[Distribution of CO at Lin'an station in Zhejiang Province]

Background CO mole fractions were continuously measured at Lian'an background station in Zhejiang province from September, 2010 to February, 2012 using Cavity Ring Down Spectroscopy (CRDS) system. The diurnal variation of CO was strongly influenced by anthropogenic activities with two peaks occurring at 07:00-10:00 and 19:00-20:00 (local time). The average daily mole fraction and amplitude in summer were the lowest among four seasons with values of 314.3 x 10(-9) +/- 7.6 x 10(-9) (mole fraction, the same below) and 50.1 x 10(-9) +/- 47.9 x 10(-9), respectively. The seasonal variations displayed peak values during winter-spring period and valley in summer, which roughly consisted with those observed variations at other sites located at northern hemisphere such as Jungfraujoch in Switzerland and Waliguan in China. However, the average mole fractions were much higher than those from other stations. The amplitude of monthly CO mole fractions was 286.8 x 10(-9) +/- 19.2 x 10(-9). The cluster analysis of backward trajectories and surface wind influence might suggest that the non-background CO mole fractions at Lin'an station were mainly affected by the emissions from the megacities and industrial area on the N-NNE-ENE sectors. The maximum enhancements in spring, summer and winter all occurred on ENE sector, with a maximum value of 106.3 x 10(-9) +/- 58.0 x 10(-9) in winter.