Tanja Suni

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.

Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique

Micrometeorological measurements made on single towers often underestimate nighttime respiration of terrestrial ecosystems because they cannot account for vertical and horizontal advection, thereby causing systematic errors in estimates of net ecosystem carbon exchange. We show that there is a maximum in the sum of the turbulent flux and change in storage of CO2 in the early evening, Rmax, that is in close agreement with concurrent and independent estimates of net carbon exchange from soil and plant chambers.We hypothesize that the peak occurs because there is a time delay between the onset of radiative cooling and the development of temperature gradients that are strong enough to initiate thermally-driven horizontal and vertical flows that remove the stored CO2. We propose taking advantage of this time delay to develop relationships between Rmax and soil temperature and moisture. The new parameterization leads to realistic values of nighttime respiration, and therefore to improved estimates of net ecosystem exchange.

Global observations of aerosol‐cloud‐precipitation‐climate interactions

Cloud drop condensation nuclei (CCN) and ice nuclei (IN) particles determine to a large extent cloud microstructure and, consequently, cloud albedo and the dynamic response of clouds to aerosol-induced changes to precipitation. This can modify the reflected solar radiation and the thermal radiation emitted to space. Measurements of tropospheric CCN and IN over large areas have not been possible and can be only roughly approximated from satellite-sensor-based estimates of optical properties of aerosols. Our lack of ability to measure both CCN and cloud updrafts precludes disentangling the effects of meteorology from those of aerosols and represents the largest component in our uncertainty in anthropogenic climate forcing. Ways to improve the retrieval accuracy include multiangle and multipolarimetric passive measurements of the optical signal and multispectral lidar polarimetric measurements. Indirect methods include proxies of trace gases, as retrieved by hyperspectral sensors. Perhaps the most promising emerging direction is retrieving the CCN properties by simultaneously retrieving convective cloud drop number concentrations and updraft speeds, which amounts to using clouds as natural CCN chambers. These satellite observations have to be constrained by in situ observations of aerosol-cloud-precipitation-climate (ACPC) interactions, which in turn constrain a hierarchy of model simulations of ACPC. Since the essence of a general circulation model is an accurate quantification of the energy and mass fluxes in all forms between the surface, atmosphere and outer space, a route to progress is proposed here in the form of a series of box flux closure experiments in the various climate regimes. A roadmap is provided for quantifying the ACPC interactions and thereby reducing the uncertainty in anthropogenic climate forcing.

Eddy covariance measurements of carbon exchange and latent and sensible heat fluxes over a boreal lake for a full open‐water period

was >0 W/m 2 at night and <0 W/m 2 in daytime. The latent heat flux dominated clearly over H in spring and summer; that is, the Bowen ratio was less than 1. Highermoment turbulence statistics proved to be efficient in detection of frequent nonstationary situations. Applying the statistical criteria for CO2 concentration and vertical wind speed, averaging over a 5-min period and selecting only the wind direction with longest fetch, we could obtain lake-representative CO2 fluxes. Footprint analysis based on a closure model revealed that the source areas were relatively short because of the presence of turbulence generated by the surrounding forest, compared to a larger lake with an extended smooth surface. We observed a net CO2 source of 0.2–0.4 mmol m � 2 s � 1 excluding July, when the flux was closer to zero. The results are consistent with the gradient method, based on more infrequent sampling, and both methods gave the same average flux, 0.2 mmol m � 2 s � 1 , over the whole open-water period.

Hygroscopic growth of ultrafine sodium chloride particles

The hygroscopic growth properties of ultrafine NaCl particles between 8 and 50 nm in mobility diameter have been studied using an ultrafine tandem differential mobility analyzer. The data include determination of hygroscopic growth curves, deliquescence behavior, and hysteresis. The measured growth factors are clearly smaller for the nanometer-sized particles compared with particles in the size range of 0.1 μm and larger. While this behavior of the ultrafine particles can be qualitatively predicted by accounting for the Kelvin effect, quantitative differences to the experimental data remain even after the differential mobility analyzer data have been corrected to account for the cubic shape of the particles. We conclude that the differences are related to size-dependent density and/or shape/structure of nanometer-sized NaCl crystals. The deliquescence relative humidities obtained in this work for nanometer-sized particles also show clear differences from the literature value obtained for large particles. The quantitative values of hygroscopic growth factors and deliquescence relative humidities for small particles have not been available previously, and our data are important in studies of the formation and properties of atmospheric aerosols.

Fluxes of carbon dioxide and water vapour over Scots pine forest and clearing

The carbon dioxide and water vapour fluxes were measured by the eddy covariance (EC) technique from July to September 2000 at two closely located sites in southern Finland: over a 38-year-old pine forest and over a 5-year-old forest clearing. The night-time respiration was of the same magnitude at both sites. At day-time the pine forest was a strong sink but the clearing close to CO2 balance, indicating that CO2 uptake of ground vegetation over the clearing balanced the release from the soil. The shoot scale gas exchange measurements in combination with process-based modelling were used to evaluate the measured CO2 exchange of the forest ecosystem. The forest CO2 exchange was explained by soil respiration and photosynthesis of forest canopy, while the contribution of understory and ground vegetation CO2 exchange could be neglected. During the study period the forest was a net sink of CO2 and the clearing a source. The daily average uptake of CO2 by the forest was −2.4 and − 1.7 gm −2 per day in July–August and September periods, respectively; and average release by the clearing 4.0 and 2.5 g m −2 per day during the same periods. This shows that carbon losses 5 years after clear-cutting are substantial. The evapotranspiration (ET) was higher over the forest compared to clearing as a result of transpiration from the forest canopy. The difference in ET was small during the July–August period when precipitation frequently occurred.

Observations on nocturnal growth of atmospheric clusters

In this paper, we summarize recent observations of nighttime nucleation events observed during 4 yr, from 2003 to 2006, at the SMEAR II station in Hyytiäläa, southern Finland. Formation of new atmospheric aerosol particles has been frequently observed all around the world in daytime, but similar observations in nighttime are rare. The recently developed ion spectrometers enabled us to measure charged aerosol particles and ion clusters to diameters < 1 nm and are efficient tools for evaluating cluster dynamics during nighttime. We observed clear growth of cluster ions during approximately 60 nights per yr. The newly formed intermediate ions usually persisted for several hours with typical concentrations of 100–200 cm-3. The evolution of nighttime growth events is different compared with daytime events. The mechanism behind nighttime events is still unclear, but the behaviour can be described by the hypothesis of activation of clusters.

Hygroscopic and Volatile Properties of Ultrafine Particles in the Eucalypt Forests: Comparison with Chamber Experiments and the Role of Sulphates in New Particle Formation

Simultaneous measurements of the volatile and hygroscopic properties of ultrafine particles were conducted in a Eucalypt forest in Tumbarumba, South-East Australia, in November 2006. These measurements were part of an intensive field campaign EUCAP 2006 (Eucalypt Forest Aerosols and Precursors). The particles exhibited a 2 step volatilisation with the first component starting to evaporate at temperatures above 50 degrees Celsius. With the onset of evaporation of the first component the hygroscopic growth factor increased. This indicated that the particle was composed of a less volatile, but more hygroscopic core, which was coated with a more volatile, but less hygroscopic, coating. The fraction of the more hygroscopic component was proportional to the measured maximum SO2 concentration indicating the role of gaseous H2SO4 in new particle formation. As the volatilisation temperature of the second more hygroscopic component was above that for H2SO4 it is likely that this component is partially or fully neutralised H2SO4. Comparison with pinene smog chamber experiments shows an excellent agreement with the first step volatilisation indicating its origin in the photooxidation of a monoterpene precursor.

Atmospheric aerosol and ion characteristics during EUCAP (Eucalypt Forest Aerosols and Precursors)

We measured the characteristics and dynamics of atmospheric ions, aerosol particles, and their precursors in an intensive field campaign in a Eucalypt forest in Tumbarumba, South-East Australia, in November 2006. The measured size range of ions was 0.34 to 40 nm and that of aerosol particles approximately 10 to 168 nm, and for observing their size distributions we used an Air Ion Spectrometer (AIS) and a Scanning Mobility Particle Sizer (SMPS). We also measured the hygroscopic and chemical properties of the particles with a Volatility-Humidity Tandem Differential Mobility Analyser (VH-TDMA). The total concentration of ultrafine aerosol particles was measured with a Condensational Particle Counter (CPC). Furthermore, we measured ambient concentrations of volatile organic compounds (VOC), SOx, NOx/NOy, and O3. Finally, we modelled the 96-h back trajectories of air masses arriving at the site and observed that the arrival directions varied greatly and included trajectories that travelled only over land as well as ones that travelled most of the time over the ocean. On the most polluted day, the air masses arrived approximately from the direction of greater Sydney / Newcastle coal mine area. The total concentration of ultrafine aerosol particles was approximately 3500 cm-3, and daytime aerosol formation took place on 64% of days with acceptable data. The dominant VOCs were isoprene, eucalyptol, a- and b-pinene, camphene, and limonene. The measured hygroscopic growth factors (Gh) at RH of 90% varied from 1.1 to 1.5. The smallest Gh were observed for aged accumulation mode particles in early mornings, and the largest Gh occurred for the freshly nucleated particles on Nov 10, the day with the highest concentration of SO2.