Year round measurements of O3 and CO at a rural site near Beijing: variations in their correlations

We examine seasonal variations of carbon monoxide (CO), ozone (O₃), and their relationships observed over the course of 3 yr (2005–2007) at Miyun, a rural site 100 km north of Beijing. Monthly mean afternoon mixing ratios of CO have broad maxima in winter and a secondary peak in June. Monthly mean afternoon O₃ shows a clear seasonal pattern with a major peak in June (85 ppb), a secondary peak in September (65 ppb) and minimum in winter (50–55 ppb). The seasonal cycles of O₃ and CO are associated with seasonal changes in dominant synoptic pattern. Substantial interannual variability is found for CO which is attributed to the interannual variability of meteorology and emissions from biomass burning. The seasonality and magnitude of background CO and O₃ derived at Miyun are consistent with observations at upwind remote continental sites. The O₃–CO correlation slope is about 0.07 ppb ppb⁻¹ on average in summer, significantly lower than the typical slope of 0.3 ppb ppb⁻¹ reported for developed countries. The O₃–CO correlation slope shows large gradients for different types of air masses (0.133 ± 0.017 ppb ppb⁻¹ in aged urban pollution plumes and 0.047 ± 0.008 ppb ppb⁻¹ in biomass burning plumes), suggesting that the conventional method of direct scaling the mean O₃–CO slope by CO emissions to deduce O₃ production rate is subject to large uncertainties if applied for China.