable to previous months have been observed. There is no hint that the ozone hole leads to higher atmospheric peroxide mixing ratios. Moreover, a comparison with the situation during austral autumn at similar solar incidence angles revealed that peroxide concentrations are even lower during the stratospheric ozone minimum (Fig. 6a). Most probably the reason for this striking results are the di!erent water vapour concentrations (Fig. 6b): during the ozone hole period the partial pressure of water, p(H O) was a factor of 4.5 lower compared to the situation at fall. It seems that enhanced actinic radiation and higher O mixing ratios could not compensate the e!ect of lower water vapour concentrations on the primary OH production rate. Moreover, we tentatively attribute the increase of hydroperoxide mixing ratios shortly after the ozone hole to increasing p(H O).
Fig. 6. (a) Comparison of observed peroxide mixing ratios (hydrogen peroxide drawn black line and MHP drawn grey line) and total ozone column densities from regularly launched ozonesondes (open squares) and satellite-based total ozone mapping spectrometer (TOMS, dots). For a better comparison of similar sunsets the daily sunshine duration is also depicted (thin black line). (b) Water vapour partial pressure (p(H O), drawn grey line) and surface ozone mixing ratios (black line) for the same year. The shaded areas mark the ozone hole period (271st}283rd day of the year) and a period during fall at comparable solar incidence angles (63rd}75th day of the year).
5. Conclusions Our measurements at Neumayer station provide the "rst year-round time series of gas-phase peroxide concentrations from the Antarctic continent. We observed a seasonal variation coinciding with seasonal changes in solar radiation due to photochemical formation of hydroperoxides. Generally, observed tropospheric H O and MHP concentrations are higher than expected especially during winter. At polar night long-range transport has to be considered from late fall to early spring due to increased hydroperoxide lifetimes. Our measurements indicate that distinct variations of the MHP/(MHP# H O ) ratio are most probably caused by di!erent air mass history. Di!erent MHP and H O deposition vel ocities over marine and ice covered areas could provoke
K. Riedel et al. / Atmospheric Environment 34 (2000) 5225}5234
fractional depletion of hydrogen peroxide. Instead of increasing hydrogen peroxide levels due to higher UV-B actinic #uxes during the stratospheric ozone depletion, we observed values comparable to typical winter levels. Most surprisingly, hydroperoxides during the ozone hole period are strikingly lower compared to typical mixing ratios in austral fall at similar solar incidence angles. However, in our case the partial pressure of water during the ozone hole period was a factor of 4.5 lower compared to the situation at fall. Clearly, for a more detailed interpretation and a meaningful analysis of the processes determining the observed hydroperoxide time series, photochemical model calculations paying attention to special Antarctic conditions are indispensable and will be topic of our further investigations.
Acknowledgements The authors would like to thank A. Minikin, B. MuK ller and all technicians and scientists of the over-wintering crews and summer campaigns at Neumayer station for their help and technical and logistical support. K.R. thanks the Studienstiftung des deutschen Volkes for a research stipend and M. Hutterli for a helpful evaluation program. The trajectories have been provided by the German Weather Service (DWD). This paper is contribution No. 1728 of the Alfred-Wegener Institute for Polar and Marine Research, Bremerhaven.
References Ayers, G.P., Penkett, S.A., Gillett, R.W., Bandy, B., Galbally, I.E., Meyer, C.P., Elsworth, C.M., Bentley, S.T., Forgan, B.W., 1996. The annual cycle of peroxides and ozone in marine air at Cape Grim, Tasmania. Journal of Atmospheric Chemistry 23, 221}252. Bales, R.C., Valdez, M.P., Dawson, G.A., 1987. Gaseous deposition to snow, 2. Physical-chemical model for SO de position. Journal of Geophysical Research 92, 9789}9799. Bales, R.C., McConnell, J.R., Losleben, M.V., Conklin, M.H., Fuhrer, K., Neftel, A., Dibb, J.E., Kahl, J.D.W., Stearns, C.R., 1995. Diel variations of H O in Greenland: a discussion of the cause and e!ect relationship. Journal of Geophysical Research 100, 18661}18668. Bales, R.C., Choi, J., 1996. Conceptual framework for interpretation of exchange processes. In: Wolf, E.W., Bales, R.C. (Eds.), Chemical Exchange Between the Atmosphere and Polar Snow. NATO-ASI Series 143, Springer, Berlin. Barth, M.C., Hegg, D.A., Hobbs, P.V., Walega, J.G., Kok, G.L., Heikes, B.G., Lazrus, A.L., 1989. Measurements of atmospheric gas-phase hydrogen peroxide concentrations in winter on the east coast of the United States. Tellus 41B, 61}69. Claiborn, C.S., Aneja, V.P., 1991. Measurements of atmospheric hydrogen peroxide in the gas phase and in cloud water at Mt. Mitchell, North Carolina. Journal of Geophysical Research 96, 18771}18787.
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DeMore, W.B., Sander, S.P., Golden, D.M., Hampson, R.F., Kurylo, M.J., Howard, C.J., Ravishankara, A.R., Kolb, C.E., Molina, M.J., 1994. Chemical kinetics and photochemical data for use in stratospheric modeling. Evaluation no.11, JPL Publ. 94-26, 1, Pasadena, CA. De Serves, C., 1994. Gas phase formaldehyde and peroxide measurements in the Arctic atmosphere. Journal of Geophysical Research 99, 25391}25398. Fels, M., Junkermann, W., 1994. The occurrence of organic hydroperoxides in air at a mountain site. Geophysical Research Letters 21 (5), 341}344. Fuglestvedt, J.S., Jonson, J.E., Isaksen, I.D.A., 1994. E!ects of reductions in stratospheric ozone on tropospheric chemistry through changes in photolysis rates. Tellus 46B, 172}192. Fuhrer, K., Hutterli, M., McConnell, J.R., 1996. Overview of recent "eld experiments for the study of the air snow transfer of H O and HCHO. In: Wolf, E.W., Bales, R.C. (Eds.), Chemical Exchange Between the Atmosphere and Polar Snow. NATO-ASI Series 143, Springer, Berlin. Gunz, D.W., Ho!mann, M.R., 1990. Atmospheric chemistry of hydroperoxides: a review. Atmospheric Environment 24, 1601}1633. Hauglustaine, D.A., Granier, C., Brasseur, G.P., Megie, G., 1994. The importance of atmospheric chemistry in the calculation of radiative forcing on the climate system. Journal of Geophysical Research 99, 1173}1186. Heikes, B.G., Kok, G.L., Walega, J.G., Lazrus, A.L., 1987. H O , O and SO measurements in the lower troposphere over the eastern United States during fall. Journal of Geophysical Research 97, 915}931. Heikes, B.G., Lee, M., Bradshaw, J., Sandholm, S., Davis, D.D., Crawford, J., Rodriguez, J., Liu, S., McKeen, S., Thronton, D., Bandy, A., Gregory, G., Talbot, R., Blake, D., 1996. Hydrogen peroxide and methylhydroperoxide distributions related to ozone and odd hydrogen over the North Paci"c in the fall of 1991. Journal of Geophysical Research 101, 1891}1905. Hellpointer, E., GaK b, S., 1989. Detection of methyl, hydroxymethyl and hydroxyethyl hydroperoxides in air and precipitation. Nature 337, 631}634. Hewitt, C.N., Kok, G.L., 1991. Formation and occurrence of organic hydroperoxides in the troposphere: laboratory and "eld observations. Journal of Atmospheric Chemistry 12, 181}194. Hough, A.M., 1991. Development of a two-dimensional global tropospheric model: model chemistry. Journal of Geophysical Research 96, 7325}7362. Jackson, A.V., Hewitt, C.N., 1996. Hydrogen peroxide and organic hydroperoxide concentrations in air in a eucalyptus forest in central Portugal. Atmospheric Environment 30 (6), 819}830. Jackson, A.V., Hewitt, C.N., 1999. Atmosphere hydrogen peroxide and organic hydroperoxides: a review. Critical Reviews in Environmental Science and Technology 29 (2), 175}228. Jacob, P., Klockow, D., 1993. Measurements of hydrogen peroxide in Antarctic ambient air, snow and "rn cores. Fresenius Journal of Analytical Chemistry 346, 429}434. Je!erson, A., Tanner, D.J., Eisele, F.L., Davis, D.D., Chen, G., Crawford, J., Huey, J.W., Torres, A.L., Berresheim, H., 1998. OH photochemistry and methane sulfonic acid formation in the coastal Antarctic boundary layer. Journal of Geophysical Reseasrch 103, 1647}1656.
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Kleinmann, L.I., 1986. Photochemical formation of peroxides in the boundary layer. Journal of Geophysical Research 91, 10889}10904. Kleinmann, L.I., 1994. Low and high NO tropospheric V photochemistry. Journal of Geophysical Research 99, 16831}16838. KoK nig-Langlo, G., Herber, A., 1996. The meteorological data of the Neumayer station (Antarctica) for 1992, 1993, and 1994. Reports on Polar Research 187, 1}101. KoK nig-Langlo, G., King, J.C., PettreH , P., 1998. Climatology of the three coastal Antarctic stations Dumont d'Urville, Neumayer, and Halley. Journal of Geophysical Research 103, 10935}10946. Lazrus, A.L., Kok, G.L., Gitlin, S.N., Lind, J.A., 1985. Automated #uorometric method for hydrogen peroxide in atmospheric precipitation. Analytical Chemistry 57, 917}922. Lazrus, A.L., Kok, G.L., Lind, J.A., Gitlin, S.N., Heikes, B.G., Shetter, R.E., 1986. Automated #uorometric method for hydrogen peroxide in air. Analytical Chemistry 58, 594}597. Lee, M., Heikes, B.G., Jacob, D.J., Sachse, G., Anderson, B., 1997. Hydrogenperoxide, organic hydroperoxide, and formaldehyde as primary pollutants from biomass burning. Journal of Geophysical Research 102, 1301}1309. Lee, M., Heikes, B.G., Jacob, D.J., 1998. Enhancements of hydroperoxides and formaldehyde in biamass burning impacted air and their e!ect on atmospheric oxidant cycles. Journal of Geophysical Research 103, 13201}13212. Lightfoot, P.D., Cox, R.A., Crowley, J.N., Destriau, M., Hayman, G.D., Jenkin, M.E., Moortgat, G.K., Zabel, F., 1992. Organic peroxy radicals: kinetics, spectroscopy and tropospheric chemistry. Atmospheric Environment 26, 1805}1961. Lind, J.A., Kok, G.L., 1994. Correction to Henry's law determinations for aqueous solutions of hydrogen peroxide, methylhydroperoxide, and peroxyacetic acid. Journal of Geophysical Research 99, 21}119. Logan, J.A., Prather, M.J., Wofsy, S.C., McElroy, M.B., 1981. Tropospheric chemistry: a global perspective. Journal of Geophysical Research 86, 7210}7254. McConnell, J.R., Winterle, J.R., Bales, R.C., Thompson, A.M., Steward, R.W., 1997a. Physically based inversion of surface snow concentrations of H O to atmospheric concentra tions at South Pole. Geophysical Research Letters 24 (4), 441}444. McConnell, J.R., Bales, J.C., Winterle, J.R., Kuhns, H., Stearns, C.R., 1997b. A lumped parameter model for atmosphere-
to-snow transfer function for hydrogen peroxide. Journal of Geophysical Research 102, 26809}26818. Neftel, A., Jacob, P., Klockow, D., 1984. Measurements of hydrogen peroxide in polar ice samples. Nature 311, 43}45. Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 1st Edition. Wiley, New York. Sigg, A., Sta!elbach, T., Neftel, A., 1992. Gas phase measurements of hydrogen peroxide in Greenland and their meaning for the interpretation of H O records in ice cores. Journal of Atmospheric Chemistry 14, 223}232. Slemr, F., Tremmel, H.G., 1994. Hydroperoxides in the marine troposphere over the Atlantic Ocean. Journal of Atmospheric Chemistry 19, 371}404. Sta!elbach, T.A., Kok, G.L., Heikes, B.G., McCully, B., Mackay, G.I., Karecki, D.R., Schi!, H.I., 1996. Comparison of hydroperoxide measurements made during the Mauna Loa observatory photochemistry experiment 2. Journal of Geophysical Research 101, 14729}14739. Thompson, A.M., Chappellaz, J.A., Fung, I.Y., Kucsera, T.L., 1993. The atmospheric CH increase since the last glacial maximum, 2, Interactions with oxidants. Tellus 45B, 242}357. Tolu, K., 1993. Untersuchung am enzymatisch-#uorometrischen Me{geraK t fuK r Peroxide (Investigations at an enzymatic #uorometric instrument for peroxides). Diploma Thesis, Fachhochschule Isny, Germany. Tremmel, H.G., 1992. Die Verteilung von Peroxiden in der unteren TroposphaK re. Ph.D. Thesis, Fraunhofer Institut fuK r AtmosphaK rische Umweltforschung, Garmisch-Partenkirchen. Van Ommen, T.D., Morgan, V., 1996. Peroxide concentrations in the Dome Summit south ice core, Law Dome, Antarctica. Journal of Geophysical Research 101, 9139}9147. Weller, R., Schrems, O., 1993. H O in the marine troposphere and seawater of the Atlantic Ocean. Geophysical Research Letters 20 (2), 125}128. Weller, R., Schrems, O., 1996. Photooxidants in the marine Arctic troposphere in summer. Journal of Geophysical Research 101, 9139}9147. Weller, R., Schrems, O., Boddenberg, A., GaK b, Gautrois, M., 2000. Meridional distribution of hydroperoxides and formaldehyde in the marine boundary layer of the Atlantic (483N}353S) measured during the ALBATROSS campaign. Journal of Geophysical Research, 105, 14401}14412.
Variability of tropospheric hydroperoxides at a coastal surface site in Antarctica Katja Riedel, Rolf Weller*, Otto Schrems, Gert KoK nig-Langlo Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany Received 13 December 1999; received in revised form 13 March 2000; accepted 13 June 2000
Abstract The annual cycles of hydrogen peroxide (H O ) and methylhydroperoxide (MHP) have been investigated at a remote site in Antarctica in order to study seasonal variations as well as chemical processes in the troposphere. The measurements have been performed from March 1997 to January 1998 and in February 1999 at the German Antarctic research station Neumayer which is located at 70339 S, 8315 W. The obtained time series for hydrogen peroxide and methylhydroperoxide in near-surface air represents the "rst all-year measurements in Antarctica and indicates clearly the occurrence of seasonal variations. During polar night mean values of 0.054$0.046 ppbv (range(0.03}0.11 ppbv) for hydrogen peroxide and 0.089$0.052 ppbv (range(0.05}0.14 ppbv) for methylhydroperoxide were detected. At the sunlit period higher Mixing ratios were found, 0.20$0.13 ppbv (range(0.03}0.91 ppbv) for hydrogen peroxide and 0.19$0.10 ppbv (range(0.05}0.89 ppbv) for methylhydroperoxide. Occasional long-range transport of air masses from mid-latitudes caused enhanced peroxide concentrations at polar night. During the period of stratospheric ozone depletion we observed peroxide mixing ratios comparable to typical winter levels. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen peroxide; Methylhydroperoxide; Seasonal variation; Photooxidants; Ozone depletion
1. Introduction Photooxidation is the most dominant sink for many of the environmentally important natural and anthropogenic atmospheric trace gases such as CH , CO, SO , and DMS (dimethyl sul"de). Besides hydroxyl radicals (OH) and ozone (O ), hydroperoxides are the main oxidising compounds in the troposphere (see review article by: Jackson and Hewitt, 1999, and literature therein). Hydroperoxides in#uence the oxidising capacity of the atmosphere and represent a major reservoir for OH radicals. The whole complexity of the atmospheric chemistry of peroxides was described by numerous former publications (see review article by: Gunz and Ho!mann, 1990, and literature therein). In the Antarctic troposphere
* Corresponding author. Tel.: #49-471-4831-1253; fax: #49-471-4831-1425. E-mail address: [email protected] (R. Weller).
the peroxide chemistry is characterised by a limitation to some important sources and sinks on which we want to concentrate in the following. At remote polar sites hydroperoxides are mainly produced by recombination of HO and RO radicals HO #HO PH O #O , (1) RO #HO PROOH#O . (2) Peroxy radicals on their part are mostly formed by photooxidation of hydrocarbons and CO initiated by OH radical attack (Lightfoot et al., 1992). The dominant source of OH radicals is photolysis of O followed by reaction of the formed O(D) atoms with water vapour. Hydroperoxides are also involved in the odd-hydrogen (H, OH, HO ) and odd-oxygen (OH, HO , CH OO) chemistry of the troposphere. They can be used as a diagnostic tool for the extent of HO free radical chemistry V occurring in the troposphere (Kleinmann, 1994). The main sinks for hydrogen peroxide are dry and wet
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 3 2 2 - 8
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deposition, reaction with OH radicals (3) and photolysis (4) (Logan et al., 1981): H O #OHPH O#HO , (3) H O #hlP2 OH(j(350 nm). (4) In contrast to the short living radicals (OH, HO ), H O itself is rather stable in the atmosphere. At mid latitudes, the overall lifetime within the planetary boundary layer is presumed to be around q"30 h (Kleinmann, 1986). In polar regions the in#uence of the local photochemistry on the production of photooxidants is very small, especially during the polar night when temperatures and solar radiation are low or absent, respectively. Dry deposition to ice and snow surfaces is not signi"cant even for water-soluble species (Bales et al., 1987). Thus, longer lifetimes for hydrogen peroxide could be expected in polar regions (q'2}3 d). Consequently, e!ects due to long-range transport have to be included in the discussion of peroxide mixing ratios in Antarctica. Whereas in polluted air various organic hydroperoxides such as methylhydroperoxide (MHP, CH OOH), ethylhydroperoxide (EHP, CH CH OOH), hy droxyethylhydroperoxide (HEHP, CH CH(OH)OOH) and hydroperoxides of other hydrocarbons can be detected (Hewitt and Kok, 1991), at remote marine and polar regions methane is the only signi"cant organic peroxy radical precursor. Reaction of methane with hydroxyl radicals produce methylperoxy radicals: CH #OHPH O#CH , (5) CH #O #MPCH O #M. (6) Ship-based HPLC investigations indicated that no other organic hydroperoxides than methylhydroperoxide could be detected in remote marine areas (Weller et al., 2000). In comparison to hydrogen peroxide the organic hydroperoxides have lower Henry law constants, for example K "304 M atm\ and K "1.02;10 +&. &M atm\ at 298 K (Lind and Kok, 1994). Thus, wet deposition of organic hydroperoxides is less important, which is supported by higher gaseous organic peroxide/hydrogen peroxide ratios during or after rainfalls (Weller and Schrems, 1993) and an enrichment of H O in rainwater (Hellpointer and GaK b, 1989). Since hydrogen peroxide seems to be preserved for millenia (Neftel et al., 1984) in glacial ice and snow, investigations of hydroperoxides in polar regions have received special attention due to the importance of polar ice caps as inexhaustible climate archives. Atmospheric hydrogen peroxide washed out by precipitation and deposited in snow and ice may potentially provide valuable information concerning the oxidative capacity of the ancient troposphere (Thompson et al., 1993). However, there is still a vital lack of information for tropospheric photochemistry transport mechanisms of hydroperox-
ides in polar regions, about peroxide deposition, postdepositional air}snow exchange and possible chemical reactions in buried snow. Hence, one aim of our investigations was to obtain basic experimental data which are needed to understand peroxide photochemistry in the pristine polar and marine troposphere free of any anthropogenic in#uence. The crucial point of our investigations was to measure for the "rst time the seasonal variations of tropospheric H O and MHP mixing ratios in Ant arctica. Analysis of the obtained data and interpretation of the results have been performed along with parameters characterising the photochemical activity, transport and deposition processes.
2. Experimental The measurements were carried out at the German Antarctic station Neumayer (70339 S/08315 W) during the overwintering season 1997/98 and during a summer campaign which was performed from January to March 1999. The station is located on the EkstroK m Ice Shelf at a distance of 7 km from the southeast part of Atka Bay, 42 m above sea level (see Fig. 1). The instruments were installed in the Air Chemistry Observatory of the Neumayer station which is located 1500 m south of the main base. Since northerly winds are hardly ever observed there, no serious contamination problem due to the main station existed. The mean annual temperature at Neumayer is !153C. The maximum solar incidence angle is 42.83. The sun stays permanently above the
Fig. 1. Map of local area around Neumayer station on the EkstroK m Ice shelf. Continental areas are in black, ice shelf areas in grey.
K. Riedel et al. / Atmospheric Environment 34 (2000) 5225}5234
horizon from 19 November to 24 January and permanently below the horizon from 19 May to 27 July (KoK nig-Langlo et al., 1998). A #uorometric two-channel technique, described in detail by Lazrus et al. (1985), was applied for continuous quanti"cation of hydroperoxides in ambient air. An air stream of 1000 cm min\, was sucked through the sampling line, a 2.5 m long Te#on hose with an inner diameter of 4 mm. This leads to a residence time of 1.9 s for the air sample in the tube. In the instrument (Aerolaser H O analyser, Model AL2002) the incoming air #ow was led into a coil where H O was stripped from the air by a bu!er solution (pH"5.8}6.0). The stripping coil was an approximately 53 cm long spiral glass tube with an inner diameter of 2 mm. The #ow rate of the stripping solution was regulated by a peristaltic pump at 0.42 cm min\. In the "rst channel, the #uorimetric reagent (para-hydroxyphenylacetic acid, POPHA) and horseradish peroxidase was added to determine the total amount of hydroperoxides. POPHA forms a dimer when it reacts with peroxidase and organic hydroperoxides or H O . This dimer can be excited with a cadmium vapour lamp at 320 nm. The #uorescence signal at 415 nm is linearly dependent on the peroxide concentration. A distinction between hydrogen peroxide and organic hydroperoxides was achieved by selective destruction of H O with the enzyme catalase in the second parallel channel. Weller et al. (2000) showed by HPLC analysis that no other organic peroxide than methylhydroperoxide (MHP, CH OOH) could be detected in remote mar ine areas. In this paper, we assume that the signal from the catalase channel corresponds to MHP. Thus, a continuous quantitative determination of MHP mixing ratios with the Lazrus method was feasible. However, a fraction of the methylhydroperoxide is inherently destroyed by catalase, making a correction necessary. The evaluation of the raw data were carried out similarly as described in literature (Claiborn and Aneja, 1991; Ayers et al., 1996; Sta!elbach et al., 1996; Weller et al., 2000). The raw data from the peroxidase channel S and the signal of the catalase channel S were processed by the following expression: aI S !S S !a S [CH OOH]" , [H O ]" aI !a e(aI !a ) where e is the collection e$ciency for MHP and a is the residual fraction of H O remaining in the catalase channel (ratio of response in the catalase channel to that of the total hydroperoxides channel to span solutions of H O ). The destruction e$ciency of the catalase solution was determined twice a day and adjusted to be between 70 and 90%, corresponding to a "0.1}0.3. Our laborat ory experiments showed that at catalase e$ciencies between 90 and 95% the MHP decay was around 5}7%. Lower destruction e$ciencies (70}75%) lead to MHP losses of 2}3% (Tolu, 1993). The ratio between pseudo
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"rst-order reaction rate coe$cients for the reaction of catalase with a mixture of hydroperoxides and for the reaction with pure hydrogen peroxide is given by k"k /k . Based on laboratory investigations with pure G MHP we presumed a value of k"0.017. This ratio was also used for all calculations in this work. Another crucial factor is the collection e$ciency e of the stripping coil, which must be carefully determined since Henry's law coe$cients for MHP and H O show a strong temper ature dependence (Lind and Kok, 1994). For hydrogen peroxide and MHP the Henry's law constant K is given by K "e 2\ K "e 2\ . &+&. In Neumayer's Air Chemistry Observatory the temperatures varied during the whole year over the range of 18}263C (minimum and maximum temperatures of 13 and 363C were also observed). Calculations according to Lazrus et al. (1986) showed that under the given conditions, viz., 1000 cm min\ air#ow and 0.42 cm min\ stripping solution #ow, equilibrium is reached in the stripping coil. The sampling e$ciency for hydrogen peroxide was always better than 99% independent of temperature, whereas the sampling e$ciency e of methylhydroperoxide showed strong temperature dependence. It was calculated to be 0.82 at 183C and 0.75 at 263C. Uncertainty in the collection e$ciency accounted for the largest contribution of estimated uncertainty in the reported mixing ratios of MHP. Measurements of hydroperoxides in air require a periodic determination of the systems sensitivity by using a calibration source. Thus, an automatic gas calibration was performed every 12 h. Signals of zero air and of an internal hydrogen peroxide standard were compared and used to determine the sensitivity of the instrument. Zero air was generated internally by passing ambient air through a zero trap with manganese-dioxide-coated charcoal. The internal gas phase H O source consisted of a short piece of polyethylene tube in a small bottle "lled with 30% H O . The temperature of the device was stabilised to approximately 253C. The permeation source itself was calibrated every week by standard solutions of 10\ mol l\ H O , freshly prepared from a stock solu tion of 10\ mol l\ H O . The absolute permeation rate is calculated by comparing the permeation signal with the signal of the prepared liquid standards. The concentration of the 10\ M hydrogen peroxide stock solution was determined every three months by titration with KMnO . MHP calibration was performed in our home laboratory with aqueous solutions of di!erent MHP concentrations before the over-wintering period. Due to the explosive nature of MHP, calibration was not repeated during the expedition. Unfortunately no suitable permeation sources are available for MHP. Especially during winter season, H O and MHP values were sometimes extremely low and thus close to the detection
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limit of the instrument. Therefore, regular zero measurements were carried out. An external zero trap "lled with manganese-dioxide-coated charcoal was used to determine the daily zero value. The trap was mounted between the instrument and inlet tube for 30 min each day. The main air inlet was installed on top of the Air Chemistry Observatory 8 m above the ground. The inlet was protected against drifting snow with a specially designed tumbler. The sampling e$ciency of the Te#on inlet system was determined to be in the range of 0.85}0.95 by comparing measurements of indoor air (contaminated with hydroperoxides). For some minutes indoor air was directly sucked into the instrument and the results were compared with the peroxide concentrations obtained with the te#on tube. All solutions were prepared with ultra pure water (milliQ-water) and reagent grade chemicals. Under these conditions a detection limit of 50 pptv is achievable. This corresponds with 3p of the standard deviation of the noise level. The accuracy of the instrument has been calculated to be in the order of 15}20% at [H O ]) 500 pptv and 12}15% at [H O ]"500}1000 pptv. The time constant for a change from 10 to 90% signal is 30 s with a delay time of 300 s. Further supporting data were provided by the long-term measuring programme established at the Air Chemistry and Meteorological Observatory at Neumayer station. UV solar radiation measurements were carried out with an UV radiometer (selenium photocell pyranometer, Eppley, USA, 300}370 nm). Relative humidity was measured with two pernix hair hygrometers (Lambrecht 800L100) (KoK nig-Langlo and Herber, 1996). Ozone column density data were obtained by ozonesondes launched weekly at Neumayer station. Additionally data from satellite-based total ozone mapping spectrometer (TOMS) were used, available in the world wide web: http://jwocky.gsfc.nasa.gov/. Daily 5day-back trajectories have been made available by the German Weather Service (DWD).
Fig. 2. Annual variation of hydrogen peroxide (drawn black line) and methylhydroperoxide (drawn grey line): Five day running means of hydroperoxides gas-phase concentrations (in ppbv) are plotted against day of the year (1"1 January and 365"31 December). The presented time series is a result of two di!erent campaigns: March 1997 to January 1998 and February 1999. The thin line shows annual variations in UV solar radiation.
3. Results Fig. 2 presents our results of the "rst all-year-round investigations of hydroperoxides in the Antarctic troposphere. The seasonal variations of hydrogen peroxide and methylhydroperoxide gas-phase mixing ratios at Neumayer station are compared with UV solar radiation for the same time period. A clear seasonal variation is evident. During the polar night we observed mixing ratios of 0.054$0.046 ppbv (range 0.03}0.40 ppbv) for hydrogen peroxide and 0.089$0.052 ppbv (range 0.05}0.35 ppbv) for methylhydroperoxide. Most values (97% of the measured H O and 86% of the measured MHP values) are in the range from below 0.05 to 0.150 ppbv. Fig. 3 illustrates the frequency distribution of
Fig. 3. Frequency distribution of measured peroxide concentrations. Frequency (in %) is depicted versus concentration intervals of 0.05 ppbv: (a) distribution of hydrogen peroxide, and (b) methylhydroperoxide, grey columns represent winter values (1 April to 15 September, 91}258 day of the year, total number of measurements N+11000), black columns represent summer values (15 September to 1 April, total number of measurements N+7800).
K. Riedel et al. / Atmospheric Environment 34 (2000) 5225}5234
measured peroxide mixing ratios during winter (1 April to 15 September, 91}258 day of the year) and summer season (15 September to 1 April, 259}90 day of the following year). As depicted in Fig. 3 of the winter data are close to, respectively, below the detection limit of 0.05 ppbv. An increase of peroxide mixing ratios could be observed in the middle of polar night (H O "0.15 ppbv and MHP"0.11 ppbv (see Fig. 2). Due to that the winter mean values are slightly shifted to higher levels. Calculation of the winter means without these values leads to 0.048$0.046 ppbv for hydrogen peroxide and 0.084$0.055 ppbv for methylhydroperoxide. During summer, higher peroxide mixing ratios were found, i.e. 0.20$0.13 ppbv (range(0.03}0.91 ppbv) for atmospheric hydrogen peroxide and 0.19$0.10 ppbv (range(0.05}0.89 ppbv) for methylhydroperoxide.
4. Discussion 4.1. Seasonal variations Since production of hydroperoxides is closely linked with the HO chemistry, variation in the solar radiation V should entail changes in atmospheric peroxide concentrations. Fig. 2 illustrates the relation between UV solar radiation and observed peroxide mixing ratios. As expected, the decline of atmospheric peroxide concentrations coincide with the drop of solar radiation to 0.0 W/m during polar night (139}208th day of the year) since photochemical oxidation is the only relevant source of atmospheric hydroperoxides. Nevertheless surprisingly high atmospheric concentrations could be observed during polar night when the sun stays permanently below the horizon with a minimum solar incidence angle of !433 at 26 June (177th day of the year). Obviously in this case local photochemistry could not contribute to the peroxide production. The South Pole photochemical models (McConnell et al., 1997a) predict mixing ratios of around 1 pptv for hydrogen peroxide during polar night. A reasonable explanation for these signi"cant di!erences might be an underestimation of transport processes (Bales and Choi, 1996). Previous investigations of hydrogen peroxide in polar regions were carried out almost exclusively during summer season (Table 1). Generally the values for hydrogen peroxide mixing ratios are in the same order of magnitude. However, in our time series concentration levels up to 1 ppbv are only reached sporadically. In comparison with "ndings of Jacob and Klockow (1993), we observed lower H O concentrations around 0.24$0.12 ppbv in December and 0.16$0.09 ppbv in January. The only measurements performed at polar night so far were carried out by de Serves (1994) at Alert, Canada. From January to February 1992 he observed total peroxide concentrations from below detection limit (0.01 to
5229
0.04 ppbv. Our measurements of total hydroperoxides indicate higher values between (0.05 and 0.26 ppbv with a mean of 0.13$0.07 ppbv during winter. Several approaches from di!erent groups were made to estimate atmospheric hydrogen peroxide mixing ratios from the measured concentration values in snow pits and ice cores (van Ommen and Morgan, 1996; McConnell et al., 1997b). This method provides a suitable possibility to reconstruct atmospheric concentration signals: For Law Dome, Antarctica van Ommen and Morgan (1996) predict hydrogen peroxide mixing ratios of 0.01 ppbv at winter and between 0.12 and 0.80 ppbv at summer. Obviously the derived winter values are signi"cantly lower than our measurements. Most probably uncertainties concerning post deposition processes and air}snow transfer coe$cients are responsible for this discrepancy. In comparison to H O , there are only very few "eld measurements of organic hydroperoxides reported in literature. In polar regions, as far as we know, our investigations represent the "rst measurements at all. As pointed out in the experimental section, we attribute the catalase channel signal to methylhydroperoxide (MHP). Mean concentrations of 0.19$0.10 ppbv MHP at summer and 0.09$0.05 ppbv at winter were observed. In comparison with the measurements conducted in the continental troposphere by other groups (Heikes et al., 1987; Barth et al., 1989, Jackson and Hewitt, 1996), our investigations exhibit a higher MHP/(H O #MHP) ratio of 0.57$0.26 (range 0.1}1.0). Ship-based investigations across the Atlantic from northern mid-latitudes to southern mid-latitudes revealed ratios between 0.1 and 0.6 (mean: 0.32$0.12) (Weller et al., 2000) and between 0.17 and 0.98 (mean: 0.48$0.14) (Slemr and Tremmel, 1994). Airborne measurements over the western North Paci"c report about ratios of 0.58 (Heikes et al., 1996) while in the continental boundary over the United States much lower ratios of 0.1 have been found (Heikes et al., 1987; Barth et al., 1989). Investigations at the European continent show slightly higher MHP/(H O #MHP) ratios. Fels and Junkermann (1994) observed ratios of 0.35}0.58 at a mountain site in southern Germany and Jackson and Hewitt (1996) report on ratios of 0.05}0.37 in central Portugal. Airborne measurements over the South Atlantic, Brazil and southern Africa during TRACE-A (Lee et al., 1997, 1998) exhibit distinct di!erences according to air mass origin. MHP/(H O # MHP) ratios of 0.2 were found over continents while in marine air masses the ratio was typically twice as high. Obviously, the highest MHP/(H O #MHP) ratios were found at Neumayer station and in remote marine regions. The overall deposition rates for MHP and H O , including dry and wet deposition should be very di!erent over land, ocean, snow and ice and within the planetary boundary layer compared to the free troposphere. Especially dry deposition on the ocean surface, which is an important removal process of water soluble
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Table 1 Comparison of previous measurements of tropospheric hydroperoxides concentrations in polar regions with our recent results Measuring site
Reference
Measuring period
Atmospheric H O concentrations (ppbv)
East Greenland sea (703}803N) Alert (Canada, 82.53N, 62.33W)
Weller and Schrems (1996)
July}August 1994
de Serves (1994)
Feb.}April 1992
Summit (Greenland, 72.23N, 37.83W) Summit (Greenland, 72.23N, 37.83W) Summit (Greenland, 72.23N, 37.83W) Cape Grim (Tasmania, 413S, 1453E)
Sigg et al. (1992)
June}July 1990
Mean: 0.42$0.26 Range: (0.1}1.0 Total peroxides Dark period: (0.01}0.04 Sunlit period: 0.10}0.40 0.3}3.5
Bales et al. (1995)
May}July 1993
McConnell et al. (1997b)
April}July 1995
Ayers et al. (1996)
Feb. 1991}March 1992
Weller and Schrems (1993)
Jan. 1992
Total peroxides Summer: 0}2 (mean Dec.: 1.4) Winter: 0}1.2 (mean July: 0.16) 0.47}0.86
McConnell et al. (1997a) Fuhrer et al. (1996)
Nov.}Dec. 1994 Jan. 1996 Dec. 1993}Jan. 1994
0.20}0.25 0.90}1.00 ( 0.5
Jacob and Klockow (1993)
Dec. 1989}Jan. 1990
This work
April}Sept. 1997 Sept. 1997}March 1998 and Feb. 1999
Mean: 0.394 Range:(0.10}1.05 Mean: 0.054$0.046 Range:( 0.03}0.40 Mean: 0.197$0.130 Range:(0.03}0.90
Northwest Weddell Sea, Brans"eld Street, (61}633S, 51}593W) South Pole (Antarctica, 903S) Dronning Maud Land (Antarctica, 73}763S, 12}93W) Neumayer (Antarctica, 70.13S, 8.03W) Neumayer (Antarctica, 70.13S, 8.03W)
trace gases in the marine boundary layer, leads to a preferential depletion of H O (Weller and Schrems, 1993). Di!erent air mass histories may explain the depicted variations measured at Neumayer station. Unfortunately, literature data even for dry deposition rates of MHP and H O over di!erent surfaces are disturbingly incon sistent (Hauglustaine et al., 1994; Hough, 1991). Our data indicate that deposition of H O is favoured relative to MHP over the ocean: Air masses passing the sea before reaching Neumayer station showed a signi"cantly higher MHP/(H O #MHP) ratio. On 7 September when a MHP/(H O #MHP) ratio between 0.3 and 0.7 was found, the corresponding trajectory indicates advection of marine air masses, passing a frontal system of a depression over the southern Atlantic masses (Fig. 4: 6 (circles), 7 (squares) and 8 September (triangles)). Most probably, wet deposition occurred along the trajectory.
Atmospheric MHP concentrations (ppbv)
Mean: 1.0$0.4 Range: 0.4}2.0 0.50}1.75
Range:( 0.05}0.35 Mean: 0.089$0.052 Mean: 0.191$0.103 Range:(0.05}0.89
In contrast, continental air masses caused lower MHP/(H O #MHP) ratios during the day before and after this event. 4.2. Long-range transport ewects and tropospheric lifetimes of hydroperoxides A striking increase of peroxide mixing ratios was found in the middle of the polar night (177th day of the year, of 26 June). Local photochemistry could not contribute to the production of photooxidants, because of missing solar radiation. Thus, long-range transport of hydroperoxides from sunlit areas to Antarctica seems to be the most plausible explanation. Analysis of the 5-day-back trajectories con"rms this assumption. Fig. 5 shows trajectories from 25 (circles), 26 (squares) and 27 June (triangles). While air parcels reaching Neumayer station on 25 and
K. Riedel et al. / Atmospheric Environment 34 (2000) 5225}5234
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ing such long-range transport, atmospheric lifetimes have to be considered. Former estimations of hydrogen peroxide lifetimes vary between q"1 d in polluted atmosphere (Kleinmann, 1986), 10 d as e!ective tropospheric lifetime (Logan et al., 1981) and a `rainouta lifetime of q"50 d, calculated with a simple box model and observed meteorological conditions at Summit, Greenland (Sigg et al., 1992). All these calculations assume three main H O removing processes: Reaction with OH radicals (3), photolysis (4) and deposition. First of all, we will estimate and compare the H O lifetime over ice for polar day and night. During polar day the H O lifetime q (H O ) can be calculated by the expression q (H O )"(k #k [OH]#J )\.
Fig. 4. 5-day-back trajectories reaching Neumayer station at surface level on September the 6th (circles), 7th (squares) and 8th (triangles). All trajectories showed only marginal vertical movements within the planetary boundary layer.
The H O deposition rate over ice (k ) was esti mated with the following considerations: Deposition should occur inside the mixed layer which is then identical with the Ekman layer. This layer extends to a height of from 300 to 500 m depending on the surface roughness of the terrain (Seinfeld and Pandis, 1998). In the case of ice the surface roughness is typically small. Therefore, a mixing height of 300 m was assumed. With k\ "h v\, an adopted dry deposition velocity v for hydrogen peroxide over ice of 0.05 cm s\ (Hough, 1991), and a mixing height h"300 m, a peroxide lifetime k of 1.6;10\ s\ results. J is the photolysis frequency of H O , estimated according to Tremmel (1992) to be around 2;10\ s\, and k "1.6;10\ cm\s\ (DeMore et al., 1994), is the rate coe$cient of reaction (3) at !103C. With a mean 24 h OH concentration of 1.1;10 cm\ (Je!erson et al., 1998), q (H O ) is roughly 3 d. During polar night, we can neglect J and k [OH], but take into account the reaction H O #NO PHO #HNO (5), thus q (H O )"(k #k [NO ])\.
Fig. 5. 5-day-back trajectories reaching Neumayer station at surface level on the 25 (circles), 26 (squares) and 27 June (triangles). Origin of the air parcel arriving on 26 is a sunlit area east of South America (533S, 433W). Again, trajectories showed only marginal vertical movements within the planetary boundary layer.
27 June originated from inner Antarctica, the source region on June 26 was a marine area east of South America (533S, 433W). The corresponding local sunshine duration in this region was around 7.5 h with a maximum solar incidence angle of 13.73. Photochemical production of hydrogen peroxide and methylhydroperoxide may have occurred enhancing the atmospheric peroxide concentrations at Neumayer station "ve days later. Regard-
The steady-state NO concentration was derived con sidering the main source and loss reactions NO #O PNO #O (6) and NO #NO PN O (7), respectively, to be [NO ]"k [O ]/k + 2.7;10 cm\ (k "6;10\ cm\ s\ and k " 1.7;10\ cm\ s\ at !253C (DeMore et al., 1994), measured O mixing ratio: 32 ppbv). Unfortunately, to our knowledge there exists no data for the rate coe$cient of the reaction H O #NO , but typically hydrogen abstraction reactions by NO are 3}4 orders of magni tude lower compared to the corresponding OH reaction (DeMore et al., 1994). As a consequence, also the reaction H O #NO can be neglected, yielding q (H O )+ 7 d. Concerning the situation at 26 June, polar night predominated in the region south of 66330 S and deposition should be the dominant sink for atmospheric hydrogen peroxide, indicating that e!ective H O advection by long-range transport could occur.
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4.3. Impact of stratospheric ozone depletion Reductions in stratospheric ozone (O ) cause in creased penetration of ultraviolet-B (UV-B, 280}315 nm) radiation to the troposphere, and therefore increase the photochemical activity. Radical production in the atmosphere should react very sensitively to enhanced tropospheric UV-B actinic #uxes. Since at remote polar sites hydroperoxides are mainly produced by recombination of HO and RO radicals the in#uence of enhanced actinic #uxes on the formation of these radicals, must be regarded. Photochemistry is basically initiated by OH radicals generated by O photolysis and subsequent re action of O(D) with water vapour O #hlPO(D)#O (j(350 nm), (7) O(D)#H OP2 OH. (8) Due to higher photolysis frequencies of tropospheric O , signi"cantly higher HO mixing ratios might be V expected. Model calculations (Fuglestvedt et al., 1994) suggest that tropospheric H O concentrations should strikingly increase in parallel to the predicted increases in HO . However, it is obvious that OH production by V reaction (9) also depends on the O and the water vapour concentration which has to be considered when dealing with the in#uence of the ozone hole on local photochemistry. In the following we will focus on the observed hydroperoxide mixing ratios during the ozone hole period (271st}283rd day of the year), shortly after this event (284}300 day of the year) and during fall at comparable solar incidence angles (63rd}75th day of the year). Fig. 6a compares the peroxide mixing ratios with measured total ozone column densities from regularly launched ozonesondes and satellite-based total ozone mapping spectrometer (TOMS). Due to its position at 703S latitude Neumayer station could be inside as well as outside of the polar vortex. Thus, ozone column densities show strong #uctuations. During stratospheric ozone depletion, peroxide mixing ratios comparable to previous months have been observed. There is no hint that the ozone hole leads to higher atmospheric peroxide mixing ratios. Moreover, a comparison with the situation during austral autumn at similar solar incidence angles revealed that peroxide concentrations are even lower during the stratospheric ozone minimum (Fig. 6a). Most probably the reason for this striking results are the di!erent water vapour concentrations (Fig. 6b): during the ozone hole period the partial pressure of water, p(H O) was a factor of 4.5 lower compared to the situation at fall. It seems that enhanced actinic radiation and higher O mixing ratios could not compensate the e!ect of lower water vapour concentrations on the primary OH production rate. Moreover, we tentatively attribute the increase of hydroperoxide mixing ratios shortly after the ozone hole to increasing p(H O).
Fig. 6. (a) Comparison of observed peroxide mixing ratios (hydrogen peroxide drawn black line and MHP drawn grey line) and total ozone column densities from regularly launched ozonesondes (open squares) and satellite-based total ozone mapping spectrometer (TOMS, dots). For a better comparison of similar sunsets the daily sunshine duration is also depicted (thin black line). (b) Water vapour partial pressure (p(H O), drawn grey line) and surface ozone mixing ratios (black line) for the same year. The shaded areas mark the ozone hole period (271st}283rd day of the year) and a period during fall at comparable solar incidence angles (63rd}75th day of the year).
5. Conclusions Our measurements at Neumayer station provide the "rst year-round time series of gas-phase peroxide concentrations from the Antarctic continent. We observed a seasonal variation coinciding with seasonal changes in solar radiation due to photochemical formation of hydroperoxides. Generally, observed tropospheric H O and MHP concentrations are higher than expected especially during winter. At polar night long-range transport has to be considered from late fall to early spring due to increased hydroperoxide lifetimes. Our measurements indicate that distinct variations of the MHP/(MHP# H O ) ratio are most probably caused by di!erent air mass history. Di!erent MHP and H O deposition vel ocities over marine and ice covered areas could provoke
K. Riedel et al. / Atmospheric Environment 34 (2000) 5225}5234
fractional depletion of hydrogen peroxide. Instead of increasing hydrogen peroxide levels due to higher UV-B actinic #uxes during the stratospheric ozone depletion, we observed values comparable to typical winter levels. Most surprisingly, hydroperoxides during the ozone hole period are strikingly lower compared to typical mixing ratios in austral fall at similar solar incidence angles. However, in our case the partial pressure of water during the ozone hole period was a factor of 4.5 lower compared to the situation at fall. Clearly, for a more detailed interpretation and a meaningful analysis of the processes determining the observed hydroperoxide time series, photochemical model calculations paying attention to special Antarctic conditions are indispensable and will be topic of our further investigations.
Acknowledgements The authors would like to thank A. Minikin, B. MuK ller and all technicians and scientists of the over-wintering crews and summer campaigns at Neumayer station for their help and technical and logistical support. K.R. thanks the Studienstiftung des deutschen Volkes for a research stipend and M. Hutterli for a helpful evaluation program. The trajectories have been provided by the German Weather Service (DWD). This paper is contribution No. 1728 of the Alfred-Wegener Institute for Polar and Marine Research, Bremerhaven.
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