MONITORING PARTICULATE MATTER USING MAGNETIC PROPERTIES OF TREE LEAVES IN THE PORTO URBAN AREA (NORTHERN PORTUGAL)

Nowadays, pollution levels in urban areas are a threat for the human health. Particulate matter (PM₁₀) due to emissions by the road traffic and others human activities such as metallurgical, cement industries and power stations burning fossil fuels constitute a cause of cardiorespiratoire diseases. The magnetic susceptibility of tree leaves has been used as a proxy to evaluate the particulate matter quantity and the road traffic contribution. In order to evaluate pollution levels, 6 sampling zones were chosen in the urban area of Porto. Sampling was carried out in the first week of July and August 2006. Leaves from deciduous species (Nerium oleander and Quercus sp.) and evergreen species (Tilia sp. and Platanus sp.) were sampled on a total of 32 sites. Magnetic susceptibility measurements were performed within a day after sampling using an Agico Kappabridge (KLY-4S) susceptometer working at a low alternating inductive field (± 4x10^-4 T, 920 Hz) ("Petrophysics Laboratory" of Geology Department, Porto University) and the values calculated by mass. Mass susceptibility values ranges are presented in the table bellow.

Zone	Number of sites	Species	Mass susceptibility E-9 m3/kg
1	11	Nerium oleander, Platanus sp., Quercus sp., Tilia sp.	-4.47 – 94.13
2	2	Nerium oleander	11.98 – 41.45
3	2	Nerium oleander, Platanus sp	19.66 – 118.22
4	2	Nerium oleander	21.95 – 41.42
5	14	Nerium oleander, Platanus sp., Tilia sp.	0.79 – 150.0
6	1	Nerium oleander	50.11 – 85.24

Only one site reveals diamagnetic values. All the others present high positive susceptibility values which indicate the presence of anthropogenic ferrimagnetic particles.

Magnetic susceptibility of Nerium and Tilia leaves shows higher values than Platanus and Quercus leaves. In August a decrease in susceptibility values was observed, probably due to the reduction of road traffic during holidays.

These results indicate that a magnetic survey of tree leaves, which is relatively rapid and inexpensive, may be used in addition to the municipal air quality monitoring systems to identify high-polluted areas in urban environments.