Relation of particle matter on roadside tree leaves with traffic density and road dust

In Japan, air pollution due to atmospheric particulate matter has been a serious problem, especially at roadsides in megacity areas. However, it is difficult to widely cover roadside areas with a limited number of air pollution monitoring stations. Therefore we focused on a biomonitoring technique by using leaves of roadside trees. We sampled leaves of ginkgo (G. biloba) along major arteria roads in Osaka City, which is the third largest city in Japan. The particles retained on the surfaces of the sampled leaves were removed by ultrasonic cleaning in ethanol solution. The ethanol solution including particles was suctioned by an aspirator and particles were collected on a PTFE filter. Elements collected on the filter were quantified by particle induced X-ray emission (PIXE) analysis. It was found that the mass of traffic-related elements on the surfaces of sampled leaves correlated with the traffic density of large vehicles and the distance from the end of roadside to the sampling point. The ratios of mass of road dust-related elements on sampled leaves were almost similar to those of the road dust profile data. These results suggest the biomonitoring technique is an effective method to assess roadside air pollution.