Tiia Grönholm

Effect of thinning on surface fluxes in a boreal forest

[1] Thinning is a routine forest management operation that changes tree spacing, number, and size distribution and affects the material flows between vegetation and the atmosphere. Here, using direct micrometeorological ecosystem-scale measurements, we show that in a boreal pine forest, thinning decreases the deposition velocities of fine particles as expected but does not reduce the carbon sink, water vapor flux, or ozone deposition. The thinning decreased the all-sided leaf area index from 8 to 6, and we suggest that the redistribution of sources and sinks within the ecosystem compensated for this reduction in foliage area. In the case of water vapor and O 3 , changes in light penetration and among-tree competition seem to increase individual transpiration rates and lead to larger stomatal apertures, thus enhancing also O 3 deposition. In the case of CO 2 , increased ground vegetation assimilation and decreased autotrophic respiration seem to cancel out opposite changes in canopy assimilation and heterotrophic respiration. Current soil-vegetation-atmosphere transfer models should be able to reproduce these observations.

Ultrafine particle scavenging coefficients calculated from 6 years field measurements

Abstract Based on 6 years of outdoor measurements at a boreal forest site in Southern Finland, scavenging coefficients were calculated for aerosol particles having diameter between 10 and 510 nm . Median scavenging coefficients varied between 7×10−6 and 4×10 −5 s −1 in this size-range. The dependence of scavenging coefficients on rain intensity was studied, and the scavenging coefficients were parameterized as a function of particle size for particle diameters of 10– 500 nm and for rain intensities 0– 20 mm h −1 .

A review of measurement and modelling results of particle atmosphere–surface exchange

Atmosphere–surface exchange represents one mechanism by which atmospheric particle mass and number size distributions are modified. Deposition velocities (vd) exhibit a pronounced dependence on surface type, due in part to turbulence structure (as manifest in friction velocity), with minima of approximately 0.01 and 0.2 cm s-1 over grasslands and 0.1–1 cm s-1 over forests. However, as noted over 20 yr ago, observations over forests generally do not support the pronounced minimum of deposition velocity (vd) for particle diameters of 0.1–2 μm as manifest in theoretical predictions. Closer agreement between models and observations is found over less-rough surfaces though those data also imply substantially higher surface collection efficiencies than were originally proposed and are manifest in current models. We review theorized dependencies for particle fluxes, describe and critique model approaches and innovations in experimental approaches, and synthesize common conclusions of experimental and modelling studies. We end by proposing a number of research avenues that should be pursued in to facilitate further insights and development of improved numerical models of atmospheric particles.

Relative Humidity Effect on the High-Frequency Attenuation of Water Vapor Flux Measured by a Closed-Path Eddy Covariance System

In this study the high-frequency loss of carbon dioxide (CO2) and water vapor (H2O) fluxes, measured by a closed-path eddy covariance system, were studied, and the related correction factors through the cospectral transfer function method were calculated. As already reported by other studies, it was found that the age of the sampling tube is a relevant factor to consider when estimating the spectral correction of water vapor fluxes. Moreover, a time-dependent relationship between the characteristic time constant (or response time) for water vapor and the ambient relative humidity was disclosed. Such dependence is negligible when the sampling tube is new, but it becomes important already when the tube is only 1 yr old and increases with the age of the tube. With a new sampling tube, the correction of water vapor flux measurements over a Scots pine

Measurements of aerosol particle dry deposition velocity using the relaxed eddy accumulation technique

The continuous measurements of aerosol particle deposition velocity have been performed from January 2004 to January 2005 using a REA technique with dynamic deadband.We measured aerosol particle deposition velocity in the size range of 10–150 nanometer with 5–10 nanometer steps using differential mobility analyser for sizing.We were able to measure two size classes simultaneously. One size class was changed at one month intervals, another we kept constant at 30 nm to investigate the effect of seasonal and meteorological variation on deposition velocity. We found that the 80–100 nanometer size particles had the lowest deposition velocity, about 0.4 cm s−-1. Deposition velocity increased with decreasing or increasing particle diameter from 80–100 nanometer size. We also found that deposition velocity increases as a function of friction velocity.

Predicting the dry deposition of aerosol-sized particles using layer-resolved canopy and pipe flow analogy models: Role of turbophoresis

inertial‐impaction regime for many laboratory and crop experiments, but none of the forest measurements fall on this apparent scaling law; and (3) two recent models with entirely different assumptions about the representation of the particle deposition process reproduce common data sets for forests. We show that turbophoresis, when accounted for at the leaf scale in vertically resolved or multilayer models (MLMs), provides a coherent explanation for the first two findings and sheds light on the third. The MLM resolves the canopy vertical structure and its effects on both the flow statistics and the leaf particle collection mechanisms. The proposed MLM predictions agree with a recent two‐level particle‐resolving data set collected over 1 year duration for a Scots pine stand in Hyytiala (southern Finland). Such an approach can readily proportion the particle deposition onto foliage and forest floor and can take advantage of recent advances in measurements of canopy structural properties derived from remote sensing platforms.

Upward fluxes of particles over forests: when, where, why?

Of the 60% of particle number fluxes over two forests that exceed the associated uncertainty bounds, approximately one-third are upward. These ‘apparent emission’ fluxes are not solely observed during periods when other micrometeorological fluxes are ill-defined, which implies they derive from a/multiple physical cause/s. Upward fluxes are slightly more frequent at night over the Danish beech forest but do not depend on wind direction or speed. Data from the pine forest in Finland indicate no diurnal cycle in the frequency with which upward fluxes are observed, although as in data from the beech forest the magnitude of upward fluxes is higher during the day. At the pine forest local emissions may account for some of the upward fluxes but other mechanisms appear also to play a role. Entrainment of particle depleted air from above the boundary layer, analysed via use of quadrant analysis and scalar correlations, appears to be important in the occurrence of upward fluxes at both sites. The rate of upward fluxes scales with prevailing geometric mean diameter (GMD) and consistent with the hypothesis of entrainment of relatively particle-depleted air upward fluxes appear to be associated with particle ensembles characterized by larger prevailing GMD.

Hot-air Balloon Measurements of Vertical Variation of Boundary Layer New Particle Formation

In this study, we used a hot-air balloon as a platform for boundary layer particle and cluster measurements. We did altogether 11 flights during the spring of 2005 and 2006. During the spring of 2006, we observed five new particle formation days. During all days, new particle formation took place in the mixed boundary layer. During one of the days, we observed particle formation in the free troposphere, separate from that of the mixed layer. The observations showed that the concentration of freshly-formed 1.5-2 nm negative ions was several times higher than the concentration of positive ions. We also clearly observed that nucleation during one of the days, 13 March 2006, was a combination of neutral and ion-induced nucleation. During some of the days, particle growth stopped at around 3 nm, probably due to lack of condensable organic vapours. Simulations of boundary layer dynamics showed that particles are formed either throughout the mixed layer or in the lower part of it, not at the top of the layer.

Effect of Leaf Water Potential on Internal Humidity and CO2 Dissolution: Reverse Transpiration and Improved Water Use Efficiency under Negative Pressure

The pull of water from the soil to the leaves causes water in the transpiration stream to be under negative pressure decreasing the water potential below zero. The osmotic concentration also contributes to the decrease in leaf water potential but with much lesser extent. Thus, the surface tension force is approximately balanced by a force induced by negative water potential resulting in concavely curved water-air interfaces in leaves. The lowered water potential causes a reduction in the equilibrium water vapor pressure in internal (sub-stomatal/intercellular) cavities in relation to that over water with the potential of zero, i.e., over the flat surface. The curved surface causes a reduction also in the equilibrium vapor pressure of dissolved CO2, thus enhancing its physical solubility to water. Although the water vapor reduction is acknowledged by plant physiologists its consequences for water vapor exchange at low water potential values have received very little attention. Consequences of the enhanced CO2 solubility to a leaf water-carbon budget have not been considered at all before this study. We use theoretical calculations and modeling to show how the reduction in the vapor pressures affects transpiration and carbon assimilation rates. Our results indicate that the reduction in vapor pressures of water and CO2 could enhance plant water use efficiency up to about 10% at a leaf water potential of −2 MPa, and much more when water potential decreases further. The low water potential allows for a direct stomatal water vapor uptake from the ambient air even at sub-100% relative humidity values. This alone could explain the observed rates of foliar water uptake by e.g., the coastal redwood in the fog belt region of coastal California provided the stomata are sufficiently open. The omission of the reduction in the water vapor pressure causes a bias in the estimates of the stomatal conductance and leaf internal CO2 concentration based on leaf gas exchange measurements. Manufactures of leaf gas exchange measurement systems should incorporate leaf water potentials in measurement set-ups.

A Structure Function Model Recovers the Many Formulations for Air-Water Gas Transfer Velocity: Air-Water Gas Transfer

Two ideas regarding the structure of turbulence near a clear air-water interface are used to derive a waterside gas transfer velocity kL for sparingly and slightly soluble gases. The first is that kL is proportional to the turnover velocity described by the vertical velocity structure function Dww(r), where r is separation distance between two points. The second is that the scalar exchange between the air-water interface and the waterside turbulence can be suitably described by a length scale proportional to the Batchelor scale lB = ηSc−1∕2, where Sc is the molecular Schmidt number and η is the Kolmogorov microscale defining the smallest scale of turbulent eddies impacted by fluid viscosity. Using an approximate solution to the von Kármán-Howarth equation predicting Dww(r) in the inertial and viscous regimes, prior formulations for kL are recovered including (i) kL = √ 2∕15ScvK , vK is the Kolmogorov velocity defined by the Reynolds number vKη∕ν = 1 and ν is the kinematic viscosity of water; (ii) surface divergence formulations; (iii) kL ∝ Scu∗, where u∗ is the waterside friction velocity; (iv) kL ∝ Sc−1∕2 √ gν∕u∗ for Keulegan numbers exceeding a threshold needed for long-wave generation, where the proportionality constant varies with wave age, g is the gravitational acceleration; and (v) kL = √ 2∕15Sc(νgβoqo) in free convection, where qo is the surface heat flux and βo is the thermal expansion of water. The work demonstrates that the aforementioned kL formulations can be recovered from a single structure function model derived for locally homogeneous and isotropic turbulence. Plain Language Summary The problem considered here is a theoretical prediction of mass transfer across and air-water interface. This interfacial transfer phenomenon is featured prominently in global carbon balances, methane, nitrous oxides, dimethyl sulfide, and other gases. It is used to assess the metabolic health of aquatic ecosystems and to determine evasion rates of volatile organic compounds from lakes, estuaries, reservoirs, and large water treatment plants. The novelty of the approach is to link a bulk quantity reflecting the efficiency of swirling motions near the air-water interface to the sizes of eddies and their energetic content responsible for the aforementioned swirling motion. The proposed approach is shown to recover a number of equations describing gas transport across interfaces that summarize a large corpus of experiments and simulations.