Advancing the understanding of the aerosolcapture efficiencies of forest components such as leaves and needles, and of the mechanisms that underpin these efficiencies, is essential to the many related issues of forest turnover of nutrients and pollutants. For idealized collectors (such as artificial plates or cylinders) aerosolmechanics offers a means for calculating capture efficiencies. For living collectors, in particular deciduous leaves, experimental investigations become necessary to assist in formulating the sub-models of capture efficiency that are fundamental to the modelling of fluxes of aerosol-borne substances to forests. We here present wind-tunnel based methods and results for leaves of Quercus robur L. exposed to an aerosol whose mass versus aerodynamic particle size distribution is characterised by a geometric mean aerodynamic particle diameter around 1.2 lm and a geometric standard deviation around 1.8. With respect to that distribution, and founded on a specially designed leaf wash-off method, we obtained average oak-leaf capture efficiencies ranging from 0.006% of the approaching aerosol mass flux at wind-speed 2 ms -1 to 0.012% of the flux at wind-speeds 10 ms -1 , respectively. These values can be translated into deposition velocities (V d ) for a leaf ensemble with a given leaf area index (LAI). With LAI in the range 2-5 (commonly found in the field) and for windspeeds 2, 5 and 10 ms -1 , resulting V d -values would be 0.02-0.05, 0.05-0.13, and 0.2-0.6 cm/s, respectively. To the extent comparisons are possible, our capture efficiency values are at the low end of the range of values reported by other researchers. The strong wind-speed sensitivity of V d has implications for the deposition of aerosol-borne substances to forests for which wind regimes may shift as a result of climatic and land-use changes.

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Within the framework of aerosol deposition to vegetation we present a specially designed leaf wash-off method used in a wind-tunnel based study, where leaves of <i>Quercus robur</i> L. were exposed to NaCl aerosols. We summarise the principles and illustrate the method for two types of substances, the chloride ion and the sodium ion, and for two levels of aerosol exposure prior to leaf washing. On the average, in the low-exposure experiments (S1), the 1st (2nd) wash-off step provided 90% (96%) of the amount of Cl^&minus; on the leaves. In the high-exposure experiments (S2) the corresponding values were 96% and 99%. For sodium, the general dynamics resembles that of chloride, but the amounts washed off were, in both series, on the average below what would be expected if the equivalent ratio in the tunnel aerosol were to be preserved. Na⁺ showed adsorption and/or absorption at the leaf surfaces. The difference between the mean values of the amounts of chloride and of sodium washed off in S1 was not statistically significant, the mean Na⁺ to Cl^&minus; difference as a fraction of Cl^&minus; being minus 18%±27%; corresponding values for S2 were minus 16%±9%, however (<i>p</i><0.05). In the latter case, 101±57 μequiv Na⁺ per m² of leaf area were missing for the equivalent relationship 1:1 with Cl^&minus; to be met. Although uncertainties are thus large, this indicates the magnitude of the Na⁺-retention. The method is suitable not only for chloride, an inexpensive and easy-to-handle tracer, but also for sodium under exposure at high aerosol concentrations. Our findings will help design further studies of aerosol/forest interactions