Elaborations of the influencing factors on the formation of secondary inorganic aerosols in a heavily polluted urban area of China

In this study, online water-soluble inorganic ions were detected to deduce the formation mechanism of secondary inorganic aerosol in Xianyang, China during wintertime. The dominant inorganic ions of sulfate (SO₄²⁻), nitrate (NO₃⁻), and ammonium (NH₄⁺) (the sum of those is abbreviated as SNA) accounted for 17%, 21%, and 12% of PM_2.5 mass, respectively. While the air quality deteriorated from excellent to poor grades, the precursor gas sulfur dioxide (SO₂) of SO₄^2- increased and then decreased with a fluctuation, while nitrogen dioxide (NO₂) and ammonia (NH₃), precursors of NO₃⁻ and NH₄⁺, and SNA show increasing trends. Meteorological factors including boundary layer height (BLH), temperature, and wind speed also show decline trends, except relative humidity (RH). Meanwhile, the secondary conversion ratio shows a remarkable increasing trend, indicating that there was a strong secondary transformation. From the perspective of chemical mechanisms, RH is positively correlated with sulfur oxidation ratios (SOR), nitrogen oxidation ratios (NOR), and ammonia conversion ratios, representing that the increase of humidity could promote the generation of SNA. Notably, SOR and NOR were also positively related to the ammonia. On the one hand, the low wind speed and BLH led to the accumulation of pollutants. On the other hand, the increases of RH and ammonia promoted more formations of SNA and PM_2.5. The results advance our identification of the contributors to the haze episodes and assist to establish more efficient emission controls in Xianyang, in addition to other cities with similar emission and geographical characteristics.