Michel Pirre

Carbon aerosols and atmospheric photochemistry

Carbon aerosols are produced by all combustion processes. This paper investigates some possible effects of heterogeneous reduction of atmospheric constituents on carbon aerosols. Reduction of HNO 3 , NO 2 , and O 3 on carbon aerosols may be an important effect of increased air traffic that has not been considered to date. It is shown that if HNO 3 , NO 2 and O 3 are heterogeneously reduced on atmospheric amorphous carbon aerosols, then a significant, lower stratospheric ozone loss mechanism could exist. This ozone loss mechanism is almost independent of temperature and does not require the presence of sunlight. The mechanism can operate at all latitudes where amorphous carbon aerosols are present. The relative importance of the mechanism increases with nightlength. The reduction of HNO 3 on carbon aerosols could also be a significant renoxification process wherever carbon aerosols are present. Owing to the very different soot levels in the two hemispheres, this implies that there should be a hemispheric assymetry in the role of these mechanisms. The renoxification leads to simulated tropospheric HNO 3 /NO x ratios that are close to those observed. In contrast to the stratospheric response, the tropospheric production of NO x due to the reduction of HNO 3 would lead to tropospheric ozone production.

Role of the BRO + HO2 reaction in the stratospheric chemistry of bromine

The impact of new laboratory data for the reaction BrO + HO2 → HOBr + O2 has been estimated in a one-dimensional photochemical modelling of bromine/ozone stratospheric chemistry. The reported 6 fold increase in the measured value for the rate constant of this reaction significantly increases both the HOBr mixing ratio and the global ozone depletion due to bromine compounds (the calculated ozone loss increases from 1.14% to 1.45% for a 20 ppt total bromine content). The higher rate constant makes the bromine partitioning and the ozone depletion very sensitive to the branching ratio of the potential channel forming HBr in the BrO + HO2 reaction. The influence of the existing uncertainty in the photolysis rate of HOBr is also analysed.

Odin/SMR limb observations of stratospheric trace gases: Validation of N2O

The Sub-Millimetre Radiometer (Odin/SMR) on board the Odin satellite, launched on 20 February 2001, performs regular measurements of the global distribution of stratospheric nitrous oxide (N2O) using spectral observations of the J = 20R 19 rotational transition centered at 502.296 GHz. We present a quality assessment for the retrieved N2O profiles (level 2 product) by comparison with independent balloonborne and aircraftborne validation measurements as well as by cross-comparing with preliminary results from other satellite instruments. An agreement with the airborne validation experiments within 28 ppbv in terms of the root mean square (RMS) deviation is found for all SMR data versions (v222, v223, and v1.2) under investigation. More precisely, the agreement is within 19 ppbv for N2O volume mixing ratios (VMR) lower than 200 ppbv and within 10% for mixing ratios larger than 150 ppbv. Given the uncertainties due to atmospheric variability inherent to such comparisons, these values should be interpreted as upper limits for the systematic error of the Odin/SMR N2O measurements. Odin/SMR N2O mixing ratios are systematically slightly higher than nonvalidated data obtained from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II). Root mean square deviations are generally within 23 ppbv (or 20% for VMR-N2O > 100 ppbv) for versions 222 and 223. The comparison with data obtained from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the Envisat satellite yields a good agreement within 9-17 ppbv (or 10% for VMR-N2O > 100 ppbv) for the same data versions. Odin/SMR version 1.2 data show somewhat larger RMS deviations and a higher positive bias.

Diurnal and nocturnal distribution of stratospheric NO2 from solar and stellar occultation measurements in the Arctic vortex: Comparison with models and ILAS satellite measurements

NO2 mixing ratio profiles were measured at sunset between 14 and 30 km using the Limb Profile Monitor of the Atmosphere (LPMA) experiment and during the night between 13 and 31 km using the Absorption par Minoritaires Ozone et NOx (AMON) experiment inside the Arctic vortex, both on February 26, 1997. Coinciding profiles measured by the Improved Limb Atmospheric Spectrometer (ILAS) instrument on board ADEOS have been used to check the consistency between the satellite and balloon profiles for NO2, O3, and HNO3. A box model has been used for the photochemical correction of the LPMA NO2 profiles at sunset. The resulting NO2 balloon-borne profiles of LPMA and AMON are compared to each other after accounting for the day/night photochemical variation using the box model initialized with measurements. The comparisons thus performed show an average difference less than 9% between the two measurements (considered to sample similar air masses) when the box model is initialized with little chlorine activation (i.e., when the major burden of chlorine is stored in ClONO2) for a 1 day integration. The comparison with the Reprobus 3-D chemistry transport model (CTM) seasonal simulations clearly confirms an underestimation of NO2 by the model below 25 km, in the altitude range where aerosols lead to a complete removal of NOx in the model. Recent updates of rate coefficients for conversion of HNO3 into NO2 only slightly improve the NO2 model results in vortex conditions. These results suggest that a source of NO2 is still lacking in the CTM.

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.

SALOMON: a new, light balloonborne UV–visible spectrometer for nighttime observations of stratospheric trace-gas species

A new, light balloonborne UV-visible spectrometer, called SALOMON, is designed to perform nighttime measurements of stratospheric trace-gas species by using the Moon as a light source. The first flight, performed on 31 October 1998 at mid-latitude with a float altitude of 26.7 km, allowed the performance of the pointing system to be checked and vertical profiles of ozone, NO(2), NO(3), and possibly OBrO to be obtained. First the instrument and then the performance of the pointing system and the detector are described. Finally the vertical profiles are compared with other profiles obtained at the same location five years before with the heavier balloonborne spectrometer AMON, which uses a star as the light source.

Role of lee waves in the formation of solid polar stratospheric clouds: Case studies from February 1997

Recent theories of solid polar stratospheric clouds (PSCs) formation have shown that particles could remain liquid down to 3 K or 4 K below the ice frost point. Such temperatures are rarely reached in the Arctic stratosphere at synoptic scale, but nevertheless, solid PSCs are frequently observed. Mesoscale processes such as mountain-induced gravity waves could be responsible for their formation. In this paper, a microphysical-chemical Lagrangian model (MiPLaSMO) and a mountain wave model (NRL/MWFM) are used to interpret balloon-borne measurements made by an optical particle counter (OPC) and by the Absorption par Minoritaires Ozone et NOx (AMON) instrument above Kiruna on February 25 and 26, 1997, respectively. The model results show good agreement with the particle size distributions obtained by the OPC in a layer of large particles, and allow us to interpret this layer as an evaporating mesoscale type Ia PSC (nitric acid trihydrate) mixed with liquid particles. The detection of a layer of solid particles by AMON is also qualitatively reproduced by the model and is interpreted to be frozen sulfate acid aerosols (SAT). In this situation, the impact of mountain waves on chlorine activation is studied. It appears that mesoscale perturbations amplify significantly the amount of computed ClO, as compared to synoptic runs. Moreover, MiPLaSMO chemical results concerning HNO3 and HCl agree with measurements made by the Limb Profile Monitor of the Atmosphere (LPMA) instrument on February 26 at a very close location to AMON, and explain part of the differences between LPMA measurement and Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) model outputs.

The possible detection of OBrO in the stratosphere

The analysis of spectral residua recorded at night by the balloon-borne AMON (Absorption par Minoritaires Ozone et Nox) UV-visible spectrometer during five stratospheric flights at middle and high latitudes shows that some absorption features remain in the 475–550 nm range, while the Rayleigh, aerosol, ozone, and NO2 contributions are subtracted. The data reduction relating to these spectral lines is presented for the flight of February 26, 1997, at Kiruna (Sweden) inside the polar vortex. A good agreement exists between these unknown absorption features and those attributed to OBrO during recent laboratory measurements. The results of measurements from the other AMON flights are also presented. Assuming a OBrO cross section maximum similar to that of OClO, an upper limit for the OBrO mixing ratio is found to be around 20 pptv at midlatitude, implying that OBrO would be the principal bromine species at night in the middle stratosphere. At high latitude the OBrO mixing ratio decreases, particularly in the presence of OClO (also measured by AMON). The results are contradictory to current knowledge and, if confirmed, could argue for major revision of the assumed bromine chemistry in the stratosphere.

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.