Interaction of vehicles produced aerosols and cold clouds in the laboratory

Particulate pollutants in urban areas affect human health, environment clouds and climate. With rapid growth of the population and increase in the number of vehicles, in urban areas, air pollution also in large cities has increased substantially. Mobile sources of air pollution have a major contribution to the pollutant levels in urban areas. For example, motor vehicles (gasoline and diesel) play a major role in air pollution in large cities such as Tehran and in fact, they are the main sources of primary aerosols emissions. Aerosols can act as cloud condensation nuclei (CCN) and ice nuclei (IN) and impact on precipitation of different regions. Precipitation can also supply water resources and help to improve the air quality of cities. In Iran, most clouds are cold clouds and ice crystals play a major role in cold precipitation. Using field emission scanning electron microscopy and energy dispersive X-ray spectroscopy (FESEM/EDX) methods, morphology and elements composition of aerosols from the vehicle exhaust pipes were investigated. In this study, we injected such aerosols into a cold cloud chamber in laboratory to get a better understanding of microphysics processes on interaction of urban aerosols and clouds. The experimental simulations include growth of graupels through rotating rods mechanism in the presence of vehicles produced aerosols and supercooled droplets. In order to obtain size distribution of supercooled droplets we used replicas method. Also we focused on ice crystals and the impact of pollutant particles on the ice crystals in cloud microphysics. The size distribution of ice crystals was evaluated from images of fallen crystals on a lamella under a microscope. By creating an electric field inside the cloud, the effect of these particles on the electrostatic torque of the ice crystals has been tested. It can provide a better view of predicting the short term lightning. The results show that vehicles produced aerosols are mainly soot and ash and in the presence of these particles, the average diameter of supercooled droplets decreases. By decreasing diameter of supercooled droplets, the growth of the graupel also decreases. Besides, the average diameter of the ice crystals in the presence of vehicles produced aerosols was reduced. However, pollutant particles do not have much effect on the electrostatic torque of ice crystals. The shape of the observed crystals in the experiments included hexagonal, stellar and sectored plates, solid and capped columns, prisms, triangular shapes, needles and dendrites. Because of the different thickness of various parts of the ice crystal exposed to light, different wavelengths of dispersed visible light with different colors appear and then, it can be seen in different colors (like a soap bubble exposed to light). When the crystals are grown large enough, they appear white. With more sophisticated mentoring system, one may distinguish between the formed crystals in different aerosol types.