1.the scientists (to discuss) this problem already.2.scientists (to develop) air pollution dispersion models to evaluate the concentration of a pollutant at a receptor or the impact on overall air quality from vehicle exhausts and industrial flue gas stack emissions

1.The scientists has already discussed this problem. 2.Scientists are developing air pollution dispersion models to evaluate the concentration of a pollutant at a receptor or the impact on overall air quality from vehicle exhausts and industrial flue gas stack emissions