Direct measurement of turbulent particle exchange with a twin CPC eddy covariance system

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Abstract

Direct measurements of particle number fluxes by eddy covariance (EC) were carried out in the years 2001 and 2002 during the BEWA2000 field experiments. An EC system combining a sonic anemometer and two condensation particle counters was set up and successfully applied above a Norway spruce forest in NE Bavaria, Germany. Particle deposition clearly dominated over emission, with the strongest deposition fluxes occurring during particle formation events identified from submicron particle size distributions. Typical deposition velocities derived from these measurements ranged from −37 to +23 mm s−1. The ultrafine particle fraction (UFP) (3–11 nm diameter) showed different concentration patterns and larger deposition velocities as compared to the particle fraction with larger diameters. Also, particle deposition occurring before noon could be attributed mainly to the UFP, whereas larger particles contributed to turbulent deposition in the afternoon.

Introduction

Vertical exchange of gases and particles between the surface and the boundary layer is mainly established through atmospheric turbulence. Quantitative knowledge about this turbulent exchange is a key prerequisite to understand ecosystem budgets of nutrients and pollutants. It is also fundamental for the dynamics of aerosol populations and their effect on atmospheric properties and processes. However, the quantification of turbulent exchange, especially of particles, is complex and still a challenging endeavor (Wesely and Hicks, 2000). Various approaches have been suggested to measure and estimate particulate fluxes between the atmosphere and the surface. One straightforward direct approach is sampling and analyzing particles on surrogate surfaces (e.g. Franz et al., 1998). In most cases, a large disadvantage of this method is the rather poor imitation of the real deposition surface and flow regime. Atmospheric deposition of gases and particles above forest stands is commonly determined by comparison of bulk precipitation and throughfall measurements (e.g. Matzner et al., 2004). This method is limited by unaccounted canopy processes affecting the measured fluxes. The “gradient” method (e.g. Fowler et al., 2001) is another widely used indirect technique. Here, the vertical flux is calculated as the product of the measured vertical concentration gradient and the so-called eddy diffusivity which is estimated from proxy measurements. However, in a well-developed turbulence regime, only small gradients are established, thus limiting the gradient method depending on the quality of the chemical analysis system. In the inferential approach, the deposition flux F is calculated as the product of the measured scalar concentration, c, and the so-called deposition velocity, vd,F=-vd·c.

The deposition velocity combines all micro-physical mechanisms contributing to the dry deposition in a single parameter. For this reason, the quality of the inferential approach depends mainly on the parameterization of the deposition velocity.

Direct micrometeorological approaches such as the “eddy covariance” (EC) or the “eddy accumulation” (EA) methods (e.g. Stull, 1999) yield promising results, however, in contrast to many gaseous species, the application of these techniques to particle flux measurements is still a field of research. Relaxed eddy accumulation (REA) has been applied to measure particle fluxes in a semi-arid environment (Schery et al., 1998), in an urban environment (Nemitz et al., 2001), and for size-resolved measurements (Gaman et al., 2004).

The vertical particle number flux may be directly measured by EC. In this approach, particle number concentrations and the vertical wind velocity are measured with high time resolution in order to obtain the mean value and the turbulent fluctuations of these scalars. Thus, the turbulent flux may be calculated as the covariance of the particle concentration and the vertical wind velocity.

Fluctuations of the vertical wind are routinely measured using sonic anemometers with time resolutions of 10–20 Hz. Atmospheric particles may be detected using electrical or optical counters. Flux experiments using electrical counting devices were limited to particle sizes >100 nm diameter (e.g. Lamaud et al., 1994). Early studies using optical particle counters in EC systems were limited by the slow response of the instruments (Katen and Hubbe, 1985; Duan et al., 1988). Nevertheless, recent work by Gallagher et al. (1997) and Nilsson et al. (2003) yielded interesting results for high particle concentrations. Optical counting of individual particles due to light scattering is also limited to particles >100 nm diameter. Below this threshold, the scattering intensity becomes too small to be detected (Flagan, 1998).

The BEWA joint project within the German atmospheric research program AFO2000, however, aimed to study the role of biogenic volatile organic compounds and their reaction products in particle formation processes (Steinbrecher et al., 2004). When studying atmospheric particle formation it is essential to use particle counters with detection limits below 100 nm diameter. This led to the use of condensation particle counters in EC systems (Buzorius et al., 1998). In this work, an EC system combining a sonic anemometer and two condensation particle counters was set up and applied above a Norway spruce forest. In particular, the turbulent vertical fluxes of aerosol particles during particle formation events were investigated. The identification and analysis of particle formation events from particle size distribution measurements are presented in Held et al. (2004). Here we describe the BEWA particle flux system, and present results of the particle flux measurements during the BEWA field campaigns.

Section snippets

Site

The BEWA field campaigns were carried out in July/August 2001 and 2002, respectively, at the “Waldstein” ecosystem research site of the Bayreuth Institute of Terrestrial Ecosystem Research (BITÖK) in the “Fichtelgebirge” mountain range. This forest site situated near the Czech/German border (50°09′N, 11°52′E) is dominated by Norway spruce (Picea abies (L.) Karst) surrounding a 30 m scaffolding tower at 776 m asl. In recent years, both EC (e.g., Klemm and Mangold, 2001; Burkard et al., 2002;

Results and discussion

Vertical turbulent fluxes of aerosol particles were measured above a Norway spruce forest using an EC system. The particle counters measured particle concentrations over a wide size range from a few nm to several μm. Thus, the resulting flux measurements yield integral number fluxes of the polydisperse particle population.

Fig. 3 shows 6 days of turbulent particle number fluxes as measured with the CPC and the UCPC systems, respectively. The CPC and UCPC flux patterns are quite similar to each

Conclusions

In this work, an eddy covariance (EC) system was set up and successfully applied above a Norway spruce forest to directly measure the vertical turbulent exchange of aerosol particles between the vegetative surface and the atmospheric boundary layer. The resulting deposition velocities on the order of 10 mm s−1 are consistent with values found in other studies over comparable surfaces (Table 3).

Considerably lower deposition velocities have been measured above open sea and low vegetation such as

Acknowledgements

We appreciate the help of T. Braun, J. Gerchau, G. Müller, T. Wrzesinsky and the support of the BEWA community. This study was funded by the German federal ministry of education and research (BMBF) through grants PT BEO 51-0339476 D (BITÖK) and PT UKF 07ATF25 (BEWA).

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    1

    Now at: Institute of Landscape Ecology (ILÖK), University of Münster, 48149 Münster, Germany.

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