A decadal time series of water vapor and D/H isotope ratios above
Mt. Zugspitze: transport patterns to Central Europe
Authors: Hausmann Petra , Sussmann Ralf , Trickl Thomas , Schneider Matthias 
Published: January 2017

Abstract. We present vertical soundings (2005–2015) of tropospheric water vapor (H2O) and its D/H isotope ratio (δD) at Mt. Zugspitze (47° N, 11° E, 2964 m a.s.l.) derived from ground-based solar Fourier transform infrared (FTIR) measurements. Beside water vapor profiles with optimized vertical resolution (degrees of freedom for signal DOFS = 2.8), the retrieval provides {H2O, δD} pairs with consistent vertical resolution (DOFS = 1.6 for H2O and δD) applied in this study. The integrated water vapor (IWV) trend of 2.4 [−5.8, 10.6] % per decade (statistically insignificant, 95 % confidence interval) can be reconciled with the 2005–2015 temperature increase at Mt. Zugspitze (1.3 [0.5, 2.1] K per decade) assuming constant relative humidity. Seasonal variations in free-tropospheric H2O and δD exhibit amplitudes of 140 % and 50 % of the respective overall means. The minima (maxima) in January (July) are in agreement with changing sea surface temperature of the Atlantic Ocean. Using extensive backward trajectory analysis, distinct moisture pathways are identified depending on observed δD levels: low column-based δD values (δDcol < 5th percentile) are associated with air masses originating in higher latitudes (62° N on average) and altitudes (6.5 km) compared to high δD values (δDcol > 95th percentile: 46° N, 4.6 km). Backward trajectory classification indicates that {H2O, δD} observations are influenced by three long-range transport patterns towards Mt. Zugspitze assessed in previous studies: (i) intercontinental transport from North America (TUS; source region: 25–45° N, 70–110° W, 0–2 km altitude), (ii) intercontinental transport from Northern Africa (TNA; source region: 15–30° N, 15° W–35° E, 0–2 km altitude), and (iii) stratospheric air intrusions (STI; source region: > 20° N, above zonal mean tropopause). The FTIR data exhibit significantly differing signatures in free-tropospheric {H2O, δD} pairs (given as mean with uncertainty of ±2 standard errors) for TUS (VMRH2O = 2.4 [2.3, 2.6] × 103 ppmv, δD = −315 [−326, −303] ‰), TNA (2.8 [2.6, 2.9] × 103 ppmv, −251 [−257, −246] ‰), and STI (1.2 [1.1, 1.3] × 103 ppmv, −384 [−397, −372] ‰). For TUS events, {H2O, δD} observations depend on surface temperature in the source region and the degree of dehydration having occurred during updraft in warm conveyor belts. During TNA events (dry convection of boundary layer air) relatively moist and weakly HDO-depleted air masses are imported. In contrast, STI events are associated to import of predominantly dry and HDO-depleted air masses. These long-range transport patterns potentially involve the import of various trace constituents to the Central European free troposphere, i.e., import of pollution from North America (e.g., aerosol, ozone, carbon monoxide), Saharan mineral dust, stratospheric ozone and other airborne species such as pollen. Our above results provide evidence that {H2O, δD} observations are a valuable proxy for the potential transport of such tracers. To validate this finding, we consult a data base of transport events (TNA and STI) covering 2013–2015 deduced by data filtering from in situ measurements at Mt. Zugspitze and lidar profiles at near-by Garmisch. Indeed, the FTIR data related to these verified TNA events (27 days) exhibit characteristic fingerprints in IWV (5.5 [4.9, 6.1] mm) and δDcol (−266 [−284, −247] ‰), which are significantly distinguishable from the rest of the time series (4.3 [4.1, 4.5] mm, −316 [−324, −308] ‰). This holds true on 1-σ level for 136 STI days (mean ± 1 standard error: 4.2 [4.0, 4.3] mm, −322 [−327, −316] ‰) with respect to the remainder (4.6 [4.5, 4.8] mm, −302 [−307, −297] ‰). Furthermore, deep stratospheric intrusions to the Zugspitze summit (in situ humidity and beryllium-7 data filtering) show a significantly lower mean value (−334 [−337, −330] ‰) of lower-tropospheric δD (3–5 km a.s.l.) on 2-σ level than the rest of the 2005–2015 time series (−284 [−286, −282] ‰). Our results show that consistent {H2O, δD} observations at Mt. Zugspitze can serve as an operational indicator for long-range transport events potentially affecting regional climate, air quality, as well as human health in Central Europe.