From clean air to severe haze: evolution of PM₂.₅ and water-soluble ions during a firework-influenced pollution episode

Abstract

A severe air pollution episode occurred in Huangshi City, central China, during February 2024, coinciding with intensive Spring Festival fireworks. To investigate the chemical processes, PM₂.₅ mass and water-soluble inorganic ions (WSIIs) were analyzed in conjunction with ionic balance, correlation, and backward trajectory models. The episode exhibited three distinct phases. In the pre-pollution stage (February 5–8), PM₂.₅ remained stable around 60 µg/m³, with secondary inorganic aerosols (NO₃⁻, SO₄²⁻, NH₄⁺) as the major components under humid and stagnant conditions conducive to secondary aerosol formation. During the pollution peak (February 9–10), concentrations approached 800 µg/m³ due to firework emissions and stagnant conditions. Ion composition shifted markedly, with sharp increases in K⁺, Cl⁻, and Mg²⁺, concurrent decreases in NO₃⁻ and NH₄⁺, and a low neutralization ratio, indicating strongly acidic aerosols dominated by fireworks-derived sulfate. Ozone depletion further suppressed photochemistry and secondary aerosol production. In the post-pollution stage (February 11–14), improved dispersion and reduced emissions lowered PM₂.₅ to background levels, while NO₃⁻ and NH₄⁺ rebounded and dust-related ions (Ca²⁺, Mg²⁺) increased. Backward trajectory clustering revealed that northern transport contributed during the clean phase, local stagnation dominated the pollution peak, and mixed inflows supported atmospheric cleansing thereafter. These findings demonstrate that episodic fireworks can significantly reshape aerosol composition and acidity, with meteorological conditions determining the severity of pollution episodes.