Water-soluble
organic carbon (WSOC), which is closely related to
biogenic emissions, is of great importance in the atmosphere for its
ubiquitous existence and rich abundance. Levoglucosan, a typical WSOC,
is usually considered to be stable and thus used as a tracer of biomass
burning. However, we found that levoglucosan can be photo-oxidized
on mineral dust, with formic acid, oxalic acid, glyoxylic acid, 2,3-dioxopropanoic
acid, dicarbonic acid, performic acid, mesoxalaldehyde, 2-hydroxymalonaldehyde,
carbonic formic anhydride, and 1,3-dioxolane-2,4-dione detected as
main products. Further, we observed the heterogeneous uptake of NH3 promoted by the carboxylic acids stemming from the photocatalytic
oxidation (PCO) of levoglucosan. The mineral-dust-initiated PCO of
levoglucosan and enhanced heterogeneous uptake of NH3,
which are highly influenced by irradiation and moisture conditions,
were for the first time revealed. The reaction mechanisms and pathways
were studied in detail by diffuse reflection infrared Fourier transform
spectroscopy (DRIFTS), high-pressure photon ionization time-of-flight
mass spectrometry (HPPI-ToF-MS) and flow reactor systems. Diverse
WSOC constituents were studied as well, and the reactivity toward
NH3 is related to the number of hydroxyl groups of the
WSOC molecules. This work reveals a new precursor of secondary organic
aerosols and provides experimental evidence of the existence of organic
ammonium salts in atmospheric particles.