Airshed impacts

Urban aerosols are complicated systems composed of material from many different sources. Achieving cost-effective air particle reductions in airsheds not meeting national ambient air quality standards requires identification of major aerosol sources and quantitative determination of their contribution to particle concentrations. Quantitative source Impact assesment, however, requires either calculation of a source s impact from fundamental meteorological principles using source oriented dispersion models, or resolving source contributions with receptor models based on the measurement of characteristic chemical and physical aerosol features. Q)... 

Presented as an overview of the State of Oregon s unique approach to particulate control strategy development, this review was prepared to provide those responsible for airshed management with new information on source impact assessment methods. (This material is available in the form of an audio-visual program suitable for presentation before public, regulatory or private Interest groups). 

Step 2 required identification of source impacts by airshed modeling. Wind speed, direction, mixing height, and emission data bases designed to represent conditions on PACS sampling days were used to insure that the CMB impact estimates could be directly compared to model predictions for each sampllne site. 

Large impacts from one of the airshed s major sources was traced to an unrealistic operating schedule. 

Point source plume trapping assumptions used in the Eugene airshed modeling were verified by comparing alternative modeling assumptions of sulfate emission impacts to measured sulfate levels. 

The development of new source apportionment methods have, for the first time, led to the development of regional particulate control strategies. Source impacts assigned using a chemical mass balance (CMB) model have been used in association with airshed dispersion models to identify emission inventory deficiencies and Improve modeling assumptions. 

Floodplain lakes can be perturbed by direct alterations to the lakes and fringing wetlands, by changes to the river with which they are associated, and by modifications to the uplands surrounding their local catchments and to their airshed. To date, human-induced impacts on floodplain lakes appear only in limited areas within the Amazon basin. As development of agriculture, mining... 

Many different types of volatile organic compounds (VOCs) are emitted into the atmosphere, where they can affect photochemical ozone formation and other measures of air quality. Because they can react in the atmospheres at different rates and with different mechanisms, the different types of VOCs can vary significantly in their effects on air quality. The effect of a VOC on ozone formation in a particular environment can be measured by its incremental reactivity , which is defined as the amount of additional ozone formed when a small amount of the VOC is added to the environment, divided by the amount added. Although this can be measured in environmental chamber experiments, incremental reactivities in such experiment cannot be assumed to be the same as incremental reactivities in the atmosphere (Carter and Atkinson, 1989 Carter et al, 1995). This is because it is not currently practical to duplicate in an experiment all the environmental factors that affect relative reactivities and, even if it were, the results would only be applicable to a single type of environment. The only practical means to assess atmospheric reactivity, and how it varies among different environments, is to estimate its atmospheric ozone impacts using airshed models. 

However, airshed model calculations are no more reliable than the chemical mechanisms upon which they are based. While the initial atmospheric reaction rates for most VOCs are reasonably well known or at least can be estimated, for most VOCs the subsequent reactions of the radicals formed are complex and have uncertainties that can significantly affect predictions of atmospheric impacts. For this reason, environmental chamber experiments and other experimental measurements of reactivity are necessary to test and... 

Two methods are generally employed to quantify the role pollutants play in forming ozone experimental and eomputational. Both types of estimation approaehes have their limitations. In the case of physical experiments, it is difficult to fully simulate ambient conditions, so the results do not have general applicability. In the case of computational approaches, uncertainties and approximations in the model for airshed conditions, in its formulation, and in the chemical mechanism cause uncertainties in the predicted ozone impacts. For these reasons, modeling predictions and experimental measurements are used together. 

Given the limitations of physical experiments to simulate atmospheric conditions, computer models have been developed to assess the impact of emissions on ozone. These models, called airshed models, are computerized representations of the atmospheric processes responsible for air pollution, and are core to air quality management. They have been applied in two fashions to assess how solvents affect ozone. One approach is to conduct a number of simulations with varying levels of solvent emissions. The second approach is to evaluate individual compounds and then calculate the incremental reactivity of solvent mix-t res. 9.2i ... 

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