Insight into winter haze formation mechanisms based on aerosol hygroscopicity and effective density measurements
Other literature type
We characterize a representative haze event from a series of periodic particulate matter (PM) episodes that occurred in Shanghai during winter 2014. Particle size distribution, hygroscopicity, and effective density were measured online, along with analysis of water-soluble inorganic ions and single particle mass spectrometry. Regardless of pollution level, the mass ratio of SNA/PM<sub>1.0</sub> (sulfate, nitrate, and ammonium) slightly fluctuated around 0.28 over the whole observation, suggesting that both secondary inorganic compounds and carbonaceous aerosols (including soot and organic matter) contributed substantially to the haze formation. Nitrate was the most abundant ionic species during hazy periods, indicating that NO<sub>x</sub> contributed more to haze formation in Shanghai than did SO<sub>2</sub>. The calculated PM concentration from particle size distribution displayed a variation pattern similar to that of measured PM1.0 during the representative PM episode, indicating that enhanced pollution level was attributable to the elevated number of larger particles. The number fraction of the near-hydrophobic group increased as the PM episode developed, indicating accumulation of local emissions. Three "banana-shape" particle evolutions were consistent with the rapid increase in PM<sub>1.0</sub> mass loading, indicating rapid size growth by condensation of condensable materials was responsible for the severe haze formation. Both hygroscopicity and effective density of the particles increased considerably with growing particle size during the banana-shaped evolutions, indicating that secondary transformation of NO<sub>x</sub> and SO<sub>2</sub> was a major contributor to the particle growth. Our results suggest that the accumulation of gas-phase and particulate pollutants under stagnant meteorological conditions and subsequent rapid particle growth by secondary processes, were primarily responsible for the haze pollution in Shanghai during wintertime.