Short-term variations of atmospheric CO2 and dominant causes in summer and winter: Analysis of 14-year continuous observational data at Waliguan, China

Using a 14-year revised dataset of atmospheric CO2 mixing ratios continuously measured at Mount Waliguan Baseline Observatory of western China during 1995–2008, the short-term variations of ambient CO2 and the dominant causes were studied. A comparison and evaluation of the revised and original data is included. Ambient CO2 in summer was usually elevated during nighttime and declined in daytime with peak-to-peak diurnal amplitude of 2.4 ± 0.3 ppm (year-to-year fluctuations). In winter, there was often a small increase during daytime with amplitude of 0.8 ± 0.2 ppm. Analysis of the local horizontal winds shows that the diurnal cycles of ambient CO2 are the combined result of changes of nearby sources/sinks and local meteorological circulation. Only in winter do we find a statistically increase trends of 0.03 ppm yr−1 for CO2 diurnal peak-to-peak amplitudes, reflecting local changes in sources and sinks of CO2 during 1995–2008 under relatively consistent meteorological conditions. In summer, atmospheric CO2 were depleted when prevailing winds came from the populated northeastern regions due to the enhanced vegetation photosynthesis which will result in low CO2; on the contrary to summer, elevated CO2 were closely associated with air parcels from populated north/northeastern or northwestern regions in winter, when the terrestrial exchanges become weak and anthropogenic emissions dominate ambient CO2. ΔCO2 and ΔCO showed significant positive correlation (r > 0.9, p < 0.01) in winter, reflecting common sources under certain synoptic meteorological conditions. But they didn't exhibit any correlation in summer. The intercepts for the linear fit of ΔCO2 and ΔCO were close to zero in winter, indicating less influence from non-CO related CO2 sources or sinks (e.g. respiration and photosynthesis), whereas the intercepts in summer were much more negative (as low as −3.7 ppm in July), reflecting enhanced biospheric CO2 uptake. The ratio of ΔCO2/ΔCO was 25–35 ppm ppm−1 in winter, which is approximately 30%–42% higher than that observed in Beijing and derived from emission inventories, reflecting less impact from human activities due to the remote location of Waliguan.