Weakly bonded pairs of water molecules (H2O)2, or water dimers (WDs), may play an important role in photochemistry and climate, but the overlap of most of its spectral features with the water monomer ...(WM) has made detection difficult. We report on WD absorption measurements by means of atmospheric long-path (18.34 kilometers) differential optical absorption spectroscopy of the near-infrared OH stretching mode $\mid 0 >_f\mid 4 >_b$ overtone transition predicted to be located near 746 nanometers. Our observation is in reasonable agreement with the known thermochemistry, calculated and measured structure, and spectroscopy (band strength, shape, and width) of the WD. The observation implies that the WD $\mid 0 >_f\mid 4 >_b$ band is located at 749.5 nanometers, with a full width at half maximum of ~19.4 wave numbers, and that its band strength ranges between$1.23 \times 10^{-22}$and$5.25 \times 10^{-22}$centimeters per molecule.
Abstract
Novel airborne in situ measurements of inorganic chlorine, nitrogen oxide species, and ozone were performed inside the lower Antarctic polar vortex and at its edge in September 2012. We ...focus on one flight during the Transport and Composition of the LMS/Earth System Model Validation (TACTS/ESMVal) campaign with the German research aircraft HALO (High‐Altitude LOng range research aircraft), reaching latitudes of 65°S and potential temperatures up to 405 K. Using the early winter correlations of reactive trace gases with N
2
O from the Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer (ACE‐FTS), we find high depletion of chlorine reservoir gases up to ∼40% (0.8 ppbv) at 12 km to 14 km altitude in the vortex and 0.4 ppbv at the edge in subsided stratospheric air with mean ages up to 4.5 years. We observe denitrification of up to 4 ppbv, while ozone was depleted by 1.2 ppmv at potential temperatures as low as 380 K. The advanced instrumentation aboard HALO enables high‐resolution measurements with implications for the oxidation capacity of the lowermost stratosphere.
Plain Language Summary
Chemistry climate models reveal large uncertainties in the future ozone projection until the end of the century in the lower polar and midlatitude stratosphere. One process that impacts the ozone lifetime during the polar winter is the formation of active chlorine from chlorine reservoir species. Here we present high‐resolution measurements performed aboard the new German research aircraft HALO (High‐Altitude LOng range research aircraft) in the lower Antarctic vortex in winter 2012. We find significant amounts of active chlorine in the lower vortex that has been transported from higher altitudes and latitudes to the flight altitude of HALO (∼14 km). This enhanced activated chlorine content has implications on the ozone lifetime in this region. Our measurements complement satellite observations but feature higher‐altitude resolution and extend to lower altitudes. With our case study we investigate the intersection of midlatitude and polar air as well as the effects of transport from the upper to the lower polar vortex. These process studies help to improve chemistry climate models.
Key Points
Newly developed HALO in situ measurements give insight in stratospheric trace gas distributions in the lower Antarctic vortex
A high amount of activated chlorine reservoir species and redistributed nitric acid in the lower Antarctic polar stratosphere was observed
Upper vortex air was transported to the flight altitude of HALO at 12 to 14 km
For the first time Lévy indices (γ) of the solar light transmitted by cloudy skies at mid latitude (50°N, 8.2°E) are reported. The Lévy index describes the dependence of the mean geometrical paths (< ...LT > ) of photons transmitted from cloudy skies as a function of the vertical cloud extension (Hc) expressed by the scaling law ∼ Hcγ. For a set of 33 individual cloudy sky < LT > measurements, reported cloud types and heights, Lévy indices are deduced. It is found that the inferred Lévy indices cluster into the range of 1 ≤ γ ≤ 2 for “all sky” observations, and into a range 1.5 ≤ γ ≤ 2 for optically very thick clouds. The observations provide evidence that the cloudy sky geometrical path lengths are Lévy distributed with the γ value depending on the cloud morphology (the shape of individual clouds and the spatial arrangement of the clouds) but also on the internal cloud inhomogeneities. Because of a particular sensitivity of our method to detect the radiative transfer (RT) caused by clouds (rather than by the clear sky parts of the atmosphere), the inferred type of the path statistics is expected to reflect mostly the path length distribution caused by cloud inhomogeneities rather than by the cloud morphology. Since the cloud inhomogeneities are caused by dynamic processes (besides other factors), the RT transfer is expected to be closely connected to atmospheric dynamics. From this it is concluded that the absorption of solar radiation in cloudy skies is connected to the dynamic state (whether it is stratified or convective) of the cloud cover. In particular, it is expected to be different (mostly larger) than calculated by naively assuming homogeneous horizontally infinite cloud covers in conventional non statistical RT models.
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