Jacques Fontan

Ozone and Aitken nuclei over equatorial Africa: Airborne observations during DECAFE 88

We determined the distribution of ozone (O3) and Aitken condensation nuclei (CN) over the rain forest in equatorial Africa during February 12–25, 1988. A pronounced O3 maximum with levels up to 70 ppbv was present in a layer between 1 and 3 km altitude throughout the period. It coincided with a CO maximum and with high levels of CO2 and gaseous organic acids. In general, the vertical distribution of CN was similar to that of O3, with number densities ranging up to approximately 3000 cm−3. Dense haze was visible within this layer. O3 and CN decreased sharply above the haze layer to values typical of the remote troposphere. Survey flights showed little change in levels of O3 and CN or in their vertical distribution over distances of hundreds of kilometers. Meteorological observations suggest that this ozone and particulate enriched layer is formed from air masses which originate in northern Africa and subsequently advect over dry tropical regions where biomass burning emits large amounts of aerosols, CO, NO, and hydrocarbons. These air masses then become trapped in the equatorial region between the near-surface monsoon flow from the southeast and the permanent easterly flow above 3–4 km. Differences in the vertical distribution of O3 and CN result from the removal of O3 by surface uptake and reactions with NO and hydrocarbons, leading to surface O3 concentrations near zero and a steep O3 gradient through the subcloud layer at night. During the day this gradient is reduced by convective mixing. CN concentrations showed no pronounced gradients in the subcloud layer, consistent with the absence of a strong sink for CN at the ground. CN gradients near the surface suggest emission of particles from the forest vegetation or from biomass burning.

Experimental Studies of the Turbulence Structure Parameters of the Convective Boundary Layer

Abstract We present a collection of experimental data concerning turbulent characteristics of the convective boundary layer. These data were obtained by means of instrumented aircraft during numerous experiments carried out above five flight areas from 1973 to 1978. Only dry convective conditions with weak dynamic instability are contained in this report. The computed quantities are vertical turbulent fluxes of sensible and latent heat, second- and third-order moments of vertical wind component, potential temperature and humidity, dissipative and spectral characteristics and length scales. All these quantities are normalized using the convective similarity hypothesis of Deardorff. A particular case is considered for humidity where it is necessary to choose between the two boundary conditions: evaporation and entrainment flux. The latter is chosen.

The Landes experiment: Biosphere-atmosphere exchanges of ozone and aerosol particles above a pine forest

An experiment was conducted in a pine forest in southwestern France during late spring 1992. The aim was fourfold: testing various flux measurement methodologies for chemically reactive species; quantifying the exchanges between the forest and the atmosphere; analyzing the involved mechanisms; and studying their influence on the chemistry of the surface boundary layer. This paper presents preliminary results obtained on the dry deposition of ozone and submicronic aerosol particles, measured using eddy correlation. Once properly normalized, the spectra and cospectra of all scalar species exhibit universal shapes over the whole frequency range. However, evidence is provided that under some meteorological conditions the time series of turbulent variables can be affected by nonstationary trends, or low-frequency fluctuations that do not contribute to vertical transfer but whose presence can induce large errors in the calculated fluxes. The time variations of the deposition velocities for ozone and aerosol particles are then presented over 2 days with different meteorological conditions. The deposition velocities are shown to be consistent with other reported studies. Dry deposition of ozone appears to be mainly governed by the stomatal resistance, whereas friction velocity and atmospheric instability in the boundary layer seem to govern the deposition of aerosol particles.

Comparison between weekend and weekday ozone concentration in large cities in France

Abstract This paper examines ozone data from five large French cities (Marseilles, Lyon, Paris, Strasbourg and Toulouse) in spring and summer over a three-year period to study the possible influence of local primary pollutant emissions. In these cities the level of traffic emission varies according to the day of the week. There is a decrease of about 25% in traffic emissions between non-consecutive Tuesdays and Sundays. Traffic emissions on Fridays are about 40% more than on non-consecutive Sundays whereas they seem to be similar for non-consecutive Tuesdays and Thursdays. Despite this variation in traffic emissions between Fridays and Sundays, 85% of daily ozone maxima are identical for all days compared; in 15% of cases, percentiles of daily ozone maxima vary by about 20% at the most. This difference is observed for the highest values of daily ozone maxima that we can find both in rural and urban sites. Marseilles is the most pollution-sensitive city; every site of this area is concerned, which gives a regional origin to ozone variability. In the less-populated Toulouse area, differences between ozone on Fridays and Sundays are less significant. Our results show the importance of advection phenomena of ozone. It calls into question strategies of local reductions in traffic during ozone episodes.

Evolution times of tropospheric negative ions

Abstract A computer program has been developed to study the evolution times of tropospheric positive ions. Twenty-eight ions have been considered and their evolution times are a function of 79 ion-molecule rate constants. These calculations have shown that the ions NO+ are created mainly from N+ and O+. At 760 Torr and water concentration of 10 per cent by vol (or 49% r.h. at 18°C) the ions H3O+, (H2O)n, n = 1, 2, 3, …, 7 are formed in less than 103s.

Monitoring of the Atmospheric Stability above an Urban and Suburban Site Using Sodar and Radon Measurements

Abstract This study presents a method of obtaining the quantitative intensity of vertical diffusion during periods of atmospheric stability. This method associates the continuous measurement of radon concentration at ground level and the use of a monostatic sodar. The value of “equivalent mixing height” he is calculated using the radon variation which can be related to the global exchange coefficient of the inversion layer. The sodar detects the thickness of the nocturnal inversion layer. Both systems operated simultaneously over several months at two sites (urban and suburban). In most cases the nocturnal layer was indicated both by sodar echoes and by a large decrease of the he value. In other cases the use of sodar alone can lead to a lack of detection of stable periods which, nevertheless, are shown clearly by radon. The comparison of measurement at two sites shows a modification of nocturnal stability above the urban site. The equivalent exchange coefficients are ∼0.3 m2 s−1 (at the urban site) and 0...

Atmospheric Particles and their Interactions with Natural Surfaces

The purpose of this short review will be to focus on those aspects of aerosol exchange which are of importance to the BIATEX community in order to aid in the assessment of acidic or alkaline deposition, from dry and wet aerosol (in the form of cloud or fog droplets) over Europe with an eye to the larger scale processes in which aerosols play a critical role. One of the important goals of the Biatex program, emphasised by its co-ordinator, has been the necessity to deliver information about these processes which are “useful” to wider interests, in particular the modelling community. In light of the co-ordinators plea for “immediately useful information” a detailed summary of parameterisations which have been forthcoming in BIATEX and related programmes for both dry and wet aerosol deposition are listed in the Appendix.

Vertical ozone profiles in a pine forest

Abstract Vertical profiles of ozone concentration have been measured in a pine forest located in the Landes region, in south-west France. In the daytime, ozone concentrations are nearly constant between ground level and an altitude of 35 m, the higher level of measurement. At night, important vertical variations of ozone concentration are observed in relation with temperature profiles. Analysis of the ozone profiles shows that the main ozone sink is the soil. When the ozone concentration near ground level is around zero, the ozone profile changes slowly, even in the main vegetation layer between 15 and 25 m height. This means that the chemical destruction of ozone at night is weak in comparison with its dry deposition at ground level.

UNE METHODE DE MESURE DE LA STABILITE VERTICALE DE L'ATMOSPHERE PRES DU SOL

For a ground-level continuous release of a trace substance like 222Rn, for which the vertical profile of concentrations is exponential, a quantity, ‘equivalent mixing height’ h, is defined in this paper in the following way. If the substance were to be redistributed uniformly through a layer such that the concentration were to be equal to that at the surface, then the top of the layer would be h.Relative values of h can be obtained from surface measurements of Radon concentrations, but absolute values require estimates of the vertical fluxes of Radon. A comparison is given of the estimates of h obtained from surface measurements with estimates derived from the vertical profile of Radon from the surface to 100 m.Practical methods are proposed for estimating the surface flux of Rn.

Measurements and parameterization of aerosol dry deposition in a semi-arid area

Abstract Eddy-correlation measurements of small particle deposition fluxes were made during the STARS ∗ campaign in the Sahelian area of Niger, in November 1989. The measurements showed a significant evolution of deposition velocity throughout the day, strongly influenced both by the atmospheric stability and a high friction velocity with values from 30 to 60 cm s − . Values of the deposition velocity I d , greater than 0.5 cm s −1 were found around 0100, while measurements made in neutral and stable conditions led to values lower than 0.1 cm s −1 . These results were partly confirmed by those obtained by the variance technique. A parameterization for aerosol deposition velocity derived in terms of friction velocity and the Monin-Obukhov stability length scale L , was found to be in agreement with past results obtained by the same techniques.