Photochemistry of toluene under high NOx, NH3 and humid conditions in the presence of inorganic seed particles: a smog chamber study for urban haze formation at East Asia

High–concentration particulate matter (PM) at East Asia threatens human health and potentially alters climate. High levels of SO₂, NO_x and NH₃ emissions attribute the formation of inorganics including sulfates, nitrates and ammoniums in PM. Consequently, PM contains a large fraction of these inorganics, and aerosol liquid water (ALW) is considered to promote inorganic PM formation. A thermodynamic model has been used to estimate inorganic concentrations and a pH of PM because PM can be viewed as an ammonium-sulfate-nitrate-water system, which maintains thermodynamic equilibriums between the gas and particle phase and in the aqueous phase within particles. However, gas–particle partitioning of semivolatile inorganic species (i.e., NH₃–NH₄ ⁺ and HNO₃–NO₃^-) particularly influenced by organics and aqueous-phase secondary organic aerosol (aqSOA) formation in PM through multiphase chemistry are not well understood.

We conducted smog chamber experiments for OH-radical initiated reactions of toluene in the presence of ammonium sulfate seed particles under high NO_x, NH₃ and humid conditions, which were similar to high-concentration haze conditions at Seoul, Korea. Measurements of inorganic concentrations in particles agree well with outputs of thermodynamic model simulations. The nitrate increase in seed particles is most prominent because ALW enhances the uptake of total HNO₃ photochemically formed from NO_x. We identified methylglyoxal as a precursor for aqSOA formation. It appears that organics attribute ALW formation under deliquescence relative humidity for inorganic salts. We further investigated the response of particle mass concentrations to various NO_x concentrations, which can be useful for NO_x controls for PM reduction.