Variations in PM_2.5, TSP, BC, and trace gases (NO₂, SO₂, and O₃) between haze and non-haze episodes in winter over Xi'an, China

To investigate chemical profiles and formation mechanisms of aerosol particles in winter haze events, daily PM_2.5 and TSP, 5-min BC, and 15-min trace gases (SO₂, NO₂, and O₃) were measured continuously during Dec. 1-31, 2012 in Xi'an. Chemical analysis was also conducted for nine water-soluble inorganic ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, F^-, Cl^-, NO₃^-, and SO₄^2-), organic carbon (OC), elemental carbon (EC), and eight carbon fractions (OC1, OC2, OC3, OC4, EC1, EC2, EC3, and OP) in both PM_2.5 and TSP samples. Higher levels of TSP, PM_2.5, BC, SO₂, and NO₂, and lower levels of O₃ were observed during haze periods in comparison with non-haze days. The sum of the major secondary ionic species (NH₄⁺, NO₃^-, and SO₄^2-) in PM_2.5 or TSP during haze periods was about 3 times of that during non-haze days. Ion balance calculations showed that PM_2.5 samples were acidic during haze periods and were close to neutral during non-haze days. The mean carbon levels were 52.9μgm^-3 and 82.1μgm^-3 in PM_2.5 and TSP, respectively, during haze events, which were ~1.5 times of those during non-haze days. The diurnal variations of BC during non-haze days showed a bimodal distribution with two peaks coincided with traffic rush hours. This was not the case during haze periods, which exhibited a relatively smooth pattern but with high concentration levels, providing evidence of particle accumulation. The ratios of SO₄^2-/EC, NO₃^-/EC, and NH₄⁺/EC sharply increased during haze periods, indicating the important pathway of secondary inorganic species formation through aqueous-phase transformation under high relative humidity condition. This study also highlights that wintertime secondary organic carbon (SOC) formation can be an important contributor to carbonaceous aerosol, especially during haze periods.