Identification of chemical compositions and sources of atmospheric aerosols in Xi'an, inland China during two types of haze events

High time resolution (1 h) of TSP filter samples was collected in Xi'an in inland China from December 5 to 13, 2012, during which a 9-day long of haze episode occurred. The hazy days were classified as two types, i.e., Light-haze period with moderate degradation in visibility (5-10 km) and relatively dry conditions (RH: 53 ± 19%) and Severe-haze period with a daily visibility less than 5 km and humid conditions (RH: 73 ± 14%). TSP in the two periods (415 ± 205 and 530 ± 180 μg m^-3 in Light-haze and Severe-haze periods, respectively) was comparable, but crustal Fe and Ca elements presented higher concentrations and strong correlation (R² = 0.72) with TSP in Light-haze period. SO₄^2-, NO₃^- and NH₄⁺ in Light-haze period were 16 ± 5.9, 12 ± 6.7 and 4.1 ± 2.8 μg m^-3, respectively, and increased dramatically to 51 ± 15, 44 ± 9.7 and 23 ± 5.6 μg m^-3 in Severe-haze period. Contributions of Fe and Ca to TSP decreased from 9.2% in Light-haze period to 5.3% in Severe-haze period, but those of SO₄^2-, NO₃^- and NH₄⁺ increased from 3.8%, 2.9% and 1.0% in Light-haze period to 9.6%, 8.3% and 4.4% in Severe-haze period, respectively. These results suggest that dust-derived particles were more significant in Light-haze period while secondary aerosols were more important in Severe-haze period. Hopanes (33 ± 24 and 38 ± 29 ng m^-3 in Light-haze and Severe-haze periods, respectively) during the two types of haze periods are comparable, indicating that differences in contribution of primary organic aerosols from fossil fuel combustions to TSP were insignificant. In contrast, the ratio of secondary organic aerosols (e.g., o-phthalic acid) to EC was much higher in Severe-haze period (5.8 ± 2.7 ng μg^-1) than in Light-haze period (3.4 ± 2.1 ng μg^-1), probably indicating that the humid conditions in Severe-haze period are favorable for secondary organic aerosol formation.