Timothy J. Lueker

Ocean pCO2 calculated from dissolved inorganic carbon, alkalinity, and equations for K1 and K2: validation based on laboratory measurements of CO2 in gas and seawater at equilibrium

Abstract The partial pressure of carbon dioxide in the oceans surface waters, precisely expressed as the fugacity ( f CO 2 ) is determined from dissolved inorganic carbon (DIC) and total alkalinity (TA), and the first and second dissociation constants of carbonic acid, K 1 and K 2 . The original measurements of K 1 and K 2 reported by Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E., Pytkowicz, R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnol. Oceanogr. 18, 897–907] are reformulated to give equations for p K 1 and p K 2 (p K =−log 10 K ) as a function of seawater temperature and salinity, consistent with the “total hydrogen ion” concentration scale: p K 1 =3633.86/T−61.2172+9.67770 ln T−0.011555 S+0.0001152 S 2 p K 2 =471.78/T+25.9290−3.16967 ln T−0.01781 S+0.0001122 S 2 By equilibrating solutions of seawater with gas mixtures of known composition, we demonstrate that the above formulations of K 1 and K 2 give calculated f CO 2 values that agree with equilibrated values to 0.07±0.50% (95% confidence interval, f CO 2 up to 500 μatm). Formulations of K 1 and K 2 based on other studies resulted in calculated f CO 2 values approximately 10% lower than the measurements. Equilibrations at f CO 2 above 500 μatm yielded measured f CO 2 values higher than calculated values by on average 3.35±1.22% (95% confidence interval). The cause for the f CO 2 dependence of the results is not known. The uncertainties in p K 1 and p K 2 were combined with the analytical uncertainties typical of contemporary measurements of DIC and TA to reveal the expected reliability of seawater f CO 2 calculated from these parameters. For example, an uncertainty of 1.0 μmol kg −1 in DIC and 2 μmol kg −1 in TA (1 standard deviation (s.d.)) will result in uncertainty of the calculated f CO 2 of 1% or ±3.5 μatm at 350 μatm (1 s.d.).

Spatiotemporal patterns of carbon‐13 in the global surface oceans and the oceanic suess effect

A global synthesis of the 13C/12C ratio of dissolved inorganic carbon (DIC) in the surface ocean is attempted by summarizing high-precision data obtained from 1978 to 1997 in all major ocean basins. The data, mainly along transects but including three subtropical time series, are accompanied by simultaneous, precise measurements of DIC concentration and titration alkalinity. The reduced isotopic ratio, δ13C, in the surface ocean water is governed by a balance between biological and thermodynamic processes. These processes have strongly opposing tendencies, which result in a complex spatial pattern in δ13C with relatively little variability. The most distinctive feature in the spatial distribution of δ13C seen in our data is a maximum of δ13C near the subantarctic front with sharply falling values to the south. We attribute this feature to a combination of biological uptake of CO2 depleted in 13C (low δ13C) and air-sea exchange near the front and upwelling further south of waters with low δ13C resulting from the remineralization of organic matter. Additional features are maxima in δ13C downstream of upwelling regions, reflecting biological uptake, and minima in the subtropical gyres caused by strongly temperature dependent thermodynamic isotopic fractionation. At the time series stations, two in the North Atlantic Ocean and one in the North Pacific, distinct seasonal cycles in δ13C are observed, the Pacific data exhibiting only about half the amplitude of the Atlantic. Secular decreases in δ13C caused by the invasion of isotopically light anthropogenic CO2 into the ocean (the 13C Suess effect) have been identified at these time series stations and also in data from repeated transects in the Indian Ocean and the tropical Pacific. A tentative global extrapolation of these secular decreases yields a surface oceanic 13C Suess effect of approximately −0.018‰ yr−1 from 1980 to 1995. This effect is nearly the same as the 13C Suess effect observed globally in the atmosphere over the same period. We attribute this response to a deceleration in the growth rate of anthropogenic CO2 emissions after 1979, which subsequently has reduced the atmospheric 13C Suess effect more than the surface ocean effect.

The 13C Suess Effect in the world surface oceans and its implications for oceanic uptake of CO2: Analysis of observations at Bermuda

Surface ocean water δ13C measurements near Bermuda are examined in an attempt to find the annual decrease caused by the addition of anthropogenic CO2 to the atmosphere. We refer to this trend as the surface ocean 13C Suess effect. Interannual variability, which may be related to the El Nino - Southern Oscillation in the Atlantic Ocean, is apparent. We try to correct the data for this variability so as to better determine the trend. The trend has implications for the uptake of anthropogenic CO2 by the oceans. We employ a three-dimensional model of ocean chemistry to relate the trend at Bermuda to the average ocean trend, then use the average ocean trend to estimate the vertical diffusivity K in a one-dimensional ocean model, and finally use this model to calculate the oceanic uptake of CO2. Uncertainties associated with the estimation of the Suess effect at Bermuda and in the analysis procedure preclude a firm estimate of the oceanic uptake of CO2. Results are, in general, consistent with the low side of the Intergovernmental Panel on Climate Control estimation of 2.0 ± 0.8 GtC yr−1. With a longer record at Bermuda and δ13C observations at additional locations, we believe this approach will lead to a useful estimate of oceanic uptake.

The oxygen to carbon dioxide ratios observed in emissions from a wildfire in northern California

At Trinidad, California we observed elevated CO2 concentrations and concomitant lowered O2 levels coincident with forest fires 70 km distant (from 10/8/99 to 10/21/99). The precision of our O2 data, ±1 µmol O2/mol dry air, revealed the reduction of atmospheric oxygen resulting from the combustion of biomass, and the stoichiometric ratios (−O2/CO2) of the wildfire emissions. Estimates of daily −O2/CO2 ratios were obtained by regression of CO2 against corresponding O2 data (R² 0.86 to 0.96). Daily −O2/CO2 ratios changed from 1.15 to 1.41 on a particularly smoky day that coincided with elevated levels of CH4 and increased CH4/CO2 ratios. The change to a higher ratio during smoky conditions illustrates the association between changing emissions and −O2/CO2 ratios, possibly due to changing wildfire dynamics.

Correction to “Trace gas and particulate emissions from the 2003 southern California wildfires”

[1] In the paper ‘‘Trace gas and particulate emissions from the 2003 southern California wildfires’’ by J. Muhle et al. (Journal of Geophysical Research, 112, D03307, doi:10.1029/2006JD007350, 2007), the units for the emission ratios (ER) of methyl chloroform (CH3CCl3) versus carbon dioxide (CO2) in paragraph 27 were mistyped as mmol/mol, but have to be nmol/mol. We thank Paul J. Fraser for pointing this out. None of the conclusions are affected. The corrected sentence is given below: [2] The measuredDCH3CCl3/DCO2 ER of 48 ± 10 nmol/ mol is about 10 times smaller than the ER reported by Rudolph et al. [1995], but within the range of 3–300 nmol/ mol later reported for different fuel types and fire conditions [Rudolph et al., 2000]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D04305, doi:10.1029/2008JD009895, 2008