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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L04402, doi:10.1029/2007GL032357, 2008

Seasonal variations in hydrogen deposition to boreal forest soil in southern Finland M. Lallo,1 T. Aalto,1 T. Laurila,1 and J. Hatakka1 Received 15 October 2007; revised 19 December 2007; accepted 11 January 2008; published 16 February 2008.

[1] In this study deposition velocity (vd) of atmospheric hydrogen to mineral and peat soils was measured in boreal forest environments in southern Finland using soil chamber measurement technique. vd was largest during the snow-free season (0.04– 0.07 cm/s) and smallest during winter (0 – 0.04 cm/s). Velocities decreased when soil temperature fell below 5°C, but deposition was observed also in near-zero temperatures. Deposition velocities to organic soil forest floor were larger than to mineral soil, but it was unclear whether this was due to the effect of carbon or the effect on soil porosity. Fluxes to both mineral and peat soils had similar temperature and soil moisture responses. In very dry and moist conditions vd decreased rapidly. Optimum soil moisture ranged from about 6 to 50 % of water by volume. The magnitude of vd was similar at urban Helsinki and at rural Loppi sites. Citation: Lallo, M., T. Aalto, T. Laurila, and J. Hatakka (2008), Seasonal variations in hydrogen deposition to boreal forest soil in southern Finland, Geophys. Res. Lett., 35, L04402, doi:10.1029/2007GL032357.

1. Introduction [2] Production of energy by hydrogen-based systems appears to be an attractive alternative in the future global economy. Environmental impacts of the change to hydrogen economy have been studied recently assessing hydrogen leakages, reduction of NOx and CO emissions and their effects on OH, methane and ozone concentrations and finally on global warming [Schultz et al., 2003; Tromp et al., 2003; Warwick et al., 2004; Derwent et al., 2006]. [3] Modelling of the changes in the tropospheric hydrogen budget requires knowledge of hydrogen chemistry in air and its surface sinks and sources [Novelli et al., 1999; Sanderson et al., 2003]. A major but relatively little studied component of the global hydrogen budget is the deposition of hydrogen to soil, where both magnitude of H2 sink and actual deposition mechanisms deserve further attention. Deposition rates have been studied in the laboratory [Conrad, 1999; Go¨dde et al., 2000; Smith-Downey et al., 2006] as well as in field conditions [Conrad and Seiler, 1985; Yonemura et al., 1999; Yonemura et al., 2000a; Rahn et al., 2002] using soil chambers and concentration measurements combined with known fluxes of other trace gases. The deposition fluxes were shown to be connected with the moisture and temperature of the soil surface layer and soil management practices. Air

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Climate and Global Change Research, Finnish Meteorological Institute, Helsinki, Finland. Copyright 2008 by the American Geophysical Union. 0094-8276/08/2007GL032357

filled porosity of the soil is also an important factor in H2 diffusion process [Yonemura et al., 2000b]. Rhee et al. [2006] concluded that snow-cover extent seemed to be important as a regulating factor. Consumption of H2 is considered to be catalyzed by soil hydrogenases not bound to living microorganisms [e.g., Conrad, 1996], while soil organic carbon content may also be important factor [e.g., Popelier et al., 1985; Smith-Downey et al., 2006]. [4] The only few measurements of hydrogen deposition to boreal forest soils are from Alaska [Rahn et al., 2002; Smith-Downey et al., 2006] and according to current knowledge no measurements exist in European boreal zone, and none through a snow cover. In this work soil chamber deposition studies were made in organic and mineral boreal forest soils in a rural site in southern Finland and in urban sites in Helsinki. Measurements were carried out in all seasons through 19 months bringing up information of the actual length of the active H2 deposition period, considered to be limited by snow cover and freezing temperatures. Results were supplemented by laboratory studies of soil samples maintained in controlled environmental conditions.

2. Materials and Methods 2.1. Field Measurements [5] Measurements were made in southern Finland. Two sites were located in the urban area at Helsinki (60°120N, 25°30E). Herttoniemi site (H) was set up in a forest restricted to industrial and residential areas. The vegetation was dominated by mixed broad-leaved and coniferous trees. The second urban site was located in Kumpula (K), which is a forested area near the center of Helsinki, mainly inhabited by broad-leaved trees. Both sites were under influence of local traffic and industrial emissions. The rural measurement site was located in Loppi (L) (60°3804900N, 24°210800E), about 60km north from Helsinki. Four measurement points abbreviated as M1 to M4 were set up in a drained pine bog (histosol) growing Scots pines. The M3 point was placed in a moist forest trench covered with mosses. Two measurement points M5 and M6 were placed in a mineral soil (haplic podzol) growing mixed coniferous and deciduous trees. The vegetation inside the chambers was mostly mosses and lichens. The surrounding vegetation of the chambers included blueberry (Vaccinium myrtillus) and marsh tea (Ledum palustre) brushes. Chamber M6 and those in Helsinki were inhabited by low grass species. Soil analysis showed that the percentage of organic carbon is about 50% (Table 1) through all layers of peat soils, while in mineral soils only the 0– 2 cm surface layer had 30– 40 % organic material. Deeper mineral soil layers had only a few percent of organic carbons. The amount of total nitrogen

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LALLO ET AL.: SEASONAL VARIATIONS IN HYDROGEN DEPOSITION

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Table 1. Soil Analysis Results for Loppi Peat and Mineral Soils and Helsinki Sites Kumpula and Herttoniemi Locationa

L(p)

L (p)

L (p)

L (m5)

L (m5)

L (m5)

L (m6)

L (m6)

L (m6)

K

K

H

Layer (cm) Layer typeb Acidity (pH) Organic C (%) Total N (%)

surface

0 – 10

10 – 20

0–2 H 4.6 29 0.98

2–4 L 5.9 3.2 0.32

4 – 10 E 5.5 1.6 0.10

0 – 10

3.4 39 1.01

4 – 10 E 5.4 1.2 0.10

7 – 20

3.5 44 1.11

2–4 L 4.0 4.7 0.21

0–7

49 1.18

0–2 H 4.0 40 1.21

4.7 4.1 0.23

5.8 2.4 0.22

5.0 2.4 0.28

a

Loppi peat, p; mineral, m. humus, H; leachete, L; enrichment layers of podzolic soil, E.

b

varied in a similar way (see Table S1 of the auxiliary material)1. [6] The field measurement system included a stainless steel chamber (60 cm 60 cm), with an aluminum cover about 30 cm above the soil surface. A small batteryoperated fan ensured the proper mixing of air in the chamber. Air samples were taken through a short silicon tube fitting which was capped after use. The sample interval varied between 2 to 5 minutes, while first air sample was taken immediately after lowering the cover. BD Plastipak plastic syringes (20 mL) with three-way stopcock valves were used for sampling and the total length of the measurement period was about 15– 20 minutes. [7] Volumetric soil moisture concentration was measured by ThetaProbe ML2x. The sensor signal was calibrated weighing soil samples from the site in varying moisture conditions. Soil temperature was measured by thermistors or thermocouples. Soil water table level was measured at Loppi and snow depth at an open field at 18 km distance from the site. 2.2. Laboratory Measurements [8] Soil samples for laboratory measurements were taken from Loppi peat and mineral soils (9 samples each) in 10 cm columns keeping the soil layers intact. The water holding capacity was determined by submerging dry soil samples to water and letting them drain about 20 minutes [e.g., Smith-Downey et al., 2006], allowing the estimation of mass of water retained in the sample. The saturation was achieved at 0.76 g(H20)/g(wet soil) for peat and 0.41 g(H20)/g(wet soil) for mineral soil. [9] Stainless steel cuvette (radius 9.5 cm, height 27 cm) was used for deposition studies. The syringe sampling volume was decreased to 10 ml. The deposition velocities were determined from the concentration decrease inside the closed chamber, assuming that the deposition onto soil follows first-order kinetics for gas concentrations at atmospheric levels [Yonemura et al., 2000a]. An exponential function was fitted to the concentration record and deposition velocity was determined from the rate of decay. Air tightness of the steel chamber was tested in the laboratory using the working gas standard which was about two times higher than normal atmospheric concentration in hydrogen. Concentration was not changed inside the steel cuvette during 20 minutes test. Plastic syringes were also tested and the maximum leakage rate was found to be 4 ppbv/hr. Hydrogen samples were analyzed during the same day for minimizing leakage and avoiding possible chemical reactions in the syringe surfaces and air. 1 Auxiliary materials are available in the HTML. doi:10.1029/ 2007GL032357.

[10] Concentration analysis was carried out using Peak Laboratories, Peak Performer 1 (PP1) instrument and modified Trace Analytical RGA5. We tested repeatability of analysis measuring several times test samples across the concentration range investigated. Average standard deviations were 1.3 % and 3.5 for the PP1 and the modified RGA5, respectively. The calibration gas was 103 ± 2 ppm H2 in synthetic air (Messer, Air Liquide) diluted to atmospheric concentration. Working standard was used in sample analysis.

3. Results 3.1. Hydrogen Deposition at Field Sites [11] At Loppi, the deposition velocities (vd) were measured from September 2005 to March 2007. During the snow-free autumn period vd was relatively high according to all chambers situated in mineral and peat soils. Chamber M3 showed near-zero vd due to its location in a forest trench with saturating water table level. Generally, the hydrogen deposition velocity values were in the range 0.046 to 0.066 cm/s, in case of peat soil, while for mineral soils the respective values were 0.044 to 0.055 cm/s provided that soil temperature were above zero. The average hydrogen uptake was higher in peat soil (0.057 cm/s) than in mineral soil (0.050 cm/s) among the observations when soil tem-

Figure 1. Field measurements of average hydrogen deposition velocities to forest soil in Loppi, and in urban parks in Helsinki Kumpula and Herttoniemi. Average standard deviation for peat was 0.010 cm/s and for mineral 0.006 cm/s in Loppi.

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vs. 0.026cm/s for 28 October 2005, one day experiment). However, deposition velocities measured in Helsinki were not significantly different from Loppi, values being roughly between the Loppi peat and mineral soil results. [13] The dependency of deposition velocity on soil temperature is shown in Figure 2. In near zero temperatures the gas deposition rates onto surface decrease towards lower temperatures. Deposition velocities increased to 0.050 – 0.060 cm/s at temperatures of 10 – 16°C. The results can be converted to deposition fluxes by multiplying vd with ambient hydrogen concentration. The unpolluted background concentration in Finland is of the order of 440– 530 ppbv (Pallas site, NOAA/ESRL/CCGG, http:// www.cmdl.noaa.gov/ccgg/index.html). Thus, a deposition velocity of 0.05 cm/s corresponds roughly to a flux of 10 nmol m2 s1 at 500 ppbv H2 in NTP conditions.

Figure 2. Hydrogen deposition velocities as a function of soil temperature in (a) Loppi and in (b) Helsinki. The R2 for the Loppi data is 0.44, and for Helsinki 0.69. Temperature relationships at both sites are statistically significant below the 0.01 level. perature exceeded 5°C. The result is statistically significant on the 0.05 level. On late October 2005 (Figure 1), deposition to peat and mineral soils was diminished due to a temporary snow cover (marked as black bars in Figure 1), while the soil was not yet frozen. During March 2006 small vd values of 0.012 to 0.023 cm/s were observed when the air (9°C) and soil temperatures were freezing and the soil was covered with about 30– 40 cm of snow. Intermediate vd values (0.025 to 0.045 cm/s) were observed in April and November to December 2006, when the soil temperature was slightly above zero. High vd values (over 0.055 cm/s) were observed in 2005 August – September and again in 2006 June to July and November. A cold spring period in 2007 rapidly reduced values to a lower level. Water table level was decreased after a warm and dry period in July – August 2006. Even M3 showed a deposition velocity of 0.015 cm/s. [12] At Herttoniemi and Kumpula, mineral soil sites in Helsinki the deposition velocities to snow free soil at above zero temperature, generally varied between 0.023 to 0.055 cm/s (Figure 1). Relatively high values were observed until December and again from beginning of April. In December, air and soil temperatures had decreased to near zero values, but hydrogen deposition onto soil was still clearly observable. Only after a long cold period in January 2006, the soil was properly cooled down to below freezing point, inhibiting the gas exchange and thus negligible deposition velocities were measured at Kumpula. Near-zero vd values were observed during January, February and March until April, when the soil started to thaw. Deposition velocity values at Herttoniemi were higher than those at Kumpula (0.043 cm/s

3.2. Laboratory Studies of Hydrogen Deposition [14] Laboratory measurements were performed in order to give better explanations for the field results, namely to separate the influences of soil temperature and moisture in the observed soil fluxes. Laboratory chamber measurements at optimum soil moisture conditions indicated that the hydrogen deposition rates were relatively constant in the typical growing season temperatures, but decreased in cold temperatures. However, near zero soil temperatures were not low enough to totally inhibit deposition (Figure 3). Across the whole soil temperature range, similar deposition behavior was observed at laboratory experiments than in field chambers. Deposition to peat soil was not significantly more efficient than to mineral soils, but the study material was

Figure 3. (a) The temperature dependency of hydrogen deposition to soil. Deposition velocities were measured in the laboratory in optimum soil moisture conditions. (b) Soil moisture dependency of hydrogen deposition. Deposition velocities were measured in 20°C temperature in laboratory.

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not extensive enough to draw a definitive conclusion. Soil moisture response was measured in room temperature by adding water to the sample until saturation was achieved, and measuring flux after every addition. Results showed that the optimum range for soil water content was between 6 to around 50– 60 vol-% with decreasing trend towards saturation.

4. Discussion [15] The hydrogen deposition velocities were measured in Loppi and Helsinki, covering over a year of soil chamber observations. Results showed relatively high values during the time period from spring to autumn and smaller or nearzero values during winter. The deposition rates were similar to those found in literature. For example, Rahn et al. [2002] presented deposition fluxes of 2 – 12 nmol m2 s1 for boreal forest soil in Alaska. In terms of deposition velocities, similar results were presented for temperate forest soil in Japan by Yonemura et al. [2000a]. Our results indicate that optimum conditions for hydrogen consuming processes existed when both air and soil temperature were above 5°C, and soil volumetric water content was in the range between 0.06 to around 0.50 –0.60 m3/m3. Saturating water table level inhibited deposition, which was shown for Loppi M3 chamber. Snow cover also reduced deposition, however, exact quantification of the effect is difficult due to the large and uncharted variability in snow depth near the chambers and also due to the changes in the water content of the snow. [16] During late winter freezing temperatures and snow cover, however, non-zero deposition velocities were observed at Loppi mineral and peat soils. An explanation for this may be related with the amount of soil water. The soil water table level at Loppi, measured continuously throughout the year, showed decrease during winter. The hydrogen deposition may be caused by emptying of the water filled soil pores thus allowing continuation of hydrogen consuming processes, provided that soil temperature were above zero. The deposition rates are dependent on soil porosity, which affects the gas permeability. In higher density mineral soils deposition rate decreases, especially when soil is saturated with water [Rahn et al., 2002; Yonemura et al., 1999, 2000a, 2000b] forming a water seal. At high soil moisture levels, the frozen soil surface inhibits the gas permeability to lower soil layers as the ice seals the gas channels. According to our field studies, this was the common condition in Helsinki, where deposition velocities were near zero during the coldest winter months. Later during spring and also in laboratory studies, we observed moderate deposition velocities in near melting state soils retained in optimum moisture conditions. Similar results have also been reported by Smith-Downey et al. [2006]. They have also reported broad optimum soil moisture and temperature region and rapidly decreasing deposition rates beyond optimum limits. [17] According to our field studies, higher deposition velocities were observed to organic peat than mineral soils. Peat soil at Loppi has a high organic carbon (50%) and total nitrogen (1%) content in comparison to mineral soil, which may correlate with higher deposition velocities [e.g., SmithDowney et al., 2006]. Higher peat soil porosity may also contribute. At Helsinki the difference between the two sites was not connected to the soil carbon or nitrogen content. Helsinki results were very similar to Loppi results, suggesting that the Helsinki soil was not saturated with respect to

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hydrogen, i.e. traffic and industrial air pollution did not have a dominating role on regulating the deposition velocities. [18] In summary, according to this study the boreal forest soil consumed atmospheric hydrogen. A seasonal cycle was observed where deposition rates were low during winter. Temperatures well below freezing point and high soil moisture hindered the gas exchange, while optimal soil moisture with more air filled pores helped the gases to permeate. The future work will be focused on deposition measurements at new sites and connected more closely with continuous measurements of atmospheric hydrogen concentration. [19] Acknowledgments. This work was supported by the Tor and Maj Nessling Foundation, the Academy of Finland, and by EU-project Eurohydros. We would like to thank Jukka Pumpanen, University of Helsinki, Department of Forest Ecology, Timo Penttila¨, Finnish Forest Research Institute and Annalea Lohila, Finnish Meteorological Institute for their valuable collaboration.

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T. Aalto, J. Hatakka, M. Lallo, and T. Laurila, Climate and Global Change Research, Finnish Meteorological Institute, P.O. Box 503, FI-00101 Helsinki, Finland. ([email protected])

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