In urban areas, a highly variable mixture of pollutants is deposited as particulate matter. The concentration and bioavailability of individual pollutants within particles need to be characterised to ascertain the risks to ecological receptors. This study, carried out at two urban parks, measured the deposition and water-solubility of metals to four species common to UK urban areas. Foliar Cd, Cr, Cu, Fe, Ni, Pb and Zn concentrations were elevated in at least one species compared with those from a rural control site. Concentrations were, however, only affected by distance to road in nettle and, to a lesser extent, birch leaves. Greater concentrations of metal were observed in these species compared to cypress and maple possibly due to differences in plant morphology and leaf surfaces. Solubility appeared to be linked to the size fraction and, therefore, origin of the metal with those present predominantly in the coarse fraction exhibiting low solubility. High density traffic resulted in elevated metal concentrations on vegetation, which were related to distance from road and plant species.

The effects of ozone in four maple species, Acer campestre, A. opalus subsp. granatense, A. monspessulanum and A. pseudoplatanus were studied in OTC under two different experimental conditions: in charcoal filtered air (CF), and in non filtered air plus 30 ppb ozone (NF+30). The four species of maple showed contrasting sensitivity to ozone as demonstrated by visible injury development, gas exchange and chlorophyll a fluorescence, and growth measurements. Plant injury index (i.e. a combination of percentage of injured leaves and leaf surface affected) was more consistently related with physiological measurements than the onset of first symptom of visible injury. Differences in ozone sensitivity among species may be partly related to higher stomatal conductances in A. opalus and A. pseudoplatanus. In these two species, ozone produced significant reductions in CO₂ assimilation under saturating light conditions (A sat), stomatal conductance (g s), transpiration rate (T r) and Water Use Efficiency (WUE) (the latter also significantly declined in A. campestre) towards the end of summer, while intercellular CO₂ concentrations (C i) increased significantly. In asymptomatic leaves of A. opalus, neither stomatal limitation nor photoinhibitory damage (F v/F m) could explain the observed decline of A sat, and photosynthesis was down regulated by reducing the proportion of absorbed energy used in photochemistry (Φ PSII) at expenses of the energy dispersed non-photochemically (NPQ). Leaf N content also declined significantly in A. pseudoplatanus. Plants exposed to ozone showed a tendency to decrease growth, but it was not significant within the exposure period for any of the four species. The most sensitive species were A. opalus and A. pseudoplatanus, while the species with the smallest and more coriaceous leaves, A. monspessulanum, was the most resistant.