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. Despite the high abundance of secondary aerosols in theatmosphere, their formation mechanisms remain poorly understood. In thisstudy, the MasterChemical Mechanism (MCM) and the Chemical Aqueous-Phase Radical Mechanism (CAPRAM) are used to investigate the multiphase formationand processing of secondary aerosol constituents during the advection of airmasses towards the measurement site of Mt. Tai in northern China. Trajectorieswith and without chemical–cloud interaction are modeled. Modeled radical andnon-radical concentrations demonstrate that the summit of Mt. Tai, with analtitude of ∼1.5  km a.m.s.l., is characterized by a suburbanoxidants budget. The modeled maximum gas-phase concentrations of the OH radicalare 3.2×106 and 3.5×106  molec. cm −3 in simulations with and without cloud passages in theair parcel, respectively. In contrast with previous studies at Mt. Tai, thisstudy has modeled chemical formation processes of secondary aerosolconstituents under day vs. night and cloud vs. non-cloud cases along thetrajectories towards Mt. Tai in detail. The model studies show that sulfate ismainly produced in simulations where the air parcel is influenced by cloudchemistry. Under the simulated conditions, the aqueous reaction of HSO 3 - with H2O2 is the major contributor to sulfateformation, contributing 67 % and 60 % in the simulations with cloudand non-cloud passages, respectively. The modeled nitrate formation ishigher at nighttime than during daytime. The major pathway is aqueous-phase N2O5 hydrolysis, with a contribution of 72 % when cloudpassages are considered and 70 % when they are not. Secondary organic aerosol(SOA) compounds, e.g., glyoxylic, oxalic, pyruvic and malonic acid, are foundto be mostly produced from the aqueous oxidations of hydrated glyoxal,hydrated glyoxylic acid, nitro-2-oxopropanoate and hydrated 3-oxopropanoicacid, respectively. Sensitivity studies reveal that gaseous volatile organic compound (VOC) emissionshave a huge impact on the concentrations of modeled secondary aerosolcompounds. Increasing the VOC emissions by a factor of 2 leads to linearlyincreased concentrations of the corresponding SOA compounds. Studies usingthe relative incremental reactivity (RIR) method have identified isoprene,1,3-butadiene and toluene as the key precursors for glyoxylic and oxalicacid, but only isoprene is found to be a key precursor for pyruvic acid.Additionally, the model investigations demonstrate that an increased aerosolpartitioning of glyoxal can play an important role in the aqueous-phaseformation of glyoxylic and oxalic acid. Overall, the present study is thefirst that provides more detailed insights in the formation pathways ofsecondary aerosol constituents at Mt. Tai and clearly emphasizes theimportance of aqueous-phase chemical processes on the production ofmultifunctional carboxylic acids.

Multiphase MCM–CAPRAM modeling of the formation and processing of secondary aerosol constituents observed during the Mt. Tai summer campaign in 2014