MAGNETIC PROPERTIES OF PARTICULATE MATTER COLLECTED BY TREE LEAVES IN BELLINGHAM, WA: IMPLICATIONS FOR AIR QUALITY MONITORING IN URBAN ENVIRONMENTS

Particulate matter pollution is recognized as a significant health hazard; ultra-fine particles (less than 10 µm or 2.5 µm diameter, referred to as PM10 and PM2.5, respectively) may warrant additional study and monitoring as these particles can be inhaled deeply into lung tissue. Studies also point to additional adverse health effects that correlate with metallic particulate matter exposure. In urban environments PM10 and PM2.5 originate from a variety of sources including natural dust, fly-ash produced by fossil fuel combustion, and motor vehicle related particles (mainly brake, tire, and road material). Most air quality monitoring programs rely on automated particle-size analyzers, and do not routinely collect samples of particulate matter for compositional analyses. Studies of metallic PM concentrations and sources are relatively rare so this aspect of air quality is poorly understood.

To assess sources of metallic PM in an urban environment, we collected samples of leaves from trees (mainly Bigleaf Maple (Acer macrophyllum) growing adjacent to several streets in Bellingham, WA that have mixed traffic densities and differing types of vehicles. The leaves were collected during spring and summer. In addition, air sampler filters from the Bellingham air quality monitoring station (collected in 2003 and 2004) were measured. The leaf and filter samples were weighed, and magnetic measurements were made using a vibrating sample magnetometer.

Magnetic hysteresis properties of both leaves and air-filter samples are similar, and indicate a mix of MD-SD and lesser SP magnetic sources. We found no simple correlation between motor vehicle traffic counts and the magnetization of the leaf samples, but in general the highest Ms and Mrs values were from areas with the highest motor vehicle traffic. Vehicle types account for some of the largest variation in magnetic particulate concentration- leaves from streets with heavy bus traffic, and near an active railway line, had enhanced Ms and Mrs values that we attribute to diesel engine emissions. We conclude that the spatial distribution of fine metallic particulate matter in urban environments is highly variable, and magnetic measurements of tree leaves can be useful for determining locations where variations in particulate matter sources and concentrations are significant.