Seasonal variations of volatile organic compounds in the coastal Baltic

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Volatile organic compounds (VOCs) play a significant role in the global climate ..... Seasonal patterns (daily mean based on hourly data) of environmental ...
Research Paper

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A. Orlikowska and D. E. Schulz-Bull, Environ. Chem. 2009, 6, 495–507. doi:10.1071/EN09107

Seasonal variations of volatile organic compounds in the coastal Baltic Sea Anna OrlikowskaA,B and Detlef E. Schulz-BullA A Department

of Marine Chemistry, Leibniz Institute for Baltic Sea Research Warnemünde (IOW), D-18119 Rostock, Germany. B Corresponding author. Email: [email protected]

Environmental context. Volatile organic compounds (VOCs) play a significant role in the global climate and are engaged in several atmospheric reactions. Relatively large amounts of VOCs are emitted from coastal waters, which is why these zones are expected to have significant impact on the atmospheric chemistry. The abundance of a single compound depends on its source and removal processes as well as on environmental parameters. Thus, seasonal changes can greatly affect the occurrence and behaviour of these trace gases. Abstract. In order to investigate temporal changes in combination with the influence of different environmental parameters on the concentration and the composition of volatile organic compounds (VOCs), seawater samples from the coastal Baltic Sea were weekly measured from January to November 2008. In most cases, concentrations of VOCs varied seasonally and were influenced by changes in temperature and light conditions or biological species composition. A nearly two-fold increase in the mean concentration was noticed for isoprene, iodomethane and bromoform in the season with higher water temperature. The strongest flux of dimethylsulfide to the atmosphere appeared in May and July. Its high production was related to the presence of Prymnesiophyceae. The highest concentrations of diiodomethane and chloroiodomethane were observed with the spring and autumn phytoplankton bloom; their distribution was strongly controlled by light intensity. Flux calculations showed that coastal regions can affect local atmosphere, especially during biologically active periods. The strongest emission of bromoform and iodomethane was in July and August. The data presented here highlights the need to include seasonal cycles when calculating the global budgets and modelling sea–air fluxes of trace gases. Additional keywords: DMS, isoprene, VOCs, volatile halogenated organic compounds (VHOCs), water.

Introduction Volatile organic compounds (VOCs) play an important role in the global climate and are involved in several photochemically induced atmospheric processes.[1] The global cycles of VOCs involve intense exchange in the marine boundary layer and the ocean surface is an important emitter of many VOCs to the atmosphere.[2–5] Several different kinds of compounds can be included in this group. Volatile halogenated organic compounds (VHOCs) are a strong source of highly reactive halogen oxide radicals to the atmosphere. Their importance is mainly a result of their significant contribution to both tropospheric and stratospheric ozone depletion.[1,2,6] Organohalogens originate from various chemical and biological processes. It is well known that marine algae produce several these compounds. Moreover, photolytic processes of organic precursors can be an additional source for trace gases in water.[2,6–9] Furthermore, some of them are created during many anthropogenic processes.[10,11] Another group of VOCs that also originate from man-made sources are aromatic hydrocarbons.[10,11] They are of interest as some of them, as well as some VHOCs, are toxic and carcinogenic.[11] An additional compound that is important in controlling the balance of atmospheric oxidants is isoprene.[12] It has been shown that isoprene is produced not only by terrestrial vegetation but also by macro-[12] and microalgae.[13,14] © CSIRO 2009

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DMS (dimethylsulfide) is another trace gas whose flux from surface marine waters to the air is expected to be of climatic importance. Oxidation of DMS causes an increase of sulfate aerosols which act as cloud condensation nuclei and can be significant for the radiative balance of the earth.[15] Moreover, the production of dimethylsulfide in water followed by its release into the air is one of the most important biogenic sources of sulfur to the atmosphere. Through their direct precursor dimethylsulfoniopropionate (DMSP), various types of micro- and macroalgae are the key producers of DMS.[16–18] Several VOCs have been identified in the water column. Nonetheless, data on their seasonal changes and biogenic sources is still limited since most of the studies are concerned with only a few compounds or are conducted over short periods. It should be pointed out that the seasonal cycles can have a great impact on the occurrence and behaviour of these trace gases since their abundance depends on many processes largely influenced by seasonally changing factors, e.g. temperature and light intensity as well as biological diversity. Therefore, this study focussed on the investigation of VOCs and their seasonal differences in the Baltic Sea. The relations between VOCs, water temperature, irradiance and phytoplankton occurrence were analysed. The fluxes of the naturally derived gases were estimated. These kinds of long-term field measurements are important in order to provide seasonal distribution data, which are necessary for the understanding and modelling 1448-2517/09/060495

A. Orlikowska and D. E. Schulz-Bull

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Fig. 1. Map of the German Baltic Sea coast with positions of the sampling station Heiligendamm (star), the Marnet Darss Sill station (triangle) and DWD meteorological station in Warnemünde (circle).

of sea–air fluxes and global budget calculations of trace gases. Results and discussion The concentrations of VOCs, i.e. halogenated organic compounds (iodomethane, iodoethane, chloroiodomethane, diiodomethane, bromoform, dibromomethane, dibromochloromethane, bromodichloromethane, chloroform, tetrachloromethane, 1,2-dichloroethane, 1,1,1-trichloroethane, trichloroethene, tetrachloroethene), isoprene, dimethylsulfide and monocyclic aromatic hydrocarbons (benzene, toluene, ethylbenzene, m,p-xylene, o-xylene) were measured in water samples from the coastal Baltic Sea (Fig. 1) during the period from January to November 2008. The lowest water temperature of less than 4◦ C occurred in February before increasing up to 18–19◦ C in July and August. Salinity varied between 8 and 16 (Fig. 2). This wide range is most likely caused by transport of different water bodies along the station. The occurrence of currents and circulation of surface water in the Baltic Sea strongly depends on the direction and strength of winds. The water coming from the Danish straits causes an increase of the salinity. Meanwhile, less salty waters are moved westwards through the Mecklenburg Bay. Even though the water temperature remained constant, considerable differences in salinity were measured at the sampling point and noticed in the data continuously taken at the Darss Sill station (Fig. 2). In general, salinity in the Baltic Sea decreases towards North and East. Salinity lower or close to 10 suggests outflow of water masses from the Baltic Proper, while salinity around 15 is characteristic of water of North Sea origin, entering the Mecklenburg Bay through the Kattegat. The Baltic Sea has a much lower salinity than the open ocean or the North Sea. It is a typical brackish sea. Lower salinity and high spatial variability is common for coastal aquatic systems. Annual cycles In most cases, concentrations of VOCs in the Mecklenburg Bay varied considerably during the investigated period (Figs 3, 4). 496

Iodinated compounds No uniform pattern for all four measured iodinated hydrocarbons could be seen. However, some similarities were noticed between the compounds (Fig. 3). Concentrations of iodomethane were generally lower from January until May; only at a single date in March was an increase in concentration noticed. From June the amount of iodomethane started to increase and reached its maximum at the end of July. The average concentration during the whole period was 4 pmol L−1 and the highest and lowest measured concentrations were 15 and 0.9 pmol L−1 respectively. A similar sinusoidal pattern of maximum concentration in late summer and minimum concentration in February–April was found in the western English Channel (0.6–14.6 pmol L−1 ).[19] Iodoethane concentrations were above the detection limit from June until October and were generally very low with an average concentration of 0.5 pmol L−1 and its maximum value of 1.1 pmol L−1 . The highest concentrations occurred somewhat similarly to those of iodomethane. Maxima in concentrations of chloroiodomethane occurred three times during the investigated period, first in March, then in May–June and next in September. In other periods the concentrations were low or, in January and February even below the detection limit. The highest concentration noted reached 50 pmol L−1 . The uppermost values of diiodomethane were observed in September, with a maximum of 84 pmol L−1 . The low concentrations appeared from July to August. Moreover, a strong decrease was noticed after the concentration peak in September. Maxima in the concentrations of diiodomethane correspond to some extent with those of chloroiodomethane. However, higher concentrations of chloroiodomethane in May were not always followed by diiodomethane. Similar observations of seasonality, with concentration peaks appearing in spring and autumn, were made by Archer et al.[19] and Klick.[20] The concentrations in the western English Chanel ranged from 0.6 to 39.3 pmol L−1 for chloroiodomethane, and from undetectable levels to 15.3 pmol L−1 for diiodomethane.[19] The maxima in the concentrations of chloroiodomethane (up to 60 pmol L−1 ) and diiodomethane (up to 250 pmol L−1 ), observed by Klick at the western coast of Sweden, occurred twice a year too, in April–May and in October.[20]

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Fig. 2. Seasonal patterns (daily mean based on hourly data) of environmental parameters: wind speed (m s−1 ), irradiance (W m−2 ), air and water temperature (◦ C), salinity, precipitation (mm h−1 ) and the concentrations of chlorophyll a (µg L−1 ) and biomass (mg L−1 ) during the investigated period. (Water temperature and salinity data were obtained from MARNET Darss Sill station in the Baltic Sea and directly at the sampling point (crosses); data of wind speed, irradiance, air temperature and precipitation were obtained from DWD meteorological station in Warnemünde; for the locations see Fig. 1. Phytoplankton data were provided by Dr N. Wasmund, Leibniz Institute for Baltic Sea Research Warnemünde (IOW) (X indicates times when microscopic analysis was not possible).)

18 pmol L−1 ) and dibromomethane (mean 2 pmol L−1 ) was noticed in Skagerrak, Danish strait, in September, November and April.[21] Dibromochloromethane and bromodichloromethane were not detected during winter months. The increase in concentration of these compounds (2.3 and 1.3 pmol L−1 respectively) was seen together with increase of bromoform.

Brominated compounds The average concentration of bromoform over the whole period was 16 pmol L−1 . The concentrations were generally higher in summer, with a maximum of 41 pmol L−1 at the end of July. Surprisingly, no apparent seasonal difference was observed for dibromomethane, although the concentrations over the year varied between 2 and 9 pmol L−1 (Fig. 3). An increase of bromoform in summer was also observed by Klick[20] in water from a station on the west coast of Sweden, where the concentrations were much higher and ranged from 79 to almost 2000 pmol L−1 . In Swedish coastal waters, contrary to observations from this study, dibromomethane showed a clear seasonal variation, with the highest concentrations in late August (around 86 pmol L−1 ) and its minima in January and February. Klick conducted her work within and in the vicinity of an algal belt and the difference in the seasonal changes in production of dibromomethane was more pronounced in the surroundings of macroalgae. No significant variation in concentrations of bromoform (mean

Isoprene Summer and autumn concentrations of isoprene were generally higher, with a maximum of 94 pmol L−1 observed in August (Fig. 3). Concentrations from January until the end of March were less than 20 pmol L−1 . These values are in the range of those previously observed in surface waters. Isoprene was detected in the Gulf Stream off the Florida coast[13] and in Liverpool Bay,[22] with concentrations between 10 and 51 pmol L−1 and between 10 and 210 pmol L−1 respectively. Broadgate et al.[12] observed the isoprene concentration reaching 497

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A. Orlikowska and D. E. Schulz-Bull

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Fig. 3. The concentrations (pmol L−1 or nmol L−1 ) of iodinated and brominated hydrocarbons, isoprene and DMS in surface water from the Bay of Mecklenburg over the period from January to November 2008. (When no data are shown, the concentration of a compound was below detection limit. Periods when no analyses were made are marked by X.)

up to 865 pmol L−1 in natural rockpools with macroalgae. They also noticed large diurnal variation within the macroalgae beds. Nevertheless, those were short period observations and in general seasonal and geographical variability is basically unknown.

showed that sulfur compounds, both DMS and DMSP, varied simultaneously. Chlorinated compounds There was no evidence for a strong seasonal cycle in the chloroform concentration (Fig. 4). The mean concentration was 9 pmol L−1 and only at a single date in July was a significant increase noted. Tetrachloromethane had higher values during cold periods. A similar pattern of tetrachloromethane was observed by Liss et al.[24] in the North Sea. The peak in the concentration of tetrachloroethene was seen in September and the lowest concentrations were observed during the summer months. A significant correlation was found between tetrachloromethane and 1,1,1-trichloroethane (r = 0.9, P = 0.0001). 1,2-Dichloroethane also showed a comparable pattern with higher concentrations in the months with lower water temperature.

DMS In the winter months low concentrations of DMS were observed. They rose to the first peak in mid-March which was followed by a sharp increase in concentration in May. High concentrations were measured also in July and on one single day of September. From that date, the concentrations decreased and were below 0.3 nmol L−1 in November (Fig. 3). A large increase in the DMS concentration in May (mean monthly concentration ∼25 nmol L−1 ) was also seen in the North Sea[23–25] and in the north-eastern Baltic Sea.[26] In the Baltic the highest concentrations (DMS > 6 nmol L−1 ) were observed in July and August. Michaud et al.[4] also detected the highest concentrations (DMS > 14 nmol L−1 ) of DMS in June–August, in the St. Lawrence Estuary. Furthermore, they

Monocyclic aromatic compounds The concentrations of monocyclic aromatic compounds such as toluene, ethylbenzene and xylenes varied greatly during the 498

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Fig. 4. The concentrations (pmol L−1 ) of chlorinated hydrocarbons (tetrachloromethane; tetrachloroethene, TeCE; 1,1,1trichloroethane, 1,1,1-TCA; trichloroethene, TCE; chloroform; and 1,2-dichloroethane, 1,2-DCA) and monocyclic aromatic compounds in surface water from the Bay of Mecklenburg over the period from January to November 2008. (When no data are shown, the concentration of a compound was below the detection limit. Periods when no analyses were made are marked by X.)

investigated period. Their highest values were observed in May and July (Fig. 4). Significant correlations were found between all these compounds (from r = 0.8 to 0.9, P = 0.0001) except for benzene, which showed a greatly different picture with its highest concentrations in February.

a cluster analysis (CA) with Ward’s method showed some relationships between several of the compounds and the specific phytoplankton taxa (Fig. 6). Regarding iodomethane, a significant correlation with temperature (r = 0.6, P = 0.0005) and an increase during the biologically active period were observed. This gas is thought to originate entirely from natural sources. It is believed to be formed in seawater by biological[27–30] or photochemical processes.[9] A similar concentration pattern of iodomethane and iodoethane in the period from June until October could indicate somewhat related sources and removal processes for both gases. The distribution of diiodomethane and chloroiodomethane in the water column is largely determined by their stability. Therefore, environmental factors like light and temperature have an influence on their behaviour and the amounts in the marine boundary layer.The photodissociation lifetime of diiodomethane is only a few minutes at midday. Most of the diiodomethane is photolysed before reaching the sea surface, where this compound is depleted with regard to the underlying water column.[31,32] The lifetime of chloroiodomethane with respect to photodecomposition in water is several hours.[33] These findings help to understand differences in patterns of both compounds found in the surface waters of the Baltic Sea. In May, when the irradiance (both over the month and at sampling time) was very high (Fig. 2), an elevated concentration of chloroiodomethane was found but this was not accompanied by high amounts

Source, environmental and biological factors As already mentioned, the sources of VOCs as well as their rates of production and removal are diverse. They are affected by various physicochemical parameters along with biological factors. In order to check a potential influence of the water temperature on the concentrations of VOCs in sea water the investigated period was divided into two seasons (separating point: 10◦ C, approximately median water temperature over a year) and an analysis of the variance was applied to see statistically important differences (Fig. 5). A significant increase of around two-fold in mean concentration in the season with higher water temperature was noticed for iodomethane, bromoform and isoprene (P < 0.005) (Fig. 5). An increase for chloroiodomethane and total measured DMS was observed too, although, the level of significance was lower (P > 0.01). In case of diiodomethane and dibromomethane no significant change in mean concentration was seen between both seasons (Fig. 5). In this study no correlations (r < 0.4, P < 0.05) between chlorophyll a or biomass and VOCs were noticeable. However, 499

A. Orlikowska and D. E. Schulz-Bull

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Fig. 5. The mean concentrations (pmol L−1 or nmol L−1 ) of several of VOCs measured in the coastal Baltic Sea in warm (water temperature > 10◦ C) and cold (water temperature < 10◦ C) seasons. (Mean value is shown by dark square, box represents 0.95 confidence interval and bars show standard deviation. Analysis of variance (ANOVA) – value of F test and P level of significance – is presented.)

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Fig. 6. The cluster analysis (CA) (with Ward’s method and Euclidean distance) for several of VOCs (diiodomethane, CH2 I2 ; chloroiodomethane, CH2 ClI; dibromomethane, CH2 Br2 ; dimethylsulfide, DMS; bromoform, CHBr3 ; iodomethane, CH3 I; and isoprene, isop), biomass, chlorophyll a (chl-a) and phytoplankton taxa (Craspedophyceae, Cras; Dictyochophyceae, Dict; Diatomophyceae, Diat; Dinophyceae, Dino; Euglenophyceae, Eugl; Cryptophyceae, Cryp; Ciliophora, Cili; Ebriidea, Ebri; Chrysophyceae, Chry; Nostocophyceae, Nost; Prymnesiophyceae, Prym; and others, unid).

layer can be calculated. Whether diiodomethane is completely photolysed depends on its residence time in the surface layer, which in turn depends on a mixed layer depth, turbulence and the attenuation coefficient of the sea water.[33] Assuming that chloroiodomethane is entirely produced by photodecomposition of diiodomethane, new calculated values represent the concentrations of photolysed diiodomethane. Its new seasonal pattern, which is very similar to that of chloroiodomethane, becomes visible (Fig. 7). These calculations do not take into account losses as a result of the sea–air exchange of the compounds. Chloroiodomethane can also be produced by some algae species,[34] so that the calculated values of diiodomethane may be overestimated.

of diiodomethane. The low values of diiodomethane, down to below the detection limit, were most likely caused by its photolysis in the surface water. On the contrary, in the middle and end of September, when irradiance was low, high concentrations of chloroiodomethane and much higher concentrations of diiodomethane were observed. Jones and Carpenter[33] showed that chloroiodomethane was produced during photolysis of diiodomethane (with a molar yield of 35 ± 20%). Thus, a significant proportion of chloroiodomethane in surface seawater may arise from diiodomethane. When the relation between diiodomethane photodecomposition and chloroiodomethane production[33] is used, the approximate values of diiodomethane concentration photolysed to chloroiodomethane in the surface 500

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150

Fig. 7. The concentrations (pmol L−1 ) of chloroiodomethane (crosses), diiodomethane (triangles) and the concentrations of photolysed diiodomethane (squares), calculated using the relationship between CH2 I2 photodecomposition and CH2 ClI production from Jones and Carpenter,[33] in surface water from Bay of Mecklenburg over the period from January to November 2008. (When no data are shown, the concentrations of compounds were below the detection limit. Period when data were not analysed are marked with X.)

Algae are the most probable source of diiodomethane and chloroiodomethane. Their production by micro- and macroalgae has been previously demonstrated.[5,35–37] In coastal regions, macroalgae are thought to be the largest source of diiodomethane.[6] Diverse types of macrophytes, i.e. colonies of brown algae, different species of drifting red algae and some filamentous green algae as well as seagrass were observed in the Mecklenburg Bay.[38,39] The sampling site from this study is not located in the vicinity of large algal beds. The blooms of specific microalgae species can contribute considerably to the amounts of these gases in water.[34] In this study the increase in concentrations of both gases was observed along with the spring and autumn phytoplankton blooms. The dendrogram (Fig. 6) clearly divides diiodomethane and chloroiodomethane from some other trace gases like iodomethane, bromoform or isoprene, implying different algae species as their source. A lack of correlation between brominated and iodinated halocarbons which was observed during this study, has also previously been reported. The incubation studies of Moore et al.[34] showed that diiodomethane and chloroiodomethane were commonly produced by organisms that did not produce bromoform, and vice versa. As mentioned before, bromoform and dibromomethane are known to be primary products of the biological metabolism of marine organisms. They are produced by many species of macroalgae[20,27,35,36] and microalgae,[8,34,37,40] with bromoform as the main brominated compound. Cluster analysis (Fig. 6) shows, however, that in the Mecklenburg Bay these two compounds were produced by different species and different pathways. This type of clustering can also have its reason in additional macroalgae production in coastal waters, which is thought to be the major marine source of bromoform.[6,27] Quack et al.[41] suggested that both compounds, at least in part, have different sources and fates. Similar conclusions could be drawn from the data presented in this work. Not only these brominated hydrocarbons but also mixed chlorobromomethanes were found during incubation experiments with micro and macroalgae.[27,36,40,42] It is possible that the mixed chlorobromomethanes are formed in seawater from bromoform by nucleophilic substitution of bromide by chloride. The concentration of bromodichloromethane is expected to be lower than that of dibromochloromethane because the second substitution step is less probable.[37] In this study, higher concentrations of dibromochloromethane than of bromodichloromethane were noticed and the concentrations were rising with increasing amounts of bromoform.

Isoprene in water was previously shown to be produced by microalgae[13,14,43] and macroalgae.[12] In this study, an increase of isoprene concentration in seawater was seen during a biologically active season and significant correlation with temperature (r = 0.7, P = 0.0005) was observed. The correlation with irradiance was weaker but still significant (r = 0.5, P = 0.001). Broadgate et al.[12] noted that production of isoprene by macroalgae is temperature dependent and related to light availability. Moreover, similar observations were made by Shaw et al.,[43] when the production by phytoplankton was investigated. Diatoms were shown to be strong emitters of this gas during laboratory studies.[14] In this study though, isoprene has a similar seasonal trend to a group of phytoplankton, which could not be identified by microscopic analysis (marked as unid, Fig. 6), rather than any particular class of phytoplankton. At the end of June this ‘unidentified’ group was the dominating fraction and consisted mainly of small coccoid cyanobacteria ( 0.1) in the mean concentration between two seasons was observed, while for other chlorinated compounds, i.e. tetrachloromethane, 1,2-dichloroethane (Fig. 5) and 1,1,1trichloroethane, the mean concentration in the months with lower water temperature was significantly higher (P < 0.005). Liss et al.[24] suggested that concentration levels of tetrachloromethane during their study in the North Sea were somewhat controlled by its aqueous phase solubility, with colder water being able to dissolve more atmospheric tetrachloromethane. To some extent this explanation seems to match our observations. The principal component analysis (PCA) (Fig. 8) clearly showed a negative correlation between temperature and tetrachloromethane, 1,2-dichloroethane and 1,1,1-trichloroethane. Moreover, precipitation seems to be an important factor affecting the distribution of chloroform and tetrachloroethene during this study. The removal of organohalogen compounds from the atmosphere by precipitation can range from 10 to 100% and depends on the initial concentration and the nature of the analyte.[46] Different emission gradients of the compounds or source areas as well as physical properties like half life in the atmosphere can be the reason for this kind of grouping. Chlorinated hydrocarbons are considered to be mostly anthropogenic,[10,11] and their presence in seawater can be due to atmospheric transport or movement of different water bodies. 502

0.1 × 103 (2.2) 1.8 (2.4)