Graciela B. Raga

Flood or Drought: How Do Aerosols Affect Precipitation?

Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model that explains this apparent dichotomy.

The Epic 2001 Stratocumulus Study

Abstract Overlaying the cool southeast Pacific Ocean is the most persistent subtropical stratocumulus cloud deck in the world. It produces a profound affect on tropical climate by shading the underlying ocean and radiatively cooling and stirring up turbulence in the atmosphere. In October 2001, the East Pacific Investigation of Climate undertook an exploratory cruise from the Galapagos Islands to Chile. The cruise gathered an unprecedented dataset, integrating radiosonde, surface, cloud remote sensing, aerosol, and ocean measurements. Scientific objectives included measuring the vertical structure of the ABL in this region, understanding what physical processes are determining the stra-tocumulus cloud albedo, and understanding the fluxes of heat and water that couple the atmosphere and ocean in this region. An unexpectedly well-mixed stratocumulus-capped boundary layer as a result of a strong inversion was encountered throughout. A strong diurnal cycle was observed, with thicker clouds and substantial dri...

Convective Forcing in the Intertropical Convergence Zone of the Eastern Pacific

Abstract One of the goals of the East Pacific Investigation of Climate, year 2001 process study (EPIC2001), was to understand the mechanisms controlling the forcing of deep atmospheric convection over the tropical eastern Pacific. An intensive study was made of convection in a 4° × 4° square centered on 10°N, 95°W in September and October of 2001. This is called the intertropical convergence zone (ITCZ) study region because it encompasses the eastern Pacific intertropical convergence zone. Starting from an analysis of the theoretical possibilities and a plethora of dropsonde, in situ, radar, and satellite data, it is found that newly developing convection occurs where a deep layer of air (of order 1 km deep or deeper) is conditionally unstable with only weak convective inhibition. Shallower conditionally unstable layers are associated with numerous small clouds, but do not seem to produce deep convection. The occurrence of deep convection over the ITCZ study region is presumably related to the propensity ...

Mexico City air quality: a qualitative review of gas and aerosol measurements (1960-2000)

Mexico City, one of the largest cities in the world, has a major problem with high levels of anthropogenic gases and aerosols. Some facets of this problem have been studied through measurements made during the past 40 years. These measurements are reviewed and evaluated with respect to physical processes that underlie the primary and secondary formation of gases and aerosols, their spatial and temporal evolution as well as their potential impact on the local and regional environment. Past measurements are heavily biased towards certain locations and time periods, and are of limited use for understanding fundamental processes that govern the formation and evolution of the principal pollutants. Recommendations are made whereby the measurement database could be expanded to better represent the characteristics of Mexico City gases and aerosols and to contribute to mitigation strategies that would lessen the environmental impact of these pollutants. # 2001 Elsevier Science Ltd. All rights reserved.

On the evolution of aerosol properties at a mountain site above Mexico City

Size distributions, scattering and absorption coefficients, and the bulk chemical composition of aerosols have been measured at a mountain site 400 m above the southwest sector of the Mexico City basin during a two-week period in November 1997. Variations in these properties are primarily related to local meteorology, i.e., wind direction and relative humidity; however, a link was found between carbon monoxide and ozone and the partitioning of aerosols between Aitken and accumulation mode sizes. Relative humidity was also found to affect this partitioning of aerosol size and volume. In addition, the fraction of sulfate in the aerosols was much higher on a high-humidity day than on a very low humidity day; however, the fraction of the mass contained in organic and elemental carbon was the same regardless of humidity levels. The daily variations of aerosol properties are associated with the arrival of new particles at the research site transported from the city basin and their subsequent mixture with aged aerosols that remain in the residual layer from the night before.

ON THE NATURE OF AIR POLLUTION DYNAMICS IN MEXICO CITY-I. NONLINEAR ANALYSIS

Abstract We have applied a nonlinear dynamic analysis to air quality data (ozone, carbon monoxide, nitrous oxide and sulfur dioxide) obtained at 13 of the surface stations of the automated monitoring network in Mexico City, following the work of Li et al. (1994, Atmospheric Environment28, 1707–1714). The time series used consisted of hourly data obtained during 1993. The analysis has helped to identify the nature of the dynamics of ozone production and we have determined the number of degrees of freedom of the system. Interestingly, this value varied with location within the city, ranging from two (at a station located downtown) to eight. We suggest that this difference may be linked to variations in local orography that forces a vortex-type circulation within the basin throughout the year.

Evidence for volcanic influence on Mexico City aerosols

In situ measurements of sulfur dioxide (SO2), carbon monoxide (CO) and sulfate mass provide evidence that aerosol composition in Mexico City is affected by emissions from the neighboring volcano, Popocatepetl. The data collected over a two-week period in November 1997 suggest that there are two distinct pathways by which SO2 is incorporated into particulates. Periods of high humidity, fog, and rain are accompanied by elevated sulfate mixing ratios, attributed to aqueous chemistry. Similarly, elevated sulfate concentrations in low humidity periods appear to be a result of adsorption onto existing particles. These two mechanisms are important for understanding the processes associated with particle formation in this highly polluted urban area. Under the influence of volcanic emissions, SO2 concentration can reach more than four times the monthly mean daily average (20ppbv) and particulate sulfate can be a factor of 2 larger.

Midsummer Gap Winds and Low-Level Circulation over the Eastern Tropical Pacific

Abstract The low-level seasonal and intraseasonal wind variability over the northeastern tropical Pacific (NETP), its relationship with other variables, and the connection with large- and middle-scale atmospheric patterns are analyzed using a suite of datasets. Quick Scatterometer (QuikSCAT) wind data show that the low-level circulation over the NETP is mainly affected by the northerly trades, the southerly trades, and the wind jets crossing through the Tehuantepec, Papagayo, and Panama mountain gaps. The seasonal and intraseasonal evolution of these wind systems determines the circulation patterns over the NETP, showing predominant easterly winds in winter and early spring and wind direction reversals in summer over the central region of the NETP. During summer, when southerly trades are the strongest and reach their maximum northward penetration, weak westerlies are observed in June, easterlies in July–August, despite that strong southerlies tend to turn eastward, and again westerlies in September–Octob...

On the formation of an elevated ozone peak in Mexico City

High ozone concentrations have been a major pollution problem in the Mexico City basin for at least 10 years, exceeding 110 ppbv (Mexican standard) more than 300 days per year. Aircraft observations obtained during February 1991 showed for the first time that vertical profiles of aerosol particles and ozone presented peak concentrations close to the top of the mixed layer. We hypothesize that the interplay between aerosol particles, solar radiation and photochemical processes is the key to the development of the elevated ozone peak. This peak is reproduced when a layer of highly absorbing particles, representative of Mexico City aerosols, is included in a 1-D turbulent transport and chemistry model of the boundary layer. The results suggest that the presence of large concentrations of absorbing particles within the mixed layer is likely inhibiting ozone formation that would otherwise reach even higher levels.

Science-Policy Dialogues for Water Security: Addressing Vulnerability and Adaptation to Global Change in the Arid Americas

www.EnvironmEntmagazinE.org voLUmE 54 nUmBEr 3 Climate change and watersupply uncertainty coupled with mounting human demands for water are straining the availability and quality of freshwater in much of the world. These twin forces cause a palpable rise in societal vulnerability, here considered as susceptibility to adverse effects of global environmental change.1 The vulnerability of water supplies (or, water vulnerability) places human communities at risk for exposure,2 or change,3 and thereby creates huge adaptation challenges. The actions being taken to reduce risks and capitalize on opportunities are considered adaptation or adaptive strategies.4 The most sensitive and vulnerable communities are those that face the greatest exposure and are most limited in their capacity to adapt. Rapidly growing and ever wealthier urban populations, expanding agribusinesses, diverse industries, extensive mining, power generation, and tourism often deprive water from or degrade its quality for use by marginalized populations of smallholder farmers and the urban poor, as well as for ecosystems along streams, lakes, and coasts recognized as biodiversity hotspots in the arid landscape. The arid5 Americas—as characterized by the southwestern United States, northwestern Mexico, north-central Chile and Argentina, and northeastern Brazil—manifest the just-described challenges especially well.6 This article focuses on two areas where our research team has been developing science-policy adaptation strategies: (1) the Sonora-Arizona drylands shared by Mexico and the United States (See map at right), and (2) the drylands east and west of the Central Andes in Chile and Argentina (see map, page 32). In these areas water remains acutely limited even as drought and flood extremes increase, ecosystems are under growing pressure, and economic globalization drives water demand. These global-change conditions threaten the security of access to water. Yet the foregoing conditions prevail—with little regard for constraints to supply, insufficient understanding of vulnerability, and inadequate attention to adaptive measures.7 To the extent that such problems are attributable to human agency,8 there is evidence that effective policies and actions can alleviate some of the harm.9 Our article describes two interactive Science-Policy Dialogues for Water Security: