Juha Hatakka

Spring initiation and autumn cessation of boreal coniferous forest CO2 exchange assessed by meteorological and biological variables

We studied the commencement and finishing of the growing season using different air temperature indices, the surface albedo, the chlorophyll fluorescence (Fv/Fm) and the carbon dioxide (CO2) tropospheric concentration, together with eddy covariance measurements of CO2 flux. We used CO2 flux data from four boreal coniferous forest sites covering a wide latitudinal range, and CO2 concentration measurements from Sammaltunturi in Pallas. The CO2 gas exchange was taken as the primary determinant for the growing season to which other methods were compared. Indices based on the cumulative temperature sum and the variation in daily mean temperature were successfully used for approximating the start and cessation of the growing season. The beginning of snow melt was a successful predictor of the onset of the growing season. The chlorophyll fluorescence parameter Fv/Fm and the CO2 concentration were good indicators of both the commencement and cessation of the growing season. By a derivative estimation method for the CO2 concentration, we were also able to capture the larger-scale spring recovery. The trends of the CO2 concentration and temperature indices at Pallas/Sammaltunturi were studied over an 11-yr time period, and a significant tendency towards an earlier spring was observed. This tendency was not observed at the other sites.

Tropospheric carbon dioxide concentrations at a northern boreal site in Finland: basic variations and source areas

Abstract Diurnal and annual variations of CO2, O3, SO2, black carbon and condensation nuclei and their source areas were studied by utilizing air parcel trajectories and tropospheric concentration measurements at a boreal GAW site in Pallas, Finland. The average growth trend of CO2 was about 2.5 ppm yr−1 according to a 4-yr measurement period starting in October 1996. The annual cycle of CO2 showed concentration difference of about 19 ppm between the summer minimum and winter maximum. The diurnal cycle was most pronounced during July and August. The variation between daily minimum and maximum was about 5 ppm. There was a diurnal cycle in aerosol concentrations during spring and summer. Diurnal variation in ozone concentrations was weak. According to trajectory analysis the site was equally affected by continental and marine air masses. During summer the contribution of continental air increased, although the southernmost influences decreased. During daytime in summer the source areas of CO2 were mainly located in the northern parts of the Central Europe, while during winter the sources were more evenly distributed. Ozone showed similar source areas during summer, while during winter, unlike CO2, high concentrations were observed in air arriving from the sea. Sulfur dioxide sources were more northern (Kola peninsula and further east) and CO2 sources west-weighted in comparison to sources of black carbon. Source areas of black carbon were similar to source areas of aerosols during winter. Aerosol source area distributions showed signs of marine sources during spring and summer.

Source areas of airborne 7Be and 210Pb measured in Northern Finland.

Daily aerosol samples were collected at Sodankylä, Northern Finland, from July 1995 to June 1997. The filter samples were analyzed for 7Be by semiconductor gamma spectrometry and for 210Pb by alpha counting of the in-grown 210Po. The concentrations were lognormally distributed with median concentrations of 2,500 and 160 &mgr;Bq m−3 for 7Be and 210Pb, respectively. The trajectory analysis performed showed that the highest 210Pb activity concentrations were associated with continental air masses originating in Central Russia. High 7Be activity concentrations were found in air masses from Central Russia and, during springtime, also in air masses coming from southwest. The latter cases were attributed to the transfer of stratospheric air masses into the troposphere along the polar front. Slightly elevated 7Be and 210Pb concentrations were found in air masses coming to Sodankylä from north-west between 70th and 80th latitudes. This was attributed to the long-range transport from continental North America.

Stable carbon isotope signatures of methane from a Finnish subarctic wetland

ABSTRACT Methane emissions from Lompolojänkkä, a Finnish aapa mire within the Arctic Circle, were studied by non-intrusive Keeling plot methods, to place better constraints on the seasonal variations in isotopic signature of methane (δ13CCH4) emitted from Arctic wetland. Air samples were collected in Tedlar bags over the wetland at heights of 42 and 280 cm between May and October 2009 and in August 2008. The mixing ratio and δ13C of the methane in the samples were incorporated into Keeling plot analyses to derive bulk δ13CCH4 signatures for the methane inputs to the air above the wetland. The results show an unexpected consistence in δ13CCH4 from early to late summer, clustered around −68.5±0.7‰, but during spring thaw and autumnal freezing, δ13CCH4 is enriched by approximately 2 and 4‰, respectively. The techniques reported in this paper are simple and economical to employ, and give a bulk source signature for the methane inputs to the air above the entire wetland that can be extrapolated to a larger regional area.

Observational signature of the direct radiative effect by natural boreal forest aerosols and its relation to the corresponding first indirect effect

By using a screened set of long-term aerosol measurement data, the contribution of natural boreal forest aerosols to the direct radiative effect (DRE) was observed at a remote continental site in n ...

Atmospheric transport of carbon dioxide to a baseline monitoring station in northern Finland

Interannual and seasonal variations in atmospheric transport to a baseline monitoring station at Pallas (67°58’N, 24°07’E) in northern Finland were examined. The transport was analysed through cluster analysis of three-dimensional 5-d back-trajectories during the period 1997–2003. The trajectory climatology shows that air mass advection from the north is most frequent—mostly at high wind speeds across the Arctic Basin and from northern Siberia—but during summer more stagnant flows from the Norwegian Sea are common as well. Western and Central Europe were found to be the second most important regions of influence for air arriving at Pallas, followed by atmospheric transport from west Russia and the Atlantic, respectively. The trajectory clusters were combined with measurements of carbon dioxide (CO2) in order to examine the linkage between atmospheric large-scale circulation and CO2concentration at Pallas. The Atlantic and Arctic air masses were associated with relatively small annual CO2 amplitudes at Pallas. In contrast, large concentration differences between the summer minimum and winter maximum were observed during periods of continental air mass transport from the south and the east. In particular the air masses originating from west Russia were associated with very low CO2 concentrations during summer, indicating high photosynthetic activity of the terrestrial biosphere in this region. We analysed how the vertical motion of the trajectories affects the observed CO2 at Pallas. The largest difference in CO2 concentration between air parcels moving at low and high altitudes, respectively, was found during air mass advection from Europe and west Russia. This was especially true during the winter months when large CO2 emissions in these areas, i.e. from fossil fuel combustion and the decomposition and respiration of the vegetation, in combination with stable stratification can give rise to very high CO2concentrations in air parcels transported close to the surface. The CO2 time-series from Pallas was compared with CO2 measurements made at the Mount Zeppelin station on Svalbard, illustrating the different characteristics—boreal and maritime, respectively—of the regions affecting the two monitoring sites.

Large contribution of boreal upland forest soils to a catchment‐scale CH4 balance in a wet year

Upland forest soils affect the atmospheric methane (CH4) balance, not only through the soil sink but also due to episodic high emissions in wet conditions. We measured methane fluxes and found that during a wet fall the forest soil turned from a CH4 sink into a large source for several months, while the CH4 emissions from a nearby wetland did not increase. When upscaled to the whole catchment area the contribution of forests amounted to 60% of the annual CH4 emission from the wetlands, while in a normal year the forest soil consumes 10% of the wetland emission. The period of high upland soil emission was also captured by the nearby atmospheric concentration measurement station. Since the land cover within the catchment is representative of larger regions, our findings imply that upland forests in the boreal zone constitute an important part in the global CH4 cycle not previously accounted for.

Seasonal variation and source areas of airborne lead-210 at Ny-Ålesund in the High Arctic

High-volume aerosol particle samples were collected onto glass-fibre filters at Mount Zeppelin Global Atmosphere Watch station, Ny-ÅAring;lesund, Svalbard, in 2001–05. The filters were assayed for lead-210 (210Pb) by measuring the alpha particles of its in-grown daughter nuclide polonium-210 (210Po). The observed 210Pb activity concentrations at Mount Zeppelin vary between <4 and 1060 μBq m-3, with an arithmetic mean of 130 μBq m-3 and a median of 74 μBq m-3. The lowest 210Pb activity concentrations are found during summer and the highest are found in winter. This variation is caused by seasonal differences in the mixing conditions of the troposphere, the level of precipitation and the speed of atmospheric chemistry induced by solar radiation. The performed source area analysis, which is based on air mass back trajectories, indicated that in summer, 210Pb can be used as a tracer for air masses coming into contact with land areas within the past 5 days. In winter this cannot be performed because of the accumulation of 210Pb-carrying aerosol particles into the Arctic atmosphere during the Arctic night. But even in winter a low 210Pb activity concentration indicates that the associated air mass has had little if any contact with land areas.

Measurement of particulate carbon in atmospheric aerosol in Helsinki

By means of a simple optical apparatus the levels and variations of concentrations of elemental carbon in particles were measured. Additionally, relations between variations of elemental carbon, lead, sulfate, total suspended particle, and nitrate concentrations were studied. Calibration was performed on real samples analyzed by a wet chemical method.

Interpreting eddy covariance data from heterogeneous Siberian tundra: land cover-specific methane fluxes and spatial representativeness

The authors present a study that aims at decomposing the flux signals captured by an eddy-covariance flux system into flux signatures for individual land cover classes (LCCs) within the heterogeneous terrain surrounding the tower. In a first step, the landscape is mapped at highest resolution based on remote sensing datasets supported by ground-trothing, yielding gridded maps of LCC, elevation, NDVI and wetness. Based