FEASIBILITY OF APPLYING A STABLE ISOTOPIC TRACER FOR DIRECT DETERMINATION OF DRY PARTICULATE DEPOSITION TO SOYBEAN PLANTS

A stable rare-earth isotopic tracer was used to measure the deposition of KNO3 particles on soybean leaves by direct measurement of the trac er on the plant surfaces by thermal-ionization mass spectrometry. Subm icrometer particles, made from a solution containing 3 mu g mL(-1) neo dymium isotope (Nd-148, 87.9%) and 1,000 mu g mL(-1) KNO3, were disper sed with a two-fluid nozzle and released upwind of a soybean field. To tal suspended- and size-fractionated-aerosol particles were collected on an open-face filter and in a micro-orifice impactor, respectively, at a distance of 40 m from the release point. Soybean leaves exposed t o the plume were collected at distances ranging from 25 to 100 m. As l ittle as 5.5 pg of the tracer (i.e., excess Nd-148) was detected in so ybean leaves at signal-to-noise ratios ranging from 7,500 to 240,000, in the presence of 200 to 2,700 pg of naturally occurring Nd. The dry- particle deposition velocity, determined from the ratio of the aerial concentration and directly deposited aerosol (geometric mass mean diam eter, 0.20 mu m) flux, and its corresponding analytical uncertainty we re 0.30 cm sec(-1) and 2.5%, respectively.