Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

Samples of ambient air were simultaneously collected at three different sites of Mexico City in March of 1997 in order to quantify the most abundant volatile aromatic compounds and estimate the source contributions by application of the chemical mass balance model (CMB). Volatile aromatic compounds were around 20 of the total of non-methane hydrocarbons present in morning air samples. The most abundant volatile aromatic species in urban air were toluene and xylenes followed by 1,2,4 trimethylbenzene benzene ethylbenzene, metaethyltoluene, 1,3,5 trimethylbenzene, styrene, npropylbenzene, and isopropylbenzane. Sampling campaigns were carried out at crossroads, a bus station, a parking place, and areas where solvents and petroleum distillates are used, with the objective of determining people's exposure to volatile aromatic compounds. The CMB was applied for estimating the contribution of different sources to the presence of each one of the most abundant aromatic compounds. Motor vehicle exhaust was the main source of all aromatic compounds, especially gasoline-exhaust, although diesel exhaust and asphalt operations also accounted for toluene, xylenes, ethylbenzene, propylbenzenes, and styrene. Graphic arts and paint application had an important impact on the presence of toluene.

A diagnostic analysis of a Mediterranean system and the associated tropopause folding for the period 27 February to 10 March, 1987 is presented. Geopotential height, potential vorticity (PV) and relative humidity distributions were diagnosed. The analysis indicates clear correlation between the development of the cut-off low and the tropopause folding. A series of vertical cross-sections at the ends of the jet streaks demonstrated that a fold could be captured using potential vorticity and relative humidity. Q-vectors were employed to investigate vertical motion in the vicinity of the fold and showed the exact positions of descent corresponding to the fold along the entire length of the jet streak. The analysis also shows that the strong correlation between total ozone and column integrated potential vorticity holds well for all levels. As both quantities are integrals through the atmosphere this result is consistent with, but does not prove, a high independent linear dependence between ozone and PV . More case studies are needed to assure the high linear dependence between ozone and PV . The maximum transport of ozone from the stratosphere to the troposphere is coinciding with the maximum developing system, and also with the maximum values of PV .

The investigation described in the present paper is concerned with the structure of atmospheric waves in the middle latitudes. The typical wave in the troposphere has, according to observational studies, a difference between the thermal field and the geopotential field in such a way that the trough and the ridge in the temperature field are located to the west of the corresponding curves for the geopotential field. A theory using a linear version of the two-level, quasi-nondivergent model including heating and dissipation is developed to explain these observed facts. Such a model will contain the possibility of baroclinic instability for certain wavelengths for a sufficiently large vertical windshear. It is thus necessary to treat the stable and the unstable cases separately. The applied techniques include both a numerical integration to steady state in the stable cases and a detailed treatment of the perturbation equations in the unstable case. A calculation of the vertical velocity is included in order to show that these waves convert eddy available potential energy to eddy kinetic energy. The model equations are simplified in such a way that the independent variables are the variations both in the zonal direction and in time. The model permits a calculation of the meridional transport of sensible heat, but due to its simplicity the meridional transport of momentum is excluded. The calculated structure of the wave is in good agreement with the observed atmospheric waves in middle latitudes.

A Fourier Transform Infrared (FTIR) spectrometer was used to analyze the composition of ambient air at a specific site in the Mexico City metropolitan area. A continuous flow of air was passed through a multi-pass gas cell and the absorption spectra were collected over a period of two weeks Quantitative analysis was performed by means of the classical-least square (CLS) method using synthetically generated spectra as references and calibration sources. Ambient levels of CO, CO2, CH4 and N2O are reported with a time resolution of five minutes for September 2001, showing interesting results in their diurnal patterns Comments on the precision, detection limits and signal-to-noise of the instrument are included for the evaluation of this technique. Water concentrations were estimated and compared with those obtained with a relative-humidity sensor. The technique of extractive FTIR for ambient trace-gas monitoring was utilized in Mexico for the first time and some potential applications are given.

In this paper an objective analysis scheme using a three–dimensional numerical variational technique has been developed over India and the surrounding region based on the assumption of quasi-geostrophic and thermal wind conditions. Analyses of height, wind and temperature fields at different pressure levels have been made for the depression case which is formed over the Bay of Bengal during the period 26 to 31 July 1991. The analysis scheme is able to capture the system and to bring out the importance of upper tropospheric warming before the formation of the monsoon depression.

A bistatic continuous wave Argon ion lidar system has been used to collect information on the variation of laser return signal strength from the lower atmosphere with scattering angles at a fixed altitude of scattering. Lidar data collected on 50 clear sky days during the period of April 1987 - December 1995 at a continental tropical urban station have been used for the study. The experimental data have been compared with the theoretically computed differential Mie scattering cross sections at the common scattering angles to determine the most probable value of size index, n, and the complex index of refraction, m, applicable to the environment over the observing site by adopting a library-search method and simple statistical tools. The study showed that the most frequently occurring aerosol type at the location is dust-like with a most probable value of size index 4.5. Retrieval of aerosol number concentration, using this information together with the experimental values of scattered signal strength, yields an average concentration value in the range of 2 - 3 x 103 cm-3 in the atmospheric layer close to the surface.

An objective classification of sequence patterns of 1000 hPa and 500 hPa geopotential heights associated with cold surges over Central Argentina was carried out. Polar Outbreaks during wintertime in Central Argentina (May to September) cause important damage to the regional agriculture and sometimes affect the energy supply. In particular, the consumption of natural gas is strongly dependent on significant temperature descents. The rotated Principal Components Analysis (PCA) was used to obtain the basic patterns of sequence of 1000 hPa and 500 hPa geopotential heights. The winter cold surges during the 1979-1993 period occur in seven patterns at 1000 hPa and three modes at 500 hPa. The combination of both levels shows three typical situations related to cold surges over Central Argentina. The most classic pattern shows an important ridge west of the Pacific coast of the continent at its upper level and a postfrontal anticyclone at its lower level producing cold air advection over Central Argentina. The other two patterns present a long wave trough at upper levels affecting the continent, one of them with the postfrontal anticyclone moving over middle and low latitudes, and the other with the migratory anticyclone affecting lower latitudes.

This study presents a synoptic and dynamic analysis of a cyclogenesis event that occurred between July 8 and 9, 1996, over Uruguay, in the region of maximum frequency of cyclogenesis as detected in previous studies. The study is based on isobaric and isentropic analysis of meteorological variables, and also, on potential vorticity perspective. The development of the cyclone occurred as a pre-existent midtropospheric cut-off low, over the Pacific Ocean, which propagated crossing over the Andes at the same time as a strong thermal advection was observed to the west of the mountain ridge. The synoptic analysis of this case revealed that this event can be “classified” as a lee cyclogenesis, since the existence of the mountain barrier was fundamental for the intensification of the surface baroclinicity, which, in turn, led to the formation of the extratropical cyclone. Also, the Andes defined the vertical tilt of the system to the west, favoring the cyclonic vorticity advection at higher levels above the warm thermal advection at low levels, and it defined the final baroclinic configuration that fed the system in its greater development stage. By using potential vorticity inversion, it was possible to evaluate quantitatively the relative contributions of upper- and low-level potential vorticity, as well as bottom temperature anomalies. It was observed that the bottom temperature anomaly was important in the initial development stages, contributing to almost 100% of the total circulation. As the surface cyclone deepened, the contributions of low- and upper- level anomalies became greater. In the stage of maximum development of the system, a phase-locking at low and upper levels seems to occur, where the upper-level advection of cyclonic vorticity, inducing northeasterly flow at surface, had the greater contribution.

The evolution of tropical cyclonic vortices on the eastern North Pacific is examined by means of a barotropic model with an idealized continental topography. The aim of the study is to investigate the trajectories of cyclones in this area affected by both the topographic and the planetary β effects. The topographic β effect is mainly due to the ascending slope of the orography, and induces the vortex to drift towards local “northwest” direction, which coincides with the geographical northwest (because of the topography orientation). As a result, the vortex drift is clearly enhanced when both effects are considered. The precise direction of the trajectory depends on the initial geographical position with respect to the continent. Vortices initialized at southeastern areas (around 12º N, 95º W) are deflected by the Sierra Madre del Sur more to the west, following a trajectory almost parallel to the continent. For vortices initialized at 15º N or more, their drift is mainly due to the planetary β effect, although eventually they are attracted towards the Sierra Madre Occidental at higher latitudes. These conclusions suggest the possible influence of orography on the trajectories of real tropical cyclones in this area.

Atmospheric electric activity measurements have been recorded continuously by a punctual lightning system at a tropical highland observatory from 1988 – onwards, and were analyzed to obtain lightning statistical confident results for thunderstorms occurrence on the leeward side of the southern mountain ridge of México’s Valley. Shown, as examples, are individual profiles of the atmospheric electrical activity, associated with severe storms. The results make clear that the fastest possible sequence of electrical measurements is required to obtain significant and application oriented data in connection with a whole series of thunderstorms taking into account the mean time variation of the atmospheric electricity measurements at an altitude of 2270 m a.s.l. The seasonal variation indicates that the lightning flash peak currents were found to be larger in summer with less than 10% occurring in the autumn and winter. With rainfall data from a network of 66 stations, we obtained a significant correlation with the lightning frequency. Special attention was undertaken concerning the question of the atmospheric electrical activity and climate at the Valley of México.