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

Abstract. We present vertical soundings (2005–2015) of tropospheric 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 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] % decade⁻¹ is statistically insignificant (95 % confidence interval). Under this caveat, the IWV trend estimate is conditionally consistent with the 2005–2015 temperature increase at Zugspitze (1.3 [0.5, 2.1] K decade⁻¹), 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 (δD_col < 5th percentile) are associated with air masses originating at higher latitudes (62° N on average) and altitudes (6.5 km)than high δD values (δD_col > 95th percentile: 46° N, 4.6 km). Backward-trajectory classification indicates that {H₂O, δD} observations are influenced by three long-range-transport patterns towards 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 (STIs; source region: > 20° N, above zonal mean tropopause). The FTIR data exhibit significantly differing signatures in free-tropospheric {H₂O, δD} pairs (5 km a.s.l.) – given as the mean with uncertainty of ±2 standard error (SE) – for TUS (VMR_H2O  =  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 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 with 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 results provide evidence that {H₂O, δD} observations are a valuable proxy for the transport of such tracers. To validate this finding, we consult a database of transport events (TNA and STI) covering 2013–2015 deduced by data filtering from in situ measurements at Zugspitze and lidar profiles at nearby 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 δD_col (−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 for 136 STI 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, 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.) than the rest of the 2005–2015 time series (−284 [−286, −282] ‰) considering uncertainty of ±2 SE. Our results show that consistent {H₂O, δD} observations at Zugspitze can serve as an operational indicator for long-range-transport events potentially affecting regional climate and air quality, as well as human health in central Europe.