Michel Chartier

SPIRALE: a multispecies in situ balloonborne instrument with six tunable diode laser spectrometers

The balloonborne SPIRALE (a French acronym for infrared absorption spectroscopy by tunable diode lasers) instrument has been developed for in situ measurements of several tracer and chemically active species in the stratosphere. Laser absorption takes place in an open Herriott multipass cell located under the balloon gondola, with six lead salt diode lasers as light sources. One mirror is located at the extremity of a deployable mast 3.5 m below the gondola, enabling the measurement of very low abundance species throughout a very long absorption path (up to 544 m). Three successful flights have produced concentration measurements of O3, CO, CO2, CH4, N2O, NO2, NO, HNO3, HCl, HOCl, COF2, and H2O2. Fast measurements (every 1.1 s) allow one to obtain a vertical resolution of 5 m for the profiles. A detection limit of a few tens of parts per trillion in volume has been demonstrated. Uncertainties of 3%-5% are estimated for the most abundant species rising to about 30% for the less abundant ones, mainly depending on the laser linewidth and the signal-to-noise ratio.

A portable infrared laser spectrometer for flux measurements of trace gases at the geosphere–atmosphere interface

A portable infrared laser absorption spectrometer named SPIRIT (SPectrometre Infra-Rouge In situ Tropospherique) has been set up for the simultaneous flux measurements of trace gases at the geosphere–atmosphere interface. It uses a continuous wave distributed feedback room temperature quantum cascade laser and a patented new optical multi-pass cell. The aim of SPIRIT field studies is to get a better understanding of land and water bodies to atmosphere exchange mechanisms of greenhouse gases (GHG). The analytical procedures to derive concentrations and fluxes are described, as well as the performances of the instrument under field conditions. The ability of SPIRIT to assess space and time dependence emissions of two GHG—nitrous oxide (N2O) and methane (CH4)—for different types of ecosystems is demonstrated through in situ measurements on peatland, on fertilized soil, and on water body systems. The objectives of these investigations and preliminary significant results are reported.

Validation of GOMOS‐Envisat vertical profiles of O3, NO2, NO3, and aerosol extinction using balloon‐borne instruments and analysis of the retrievals

The UV-visible Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument onboard Envisat performs nighttime measurements of ozone, NO 2 , NO 3 and of the aerosol extinction, using the stellar occultation method. We have conducted a validation exercise using various balloon-borne instruments in different geophysical conditions from 2002 to 2006, using GOMOS measurements performed with stars of different magnitudes. GOMOS and balloon-borne vertical columns in the middle stratosphere are in excellent agreement for ozone and NO 2 . Some discrepancies can appear between GOMOS and balloon-borne vertical profiles for the altitude and the amplitude of the concentration maximum. These discrepancies are randomly distributed, and no bias is detected. The accuracy of individual profiles in the middle stratosphere is 10 % for ozone and 25 % for NO 2 . On the other hand, the GOMOS NO 3 retrieval is difficult and no direct validation can be conducted. The GOMOS aerosol content is also well estimated, but the wavelength dependence can be better estimated if the aerosol retrieval is performed only in the visible domain. We can conclude that the GOMOS operational retrieval algorithm works well and that GOMOS has fully respected its primary objective for the study of the trends of species in the middle stratosphere, using the profiles in a statistical manner. Some individual profiles can be partly inaccurate, in particular in the lower stratosphere. Improvements could be obtained by reprocessing some GOMOS transmissions in case of specific studies in the middle and lower stratosphere when using the individual profiles.

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. II. Comparison of extinction, reflectance, polarization, and counting measurements

The physical properties of stratospheric aerosols can be retrieved from optical measurements involving extinction, radiance, polarization, and counting. We present here the results of measurements from the balloonborne instruments AMON, SALOMON, and RADIBAL, and from the French Laboratoire de Météorologie Dynamique and the University of Wyoming balloonborne particle counters. A cross comparison of the measurements was made for observations of background aerosols conducted during the polar winters of February 1997 and January-February 2000 for various altitudes from 13 to 19 km. On the one band, the effective radius and the total amount of background aerosols derived from the various sets of data are similar and are in agreement with pre-Pinatubo values. On the other hand, strong discrepancies occur in the shapes of the bimodal size distributions obtained from analysis of the raw measurement of the various instruments. It seems then that the log-normal assumption cannot fully reproduce the size distribution of background aerosols. The effect ofthe presence of particular aerosols on the measurements is discussed, and a new strategy for observations is proposed.

Measurements and simulation of stratospheric NO3 at mid and high latitudes in the northern hemisphere

Simultaneous measurements of NO 3 , along with those of O 3 , NO 2 , and aerosol extinction coefficient, have been performed during the night by the AMON instrument since 1992 at high and midlatitudes and by the SALOMON instrument since 1998 at midlatitude. Observations are conducted using the stellar and lunar occultation methods, respectively. Vertical profiles of NO 3 are obtained after inversion of the optical depth spectra recorded from 650 to 670 nm, including the 662-nm absorption line. Five profiles at midlatitude and two profiles at high latitude are presented. Comparisons with a box model constrained with measured ozone and temperatures (and NO 2 at high latitude) have been performed, taking into account the uncertainties in the rate constants of the reactions leading to NO 3 equilibrium. The modeling results can reproduce part of the observations, taking into account possible errors in the rate constants, temperature, or NO 3 absorption cross sections. Some disagreements nevertheless remain between observations and modeling outputs. In the middle stratosphere they could result from gradients of temperature. Below 30 km, other phenomena could be invoked to explain the disagreements. At high latitude the presence of solid polar aerosols induces an artifact in the data reduction. At midlatitude, large increases observed in the NO 3 concentration profiles obtained between 1992 and 1994 are real. A speculative hypothesis involving volcanic aerosols is proposed.

Remote‐sensing measurements in the polar vortex: Comparison to in situ observations and implications for the simultaneous retrievals and analysis of the NO2 and OClO species

Nighttime remote-sensing balloon observations conducted by the SALOMON instrument in the arctic polar vortex in January 2006 reveal high amounts of stratospheric NO 2 in the lower stratosphere similarly to previously published profiles. NO 2 concentration enhancements are also present in the vertical profiles observed by the GOMOS instrument on board the Envisat satellite and obtained coincidently to the balloon measurements. Such quantities are not present in in situ observations obtained by the SPIRALE instrument in similar geophysical conditions. While OClO amounts are acceptably reproduced by Chemistry Transport Model (CTM) calculations, NO 2 simulated values are well below the observed quantities. The examination of the slant column densities of NO 2 obtained at float altitude highlights unexpected strong enhancements with respect to the elevation angle and displacement of the balloon. It is shown that these fluctuations result from NO 2 spatial inhomogeneities located above the balloon float altitude. Potential vorticity maps reveal the presence of midlatitude NO 2 -rich air in the upper stratosphere or lower mesosphere as a result of the perturbed dynamical situation of the vortex. The presence of spatial inhomogeneities crossed by the lines of sight leads to artificial high concentration values of NO 2 in the vertical profile retrieved from the slant column densities assuming spatial homogeneity. A direct implication is that the differences previously observed between measurements of NO 2 and OClO and model results are probably mostly due to the improper inversion of NO 2 in the presence of perturbed dynamical conditions or when mesospheric NO x production events occur. The dynamical situation will have to be systematically analyzed in future studies involving remote-sensing observations.

Ultraviolet–visible bulk optical properties of randomly distributed soot

The presence of soot in the lower stratosphere was recently established by in situ measurements. To isolate their contribution to optical measurements from that of background aerosol, the soots bulk optical properties must be determined. Laboratory measurements of extinction and polarization of randomly distributed soot were conducted. For all soot, measurements show a slight reddening extinction between 400 and 700 nm and exhibit a maximum of 100% polarization at a scattering angle of 75 +/- 5 degrees. Such results cannot be reproduced by use of Mie theory assumptions. The different optical properties of soot and background stratospheric aerosol could allow isolation of soot in future analyses of stratospheric measurements.

A quantum cascade laser infrared spectrometer for CO2 stable isotope analysis: Field implementation at a hydrocarbon contaminated site under bio-remediation

Real-time methods to monitor stable isotope ratios of CO2 are needed to identify biogeochemical origins of CO2 emissions from the soil-air interface. An isotope ratio infra-red spectrometer (IRIS) has been developed to measure CO2 mixing ratio with δ(13)C isotopic signature, in addition to mixing ratios of other greenhouse gases (CH4, N2O). The original aspects of the instrument as well as its precision and accuracy for the determination of the isotopic signature δ(13)C of CO2 are discussed. A first application to biodegradation of hydrocarbons is presented, tested on a hydrocarbon contaminated site under aerobic bio-treatment. CO2 flux measurements using closed chamber method is combined with the determination of the isotopic signature δ(13)C of the CO2 emission to propose a non-intrusive method to monitor in situ biodegradation of hydrocarbons. In the contaminated area, high CO2 emissions have been measured with an isotopic signature δ(13)C suggesting that CO2 comes from petroleum hydrocarbon biodegradation. This first field implementation shows that rapid and accurate measurement of isotopic signature of CO2 emissions is particularly useful in assessing the contribution of contaminant degradation to the measured CO2 efflux and is promising as a monitoring tool for aerobic bio-treatment.