Observations of the chemical properties of dew on vegetation that affect the dry deposition of SO2

Abstract

Experiments were conducted to examine the chemical properties of dew (and guttation) that affect the dry deposition of SO2 to prairie grasses of tall stature and mixed broadleaf plants of short stature. For 2–3 hours after sunrise, liquid samples were extracted directly from the upper of the plant canopies and from surrogate surfaces, concurrently with selected atmospheric observations. Chemical analyses showed that aqueous concentrations of S(IV), SO42‐, and several other ions in the leaf wetness were often high compared to values typically found in rain, while NH4+ was not very abundant. Guttation produced substantial amounts of liquid, which mainly contributed Cl− to the leaf wetness. The dry deposition of S(IV) to leaf wetness was not always apparently limited by aqueous oxidation by O3, especially when aerodynamic resistances limited the maximum potential deposition velocity to less than 0.5 cm/s at a height of 8 m. A more effective oxidant than atmospheric O3 or NO2 could have been present and could have lessened the inferred substantial contributions of particle deposition to leaf wetness SO42‐. Foliar uptake of NO3‐ and SO42‐ was significant after sunrise. The pH of the leaf wetness in the broadleaf vegetation seemed to have a lower limit of about 5.3, possibly because of neutralization by substances leached from leaves, while the pH of the grasses reached values as low as 4.5 when the amount of leaf wetness was small. The potassium ion concentration in leaf wetness appeared to be an effective marker for leaf leaching.