Jean Leveau

Combined characterisation of GOME and TOMS total ozone measurements from space using ground-based observations from the NDSC

Several years of total ozone measured from space by the ERS-2 GOME, the Earth Probe TOMS, and the ADEOS TOMS, are compared with high-quality ground-based observations associated with the Network for the Detection of Stratospheric Change (NDSC), over an extended latitude range and a variety of geophysical conditions. The comparisons with each spaceborne sensor are combined altogether for investigating their respective solar zenith angle (SZA) dependence, dispersion, and difference of sensitivity. The space- and ground-based data are found to agree within a few percent on average. However, the analysis highlights for both GOME and TOMS several sources of discrepancies: (i) a SZA dependence with TOMS beyond 80° SZA; (ii) a seasonal SZA dependence with GOME beyond 70° SZA; (iii) a difference of sensitivity with GOME at high latitudes; (iv) a difference of sensitivity to low ozone values between satellite and SAOZ sensors around the southern tropics; (v) a north/south difference of TOMS with the ground-based observations; and (vi) internal inconsistencies in GOME total ozone.

Diurnal and seasonal variation of carbonaceous aerosols at a remote MBL site of La Réunion island

In this study, we characterise the variability of black carbon (BC) obtained at Sainte-Rose, a tropical marine site of La Reunion island (21.5°S, 55°E) from one and a half-year (1998–1999) semi-continuous ground-based measurements. BC diurnal variation shows low concentrations during night-time (mean ∼10–20 ng/m3, following the season) and moderate levels at daytime (mean ∼35 ng/m3), while BC peaks are observed at the beginning and end of the day. Morning (mean ∼50 ng/m3 at 0700 h) and evening (mean ∼70 ng/m3 at 1800 h) BC peaks are thought to be due to local pollution induced by anthropogenic activities, most probably car exhausts from nearby roads. Night-time BC exhibits a seasonal variation too, with maximum levels observed during autumn and winter (∼20 ng/m3) and lowest values measured during spring and summer (∼10 ng/m3). BC seasonality suggests a local-to-regional anthropogenic origin of carbonaceous aerosols following the season. These suggestions are confirmed by wind sector and BC concentrations analysis at Sainte-Rose, which show that daytime BC aerosols originate mainly from inland while night-time BC concentrations originate from northeasterly (regional inhabited areas) to southeasterly (marine) regions, following the season. Wind sector analysis also points out important seasonal shift in origin of night-time air samples. These assumptions are confirmed by trajectory analysis indicating a spatial shift in the origin of air samples following the season. Finally, it is shown that during austral spring and summer, low carbonaceous aerosol concentrations encountered at this site are weakly subjected to scavenging processes. Results point out the potential of ground-based measurements to assess the origin and causes of carbonaceous aerosol variability at remote locations under favourable conditions (minimum local anthropogenic inputs and meteorology).

Description and evaluation of a tropospheric ozone lidar implemented on an existing lidar in the southern subtropics

Rayleigh-Mie lidar measurements of stratospheric temperature and aerosol profiles have been carried out at Reunion Island (southern tropics) since 1993. Since June 1998, an operational extension of the system is permitting additional measurements of tropospheric ozone to be made by differential absorption lidar. The emission wavelengths (289 and 316 nm) are obtained by stimulated Raman shifting of the fourth harmonic of a Nd:YAG laser in a high-pressure deuterium cell. A mosaic of four parabolic mirrors collects the backscattered signal, and the transmission is processed by the multiple fiber collector method. The altitude range of ozone profiles obtained with this system is 3¿17 km. Technical details of this lidar system working in the southern tropics, comparisons of ozone lidar profiles with radiosondes, and scientific perspectives are presented. The significant lack of tropospheric ozone measurements in the tropical and equatorial regions, the particular scientific interest in these regions, and the altitude range of the ozone measurements to 16¿17 km make this lidar supplement useful and its adaptation technically conceivable at many Rayleigh-Mie lidar stations.

LIDAR observations of lower stratospheric aerosols over South Africa linked to large scale transport across the southern subtropical barrier

Abstract The study of the variability of stratospheric aerosols and the transfer between the different atmospheric regions improves our understanding of dynamical processes involved in isentropic exchanges that take place episodically in the lower stratosphere through the subtropical barrier. One useful approach consists in combining in situ ground-based and global measurements with numerical analyses. The present paper reports on a case study of a horizontal transfer evidenced first by Rayleigh–Mie LIDAR observations over Durban (29.9°S, 31.0°E, South Africa). Additional data from MeteoSat and SAGE -2 experiments, and from ECMWF meteorological analysis have been used in this study. Contour advection maps of potential vorticity from the MIMOSA model derived from ECMWF fields, were also used. By the end of April, 1999, LIDAR observations showed that aerosol extinction, in the lower stratosphere, has increased significantly and abnormally in comparison with other LIDAR and SAGE -2 observations recorded for the period from April 20 to June 14, 1999. The dynamical context of this case study seems to exclude the possibility of a local influence of the subtropical jet stream or tropical convection, which could inject air masses enriched with tropospheric aerosols into the stratosphere. On the contrary, a high-resolution model based on PV advection calculations and ECMWF meteorological analyses shows that air masses are isentropically advected from the equatorial zone close to Brazil. They cross the southern barrier of the tropical reservoir due to laminae stretching and reach the southern subcontinent of Africa 5–6 days later.

Vertical short-scale structures in the upper tropospheric and lower stratospheric temperature and ozone at la Réunion Island (20.8°S 55.3°E)

The distribution and the nature of vertical short-scale structures observed in ozone and temperature are investigated in the upper troposphere and the lower stratosphere at La Reunion Island located in the vicinity of the southern subtropical barrier by using wavelet-based methods. A climatology of dominant wavelike patterns with short vertical wavelengths reveals the presence of localized structures on both the ozone and the temperature perturbations, extracted from ozonesonde and temperature data, up to the middle stratosphere. Some case studies are presented to identify the nature of short-scale structures with 1- to 5-km vertical wavelengths in the troposphere and the stratosphere. A climatology of short-scale structures induced by gravity waves and the horizontal advection shows that short-scale structures are mainly detected in the middle and upper troposphere and in the lower stratosphere. The weak value of the coefficient R(z) that links the ozone and temperature perturbations induced by gravity waves is a major limit to detecting such short-scale structures above 21-km altitude. Some structures with vertical wavelengths ranging from 1 to 5 km are attributed to gravity waves produced by convection in summer and the subtropical jet in winter, or quasi-horizontal large-scale motions from both sides of the subtropical barrier.

Chemical effect of carbonaceous aerosols on the diurnal cycle of MBL ozone at a tropical site: measurements and simulations

During late austral summer and winter 1998, black carbon (BC) aerosols were monitored with an Aethalometer at 2 sites of La Réunion Island (Indian Ocean): Saint-Denis, the main city and Sainte-Rose, a quite uninhabited region situated at the east coast. BC concentration data at Saint-Denis show a marked diurnal cycle, which may be primarily attributed to traffic. The background data found at night-time display average BC concentrations, ranging from about 80 to 250 ng/m3 whereas during the day, BC concentrations increase by a factor of at least 4. In comparison, BC concentrations vary in the range of 10 to 60 ng/m3 at Sainte-Rose. Ozone concentration was also measured at Saint-Denis using a Dasibi photometer and found to be at significant levels (means: 16.5–23 ppbv in April and 28.5–34 ppbv in September). A noticeable increase of ozone concentrations during the day points out the build-up of pollutants enhancing photochemical transformations. However, during traffic pollution peaks, ozone concentration displays systematic depletion. The comparison of ozone and BC measurements at both seasons points to some possible effects of heterogeneous interaction of ozone and its precursors with BC particles. These interactions were also simulated with a 0D time-dependent chemistry model using conditions of a polluted site. The measured ozone concentration characteristics (mean concentration and range of variation) are well simulated in the presence of BC. Our model results show that at La Réunion Island adsorption of ozone and its precursors onto BC aerosol particles could be one of the important steps determining ozone concentration characteristics, especially in absence of photochemistry during night-time.

Variability of carbonaceous aerosols, ozone and radon at Piton Textor, a mountain site on Réunion island (south-western Indian Ocean)

Black carbon (BC) was monitored during 1997–1999 in the lower troposphere of the southernIndian Ocean at La Reéunion island (21.5°S, 55.5°E). BC concentrations obtained at PitonTextor, an altitude site (2150 m) representative of free troposphere, exhibited diurnal patternsand concentrations different from urban locations on the island, with maximum concentrationsobserved at daytime (~50–150 ng/m3) and minimum levels (~10–70 ng/m3) at night-time. BCdiurnal variation is anti-correlated with diurnal ozone measured semi-continuously in parallelduring 1998–1999, suggesting possible interaction of ozone and precursors (NOx, VOC, etc.)on carbonaceous aerosols, especially at night-time. Daytime BC enhancement may be explainedby dynamical processes, due to updraught of air masses from lower levels to the troposphere,while at night-time, this process is reversed. Daytime ozone depletion is governed by photochemicalprocesses, due to low precursor levels, while night-time ozone recovery is mainly driven bydynamical processes from upper tropospheric layers. Night-time BC and ozone in the lowertroposphere show a marked seasonal pattern too, with minimum levels during austral summer(~15 ng/m3, 22 ppbv), secondary peaks in autumn and spring (~35 ng/m3, 36 ppbv) and maximumvalues during austral winter (~70 ng/m3, 41 ppbv) respectively. Night-time BC and ozoneseasonalities are concordant with night-time radon seasonal trend in the lower troposphere,indicating that sampled air masses have mainly a marine origin in summer, off the Africanbiomass burning season, and a continental origin in austral winter and spring. Winter andspring BC and ozone enhancement corroborate with fire-count maximum peaks observed overAfrica and Madagascar, suggesting that the main cause is combustion products long-rangetransported in stable layers evidenced by thermodynamic analysis using 1996–1999 PTU soundings.These assessments are confirmed by 5-day backtrajectories, which show important seasonalshift in origin of air masses arriving in the lower troposphere of the south-western Indian Ocean.

Climatology of the upper troposphere with lidar

The upper troposphere should play an important role on the climate. Several greenhouse gazes as well as cirrus clouds present in this region a large variability in both temporal and spatial scales. This variability is strongly connected with processes and air mass exchanges with the lower altitude as well as the stratosphere. The upper troposphere is not easy to probe nor from space nor from the ground. Lidar appears to be a good candidate for probing the upper troposphere. Ozone, water vapor, cirrus clouds, and temperature can be measure with lidar. All those measurements have been deployed in two sites. One is located in south of France at mid-latitude of the Northern Hemisphere and the other at the tropical site of the Southern Hemisphere: La Reunion. The technology has been improved to make such reliable measurements on a routine basis and to implement those measurements on a single lidar. Due to the episodic nature of the variability (in comparison with wave type variability), some questions about lidar signal integration need to be addressed. First climatologies of each parameter independently have been already obtained. Future strategy will be discussed.

Raman water-vapor lidar implemented on an existing lidar system in the southern tropics

A Raman lidar dedicated to night-time tropospheric water-vapor high-resolution measurements is currently being developed at Réunion island in the south-western Indian Ocean. To our knowledge, it is the first permanent instrument of its kind in this tropical region. The geophysical and instrumental interests and issues on the radiative, dynamical and chemical plans for such a measurement, specially in the tropics, are obvious. The choice of a visible laser excitation wavelength was initially a constraint, in view of the weakness of the Raman scattering process that is the basis of the development of this instrument, but many arguments also plead for such a choice. After describing the water-vapor measurement method of this lidar, which is straightforward in principle, we stress on the main delicate underlying issues related to this method. A precise description of the optical parts of the lidar system is then given that emphasizes the importance of the rejection of the elastically backscattered signals in the Raman channels. Finally, we list the most important future works concerning the validation and calibration stages of this instrument that is intended to become an atmospheric surveillance instrument on a medium term.

Ground-based solar absorption FTIR spectroscopy at the Reunion Island

This paper gives preliminary results of FTS atmospheric measurements at the Reunion Island. A special emphasis is given to the retrievals of CH4, CO, O3, N2O, HCl, HF and HNO3