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We present vertical soundings (2005–2015) oftropospheric water vapor (H2O) and its D ∕ H isotope ratio (δD)derived from ground-based solar Fourier transform infrared (FTIR)measurements at Zugspitze (47° N, 11° E, 2964 m a.s.l.).Beside water vapor profiles with optimized vertical resolution (degrees of freedom for signal, DOFS,  =  2.8), {H2O, δD} pairs with consistent verticalresolution (DOFS  =  1.6 for H2O and δD) applied in this study.The integrated water vapor (IWV) trend of 2.4 [−5.8, 10.6] % decade−1 is statistically insignificant (95 % confidence interval).Under this caveat, the IWV trend estimate is conditionally consistent withthe 2005–2015 temperature increase at Zugspitze (1.3 [0.5, 2.1] K decade−1), assuming constant relative humidity. Seasonal variations infree-tropospheric H2O and δD exhibit amplitudes of 140 and50 % of the respective overall means. The minima (maxima) in January(July) are in agreement with changing sea surface temperature of theAtlantic Ocean. Using extensive backward-trajectory analysis, distinct moisture pathways areidentified depending on observed δD levels: low column-basedδD values (δDcol < 5th percentile) areassociated with air masses originating at higher latitudes (62° Non average) and altitudes (6.5 km)than high δD values(δDcol > 95th percentile: 46° N,4.6 km). Backward-trajectory classification indicates that {H2O, δD} observations are influenced by threelong-range-transport patterns towards Zugspitze assessed in previousstudies: (i) intercontinental transport from North America (TUS; sourceregion: 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 (STIs;source region: > 20° N, above zonal mean tropopause). The FTIR data exhibitsignificantly differing signatures in free-tropospheric {H2O, δD} pairs (5 km a.s.l.) – given as the meanwith uncertainty of ±2 standard error (SE) – 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 STIs (1.2 [1.1, 1.3]  ×  103 ppmv, −384 [−397, −372] ‰). For TUSevents, {H2O, δD} observationsdepend on surface temperature in the source region and the degree ofdehydration having occurred during updraft in warm conveyor belts. DuringTNA events (dry convection of boundary layer air) relatively moist andweakly HDO-depleted air masses are imported. In contrast, STI events areassociated with import of predominantly dry and HDO-depleted air masses. These long-range-transport patterns potentially involve the import ofvarious trace constituents to the central European free troposphere, i.e.,import of pollution from North America (e.g., aerosol, ozone, carbonmonoxide), Saharan mineral dust, stratospheric ozone, and other airbornespecies such as pollen. Our results provide evidence that {H2O, δD} observations are a valuable proxy forthe transport of such tracers. To validate this finding, we consult adatabase of transport events (TNA and STI) covering 2013–2015 deduced bydata filtering from in situ measurements at Zugspitze and lidar profiles atnearby 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) andδDcol (−266 [−284, −247] ‰), which aresignificantly distinguishable from the rest of the time series (4.3 [4.1,4.5] mm, −316 [−324, −308] ‰). This holds true for 136STI days considering uncertainties of ±1 SE (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, deepstratospheric intrusions to the Zugspitze summit (in situ humidity andberyllium-7 data filtering) show a significantly lower mean value (−334[−337, −330] ‰) of lower-tropospheric δD (3–5 km a.s.l.) than the rest of the 2005–2015 time series (−284 [−286, −282] ‰)considering uncertainty of ±2 SE. Our resultsshow that consistent {H2O, δD}observations at Zugspitze can serve as an operational indicator forlong-range-transport events potentially affecting regional climate and airquality, as well as human health in central Europe

A decadal time series of water vapor and D / H isotope ratios above Zugspitze: transport patterns to central Europe