Polar Nitrated Aromatic Compounds in Urban Fine Particulate Matter: A Focus on Formation via an Aqueous-Phase Radical Mechanism

Polar nitrated aromatic compounds (pNACs) are key ambient
brown
carbon chromophores; however, their formation mechanisms, especially
in the aqueous phase, remain unclear. We developed an advanced technique
for pNACs and measured 1764 compounds in atmospheric fine particulate
matter sampled in urban Beijing, China. Molecular formulas were derived
for 433 compounds, of which 17 were confirmed using reference standards.
Potential novel species with up to four aromatic rings and a maximum
of five functional groups were found. Higher concentrations were detected
in the heating season, with a median of 82.6 ng m–3 for Σ17pNACs. Non-negative matrix factorization
analysis indicated that primary emissions particularly coal combustion
were dominant in the heating season. While in the non-heating season,
aqueous-phase nitration could generate abundant pNACs with the carboxyl
group, which was confirmed by their significant association with the
aerosol liquid water content. Aqueous-phase formation of 3- and 5-nitrosalicylic
acids instead of their isomer of 4-hydroxy-3-nitrobenzoic acid suggests
the existence of an intermediate where the intramolecular hydrogen
bond favors kinetics-controlled NO2• nitration.
This study provides not only a promising technique for the pNAC measurement
but also evidence for their atmospheric aqueous-phase formation, facilitating
further evaluation of pNACs’ climatic effects.