Nitro‐aromatic compounds (NACs) measured in ambient aerosol are important contributors to brown carbon. Aromatic compounds are expected to be vital precursors of NACs in the presence of NOx in urban atmospheres. NACs of secondary organic aerosol (SOA) formed from photooxidation of p‐xylene with a variety of NO2 in smog chamber were investigated in this study. NACs were on‐line measured by aerosol laser time‐of‐flight mass spectrometer and verified by off‐line of ultraviolet absorption and electrospray ionization mass spectra. The results show that NO2 can promote the formation of p‐xylene SOA. Different from the carbonyls detected in p‐xylene SOA without NO2, nitrophenols, nitrocatechols, and other NACs via the nitration of phenolic products of p‐xylene are detected as the major components of SOA with NO2. The increase of electrophilic nitro groups in SOA strengthens absorption ability. The mass absorption coefficient averaged over 200–600 nm () of SOA gradually increased with the increasing of the concentration of NO2. The of SOA with 1,000 ppb NO2 was comparable to that of biomass burning organic aerosols. These provided experimental basis for studying the formation mechanism and optics of anthropogenic NACs in the background of high concentration of NOx in Chinese urban atmosphere.
Nitrated Polycyclic aromatic hydrocarbons (NPAHs) widely exist in atmospheric aerosol particles are important constituents of brown carbon. Naphthalene and other polycyclic aromatic hydrocarbons (PAHs) are vital precursors of NPAHs. The NPAHs components of secondary organic aerosol (SOA) generated by the photooxidation of naphthalene under different concentrations of NO2 in chamber are on-line measured using synchrotron radiation vacuum ultraviolet photoionization aerosol mass spectrometer (VUV-PIMS) and verified off-line with optical spectrometry in this study. The results demonstrate that NO2 can promote the generation of NPAHs and formation of SOA. Using VUV-PIMS on-line detection, and combined with off-line UV-Vis and infrared spectroscopy verification, it is determined that carboxyls and naphthols are main components of naphthalene SOA particles without NO2, and nitro-naphthalene, dinitro-naphthalene, nitro-naphthol and dinitro-naphthol via the nitration of OH-naphthalene adduct and naphthol products of naphthalene are identified as principal constituents of SOA with NO2. The increase of strong chromophore of N = O of nitro group in NPAHs strengthens the absorbing ability of naphthalene SOA particles. The averaged mass absorption coefficient (< MAC>) over 200–600 nm of naphthalene SOA particles is increases gradually with the increasing concentration of NO2. The < MAC > of naphthalene SOA particles with 1000 ppb NO2 is 688 cm2/g, which is close to to that of biomass burning organic aerosol particles. These results offer the reference for exploring the formation mechanism and optical properties of NPAHs under the background of high concentration of NOx in urban atmosphere.
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