September 1978

Document Type


Degree Name



Dept. of Environmental Technology


Oregon Graduate Center


The primary explanation of the variability of urban ozone concentrations is variable meteorology. In order to develop a better understanding of this relationship, the physics and chemistry of urban ozone formation is considered from a theoretical point of view. If it is assumed that the ratios of the emission rates among the precursors to ozone formation are constants and that the formation of reactive radicals (i.e. those species that convert NO to NO2 without the destruction of O3) is proportional to initial hydrocarbon concentration, then a simple six step reaction model for ozone formation can be solved analytically. Expressing this solution in terms of meteorological parameters results in an equation giving a rough estimate of the sensitivity of O3 concentration to wind speed, mixing height, intensity of solar radiation, precursor emission rate, background O3 concentration and location downwind of an urban center. A computer simulation of ozone formation for the Portland, Oregon area was also developed as a more realistic model. The model is characterized by a photochemical reaction mechanism of 33 specific reactions that takes into account the effect of variations in solar intensity, dilution due to lifting of the mixing height, and a non-impulsive schedule for precursor injection. This computer program gives an estimate of O3 concentration in the Portland area as a function of wind speed, mixing height, precursor emission rate, background O3 concentration, time of day, time of year, and location downwind. Computer simulation results indicate that immediately downwind of an urban center the size of Portland, O3 concentration on sunny days depends most strongly on wind speed and less strongly upon mixing height, precursor emission rate and time of year as long as the time of year is the months of April through September. In addition it appears that O3 due to precursors from Portland and upwind ozone levels cannot be simply summed to give O3 at a downwind location; that is, the relationship is more complex than additive. These results are consistent with the results derived analytically. Analysis of air quality data for sunny days from two locations in the Portland area sustains these conclusions. The two locations are relatively near (10 km ) and relatively far (30 km ) from city center. For the near location, wind speed and upwind O3 concentration are clearly the most important determinants, while wind direction and upwind O3 concentration appear to be more important for the more distant location. Background O3 levels appear to be related to the degree of stagnation of the high pressure system that characterizes the meteorology of the region on days with elevated O3 levels. This effect results in a correlation between background O3 and temperature. In addition the data give some evidence for the existence of a second order positive coupling between precursor emissions and upwind ozone concentration. A practical result of this work is a procedure for forecasting O3 levels in the Portland area. The procedure is the result of regression analysis of air quality and meteorological data using equations suggested by computer simulation results. The forecasting methodology is also based in part upon a time series analysis of the residuals of the regression analysis. The forecast appears to be fairly successful for the near location but only moderately successful for the far location.





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