Characteristics Of Water Soluble Inorganic Ions And Metal Elements In PM_2.5 At A Roadside In Wuhan

In recent years, haze occurred frequently in Chinese cities. Therefore, more and more attentions have been paid to ambient particulate matter（PM） pollution. In the newly issued ambient air quality standard（GB3095-2012）, fine particle(PM_2.5) was included as a criteria pollutant for the first time in China. As a megacity with population of over 10 million, air quality in Wuhan is not satisfying due to the effect of industrial emission, vehicle exhaust, coal burning emission, and dust. However, research on PM_2.5 in Wuhan is relatively scarce, especially for the micro-environment in the roadside. Therefore, PM_2.5 sampling was conducted in a curbside site in Wuhan for a period of around 1 year. PM_2.5 mass, water soluble inorganic ions, and selected metal elements were determined and their seasonal variations were addressed. Following results are obtained:1. The average mass concentration of PM_2.5 was 118.1±70.3μg/m3 during the sampling period, exceeding the PM_2.5 guidelines of 75μg/m3, and the occurrence rate of exceeding was about 67.6%. The seasonal variation of PM_2.5 concentrations followed the sequences of winter（130.3±50.4μg/m3）, summer（128.92±94.8μg/m3）, spring（128.87±72.6μg/m3）, autumn（90.26±37.6μg/m3）.2. The hourly variation of PM_2.5 concentration showed two peaks, occurring from 7:00 to 8:00 and from 18:00 to 19:00. Through the correlation analysis, PM_2.5 concentrations showed significant positive correlation with humidity and temperature. Moreover, the variation trend of PM_2.5 concentrations were similar to the vehicle flow rates.3.Chemical component analysis showed that the average mass concentration of water soluble inorganic ions was 42.3±21.1μg/m3, accounting for more than 30% of PM_2.5 concentration, SO42-、NO3- and NH4+ concentrations shared 36.8%、29.4% and 19.9% of total ions, respectively. From the air quality classification, secondary inorganic ions increased gradually when air quality become worse. However, their shared proportion in PM_2.5 mass dropped down, indicating that other components rose dramatically in PM_2.5, being higher than the secondary ions. The seasonal variations of NO3- and SO42- concentration showed two peaks, and the concentration of NO3- in winter was highest with the minimum value of SO42- concentration in summer. Seasonal variation of source emissions and role of secondary reaction might be the cause. The mass ratio of NO3-/SO42-（0.89） indicated more influence of stationary sources than that of mobile sources on fine particles, and the contribution of mobile vehicles increased in winter. Through the correlation analysis, these three secondary ions probably showed good correlation and the main forms of these species were（NH4）2SO4 and NH4NO3. Ion balance calculations suggested a deficiency in anions for PM_2.5 at the roadside and PM_2.5 intended to be alkaline.4. Among the measured metal elements in this study, Fe, Zn, Pb were the most abundant elements. Enrich factor（EF） estimation demonstrated that Mn was mostly from the soil dust with EF less than 10. EFs of Zn, Ni and Pb were all above 100, indicating the important role of anthropogenic emission. On the other hand, Cu was affected both by the natural and anthropogenic sources, with EF value of 79.2.