Day-Night Differences and Source Apportionment of Inorganic Components of PM2.5 During Summer-Winter in Changzhou City

Abstract

To investigate the day-night variation characteristics of inorganic components in atmospheric aerosol, PM2.5 samples were continuously collected for one month in Changzhou during summer and winter. Eleven water-soluble ions (WSⅡs) and 13 metal elements were compared in terms of their day-night character and sources. The results indicated that the day time average PM2.5 mass concentration was higher than the night time, while the percentage of the total WSⅡs in PM2.5 during the night was higher. The total WSⅡs fractions in PM2.5 were higher in winter (44%-45%) compared to summer (31%-36%), with an opposite seasonal character for metal elements (winter day 3.03%, winter night 2.29%, summer day 4.40%, summer night 4.51%). SO42-, NO3-, and NH4+, were the three main secondary ions, comprising 77%-85% of the total WSⅡs, suggesting that air pollution in Changzhou exhibits complex pollution characteristics dominated by secondary processes. The day time SO42-/WSⅡs ratio (49.0%) was slightly higher than that of the night (41.1%) due to the photochemical reaction under stronger solar radiation, while the lower NO3-(1.98 μg·m-3) in the day compared to the night (5.10 μg·m-3) was attributed to the decomposition of NH4NO3 during summer days. A good linear correlation among NH4+, SO42- and NO3-, accompanied by a ratio of predicted NH4+ to measured NH4+ near 1, illustrated that NH4+ ions mainly existed in the form of (NH4)2SO4, NH4NO3, and NH4Cl. It was concluded from the ion balance that PM2.5 was weakly alkaline in summer but neutral in winter. Fe, Al, and Zn were the largest contributors to the total metal elements, with higher concentrations of Fe and Al and lower levels of Zn in the day time. A correlation coefficient analysis and principle component analysis revealed that inorganic components come from sources that include secondary aerosol formation, suspended dust, and vehicle emissions, but there is some seasonal variation and day-night differences.