Characterization and dry deposition of carbonaceous aerosols in a wet tropical forest canopy

Carbon aerosol concentration was measured using an impactor on a 42 m tower over a wet tropical forest in northeast Costa Rica. Samples were collected at three different heights, 42, 21, and 2 m, for 2 months during the wet season in 1998. Winds originated from two directions, southeast from the Caribbean Sea and west from the continental isthmus. Concentrations were normalized by the fraction of dry sampling time during the collection. The distribution was negatively skewed for the range of aerodynamic diameter aerosols measured. The main size constituent was in the class 4.7-3.3 mm, accounting for similar to0.70 mug C mol(-1). No significant difference was found in the distribution of aerosol carbon with height, suggesting a well-mixed column of air, minimal resuspension, and that the source was from surrounding land use types. Functional relationships were developed to describe the loading of aerosols to the atmosphere and the removal by precipitation. Deposition was estimated using these relationships, combined with three different estimates of velocity deposition derived from (1) aerodynamic and canopy conductance, (2) aerodynamic and momentum conductance, and (3) traditional estimates of gravitational settling diffusion, impaction, and interception. Annual deposition estimates were 2.9, 5.0, and 9.6 kg ha(-1) yr(-1), respectively. Concentrations of carbon aerosols reported here are as much as two orders of magnitude higher than those reported elsewhere. Annual dry deposition estimates, however, were within the range of other estimates but were likely underestimated. Potential effects on deposition caused by seasonal burns and El Nino-Southern Oscillation are discussed.