CO and CO₂ isotopic composition in Spitsbergen during the 1995 ARCTOC campaign

Abstract

A set of concentration and isotope data for CO and CO₂ was obtained during the 1995 spring ARCTOC campaign in Spitsbergen. CO mixing ratios decline from 170 nmole/mole in mid April to 120 nmole/mole at the end of the 6-week period, indicating clean air conditions. CO stable isotope data show unprecedented enrichment in 13C increasing with time from —25.7‰ to —23.4‰ vs. V-PDB, whereas δ¹⁸O decreases from 8.5‰ to 2.5‰ versus V-SMOW, being higher than SH values as expected. Much of the observed changes in the isotope ratios can be explained assuming an OH sink with the kinetic isotope effect in the sink reaction governing δ¹³C and δ¹⁸O seasonality. Non-combustion sources contribute most to the total source strength in Arctic spring, but particularly oxidation of methane is only of minor importance in the NH. ¹⁴CO values, which decrease from 22 to 16 molecules/cm³ STP, show the expected seasonal decline due to increasing destruction by OH. The values are higher than SH data and agree well with the first NH ¹⁴CO data obtained by Volz in 1976–78. It is shown that during a tropospheric low ozone event, a shift in δ¹³C of CO should be induced by the reaction CH₄ + Cl. This effect is too small to be detected with the present measurements, but it would help to explain small remaining differences between expected and measured δ¹³C values. CO₂ isotope data reveal an excellent agreement with the NOAA/CMDL scale. With the onset of the growing season mixing ratios slightly decrease towards the end of our sampling period, anticorrelated to δ¹³C(CO₂) which increases, while δ¹⁸O seems to pass its seasonal maximum in late May.