Photochemical smog evaluation in an urban area for environmental management

Transcription

1 ISEE/RC'1 Fifth International Conference of the International Society for Ecological Economics (ISEE) Russian Chapter (Russian Society for Ecological Economics - "Ecological Economic Management and Planning in Regional and Urban Systems" Institute oc Control Sciences, Russian Academy of Sciences, Moscow, Russia, September 26-29, 1 Photochemical smog evaluation in an urban area for environmental management T. Tirabassi 1, M. Deserti 2, L. Passoni 2, A. Kerschbaumer 2 1 Institute ISAO of C.N.R., Bologna, Italy 2 ARPA - SMR, Bologna, Italy Abstract Two photochemical models were applied to the area of Bologna (Italy). A simple single cell variable volume flow reactor (PBM - Photochemical Box Model) was employed to simulate the relationship between pollutant emission and ambient air quality. The model represents the Bologna area as a single cell km in both length and width, with variable height representing the changing depth of the mixed layer. Moreover, the photochemical Eulerian CALGRID model was applied to predict the hourly, three-dimensional concentration fields for ozone focusing on the Bologna urban area. The influence of a reduction in NOx emission was investigated. 1 Introduction The problem of photochemical smog and, consequently, of tropospheric ozone is increasing in importance. It is, however, a problem of extreme complexity, involving as it does both atmospheric diffusion processes as well as complex chemical reactions. Once emitted into the atmosphere, pollutants are dispersed by air turbulence. Some substances react chemically, either among themselves or with other substances present in the atmosphere. In the presence of solar energy, these processes give rise to so-called "photochemical smog", which is characterised by anomalous concentrations of ozone. The intricacy of the processes leading photochemical smog formation and the nonlinear character of the phenomenon impose the need for detailed studies performed with 1

2 special measurement campaigns and appropriate models, before any remedial intervention can be attempted. In fact, the mere reduction in emissions of primary pollutants into the atmosphere would not necessarily bring about a corresponding decrease in secondary pollutants such as ozone, or to an ensuing attenuation of the phenomenon of photochemical smog. In addition, should it be possible by various abatement methods to reduce tropospheric ozone, any eventual remedial effort would have to be preceded by a costs/benefit study. In order to evaluate the three-dimensional distribution of ozone and other photochemical oxidants in the area of Bologna, for a future evaluation of abatement strategies, a realistic simulation of ozone and other chemical species involved in the smog production is necessary. In this paper, we perform a preliminary evaluation of the influence of NO 2 abatement on the O 3 concentration levels. 2 Bologna area Bologna is a metropolitan area with half a million inhabitants. It is situated in the Emilia-Romagna region of northern Italy, on the southern border of the flat Po Valley. The climate is basically continental and the entire area is subject to a series of weak local circulations, frequent inversion phenomena and ensuing high relative humidity, particularly during extended periods of anticyclonic conditions. Usually, perturbations are of northwesterly origin. Figure 1 shows the area considered in this research, along with the domains of the different models applied. The Po Valley is a flat region bordered by the Alpine chain to the north and north west, the Apennines to the south and by the Adriatic Sea and Dinaric Alps on the eastern boundary. The complex morphological structure of the region is characterized by many small valleys leading into the flat land and by a land-sea contrast on the eastern boundary of the plain. During summer anti-cyclonic conditions, local circulation is completely driven by thermal contrasts. During the sunny summer days, when the chemical air mass becomes stagnant under low wind speed conditions, ozone pollution episodes are observed over a wide area. Measurements performed by airplane reveal the existence of atmospheric layers at various altitudes in which ozone values present marked differentiation. Local maximum ozone values are observed downwind of the main urban areas, as was shown by analyzing a large number of data obtained during the MOTAP project field study [1]. 2

3 Figure 1: CALMET, CALGRID and PBM domains 3 The meteorological preprocessor model CALMET Special care was taken to provide the high quality meteorological input required by the models. The CALMET meteorological pre-processor [2] was run to interpolate surface and upper-air station data, and to calculate boundary layer turbulence and stability parameters so as to obtain a complete three-dimensional meteorological field. In order to increase the quality of the interpolation data it includes SYNOP and radiosounding data and data from all local network monitoring stations belonging to the Regional Meteorological Service of Emilia-Romagna (S.M.R.). The area where CALMET was used, shown in Figure 1, was larger than the area where concentrations were evaluated. The model rebuilds three-dimensional wind and temperature fields and parameters descriptive of the boundary layer, such as friction velocity and Monin- Obukhov length. 4 PBM model The PBM (Photochemical Box Model) [3] is able to simulate pollution in urban areas over a domain consisting of a simple square box located over the area of study. The domain is variable in volume, with the pollutants inside the box supposed to be well mixed. The model evaluates the air quality as the pollutants are transported through the walls of the box. The downward flux is due to the mixing height variation, while the primary pollutants emitted by sources present within the box are transformed by photochemical reactions into intermediate and secondary products. 3

4 Model simulation always begins at 5. AM, local standard time (LST) and continues throughout the day, typically ending at or just before sunset, not to exceed 18 hours The PBM contains 63 chemical reactions involving 37 reagent species and 8 classes of hydrocarbons, which describe the photo-oxidation cycle HC-NOx-O 3. The output concentrations include CO, NO, NO 2, and O 3. The model is based on the following equation: X h t where: C i = X Qa i + h u (Cb i -C i ) + X h t (Ca i -C i ) + X h R c,..., c ) i ( i n Qa i is the time average emission flux of pollutantants for unit area t is time c i is the concentration of species i at time t within the box X is the length of the box along the wind direction h is the depth of the box u is the time averaged wind speed through the box Ca is concentration of species i at box top boundary Cb is concentration of species i at the upwind boundary. R i is the production and/or destruction of species i from chemical reactions 5 PBM simulation The episode simulated by the Photochemical Box Model occurred in July 11, 96, during a period characterised by clear sky and wind velocity (at the box boundary) ranging from 2 to 5 m/s. The modelling system is run in a box of x km extension on the Bologna urban area (Figure 1). Meteorological input is obtained by CALMET meteorological pre-processor. The emissions are based on the CORINAIR project database; the initial and boundary concentrations of species are obtained from the S.M.R. database. Boundary conditions are considered spatially homogeneous and are equal to the arithmetic mean of the measurements available for the suburban area of Bologna. Non-methane hydrocarbon (NMHC) boundary concentrations have been evaluated by NO x measurements, keeping the NMHC/NO x ratio inside the box unchanged. Figure 2 shows the comparison between PBM simulated and measured data of ground-level ozone. In the figure, the observed data are obtained by the mean average of the hourly concentrations of all the monitors in the area of x km. Moreover, the figure reports the value measured by the monitor located in an urban park (Giardini Margherita), i.e. far from the traffic emission of Nox. The data collected in Giardini Margherita are higher because the NMHC /NO x ratio in the town indicates that we are in a hydrocarbon-limited area: a reduction in NOx can lead to an increase in maximum O 3. In order to evaluate the influence of NO 2 on the ozone peak we employed the 4

5 PBM model to simulate the same situation reported in Figure 2, but with a reduction of NO 2 emissions, and a reduction of the concentration values of NO 2 at the box boundaries (Ca and Cb). The results are shown in the figure 3, where a high sensitivity on boundary conditions has been verified. 6 CALGRID model The Eulerian photochemical model CALGRID [4] was applied to predict the hourly, three-dimensional concentration fields for ozone, focusing on the Bologna urban area. The modeling system is run in a grid-space of 185 km x 9 km extension (Figure 1), 5 km spacing, with eleven vertical levels up to a height of 5 m. The emission inventory is based on the CORINAIR project database and has been adapted spatially and temporally for the species required by the CALGRID photochemical mechanism SAPRC9 [5]. 7 CALGRID simulations A three-day episode was simulated (from July 1 to July 12, 96), which included the day simulated by the PBM. The results for ozone are shown in figures 4 and 5, for an urban and rural site, respectively. Examining the figures, it can be seen how the ozone in the rural site tends to be higher than in the urban site. However, a comparison of the model results with the observed data shows them to be in reasonable agreement, with the exception of the first simulation day in the rural site. This depends on the strong sensitivity of the CALGRID model to the initial conditions. It is interesting to note that with both the CALGRID and PBM, the ozone peak for July 11 is delayed by about two hours with respect to the one observed. As is often the case with results of photochemical models, the comparison between observed and calculated values for the precursors of ozone does not yield such good results. Figure 6 and 7 shows the comparison between the values of NO2 predicted by CALGRID, in the same temporal range, and those observed for an urban and rural site, respectively. It can be seen how, especially for the first two days, the results are not satisfactory. 8 Conclusions Figures 2, 4 and 5 show a good agreement between simulated and measured values of ground-level ozone, in both an urban and a rural observation site. 5

6 1 1 O3[ug/m3] Com.. Giardini Margherita hour Figure 2: erved () and computed (Com) ozone concentrations by PBM at July 11, 96. O3[ug/m3] hour nox_1%_boundary nox_5%_boundary actual simulation nox_5%_bound._emis. nox_1%_bound._emis. nox_5%_emissions nox_1%_emissions Figure 3: Computed ozone concentrations by PBM at July 11, 96, with different reduction of emission and/or boundary values of NO 2. 6

7 O3 [ug/m3] //96: O3 Urban Site date Figure 4: Comparison between observed () and simulated (Sim) hourly ozone by CALGRID in an urban site Sim O3 [ug/m3] O3 Rural Site 1//96:1 11//96:1 date 12//96:1 //96:1 Figure 5: Comparison between observed () and simulated (Sim) hourly ozone by CALGRID in a rural site Sim Conversely, CALGRID is less able to give realistic simulations of other pollutants, such as NO 2, as shown in figures 6 and 7. This consideration emphasises the importance of feeding a photochemical model with well-balanced concentrations of species which are not normally measured on a routine basis and whose emission rates are not estimated. 7

8 1 NO2 Urban Site 1 NO2 [ug/m3] 8 6 1//96:1 11//96:1 12//96:1 date //96:1 Sim Figure 6: Comparison between observed () and simulated (Sim) hourly nitrogen dioxide by CALGRID in an urban site NO2 [ug/m3] //96: //96:1 5 9 NO2 Rural Site 21 12//96:1 5 9 date 21 //96:1 5 9 Sim Figure 7: Comparison between observed () and simulated (Sim) hourly nitrogen dioxide by CALGRID in a rural site Finally, it was verified that the reduction in atmospheric emissions of primary pollutants (e.g. NO 2 ) does not necessarily lead to a decrease in secondary pollutants, such as ozone, or to attenuation in the phenomenon of photochemical smog. Therefore, the development of appropriate models is confirmed to be of crucial importance for a correct management of the environment. 8

9 9 References 1. Deserti M, Cattani S, Poluzzi V, Il progetto di monitoraggio dell ozono troposferico (MOTER-MOTAP) dell Agenzia Regionale per la Prevenzione e l Ambiente dell Emilia-Romagna. Ozono e smog fotochimico, eds. Poluzzi V, Deserti M, Fuzzi S, Maggioli, Rimini, pp 1-167, Scire JS, Insley EM, Yamartino R, Fernau ME, A User s Guide for the CALMET Meteorological Model, California Air Resource Board, Sacramento (CA), USA, Scere KL and Dermerjian KL, User s guide for the Photochemical Box Model (PBM). Report EPA 6/8-84/22A, Environmental Sciences, Research Laboratory, Research Triangle Park, NC (USA), Yamartino R, Scire JS, Carmichael GR, Chang YS, The CALGRID Mesoscale Photochemical Grid Model I. Model Formulation. Atmos. Environ., 26A, pp , Carter W, A detailed mechanism for the gas-phase atmospheric reactions of organic compounds. Atmos. Environ. 24A, pp ,9. 9