Multiphase MCM–CAPRAM modeling of the formation and processing of secondary aerosol constituents observed during the Mt. Tai summer campaign in 2014
Authors: Zhu Yanhong , Tilgner Andreas , Hoffmann Erik Hans , Herrmann Hartmut , Kawamura Kimitaka , Yang Lingxiao , Xue Likun , Wang Wenxing 
Published: June 2020

Abstract. Despite the high abundance of secondary aerosols in the
atmosphere, their formation mechanisms remain poorly understood. In this
study, the Master
Chemical Mechanism (MCM) and the Chemical Aqueous-Phase Radical Mechanism (CAPRAM) are used to investigate the multiphase formation
and processing of secondary aerosol constituents during the advection of air
masses towards the measurement site of Mt. Tai in northern China. Trajectories
with and without chemical–cloud interaction are modeled. Modeled radical and
non-radical concentrations demonstrate that the summit of Mt. Tai, with an
altitude of ∼1.5 km a.m.s.l., is characterized by a suburban
oxidants budget. The modeled maximum gas-phase concentrations of the OH radical
are 3.2×106 and 3.5×106 molec. cm−3 in simulations with and without cloud passages in the
air parcel, respectively. In contrast with previous studies at Mt. Tai, this
study has modeled chemical formation processes of secondary aerosol
constituents under day vs. night and cloud vs. non-cloud cases along the
trajectories towards Mt. Tai in detail. The model studies show that sulfate is
mainly produced in simulations where the air parcel is influenced by cloud
chemistry. Under the simulated conditions, the aqueous reaction of
HSO3- with H2O2 is the major contributor to sulfate
formation, contributing 67 % and 60 % in the simulations with cloud
and non-cloud passages, respectively. The modeled nitrate formation is
higher at nighttime than during daytime. The major pathway is aqueous-phase
N2O5 hydrolysis, with a contribution of 72 % when cloud
passages are considered and 70 % when they are not. Secondary organic aerosol
(SOA) compounds, e.g., glyoxylic, oxalic, pyruvic and malonic acid, are found
to be mostly produced from the aqueous oxidations of hydrated glyoxal,
hydrated glyoxylic acid, nitro-2-oxopropanoate and hydrated 3-oxopropanoic
acid, respectively. Sensitivity studies reveal that gaseous volatile organic compound (VOC) emissions
have a huge impact on the concentrations of modeled secondary aerosol
compounds. Increasing the VOC emissions by a factor of 2 leads to linearly
increased concentrations of the corresponding SOA compounds. Studies using
the relative incremental reactivity (RIR) method have identified isoprene,
1,3-butadiene and toluene as the key precursors for glyoxylic and oxalic
acid, but only isoprene is found to be a key precursor for pyruvic acid.
Additionally, the model investigations demonstrate that an increased aerosol
partitioning of glyoxal can play an important role in the aqueous-phase
formation of glyoxylic and oxalic acid. Overall, the present study is the
first that provides more detailed insights in the formation pathways of
secondary aerosol constituents at Mt. Tai and clearly emphasizes the
importance of aqueous-phase chemical processes on the production of
multifunctional carboxylic acids.