DETERMINATION OF THE SO2 NEUTRALIZING CAPACITY OF A DECIDUOUS CANOPY IN AN URBAN ENVIRONMENT

Vegetation is well known to be an effective trap for wet and dry acid deposition (SO₂), and biotic transformations by uptake, litter production, storage, and microbial decomposition influence sulfur cycling. Studies suggest that deciduous canopies at any level of acid deposition result in nearly complete neutralization. Plants remove gaseous pollutants by uptake through leaves via stomatal and non-stomatal pathways and by interception of airborne particles. Acidifying species are recycled primarily by leaf fall and mineralization. Dramatic contrast in decay of lead-lettered marble gravestones between two cemeteries in the Birmingham, UK, city center allows estimation of the canopy uptake of SO₂. Warstone Lane and Key Hill cemeteries are located in Birmingham's Jewellery Quarter, lie at the same elevation, are nearly identical in topographic relief, and are surrounded by commercial properties. However, Key Hill has a continuous canopy of 100+ year-old London plane, whereas Warstone Lane is largely open with only 25% tree cover. The effect of tree canopy on gravestone decay is remarkable with Key Hill recording up to 70% less gravestone decay and therefore acid deposition. This difference in gravestone decay is used to calculate canopy uptake of SO_2, which is generally measured as the resistance to gaseous deposition. Gas deposition velocity (v_d) = -F/C_z, where F is the flux density and C_z is the concentration of SO₂. v_d is related to the aerodynamic resistance (R_a), the quasilaminar (or diffusive) sublayer resistance (R_b), and the surface resistance (including the canopy) (R_c) by the relation v_d⁻¹ = R_a + R_b + R_c. R_a and R_b are determined for both sites and R_c is calculated. Values of R_c range from approximately 300 to 900 s m⁻¹ and are consistent with estimated and calculated values from a wide variety of studies. Canopy resistance reduces SO₂ flux to the surface by 50-70% in this urban setting.