Ana Graciela Ulke

New turbulent parameterization for a dispersion model in the atmospheric boundary layer.

A modeling approach for the dispersion of pollutants released in the atmospheric boundary layer is presented and evaluated. The model includes a continuous formulation for the transport and turbulent diffusion processes that adequately represents the mechanisms in the various regimes of the atmospheric boundary layer. The vertical diffusion coefficients consider shear and buoyant-induced turbulence in a well-behaved functional dependence. The suggested eddy diffusivity and wind speed profiles compare adequately with those reported in the literature and observational data. The evaluation of the proposed dispersion model as well as an independent method, against data from diffusion experiments, showed a satisfactory physical behavior and a good overall statistical performance of each model. An improvement is introduced over the alternative approach with the suggested continuous formulation with similar performance.

Distribution and temporal behavior of particulate matter over the urban area of Buenos Aires

Abstract This paper contributes for the first time in Buenos Aires city to the temporal and horizontal distribution of the PM 2.5 and PM 10 concentrations. Their variations and relationships with relevant variables that characterize the air pollution potential of the urban air shed are also given. The measurements were carried out for one year from May 2006 to May 2007. During this period, particulate matter (PM) was continuously measured at one reference station and alternatively for shorter time periods at six different sites. The values and temporal variations on a daily and seasonal basis were consistent with the ventilation potential of the atmosphere. The cold season, which had the lowest values of the ventilation coefficient, indicated higher probabilities of poor air quality and this was confirmed by the higher concentrations of PM 10 and PM 2.5 measured. At the reference station, the daily EU limit value for PM 10 was exceeded 36 times during one year while the Buenos Aires limit value was exceeded only once. The PM 10 annual mean value was almost 70% of the Buenos Aires annual limit. The PM 2.5 annual mean value (15 μg m -3 ) was same as the regulated one (15 μg m -3 ). The correlation between PM 10 and PM 2.5 concentrations and frequencies of wind directions showed that the highest concentrations were observed when the wind was from the city (land wind) and lowest concentrations when the wind was from Rio de La Plata (fluvial wind). The concentrations during land wind events exceeded the Buenos Aires PM 2.5 annual limit value. The ratio of PM 2.5 to PM 10 was 0.44, which indicates the coarse particles (>2.5 μm) originated from road dust, soil re-suspension and abrasion processes are the dominated fractions of PM. Results of random PM measurements at 60 sites showed that PM 2.5 was more homogeneously distributed over the city than PM 10 .

Assessment of the unified analytical solution of the steady-state atmospheric diffusion equation for stable conditions

In this work, the performance of a unified formal analytical solution for the simulation of atmospheric diffusion problems under stable conditions is evaluated. The eigenquantities required by the formal analytical solution are obtained by solving numerically the associated eigenvalue problem based on a newly developed algorithm capable of being used in high orders and without missing eigenvalues. The performance of the formal analytical solution is evaluated by comparing the converged predicted results against the observed values in the stable runs of the Prairie Grass experiment as well as the simulated results available in the literature. It was found that the developed algorithm was efficient and that the convergence rate depends on the stability condition and the considered parametrizations for wind speed and turbulence. The comparisons among predicted and observed concentrations showed a good agreement and indicate that the considered dispersion formulations are appropriate to simulate dispersion under slightly to moderate atmospheric stable conditions.

Biomass Burning in South America: Transport Patterns and Impacts

Ana Graciela Ulke1, Karla María Longo2 and Saulo Ribeiro de Freitas3 1Departamento de Ciencias de la Atmósfera y los Océanos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires 2Divisão de Geofísica Espacial, Instituto Nacional de Pesquisas Espaciais, São José dos Campos, São Paulo 3Centro de Previsão de Tempo e Estudos Climáticos, Instituto Nacional de Pesquisas Espaciais, Cachoeira Paulista, São Paulo 1Argentina 2,3Brazil

A case study of biomass burning and its smoke dispersion to Buenos Aires City, Argentina

In February and March 2000, several uncontrolled forest and grassland fires occurred in the municipalities of Campana and Zarate (100 km north of Buenos Aires, Argentina). The biomass burning emitted a large amount of smoke particulates, which caused dense fog and visibility impairment in the nearby area. From 18 to 19 March, the smoke was transported towards the Buenos Aires Metropolitan Area (BAMA), resulting in a prolonged reduction of visibility. This feature was supported by a build-up of the Aerosol Optical Thickness (AOT) and deposited particulate matter mass observed in BAMA. This paper examines the prevailing meteorological situation that produced the smoke transport towards BAMA. An anticyclone, displaced easterly over the interest area by a frontal low-pressure system, produced low-level ventilation conditions that favoured the transport and the smoke persistence in BAMA. The transport of particles and the behaviour of their normalised concentrations were simulated adequately by a regional dispersion model.

Modelling Photochemical Air Pollution in SÃo Paulo, Brazil

area, with 5km resolution. The vertical extent was 1100m, divided in five cells with varying depth. Meteorological and air quality fields were generated based on measurements, applying objective analysis procedures. The wind field on afternoon hours of the final day of the simulation along with the topography is depicted in Figure 1. Different emission scenarios were considered in order to analyze uncertainties in the emission pattern. There is a lack of accurate information related to the emissions in Sao Paulo. The 1989 actual fleet scenario (AF) considered the official emission inventory (CETESB, 1990). Alternative scenarios duplicated the nitrogen oxides contributions (NOx2) and the hydrocarbon emissions (HC2). Figures 2 to 4 present the predicted ground level ozone concentrations for Feb. 19, 15 LT. The urban core of Sao Paulo exhibited inhibition of ozone formation due to high nitrogen oxides levels, while the suburban area showed larger ozone concentrations, the product of photochemistry and transport. The ozone levels predicted by the NOx2 scenario were the smallest. The HC2 scenario led to the greatest downwind ozone concentrations. This alternative resulted also in a satisfactory agreement with observed values. The application of the model demonstrated the need of an accurate emission inventory for improved predictions of the pollutant concentrations. An alternative approach for the turbulence parameterization was introduced in the model. The scheme gives a continuous transition between the different regimes in the atmospheric boundary layer (Ulke, 2000). Figures 5 and 6 show the evolution of the vertical distributions of ozone during the two final days of the period, obtained with the original and the alternative schemes. The selected grid point (X=27.5, Y=15) represents a typical urban location. The alternative parameterization led to greater concentration levels and less uniform distributions.

Aerosol Load and Characteristics in Buenos Aires: Relationships with Dispersion Mechanisms and Sources in South America

The aim of this contribution is to study the aerosol load in Buenos Aires in order to help characterize the contribution of distant sources and the role of the flow patterns in Southeastern South America (SESA) as dispersion mechanisms. The regional pollution in SESA is mainly due to biomass burning and the related smoke plume could be frequently observed by remote sensors from space. Observations and modeling results are combined to assess the relative importance of the biomass burning contribution to the aerosol pollution in the megacity. Buenos Aires aerosol load and derived quantities have episodic contributions from biomass burning. An increase in aerosol optical depth and Angstrom coefficient and an impact in the particle size distribution are observed. A dispersion model coupled on-line with a regional atmospheric model is able to reproduce the plume pattern and evolution and the mean aerosol load.

Analysis and modelling of turbulent fluxes in two different ecosystems in Argentina

Turbulence data measured with fast response instruments at two sites with distinct conditions were available. The ability of a mesoscale model in the simulation of turbulent fluxes and boundary layer height was assessed in case studies covering periods in the Southern Hemisphere summer and winter. The energy apportionment at both locations was properly reproduced. In summer, at the site with native forest, the sensible heat flux was well estimated and the latent heat flux was underestimated. At the other site (commercial plantation) the sensible heat flux was properly represented and the latent heat flux was slightly overestimated. In winter, the best results were obtained at the implanted forest site. The performance of the model at the native forest site was very satisfactory when homogeneous initial soil moisture was considered. A model intercomparison between the boundary layer height obtained with an analytical model and the mesoscale model showed the influence of data and methodologies used in the two approaches.

Evaluation of the influence of micrometeorological data on concentration estimates

A dispersion model for continuous releases of pollutants in the atmospheric boundary layer is applied to study the influence on modelled concentrations of different estimates of the key micrometeorological variables that represent the dispersion processes. The modelling approach is based on the advection-diffusion equation and includes profiles of wind and eddy diffusivity for the entire atmospheric boundary layer. Their functional forms depend on relevant parameters that represent the turbulent mechanisms in a wide range of stabilities. The Prairie Grass experiments were considered for the evaluation. The model performed satisfactorily in the estimation of the near surface crosswind integrated concentrations. The distinct micrometeorological parameters caused, in general, minor differences in the concentrations. However, a greater impact was found during very stable or unstable conditions. In consequence, in these particular situations, the methodologies to obtain the micrometeorological parameters must be carefully analysed.

A Dispersion Model Evaluation for Buoyant Releases

The application of a dispersion model of pollutants released in the atmospheric boundary is presented. Based on the bidimensional semiempirical equation, the model incorporates the current understanding of the atmospheric boundary layer dynamics to estimate and parameterize the atmospheric stability, advection, turbulent diffusion and plume rise. The model performance has previously been analyzed against experimental data obtained in non-buoyant releases with a data base covering wide parameters ranges, and showed a correct behavior1. The present evaluation involves comparisons of model estimates with measurements from studies with buoyant releases from elevated sources in flat terrain in urban areas (Indianapolis, USA) in different stability conditions2. The comparison with tracer data is done for the predicted concentrations with the proposed model (mm) and with an updated Gaussian plume model (gm). All concentrations were normalized by source strength. Only quality 3 data (Q3) have been used. The results here include urban effects on the specification of stability. Predicted normalized crosswind-integrated, maximum arcwise and centerline concentrations were compared with observations. Current quantitative measurements and techniques of model evaluation were obtained and applied3,4. The comparison was done with the full dataset and for the data stratified by stability conditions. Confidence intervals on these performance measures were determined by the bootstrap resampling procedure. The obtained statistics for the maximum arcwise concentrations are shown in Table 1.