Long-term aerosol particle flux observations part I: Uncertainties and time-average statistics

Long-term eddy covariance particle flux measurements for the size range starting from 10 nm were performed at a boreal forest site in Southern Finland. The large variability in turbulent flux estimates is inherent to the particle flux observations and thus long-term particle flux measurements enable to obtain statistically significant results by a suitable averaging. The particle flux random errors were estimated and a parameterisation for the integral time scale of turbulent flux was proposed. Application of flux errors for classification according to statistical significance of single flux values leads to systematically different deposition estimates on ensemble average basis. This must be avoided for determination of unbiased average deposition fluxes. The role of storage term in particle deposition evaluation was analysed. It was empirically determined that the method of storage term estimation discussed by [Finnigan, J., 2006. The storage term in eddy flux calculations. Agric. Forest Meteorol., 136, 108–113.] is not sensitive to the selection of the concentration averaging window in both ends of the flux averaging period. It is argued that the storage change in real atmospheric conditions results from boundary layer development as well as source–sink activity and therefore the filtering effect arising from averaging the concentration is of less importance. Diurnal, seasonal and annual variability of particle fluxes was analysed and it was observed that particle deposition rates are higher in winter. More detailed analysis of functional dependencies of particle deposition on environmental factors as well as dependence on size will be done in the second part of the paper.     

The dependence of the beta coefficient of REA system with dynamic deadband on atmospheric conditions

We simulated the REA system with dynamic deadband to study numerical value and the effect of atmospheric conditions on the empirical constant beta which relates vertical flux to concentration difference between updrafts and downdrafts. We found that the value of beta depends only weakly on the friction velocity and atmospheric stability. In agreement with previous studies, the median value obtained for a system with dynamic deadband proportional to 0.5 times the running mean of the standard deviation of vertical wind speed was beta=0.42+/-0.03. For a single half-hour measurement one has to consider the large uncertainty of +/-0.2. According to our study, the dynamic deadband enables the use of a constant value of beta in flux calculation.     

CO2 fluxes near a forest edge: a numerical study

In contrast with recent advances on the dynamics of the flow at a forest edge, few studies have considered its role on scalar transport and, in particular, on CO2 transfer. The present study addresses the influence of the abrupt roughness change on forest atmosphere CO2 exchange and contrasts the concentration and flux fields against those of a uniform forested surface. We use an atmospheric boundary layer two-equation closure model that accounts for the flow dynamics and vertical divergence of CO2 sources/sinks within a plant canopy. This paper characterizes the spatial variation of CO2 fluxes as a function of both sources/sinks distribution and the vertical structure of the canopy. Results suggest that the ground source plays a major role in the formation of wave-like vertical CO2 flux behavior downwind of a forest edge, despite the fact that the contribution of foliage sources/sinks changes monotonously. Such a variation is caused by scalar advection in the trunk space and reveals itself as a decrease or increase in vertical fluxes over the forest relative to carbon dioxide exchange of the underlying forest. The effect was more pronounced in model forests where the leaf area is concentrated in the upper part of the canopy. These results can be useful both for interpretation of existing measurements of net ecosystem exchange of CO2 (NEE) from flux towers in limited fetch conditions and in planning future CO2 transport experiments.