Ground Based FTIR Measurements of Stratospheric Species from Harestua, Norway During Sesame and Comparison with Models

Vertical columns of HF, HCl, HNO3, ClONO2, N2O, ClO and COF2 were measured at Harestua, Norway (60.22° N, 10.75° E, Elevation 600 a.s.l.) beginning on 24 November 1994 and concluding on 1 May 1995 during Phase-III of the SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment) measurement campaign. The vertical columns of HCl, HNO3 and ClONO2 measured on 81 days were compared with columns calculated by the 3-D Cambridge model SLIMCAT. In addition the results were also interpreted by comparison with a photochemical trajectory model. Good agreement was seen for HCl while the nitrogen compounds showed larger discrepancies, especially for ClONO2. Evidence for chlorine activation was seen with 65% reduction of the chlorine reservoirs (HCl + ClONO2) while the levels of ClO were greatly enhanced. Interpretation of the loss with the trajectory model indicated condensation of chlorine on PSCs. The vertical column ratio of COF2 and HF was measured to 0.21 outside the vortex and a factor of two lower inside. The recovery of ClONO2 was seen to be much faster than that of HCl in the early spring.     

Empirical evaluation of microtremor H/V spectral ratio

The objective of this work is to perform a purely empirical assessment of the actual capabilities of the horizontal-to-vertical (H/V) spectral ratio technique to provide reliable and relevant information concerning site conditions and/or site amplification. This objective has been tackled through the homogeneous (re)processing of a large volume of earthquakes and ambient noise data recorded by different research teams in more than 200 sites located mainly in Europe, but also in the Caribbean and in Tehran. The original recordings were first gathered in a specific database with information on both the sites and recorded events. Then, for all sites close to an instrumented reference, average site-to-reference spectral ratios (“spectral ratio method” (SSR)) were derived in a homogeneous way (window selection, smoothing, signal-to-noise ratio threshold, averaging), as well as H/V ratios (“HVSRE–RF”) on earthquake recordings. H/V ratios were also obtained from noise recordings at each site (either specific measurements, or extracted from pre- or post-event noise windows). The spectral curves resulting from these three techniques were estimated reliable for a subset of 104 sites, and were thus compared in terms of fundamental frequency, amplitude and amplification bandwidth, exhibiting agreements and disagreements, for which interpretations are looked for in relation with characteristics of site conditions. The first important result consists in the very good agreement between fundamental frequencies obtained with either technique, observed for 81% of the analyzed sites. A significant part of the disagreements correspond to thick, low frequency, continental sites where natural noise level is often very low and H/V noise ratios do not exhibit any clear peak. The second important result is the absence of correlation between H/V peak amplitude and the actual site amplification measured on site-to-reference spectral ratios. There are, however, two statistically significant results about the amplitude of the H/V curve: the peak amplitude may be considered as a lower bound estimate of the actual amplification indicated by SSR (it is smaller for 79% of the 104 investigated sites), and, from another point of view, the difference in amplitude exhibits a questioning correlation with the geometrical characteristics of the sediment/basement interface: large SSR/HV differences might thus help to detect the existence of significant 2D or 3D effects.     

Physical State and Composition of Polar Stratospheric Clouds Inferred from Airborne Lidar Measurements During SESAME

The airborne lidar LEANDRE was deployed from Kiruna (Sweden) in January1995, during Phase III of the Second European Stratospheric Arctic andMid-latitude Experiment (SESAME). Polar Stratospheric Clouds (PSC) weredetected on several flights. Four lidar retrievals, on two different days, arepresented together with the thermal history of air masses, derived from 5-daybackward trajectories calculated from the TOPCAT model, using ECMWF analyses.The first PSC case was observed at the synoptic scale and at a temperatureclose to the ice frost point, with no significant depolarization, suggestingthat particles were in the liquid phase, probably liquid ternaryH₂SO₄/HNO₃/H₂Osolution forming a Type 1b cloud. Locally, on the same day, a Type II PSC (icecloud) was detected above the Scandinavian Mountains and was related tomesoscale processes. On another day, the third lidar retrieval only showedbackground aerosols signature, whereas the last one was performed at the NATequilibrium temperature and its measured depolarization ratio was consistentwith few solid state Type Ia particles, probably composed of NAT. From thethermal histories, the different behaviour of the two Type I clouds isexplained in terms of cooling rates and compositional hysteresis, driven bytemperature.     

Column Observations of Stratospheric Gases at Søndre Strømfjord, Greenland During Winter 1994-1995

As part of the Second European Stratospheric Arctic and Mid-latitude Experiment (SESAME) field campaign, observations were made during the period from October 1994 to April 1995 at Søndre Strømfjord, Greenland (67°N, 53°W). Using a Fourier transform spectrometer, high resolution (0.06 cm-1) infrared spectra were recorded with the sun as a radiation source. Column amounts of trace gases including HCl, HF, HNO3, N2O and O3 are shown for five time periods during the course of the 1994-95 Arctic winter. Results are compared with lower stratospheric potential vorticity fields to describe variations in trace gas column amounts.