Tobias Naegler

Observations and modelling of the global distribution and long-term trend of atmospheric 14CO2.

Global high-precision atmospheric Δ14CO2 records covering the last two decades are presented, and evaluated in terms of changing (radio)carbon sources and sinks, using the coarse-grid carbon cycle model GRACE. Dedicated simulations of global trends and interhemispheric differences with respect to atmospheric CO2 as well as δ13CO2 and Δ14CO2, are shown to be in good agreement with the available observations (1940–2008). While until the 1990s the decreasing trend of Δ14CO2 was governed by equilibration of the atmospheric bomb 14C perturbation with the oceans and terrestrial biosphere, the largest perturbation today are emissions of 14C-free fossil fuel CO2. This source presently depletes global atmospheric Δ14CO2 by 12–14‰ yr−1, which is partially compensated by 14CO2 release from the biosphere, industrial 14C emissions and natural 14C production. Fossil fuel emissions also drive the changing north–south gradient, showing lower Δ14C in the northern hemisphere only since 2002. The fossil fuel-induced north–south (and also troposphere–stratosphere) Δ14CO2 gradient today also drives the tropospheric Δ14CO2 seasonality through variations of air mass exchange between these atmospheric compartments. Neither the observed temporal trend nor the Δ14CO2 north–south gradient may constrain global fossil fuel CO2 emissions to better than 25%, due to large uncertainties in other components of the (radio)carbon cycle.

Excess radiocarbon constraints on air-sea gas exchange and the uptake of CO2 by the oceans

We re-assess the constraints that estimates of the global ocean excess radiocarbon inventory place on air-sea gas exchange. We found that the gas exchange scaling parameter aq cannot be constrained by the excess radiocarbon inventory alone. Non-negligible biases in different global wind speed datasets require a careful adaptation of aq to the wind field chosen. Furthermore, aq depends on the spatial and temporal resolution of the wind fields. We develop a new wind speed- and inventory-normalized gas exchange parameter aqN which takes into account these biases and which is easily adaptable to any new estimate of the ocean excess radiocarbon inventory. Our study yields an average estimate of aq of 0.32+/-0.05 for monthly mean winds, lower than the previous estimate (0.39) from Wwanninkhof (1992). We calculate a global annual average piston velocity for CO2 of (16.7+/-2.9) cm/hr and a gross CO2 flux between atmosphere and ocean of (73+/-10) PgC/yr, significantly lower than results from previous studies.

Observations of atmospheric variability and soil exhalation rate of radon-222 at a Russian forest site. Technical approach and deployment for boundary layer studies

Abstract A monitor for continuous observations of the atmospheric 222Rn daughter activity has been improved and successfully implemented in a field study in the European Taiga (Fyodorovskoye Forest Reserve). The α-activity of the short-lived 222Rn and 220Rn (212Pb) decay products, which are attached to aerosols, is accumulated on a quartz aerosol filter and assayed on line by α-spectroscopy. The α-activity from the 212Pb daughters is determined by spectroscopy and corrected for. This monitor is suitable to measure 222Rn activities at hourly resolution down to 0.5 Bq m−3 with an uncertainty well below ±20%. The prototype of this monitor is run in Heidelberg on the roof of the Institutes building about 20 m above ground. For this site, the atmospheric radioactive disequilibrium was determined between the 222Rn daughter 214Po and 222Rn, which has to be known in order to derive the atmospheric 222Rn activity with the static filter method. We derived a mean disequilibrium 214Po/222Rn = 0.704 ± 0.081 for various meteorological conditions through parallel222Rn gas measurements with a slow pulse ionisation chamber. At the Russian field site, continuous activity observations were performed from July 1998 until July 2000 with half a years interruption in summer/fall 1999. During intensive campaigns, a second monitor was installed at Fyodorovskoye at 15.6 m (July/August 1998), and at 1.8 m (July/August 1999 and October 1999) above ground. As expected, pronounced diurnal cycles of the 222Rn daughter activity were observed at all sites, particularly during summer when the vertical mixing conditions in the atmospheric surface layer vary strongly between day and night. The lower envelope of the continuous measurements at Fyodorovskoye and at Heidelberg changes on synoptic timescales by a factor of 4–10 due to long-range transport changes between continental to more maritime situations. Generally, the 222Rn activity at 26.3 m height at Fyodorovskoye is lower by a factor of 2–3 compared to Heidelberg at 20 m above ground. This unexpected result is due to considerably lower 222Rn exhalation rates from the soils measured in the footprint of the Fyodorovskoye Forest tower compared to Heidelberg. With the inverted chamber technique 222Rn exhalation rates in the range 3.3–7.9 Bq m−2 h−1 were determined at Fyodorovskoye for summer 1998 and autumn 1999 (wet conditions with water table depths between 5 and 70 cm). Only during the very dry summer of 1999 the mean222Rn exhalation rate increased by about a factor of five. All measured exhalation rates at the Fyodorovskoye Forest are considerably smaller by a factor of 2–10 compared to observations in the vicinity of Heidelberg (ca. 50–60 Bq m−2 h−1) and generally in Western Europe.

Reconciliation of excess 14C-constrained global CO2 piston velocity estimates

Oceanic excess radiocarbon data is widely used as a constraint for air–sea gas exchange.However, recent estimates of the global mean piston velocity 〈k〉 from Naegler et al., Krakauer et al., Sweeney et al. and Müller et al. differ substantially despite the fact that they all are based on excess radiocarbon data from the GLODAP data base. Here I show that these estimates of 〈k〉 can be reconciled if first, the changing oceanic radiocarbon inventory due to net uptake of CO2 is taken into account; second, if realistic reconstructions of sea surface δ14C are used and third, if 〈k〉 is consistently reported with or without normalization to a Schmidt number of 660. These corrections applied, unnormalized estimates of 〈k〉 from these studies range between 15.1 and 18.2 cmh-1. However, none of these estimates can be regarded as the only correct value for 〈k〉. I thus propose to use the ‘average’ of the corrected values of 〈k〉 presented here (16.5±3.2 cmh-1) as the best available estimate of the global mean unnormalized piston velocity 〈k〉, resulting in a gross ocean-to-atmosphere CO2 flux of 76 ± 15 PgC yr-1 for the mid-1990s.