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

Abstract Using a 14-year revised dataset of atmospheric CO 2 mixing ratios continuously measured at Mount Waliguan Baseline Observatory of western China during 1995–2008, the short-term variations of ambient CO 2 and the dominant causes were studied. A comparison and evaluation of the revised and original data is included. Ambient CO 2 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 CO 2 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 CO 2 diurnal peak-to-peak amplitudes, reflecting local changes in sources and sinks of CO 2 during 1995–2008 under relatively consistent meteorological conditions. In summer, atmospheric CO 2 were depleted when prevailing winds came from the populated northeastern regions due to the enhanced vegetation photosynthesis which will result in low CO 2 ; on the contrary to summer, elevated CO 2 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 CO 2 . ΔCO 2 and ΔCO showed significant positive correlation ( r  > 0.9, p 2 and ΔCO were close to zero in winter, indicating less influence from non-CO related CO 2 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 CO 2 uptake. The ratio of ΔCO 2 /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.