Ü. Rannik, A. Gaman, P. Aalto, P. Keronen, T. Vesala and M. Kulmala

Department of Physical Sciences, P.O.Box 64

FIN-00014 University of Helsinki, Finland.

Abstract

Measurements of particle fluxes for sizes smaller than 100 nm are very scarce. Eddy covariance (EC) method imposes requirements on the instrumentation as well as observation conditions. Measurements of particle fluxes were done at SMEAR II station (Station for Measuring Forest Ecosystem-Atmosphere Relationships), Southern Finland (61° 51¢ N, 24° 17¢ E, 181 m ASL) over a 35 years old and 14 m high Scots pine forest by the EC technique. Condensational particle counter (CPC) was used to detect particles in the size range from 10 to 500 nm.

The natural variability of aerosol particle concentrations in the atmosphere frequently does not satisfy the assumptions behind the flux measurement methods. The large variability in space and time results in non-stationary behaviour of the concentration time series. Vertical wind speed and concentration records were analysed for unusual, non-stationary behaviour. Quality tests for skewness, kurtosis, Haar mean and Haar variance, and for random flux error were applied (Vickers and Mahrt, 1997). A test for flux (in)stationarity according to Foken and Wichura (1996) was also used. The stationarity test appeared to be strongest and was applied to quality selection of flux data for further analysis.

The particle deposition as well as emission was observed over the forest. About 50 % of half-hour fluxes passed the quality test (Table). 76.7% flux estimates corresponded to particle deposition and 23.3% to emission. It is unexpected that 66 % of all cases that passed the FS test were stable to neutral stratification observations (L>=0) and only 34 % unstable. Also the fraction of deposition observations was higher during stable stratification compared to unstable. About ¼ of the fluxes that passed the quality test indicated total particle emission. Emission could occur due to particle emission from the forest, temporary particle sources and/or random error in flux estimates.

Table. Percentages of deposition (positive) and emission (negative) fluxes (F) of those, which passed the quality test. L – Obukhov stability length.

The size spectrum of aerosol particles was analysed in order to determine the size dependence of particle deposition. Aerosol size distribution (from 3 to 500 nm particles in diameter) measurements were performed using Differential Mobility Particle Sizer (DMPS) system. The aerosol was sampled from inside the forest at 2 m height. A large data set of long-term flux measurements, containing different meteorological conditions as well as particle distributions, including new particle formation events with dominating nucleation mode particles, enables to determine deposition velocities for different particle sizes.

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