Effects of ammonium sulfate on the hygroscopic properties and phase transition of aerosols with organic surrogates related to biomass burning and atmospheric oxidation

In this study, we investigated the hygroscopic properties of aerosols containing organics related to biomass burning and atmospheric oxidation mixed with (NH₄)₂SO₄ in various proportions during the hydration process using HTDMA. We extrapolated the morphology, phase transition, and potential chemical reactions based on the hygroscopic process of dried particles precipitated from the solution and findings from the literature. The levoglucosan-(NH₄)₂SO₄ mixed particles exhibited deliquescence might be due to an incompletely encapsulated microstructure by levoglucosan, while a continuous growth process with gradual levoglucosan content increase could be attributed to the formation of a core-shell structure. Newly generated substances filled the micropores of succinic acid aerosol after the addition of (NH₄)₂SO₄, but (NH₄)₂SO₄ and succinic acid existed separately after droplet drying when (NH₄)₂SO₄ was dominant. Experimental results of three-component aerosols with adjustable mass ratios reveal that hygroscopic properties and deliquescent relative humidity (DRH) significantly increased with (NH₄)₂SO₄ content. In contrast, the increase in succinic acid reduced particle hygroscopicity at lower RH while having minimal effect on DRH. Furthermore, deliquescence was suppressed by forming a core-shell structure with levoglucosan increase, leading to continuous aerosol diameter increase. Overall, this study provides insights into the effects of (NH₄)₂SO₄ on modulating the hygroscopic properties of typical organic surrogates related to biomass burning and atmospheric oxidation.