Seasonal and Spatial Variations of Carbon Fractions in PM2.5 in Ningbo and the Estimation of Secondary Organic Carbon

To investigate the seasonal and spatial variations of carbon fractions in PM2.5 in Ningbo, PM2.5 samples were collected at 5 sites in typical periods of 4 seasons from December 2012 to October 2013. The concentrations of organic carbon (OC) and elemental carbon (EC) were determined and the contribution of secondary organic carbon (SOC) was estimated. The result shows that: (1)the annual average of PM2.5 in Ningbo is 51. 6 µm.m-3. OC and EC account for 17% and 6% respectively. According to the result of the backward trajectory model, the concentrations of PM2.5 in winter and spring are higher mainly because of regional transport from inland China. (2) the OC/EC ratio and the correlation between OC and EC indicate that in summer a large sum of SOC is generated while in winter the influence of coal burning emission for heating in North China may be the main reason for high OC/EC ratio. (3) the contribution of SOC was estimated using an EC tracer method. The result shows that this method is not suitable for both winter and spring because of significant regional transport. The average concentrations of SOC in summer and autumn are 2. 5 µg.m-3 and 2. 3 µg.m-3, accounting for 42% and 28% of total OC respectively.