Does Fog Chemistry in Switzerland Change with Altitude?
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1 Does Fog Chemistry in Switzerland Change With Altitude? a b b,∗ 2 Pavel Michna , Roland A. Werner , Werner Eugster a 3 Institute of Geography, University of Bern, Hallerstrasse 12, CH-3012 Bern, Switzerland b 4 ETH Z¨urich, Institute of Agricultural Sciences, Universit¨atsstrasse 2, CH-8092 Z¨urich, Switzerland 5 Abstract 6 During two extended summer seasons in 2006 and 2007 we operated two battery driven versions 7 of the Caltech active strand cloud water collector (MiniCASCC) at the Niesen mountain 8 (2362 m a.s.l.) in the northern part of the Swiss Alps, and two devices at the Lageren¨ research 9 tower (690 m a.s.l.) at the northern boundary of the Swiss Plateau. During these two field 10 operation phases we gained weekly samples of fog water, where we analyzed the major anions 2 18 11 and cations, and the isotope ratios of fog water (in form of δ H and δ O). Dominant ions in fog + − 2− 12 water at all sites were NH4 , NO3 , and SO4 . Compared to precipitation, the enrichment factors in 13 fog water were in the range 5–9 at the highest site Niesen Kulm. We found considerably lower 14 summertime ion loadings in fog water at the two Alpine sites than at lower elevations above the 15 Swiss Plateau. Lowest ion concentrations were found at the Niesen Kulm site at 2300 m a.s.l., 16 whereas highest concentration (a factor 7 compared to Niesen Kulm) were found in fog water at 17 the Lageren¨ site. Occult nitrogen deposition was estimated from fog frequency and typical fog −1 −1 18 water flux rates. This pathway contributes 0.3–3.9 kg N ha yr to total N deposition at the −1 −1 19 highest site on Niesen mountain, and 0.1–2.2 kg N ha yr at the lower site. These inputs are the 20 reverse of ion concentrations measured in fog due to the 2.5 times higher frequency of fog 21 occurrence at the mountain top (overall fog occurrence was 25% of the time) as compared to the 22 lower Niesen Schwandegg site. Although fog water concentrations were on the lower range 23 reported in earlier studies, fog water is likely to be an important N source for Northern Alpine 24 ecosystems and might reach values up to 16% of total N deposition and up to 75% of wet N 25 deposition by precipitation. 26 Keywords: Fog chemistry, Cloud chemistry, Swiss Alps, Mountain fog, Stable isotopes in fog 27 water 1 28 1. Introduction 29 Fog and cloud water can be a significant source of water input to montane ecosystems [Walmsley 30 et al., 1996; Dawson, 1998; Bruijnzeel et al., 2005], and chemical solutes in fog and cloud water 31 can add relevant amounts of nutrients or pollutants to these ecosystems (e.g., Thalmann et al. 32 2002; Burkard et al. 2003). Although in Central Europe fog does not yield significant amounts of 33 water compared to precipitation, deposition of nutrients such as nitrogen or sulphur can lead to 34 over-nutrition, mainly in forest ecosystems [Thalmann et al., 2002]. However, it is unknown, 35 whether such findings also apply to mountain peaks, or only to lower mountains that are within 36 the anthropogenically polluted atmosphere. We addressed this question by collecting fog water 37 and analysing its composition at four sites grouped at two localities, (a) Mount Niesen, 38 Switzerland (2362 m a.s.l.), and (b) the low mountain site Lageren¨ (682 m a.s.l.) (Figure 1). 39 Many factors influence the chemical composition of fog and cloud water, namely the provenience 40 of the air mass, cloud dynamics, and microphysics [Cini et al., 2002]. In industrialized countries, 41 concentrations of solutes are generally higher than in precipitation up to a factor of 100 [Fuzzi 42 et al., 1996; Choularton et al., 1997; Thalmann et al., 2002; Burkard et al., 2003; Lange et al., 43 2003]. Acidification as a result of high SO2 emissions was especially problematic in the border 44 area of Germany, the Czech Republic and Poland [Lange et al., 2003; Błas´ et al., 2008, 2010]. 45 But also in North America fog was found to be a major source of acidification via hydrogen ion 46 deposition [Schemenauer, 1986; Schemenauer et al., 1995]. The common definition of fog is a 47 cloud in contact to ground with a meteorological visibility ≤ 1000 m. Therefore, cloud and fog is 48 the same phenomenon in the context of this study. 49 So far, the focus of research has mainly been on areas with high atmospheric pollution, and on 50 tropical montane cloud forests where high biodiversity is coincident with frequent occurrence of 51 fog. However, little is known about the relevance of chemical compounds in fog in mountain 52 areas of Switzerland, where fog is frequent during the vegetation period and where ecosystems ∗E-mail address: [email protected], +41 44 632 68 47 Preprint submitted to Atmospheric Research January 28, 2014 53 tend to be adapted to low-nutrient conditions. Cloud and precipitation chemistry at higher 54 elevations and along an altitudinal gradient in Switzerland had only been investigated in 55 1984/1985 at Mt. Rigi (1798 m a.s.l.) which is located 75 km NE of our main study site 56 [Staehelin et al., 1993; Collett et al., 1993a,b]. 57 In mountainous areas, deposition chemistry is largely influenced by topography, altitude, valley 58 orientation and exposure to main wind directions [Rogora et al., 2006]. An assessment of 59 atmospheric deposition in the Alpine region, mainly focusing on nitrogen deposition, can be 60 found in Rogora et al. [2006]. They used data from previous and ongoing projects to investigate 61 geographical variability and temporal trends of wet deposition. Although air pollution has 62 decreased in the last decades [e.g. Barmpadimos et al., 2011], the deposition of nitrogen is still 63 expected to be a critical factor at some sites. The role of cloud and fog deposition remains 64 unknown, but should not be ignored, as Rogora et al. [2006] emphasize. Previous studies have 65 shown the importance of altitudinal gradients [Miller et al., 1993], where deposition rates were 66 found to increase with altitude, which would imply highest deposition rates at the mountain tops. 67 The goal of our study hence was to quantify concentrations in fog water at a mountain in the 68 Prealps (Niesen) that is prominently exposed to northerly winds from the densely populated 69 Swiss Plateau. We hypothesize that even at such relatively remote locations substantial amounts 70 of plant-accessible nitrogen may be deposited by fog. We expect this to be most relevant at times, 71 when the Niesen mountain may be affected by pollutants originating within a few tens of km 72 distance, whereas at other times its elevation is clearly above the polluted atmospheric boundary 73 layer. To distinguish mountain top (exposed) conditions from mid-elevation conditions at the 74 mountain, and to resolve altitudinal differences in fog chemistry, measurements were carried out 75 in parallel at two sites at ≈ 2300 m and 1650 m a.s.l. (Figure 1). Moreover, additional samples 76 were taken at the lower-elevation Lageren¨ Mountain site (682 m a.s.l.) to allow for a direct 77 comparison with earlier fog water flux studies [Burkard et al., 2003]. As an outcome, the first 78 order-of-magnitude estimate of N deposition by fog for a mountain top in the Alps was attempted 79 in order to assess the relevance of fog as an N deposition pathway for alpine vegetation. 3 80 2. Material and Methods 81 2.1. Observation Sites 82 From 22 June to 6 October 2006 and from 23 April to 22 October 2007 (one device running until 83 23 November 2007) two MiniCASCC (see Section 2.2) were operated on the slope of the Niesen 84 mountain (2362 m a.s.l.) in the Swiss Alps. The Niesen mountain is located ≈ 35 km south of 85 Bern at the northern limit of the Central Alps (Figure 1). It is the first peak of the 22 km long 86 northeast–southwest running Niesen ridge, and it’s elevation is ≈ 1700 m above the bottoms of 87 the surrounding valleys. The vegetation cover is subject to an altitudinal gradient and consists of 88 mixed forests (dominated by Fagus sylvatica (L.) and Picea abies (L.) Karst.) on the southern 89 slope up to an altitude of 1400 m a.s.l., followed by coniferous forests up to 1900 m a.s.l., and 90 then by grassland and bare rocks. 91 One device (Niesen Schwandegg, S) was installed at 3 m above ground on an avalanche ◦ 0 00 ◦ 0 00 92 protection structure at 1650 m a.s.l. (46 38 32.3 N, 7 40 03.3 E) in a wood glade (coniferous 93 forest) on a south-east facing slope; the second device (Niesen Kulm, K) was installed 1.5 m ◦ 0 00 ◦ 0 00 94 above ground at 2330 m a.s.l. (46 38 41.7 N, 7 39 05.8 E; in May 2007 moved 130 m in 95 direction North-east to 2300 m a.s.l., 5 m above ground) on a south facing slope just below the 96 mountain summit (alpine grassland and rocks). From 8 August 2007 to 23 November 2007 a 97 third MiniCASCC (Niesen Kulm, B) was installed near the mountain summit (2300 m a.s.l., 98 same location as K).