Impact of roadside tree lines on indoor concentrations of traffic-derived particulate matter.

Exposure to airborne particulate pollution is associated with premature mortality and a range of inflammatory illnesses, linked to toxic components within the particulate matter (PM) assemblage. The effectiveness of trees in reducing urban PM10 concentrations is intensely debated. Modeling studies indicate PM10 reductions from as low as 1% to as high as ~60%. Empirical data, especially at the local scale, are rare. Here, we use conventional PM10 monitoring along with novel, inexpensive magnetic measurements of television screen swabs to measure changes in PM10 concentrations inside a row of roadside houses, after temporarily installing a curbside line of young birch trees. Independently, the two approaches identify >50% reductions in measured PM levels inside those houses screened by the temporary tree line. Electron microscopy analyses show that leaf-captured PM is concentrated in agglomerations around leaf hairs and within the leaf microtopography. Iron-rich, ultrafine, spherical particles, probably combustion-derived, are abundant, form a particular hazard to health, and likely contribute much of the measured magnetic remanences. Leaf magnetic measurements show that PM capture occurs on both the road-proximal and -distal sides of the trees. The efficacy of roadside trees for mitigation of PM health hazard might be seriously underestimated in some current atmospheric models.