Air humidity affects secondary aerosol formation in different pathways

Highlights: • Impact of RH and water vapor content on nitrate and sulfate formation was investigated. • Air humidity affects nitrate formation by both gaseous oxidation and aqueous reaction. • Lower temperature and higher RH more facilitate SO₄²⁻ formation. Haze pollution characteristics and PM_2.5 chemical composition were distinctive in different air humidity-dependent haze episodes in winter of North China Plain (NCP). The impact of air humidity on particulate chemical composition was investigated based on the in situ observation in winter of 2017–2018 in Tianjin. Relative humidity (RH) and absolute humidity affect the secondary aerosol generation in different ways. Particularly, nitrate changes more obviously with absolute humidity, while sulfate changes more obviously with RH. In the daytime, at certain conditions, high water vapor content, O₃ concentration and stronger solar radiation may promote the gas-phase oxidation of NO_x by the addition of OH formed though O₃ photolysis, especially during the transition periods between winter and autumn or spring. Whereas in the nighttime, temperature drop generated the high RH, which was favorable for the gas-particle portioning of HNO₃ and the occurrence of the N₂O₅ heterogeneous hydrolysis reaction. At lower temperature and higher RH (T < 0 °C, RH > 80%) condition, SO₄²⁻ mass fraction was relatively higher. Lower temperature can result in more SO₂ dissolved in equilibrium and the relatively higher initial aerosol pH, which both generate faster aqueous oxidation rate. Given the currently low SO₂ concentration in the regional scale, the meteorological condition in which the occurrence of sulfate formation through aqueous reaction may be more stringent.