Chemical characteristics and causes of airborne particulate pollution in warm seasons in Wuhan, central China

Continuous measurements of airborne particles and their chemical compositions were conducted in May, June, October, and November 2014 at an urban site in Wuhan, central China. The results indicate that particle concentrations remained at a relatively high level in Wuhan, with averages of 135.1±4.4 (mean±95% confidence interval) and 118.9±3.7μg m^-3 for PM10 and 81.2±2.6 and 85.3±2.6μg m^-3 for PM_2.5 in summer and autumn, respectively. Moreover, PM_2.5 levels frequently exceeded the National Standard Level II (i.e., daily average of 75μg m^-3), and six PM_2.5 episodes (i.e., daily PM_2.5 averages above 75μg m^-3 for 3 or more consecutive days) were captured during the sampling campaign. Potassium was the most abundant element in PM_2.5, with an average concentration of 2060.7±82.3ng m^-3; this finding indicates intensive biomass burning in and around Wuhan during the study period, because almost no correlation was found between potassium and mineral elements (iron and calcium). The source apportionment results confirm that biomass burning was the main cause of episodes 1, 3, and 4, with contributions to PM_2.5 of 46.6%±3.0%, 50.8%±1.2%, and 44.8%±2.6%, respectively, whereas fugitive dust was the leading factor in episode 2. Episodes 5 and 6 resulted mainly from increases in vehicular emissions and secondary inorganic aerosols, and the mass and proportion of NO₃^- both peaked during episode 6. The high levels of NO_x and NH₃ and the low temperature during episode 6 were responsible for the increase of NO₃^-. Moreover, the formation of secondary organic carbon was found to be dominated by aromatics and isoprene in autumn, and the contribution of aromatics to secondary organic carbon increased during the episodes.