D EPOSITION OF SUBMICRON AEROSOL ON SPRUCE NEEDLES ; WIND TUNNEL MEASUREMENTS

1. K. LAMPRECHTOVÁ, J.HOVORKA lampr@seznam.cz, hovorka@cesnet.cz Institute for Environmental Studies, Faculty of Science, Charles University in Prague, Benátská 2, 128 01 Prague 2, Czech Republic DEPOSITION OFSUBMICRON AEROSOLON SPRUCE NEEDLES;WIND TUNNEL MEASUREMENTS • Atmospheric aerosol deposition onto a plantsurface is an efficient mechanism of its removal fromthe atmosphere. Rate of aerosol deposition stronglydepends deposited aerosol, on thickness of laminar sublayer coveringplant surface. The layer thickness is altered byvegetal surface morphology and by deposited aerosol. • Deposited aerosol may increase stomatal conductance, which lead to an unwanted increase of watertranspiration during water stress periods. • Conifers are periodically subjected to water stress during winter,when water supply is blocked due to frozen soil, andincreased transpiration may affects conifer winter survival. Study objectives • to construct and to test closed circulation wind tunnel for the measurements of aerosol deposition under controlled conditions • to quantify differences in aerosol deposition rates between two coniferous species, Picea abies and Picea pungens-glauca • to quantify the influence of needle surface alteration on aerosol deposition Top view on closed circulation wind tunel Experimental conditions • closed circulation wind tunnel (Vol.-0,36 m3,Surface-3,73 m2, S/V=10.4) • working section (0,5m long, 0,18m2 cross section) • Stairmand disc: turbulent flow (Re = 13 680), honeycomb: laminar flow (Re = 608) • wind speed 1.14 m/s • testing aerosol:average GMD=0.7μm, sg= 1.3, N max. = 1200 pt/cm3(AGK 2000, Palas) • aerosol size distributions 0.524-1.0μm, integration time 6s, (APS 3321, TSI) • 32 twigs in8 rows of Picea pungens-glauca or Picea abies • alteration of needle surface by removing epicuticular wax with chloroform (washing for 60s) Measurements • experiment = 3 different cycles measured consecutively; each measurement took approx. 90 minutes • one control measurement with empty tunnel • four consecutive measurements of aerosol deposition rate with conifer twigs in the tunnel • three consecutive measurements of aerosol deposition rate with conifer twigs without wax layer Analysis • basic model:N = No exp (- βt) where N– actual number of particles, No– initial number of particles, β- deposition rate constant • b = b0 + bn where “0”or “n” coefficient are experiments for empty tunnel or with twigs • b = (S0 + Sn) / V * D/d where S0,Snistunnel/needles geometric surface V tunnel volume, D = diffusion coefficient and d = boundary layer thickness • deposition velocity Vd = V/ Sn *bn RESULTS Detail of working section (side view) b0 values for empty tunnel Contour graph of temporal changes of aerosol number size distributions Regression curves of aerosol size fractions, b determination Relative Standard Deviations – RSD’s of b values measurements and needle surface between experiments βnfor Picea Pungenswith a wax layer decreases with growing amount of deposited aerosol on needles βfor Picea pungens with waxlayer and b0 for empty tunnel βn for Picea pungens with/without wax layer *DNW - dry needle weight βnfor Picea abies with /without wax layer βnfor Picea pungens without wax varied stochastically with growing amount of deposited aerosol on needles CONCLUSIONS • low values of RSD of experimental data proved the tunnel to be suitable for such a kind of experiment • aerosol deposition velocity was on average higher for P.abies than for P.pungens by 40% and 60% respectively, for experiments with and without wax layer respectively • removing epicuticular wax caused 15% decrease of βn for P. punges regardless the aerosol sizewhile for P.abies, probably due to stronger alteration of laminar sublayer, βn changes with aerosol size • repeated deposition of experimental aerosol decreased βn for both the species near to βn values recorded for experiments without wax layer. This could be explained by filling up the waxstructure withaerosol particles • repeated deposition of experimental aerosol had no reproducible effect on values of βn for both the species for experiments without wax layer ACKNOWLEDGEMENT The study is a part of MSc. diploma work; financial support by the Institute for Environmental Studies, Faculty of Science, Charles University in Prague is greatly acknowledged.