INTRODUCTION
Investigations about urban climate conducted since the seventies in many North American and European cities have revealed new urban ecological aspects (Kuttler 1988; Jendritzky 1991; Jendritzky et al. 1994; Landsberg 1981; Oke 1986,1987; Wanner 1990). In Latin American countries, however, where the explosive population growth has led to a rather uncontrolled horizontal and vertical expansion of the urban morphology, long data series and detailed information are still rare. Pollution effects have been investigated only in a few large cities, such as Mexico City, with focus on the energy balance (Oke, Zeuner and Jauregui 1992), the urban heat island (Jauregui 1986; 1993), bioclimatic conditions (Jauregui 1991; Jauregui, Cervantes and Tejeda 1997), and air pollution (Jauregui 1989), as well as on the influence of parks on temperature and precipitation (Jauregui 1990/91). The urban climate of the tropical mega-city of São Paulo has been surveyed by Lombardo (1984). In Santiago de Chile, several studies on local climate and pollution problems have been conducted over the past decade (Aranda and Romero 1988; Ihl, Romero and Rivera 1995): regional and local circulation patterns as well as weather types were established and smog problems pinpointed. In the extensive study of Romero, Rivera, Salazar, Ihl and [end p. 61] Azocar (1996) measuring drives, air photography, satellite imagery, and geographic information systems were used in order to relate the topography of the Santiago basin, landuse changes, and urban growth to climatic parameters, such as surface radiation temperatures measured by satellites. These studies demonstrate the seriousness of the smog problem in Santiago de Chile, especially during thermal inversions in winter, and the importance of sea and mountain breezes for the reduction of heat stress and air pollution in the Santiago basin. Since the publication of the status report edited by Sandoval, Prendez and Ulriksen (1993), measures to improve the pollution situation in Santiago have been taken. Recently, Ihl (1998) completed a comprehensive study of the summer smog situation in which he also stresses the importance of the local wind systems at the Andean foothills. Tethered balloon measurements up to 200 m a.g.l. confirmed the changes in wind direction at the entrance of the Mapocho River into the basin.

In Argentina, detailed long term studies of urban climates are scarce. A few short comments about temperature and atmospheric stability conditions in the large agglomeration of Buenos Aires are given by Camilloni and Barros (1994) and Ulke and Mazzeo (1996). Navas (1998) published the results of several measuring drives through the city of San Juan. More information is available on the coastal city of Bahía Blanca (Capelli et al. 1985; 1986; 1995) and on Tucumán, the regional center of Northwest Argentina (Endlicher and Wirschmidt 1993, 1995; Endlicher and Schultz 1996; Endlicher and Hernández 1998). The latter focused on the urban heat island, local wind systems, and air pollution by suspended particulate matter. The instrumentation and methodology were similar to the Mendoza project reported here. With the exception of a few general observations on air pollution in Mendoza (Mannino 1992) and preliminary results of a larger environmental project concerned with tropospheric ozone (Grosser 1997), no detailed studies on urban climate and air pollution exist.

PURPOSE OF THE PROJECT
Mendoza is situated at the eastern foothills of the Andes (750 m. a.s.l.) at 33°S. It is a provincial capital of about 600,000 inhabitants, the center of Argentine viticulture and core of the Cuyo region. To date, climatological investigations have addressed regional problems (Capitanelli 1967) or been centered on agroclimatology (Denis 1968). On a regional scale, it would be useful to compare Mendoza with other Argentine cities, such as Bahía Blanca on the Atlantic coast or Tucumán in the summer-rain subtropics of the Northwest, so as to get a clearer picture of the atmospheric environment in other major cities of the second largest country in South America. It would also be interesting to compare the cities of Mendoza and Santiago on opposite sides of the Andes: What about the air pollution levels? What about the katabatic winds, that are funnelled through the Mapocho valley in Santiago, but not to the same extent through the Mendoza River valley? Are there anabatic winds in Mendoza of the same magnitude as the sea winds in Santiago? Finally, an effort should be made to relate air pollution levels to local circulation systems, at least in some prominent areas of the city, and to get a closer look at conditions in the city center -- where most of the population lives and works -- so that city planning can be put on a sound ecological footing.

At present, Mendoza has two reliable meteorological stations, one in the large city park General San Martín, west of downtown, and the other at the airport to the north. While none of the stations provide specific information about local climate in other parts of the city, especially not downtown, their measurements can be used to compare the microclimate of the park -- located on the glacis to the west -- with that of the plains that extend to the east and north (Figure I).

INVESTIGATION METHODS
From July of 1995 to December 1997, the urban heat island, local breezes, humidity conditions, inversion layer, and air pollution in Mendoza were investigated using different observational methods that included, (i) a special network of 4 automated and 3 conventional weather stations, (ii) 8 passive air pollution detectors SIGMA-2 that were monitored on a weekly basis, (iii) measuring drives along two transects across the city, one from west to east, and the other, from north to south, and (iv) soundings from a tethered balloon. A detailed description of the observation procedures can be found in Endlicher et al. (1998). [end p. 62]

[end p. 63] Since it was not possible to investigate all the contributors to air pollution, the study concentrated on suspended particulate matter, which seems to be the most widespread problem (Schwela 1995). The data were compiled in reference to the main weather types that occur in the Cuyo region. The typical weather situations are taken from daily weather maps of the Southern Cone released at surface level and at 500 hPa level. In this manner, particular conditions detected during field observations were related with a particular weather type, which allows one to draw more sensible conclusions than those which could be drawn from the relatively short measuring period of two and half years.

The results of these urban climate observations pertain to three categories of lower atmospheric conditions: one concerning the characteristics of the thermal layering over the city (urban heat island); a second, related to the local wind circulation; and a third, pertaining to the levels of suspended particulate matter in different city sectors. Conclusive results of the entire 30-months-study will be published later this year (Endlicher and Mikkan 1999, in preparation). This paper underscores the importance of the different weather types for the urban climate of Mendoza; it details the results that were obtained with the application of the observational methods, and makes cross-references to other Argentine and Chilean cities.

THE CITY AND ITS REGIONAL CLIMATE

The climate of the Cuyo region at latitude 33°S is dominated by anticyclonic conditions, although the effects of winter depressions from the Pacific are considerably reduced by the city's location in the rainshadow of the Andes. Meteorological data from the Observatorio Meteorológico Nacional in the General San Martín Park reveal a semi-arid climate with a mean annual precipitation of 232 mm and a pronounced summer rain regime in December, January, and February (Endlicher 1998). In the summer, the city enjoys some 300 sunshine hours a month, and absolute maximum temperatures of 40°C can be reached. In winter, particular weather events may raise temperatures as high as 30°C, while the lowest values approach 0°C, and killing frosts may occur in certain areas. Generally winds blow from east and south and are weak, but when coming from a westerly direction they are strong.

Nocturnal boundary layer inversion occurs most of the year, except during the few days of zonda trans-mountain wind. The rare rain events are not sufficient to clean the urban atmosphere through wash-out, and air contamination has become a serious problem for Mendoza.

WEATHER TYPES
The climate of Mendoza is determined by five fundamental synoptic situations:

1. The most frequent situation is that of the subtropical high dominating. During this so-called autochtonous weather type dry and calm conditions prevail, radiation processes are strong, and local beezes are well developed (Figure 2b).
2. In periods when strong low pressure systems develop over the Southern Cone, a strong meridional gradient at 500 hPa generates the zonda, a dry foehn-wind that blows from western directions, especially in winter (Figure 2a).
3. The South Atlantic High and a shallow thermo-orographic low over northwestern Argentina are responsible for the nordestadas, light winds from the northeast that bring in humid air masses in the summer and dry continental air in the winter (Figure 2c).
4. From a transient high pressure system over Patagonia may originate the sudestadas. These southeasterly flows, that run opposite to the constant upper air westerlies, carry maritime air from the South Atlantic and produce abundant orographic rainfall over the Andean foothills (Figure 2d).
5. . Finally, a large upper air trough over the South Atlantic generates southerly to southwesterly flows, the surazos , which represent dry sub-antarctic/ Patagonian air invasions. This weather type brings cold temperatures to the subtropics and may occur in association with strong pampero-cold fronts (Figure 2e).

   In Figure 1 are schematized the main effects that these synoptic situations and wind directions have on the climate of Greater Mendoza. The peculiarities of the urban climate of Mendoza will be discussed on the basis of these regional weather types as they determine particular local circulation systems. [end p. 65]

   THE URBAN HEAT ISLAND
   Thermal differenciation between a city and its surroundings is one of the basic tenets of urban climatology, but the specific thermal conditions of the various city sectors and suburbs -- which depend on the built-up structure -- are also important. In the subtropics, summer temperatures are especially crucial, since they constitute a severe human stress factor (Mayer and Hoppe 1987; Oke 1986), as do humidity, global radiation, and wind velocity. The influence of plazas and patios -- specific housing features in the subtropics of South America -- must be investigated as well as the influence of the topography, parks, and gardens (Givoni 1989). The 512 hectares area of the General San Martín Park certainly plays an important role in the generation of the local climate of western Mendoza.

   

   To approach the question of urban heat island formation and dissipation, the hourly temperature data of the city center station Casa del Maestro were compared with the data of the vineyard station of Bodega Santa Ana in the east, and measuring drives were conducted through the Greater Mendoza area. Figure 3 shows the mean daily range of the urban heat island during an autochtonous weather situation (Aug. 26- 29, 1996): maximum values reach 3 - 4 K during night time; in the morning hours the heat island dissipates and, due to the shading effect of the built-up, is replaced by what one might call a cool island when compared to the open vineyards.

   Measuring drives during similar weather conditions revealed a more detailed picture of the nocturnal situation as depicted in the W-E and N-S profiles (Figure 4). On the morning and evening drives, the urban heat island was clearly defined. The lowest temperatures, near dewpoint, were recorded at the airport and in the vineyards north and east of the city. In the cultivated fields of Godoy Cruz and in the General San Martín Park, temperatures were also low due to the night-time inversion. At Cerro de la Gloria hill, west of downtown, temperatures were higher than in the eastern and northern plains, because the hill rises above the local cold flow. The values of the heat island in the downtown area lie between 5 and 8 K, which is quite high for the winter season. Mountain breezes from southwesterly and northwesterly directions with velocities of 2 - 4 m. sec-1 were clearly recognized -- important clues for understanding aeration conditions in the built-up city area. The differences at the four ends of the profiles stem from different topoclimatic situations: to the north and east, the city borders on the large Monte-Pampa-plain which is blanketed by a huge mass of cold air. To the west and south, the glacis and the alluvial fan of the Mendoza River lie in the warm slope zone above this ground inversion layer. At night, the cold air imponded there slides downslope as a katabatic breeze.

   The intensity of the Mendoza heat island is influenced by the various weather types that occur in the region (Figure 5). During the autochtonous weather situations, the heat island is well developed. The wave likei> zonda-event leads to major differences between the city center -- situated in the path of the warm foehn air -- and the eastern surburbs in the cold air beneath the shallow inversion layer. During the rainy sudestada weather, the heat island tends to dissipate. The humid/warm air masses provoke strong atmospheric counter-radiation, thus diminishing the thermal differences between the city and the vineyards to the east. During episodes of dry Patagonian air predominance (surazos), the heat island may be intensified, but the associated strong pampero -like winds tend to sweep away the microclimatic conditions.

   LOCAL WIND SYSTEMS
   Due to the combined effects of the city's location on the precordillera glacis, the relief of the [end p. 66]

   

   [end p. 67] Andes, the proximity of the Mendoza River valley, and the prevalence of anticyclonic weather types, circulation systems, such as slope winds and mountain and valley breezes, are well developed and play an important role in determining the local climate and air pollution levels.

   Soundings from the tethered balloon obtained November 20, 1995 (Figure 6) show the normal adiabatic lapse rate and an afternoon temperature of 33°C at ground level. The three evening measurements corrobate the formation of an inversion layer and stable conditions in the lower 500 meters of the urban boundary layer at night. In the course of the afternoon, the wind changed from a northern upslope direction to the nighttime westerly downslope direction in the lower 100 meters of the profile. This mountain breeze reached velocities beween 2.5.-3.0 m.sec-1.

   The daily change in wind direction is a typical process in the Andean foothills (Figure 7). For the summer of 1996-97, the frequency distribution of the wind directions in southern Mendoza is shown at hourly intervals. The pattern of dominating northeasterly winds during the day and westerly winds during the night is observed not only during the summer months, but in all seasons. The local flows can be differentiated into daytime upslope and nighttime downslope winds (drainage flow); the latter are often reinforced by western synoptic winds from the Mendoza River valley (mountain breeze). In special winter situations, the trans-mountain wind arrives in Mendoza as the famous zonda. The nightly mountain breezes are generally welcome for their cooling effect on the overheated city center in the summer and as providers of clean air for western Greater Mendoza. The eastern sector of the city, however, usually stays under the inversion layer, engulfed by stagnant polluted air. Relief is provided only by the sudestadas and surazos.

   

   AIR POLLUTION FROM SUSPENDED PARTICULATE MATTER
   Mendoza is not only a residential and service center, but also an industrial center. A concrete plant in the north and an oil refinery in the south contribute to air pollution, as does the exhaust from individual cars and public transportation vehicles, including a large fleet of diesel powered buses that release into the air soot and rubber fragments originating from tire wear. Due to the arid surroundings of Mendoza and poor aeration conditions, the natural dust burden can also be expected to be high.

   Air contamination levels of Greater Mendoza were quantified in a two-year measuring campaign [end p. 68]

   

   [end p. 69]
   

   

   in which airborne supermicron particles were sampled at eight sites (Schultz 1990). Analysis of the particles obtained via the passive sampling technique Sigma-2 confirmed a high natural dust load, which was further found to be considerably compounded by traffic-induced particle resuspension, especially in the coarse particle range above a 12 m diameter. Particle emissions from the concrete plant north of the city and from secondary industrial establishments and agricultural activities in the east were detected as additional anthropogenic pollutants of major importance, mainly in the fine particle range below 12 m. Air quality in western Greater Mendoza benefits from the local wind system advecting air from the largely uncontaminated mountain area during the night, a process which, however, pushes polluted air into the eastern suburbs. Ion-chromatographic analysis has identified a high sulfate content in particle depositions. But since it was found that these particles correlated mainly with dust components, no serious health effects are feared. Still, the European PM 10 standards for suspended particulate matter are exeeded by a factor of 2 all over the Greater Mendoza area, something which is not to be taken lightly, not even in a semi-arid environment.

   The most polluted sites are located in the city center (Casa del Maestro) and in the northern suburbs (Las Heras), Figure 8). Dangerous levels of the black carbon fraction within the total dust load occur at the Casa del Maestro station, whereas in the eastern winegrowing area (Bodega Santa Ana), where the [end p. 70]

   

   [end p. 71]

   

   [end p. 72]

   dust load is also heavy, there are much fewer black carbon particles than organic particles. This part of Greater Mendoza is not within reach of the nocturnal downslope winds and often stays underneath the inversion layer. The San Juan values, measured in a typical backyard (patio) near the Casa del Maestro station right in the center of Mendoza, are clearly lower than those of the high-traffic area just in front of the station. The lowest particle load was found at the two stations on the western glacis (General San Martín Park and Campus of the Universidad Nacional de Cuyo). At night, both are frequently affected by katabatic drainage flows that bring in less polluted air. Here, the dust load is lessened significantly by the dense vegetation of the large Park, while the open Campus does not seem to have much impact.

   In order to discern the influence of weather types on contamination levels, the values of all the stations' weekly means were compared with those of the entire study area [1= mean value, 2 = double value] (Figure 9). Certainly, weekly mean values allow only a coarse relation to weather types and the findings must be regarded with caution. On the other hand, the 24-months measuring period is long enough to exclude purely accidental results.

   Rainy sudestada weather during weeks 31 and 50 of 1996, and 42 of 1997, lowered contamination values in the week following, especially among transparent particles, since the wash-out effect of rain lasts longer on natural, transparent particles than on anthropogenic black carbon. Also, coarse particles are more sensitive to wash-out than are finer ones, while N02 values do not react to rains at all.

   Surazo weather (e.g. weeks 5 and 36 of 1996; 6 and 26 of 1997) brings dust-rich air from Patagonia, which resulted in very high particle load measurements during those weeks. Especially pronounced was the influence of a pampero-front in week 25 of 1997, which also showed high black carbon particle levels. This means that even the dangerous fine black carbon fraction from anthropogenic sources, such as vehicle exhaust, is taken up by those high winds and redistributed. The zonda trans-mountain wind, on the other hand, which occurred in weeks 41 and 46 of 1997, caused generally higher levels of coarse transparent particles, but those were still below the values reached in the wake of surazos and nordestadas.

   Maximum fine-dust contamination is related to weather dominated by northerly flows, more common in Mendoza's dry seasons, autumn and winter (e. g. week 37 of 1996, and weeks 19 and 32 of 1997. High pressure conditions are characterized by weak regional, and well-developed local winds, during which fine dust pollution values are average, and N02 values high, due to low mixing and diffusion rates (e.g. weeks 13 and 35 of 1997).

   CONCLUSION
   From a regional perspective, the urban climate and air quality of Mendoza are determined by the city's location in the wind shadow of the Andes and the predominance of high pressure weather types most of the year, with zonda-and sudestada-weather occurences in winter and some nordestada influences in autumn (Figure 9). Still to be determined is the frequency with which the weather types occur that induce the formation of the inversion layer.

   At the local level, orographically enhanced breezes contribute to attenuate human bioclimatic conditions and air quality. These modifications, however, are socio-economically specific in that they proceed from the fact that Mendoza is subdivided into western suburbs with high quality air conditions and eastern suburbs with low quality air conditions and high pollution levels. When comparing the pollution levels of Mendoza with those of other Latin American cities in similar locations, those of the large metropolises of Mexico City and Santiago de Chile are worse (Figure 10). Mendoza is only a provincial capital at the moment, but its strategic location on an important axis of the MERCOSUR that connects Santiago with Buenos Aires and São Paulo, will stimulate its further growth. In comparison with Bahía Blanca or Buenos Aires on the ocean, Mendoza has moderate to severe contamination problems, similar to those of Tucumán, further north. In northwestern Argentina, however, wash-out events are much more frequent than in the arid climate of the oasis of Mendoza. Still, even under such favorable circumstances, the pollution level of Tucumán exceeds considerably the values of Central European cities (Endlicher and Schultz 1996). The alarming results of Ihl (1998) concerning the summer pollution level in Santiago de Chile lead one to suspect similar problems east of the Andes, but this still needs to be [end p. 73] confirmed by special investigations of tropospheric ozone formation in Greater Mendoza.

   

   There is a great urgency that the administrations of the City and Province of Mendoza should increase their efforts to actively control air pollution and improve the urban bioclimate. More effective emission control, for example, would reduce the highly problematic source of particle contamination as well as photochemical smog. For now, Mendoza is still a favorite tourist center, reputed to be one of the "greenest" cities of Argentina. This attraction may fade if air pollution increases and ecological city planning fails to achieve major priority.

   ACKNOWLEDGEMENTS

   The authors thank the DAAD, DFG, DLR, GKSS, GTZ, SMN, and UNC for funding and data supply, and their Argentine colleagues Moira Alessandro, Raul Mikkan, and Marcela Polimeni for the field work. Mrs Christiana Donauer-Caviedes reviewed the English version.

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   RESUMEN
   En el oasis piemontano de Mendoza (33°S) ubicado en Argentina occidental se han conducido investigaciones de clima urbano y contaminación atmosférica. Objectivo de estos estudios ha sido la investigación de la isla de calor urbano, sistemas locales de vientos, y contaminación por partículas suspendidas. Siete estaciones climáticas fueron establecidas, transectos fueron ejecutados y sondajes fueron conducidos. Muestras de partículas en suspensión fueron obtenidas en placas colgantes expuestas al aire. La isla de calor urbano es detectable durante la noche, pera desaparece durante las horas de día. El sistema local de vientos se caracteriza por brisas nocturnas de montaña que bajan de la precordillera al oeste de la ciudad. El análisis de partículas en suspensión muestra un alto contenido de polvo incluso durante los periodos de lluvia invernal. Fragmentos de caucho (neumáticos) y restos de combustión fueron detectados en cruces de gran congestión, revelando la influencia del tráfico motorizado sobre la calidad del aire. [end p. 76]