CO₂ and its correlation with CO at a rural site near Beijing: implications for combustion efficiency in China

Abstract. Although China has surpassed the United States as the world's largest carbon dioxide emitter, in situ measurements of atmospheric CO₂ have been sparse in China. This paper analyzes hourly CO₂ and its correlation with CO at Miyun, a rural site near Beijing, over a period of 51 months (Dec 2004 through Feb 2009). The CO₂-CO correlation analysis evaluated separately for each hour of the day provides useful information with statistical significance even in the growing season. We found that the intercept, representing the initial condition imposed by global distribution of CO₂ with influence of photosynthesis and respiration, exhibits diurnal cycles differing by season. The background CO₂ (CO_2,b) derived from Miyun observations is comparable to CO₂ observed at a Mongolian background station to the northwest. Annual growth of overall mean CO₂ at Miyun is estimated at 2.7 ppm yr⁻¹ while that of CO_2,b is only 1.7 ppm yr⁻¹ similar to the mean growth rate at northern mid-latitude background stations. This suggests a relatively faster increase in the regional CO₂ sources in China than the global average, consistent with bottom-up studies of CO₂ emissions. For air masses with trajectories through the northern China boundary layer, mean winter CO₂/CO correlation slopes (dCO₂/dCO) increased by 2.8 ± 0.9 ppmv/ppmv or 11% from 2005–2006 to 2007–2008, with CO₂ increasing by 1.8 ppmv. The increase in dCO₂/dCO indicates improvement in overall combustion efficiency over northern China after winter 2007, attributed to pollution reduction measures associated with the 2008 Beijing Olympics. The observed CO₂/CO ratio at Miyun is 25% higher than the bottom-up CO₂/CO emission ratio, suggesting a contribution of respired CO₂ from urban residents as well as agricultural soils and livestock in the observations and uncertainty in the emission estimates.