Chemical compositions and source apportionment of PM2.5 during clear and hazy days: Seasonal changes and impacts of Youth Olympic Games

Seasonal changes in chemical compositions and source apportionment of PM2.5 during clear and hazy days help to develop effective control policy, but limited information is available in megacity Nanjing. In this study, 102 PM2.5 samples were collected during clear and hazy days from 4 seasons in 2014-2015. Their chemical compositions (organic and elemental carbon, 8 water-soluble ions, and 22 inorganic elements) were determined, which were used for PM2.5 source apportionment using the PMF model. The mean PM2.5 concentration was lower during clear days than hazy days (42 vs. 122 μg m-3), so were mean concentrations of metals (0.48 vs. 0.82 ng m-3 for Co and 2.0 vs. 2.4 μg m-3 for Na), water soluble ions (0.10 vs. 0.16 μg m-3 for Mg2+ and 12 vs. 23 μg m-3 for SO42-), and carbon species (3.2 vs. 5.4 μg m-3 for elemetal C and 20 vs. 35 μg m-3 for organic C). Based on the PMF model, five main sources of PM2.5 were identified including secondary aerosols (31%), coal combustion (27%), road & construction dust (26%), oil combustion (8.5%), and iron & steel industry (5.1%) for all samples. The PM2.5 concentrations from the 5 sources were 0.01-46.5, averaging 9.8 μg m-3 during clear days (PM2.5 < 75 μg m-3), which increased to 1.83-60.1, averaging 18 μg m-3 during hazy days. However, based on their contributions to PM2.5, only secondary aerosols increased during hazy days compared to clear days in all seasons (11 vs. 42%), indicating its dominant contribution to haze in Nanjing. For different seasons, road & construction dust was a major contributor to PM2.5 in the summer, while oil combustion (4.86 vs.16.8%) contributed more in spring. However, coal combustion became the main source of PM2.5 during the summer (44-85%) due to the pollution controls for the Youth Olympic Games. Our results suggest that secondary aerosols play an important role in haze formation and season-dependent pollution measures should be implemented for effective control of air pollution.